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SPOKANE REGIONAL STORMWATER MANUAL
APRII,2008
Prepared by:
Spokane County
1026 W. Broadway Avenue
Spokane, Washington 99260
(509) 477-3600
City of Spokane
808 W. Spokane Falls Blvd.
Spokane, Washington 99201
(509) 625-6700
City of Spokane Valley
11707 E. Sprague Avenue
Spokane Valley, Washington 99206
(509) 921-1000
A special thanks to the members of the committee who dedicated their time and energy in
the development of the manual:
Gloria Mantz, City of Spokane Valley
Kris Becker, City of Spokane
Gary Nelson, Ciry of Spokane
Mike Yake, Ciry of Spokane
Colleen Little, Spokane Counry
Matt Zarecor, Spokane County
Spokane County, City of Spokane, City of Spokane Valley
Spokane Regional Stormwater Manual
TAB LE OF CONTE NTS
Title Page No.
Tableof Contents.............................................................................................................................i
Listof Figures..............................................................................................................................viii
Listof Tables .................................................................................................................................ix
Glossary.......................................................................................................................................... x
References................................................................................................................................... xxv
Chapter1. Introduction................................................................................................................ 1-1
1.1 Objective And Purpose................................................................................................. 1-1
1.2 Vision Statement........................................................................................................... 1-1
1.3 Using The Stormwater Manual..................................................................................... 1-1
1.3.1 TechnicalInformation.......................................................................................... 1-2
1.32 Policy Information............................................................................................... 1-3
1.4 Relevant Programs and Concepts ................................................................................. 1-3
1.4.1 Underground Injection Control Program Compliance......................................... 1-4
1.42 Spokane Valley-Rathdrum Prairie Sole-Source Aquifer..................................... 1-4
1.43 National Pollutant Discharge Elimination System Compliance.......................... 1-5
1.4.4 Pollutant Generating Impervious Surface............................................................ 1-6
1.5 General Requirements................................................................................................... 1-6
1.6 Standard of Care ........................................................................................................... 1-7
1.7 Severability................................................................................................................... 1-8
Chapter 2. Basic Requirements....................................................................................................2-1
2.1 Introduction...................................................................................................................2-1
2.1.1 Regulatory Threshold...........................................................................................2-1
2.12 New Development ...............................................................................................2-1
2.1.3 Redevelopment ....................................................................................................2-2
2.1.4 Exemptions ..........................................................................................................2-3
2.1.5 Generally Exempt................................................................................................2-4
2.1.6 Design Deviations................................................................................................2-5
2.2 Basic Requirements ......................................................................................................2-6
2.2.1 Basic Requirement No. 1 —Drainage Submittal..................................................2-6
2.22 Basic Requirement No. 2—Geotechnical Site Characterization .........................2-9
2.23 Basic Requirement No. 3 —Water Quality Treatment.......................................2-10
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2.2.4 Basic Requirement No. 4 —Flow Control..........................................................2-14
2.2.5 Basic Requirement No. 5 —Natural And Constructed Conveyance Systems....2-16
2.2.6 Basic Requirement No. 6 —Erosion And Sediment Control .............................2-18
2.2.7 Basic Requirement No. 7 — Source Control.......................................................2-19
2.2.8 Basic Requirement No. 8 —Operation And Maintenance..................................2-20
Chapter 3. Drainage Submittal.....................................................................................................3-1
3.1 Introducti on...................................................................................................................3-1
3.2 Applicability .................................................................................................................3-1
3.2.1 Required...............................................................................................................3-1
3.22 Generally Required.............................................................................................. 3-1
3.2.3 Generally Exempt................................................................................................ 3-2
3.2.4 Exempt.................................................................................................................3-3
3.3 Concept Drainage Report.............................................................................................. 3-4
3.3.1 In tro du c ti on..........................................................................................................3-4
3.32 Applicability ........................................................................................................3-4
3.3.3 Exemptions ..........................................................................................................3-5
3.3.4 Scope.................................................................................................................... 3-5
3.4 Drainage Report............................................................................................................ 3-7
3.4.1 Introducti on..........................................................................................................3-7
3.42 Narrative .............................................................................................................. 3-7
3.4.3 Figures.................................................................................................................. 3-9
3.4.4 Calculations........................................................................................................3-10
3.4.5 Down-Gradient Analysis ...................................................................................3-11
3.5 Road and Drainage Plans............................................................................................ 3-13
3.5.1 In tro du c ti on........................................................................................................ 3-13
3.52 Minimum Plan Elements.................................................................................... 3-13
3.53 Revisions After Plan Acceptance.......................................................................3-14
3.6 Other Submittal Elements........................................................................................... 3-14
Appendix 3A —Example Tables for Drainage Report...........................................................3-17
Appendix 3B —Standard Notes for Road and Drainage Plans .............................................. 3-19
Appendix 3C —Lot Plan Criteria and Example.....................................................................3-21
Chapter 4. Geotechnical Site Characterization............................................................................4-1
4.1 Introduction...................................................................................................................4-1
4.2 Applicability .................................................................................................................4-1
4.3 Geotechnical Site Characterization Report...................................................................4-2
4.3.1 Field and Laboratory Testing...............................................................................4-4
4.32 Sub-Level Structure Feasibility ...........................................................................4-6
Appendix 4A —Spokane 200 Method......................................................................................4-9
Appendix 4B —Full-Scale Drywell Test Method..................................................................4-13
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SPOKANE REGIONAL STORMWATER MANUAL
Appendix 4C —Test Pit Method............................................................................................4-17
Appendix 4D —Single-Ring Infiltrometer Test Method........................................................4-21
Appendix 4E—Swale Flood Test..........................................................................................4-23
Appendix 4F —Pond Flood Test............................................................................................4-25
Chapter 5. Hydrologic Analysis and Design................................................................................5-1
5.1 Introducti on...................................................................................................................5-1
5.2 Hydrologic Analysis Methods ...................................................................................... 5-1
5.3 Curve Number Method.................................................................................................5-2
5.3.1 Introducti on..........................................................................................................5-2
5.32 Curve Number Method Theory............................................................................5-2
5.3.3 Limitations........................................................................................................... 5-3
5.3.4 Design Steps.........................................................................................................5-3
5.4 Level Pool Routing Method........................................................................................ 5-19
5.5 Rational Method.......................................................................................................... 5-19
5.5.1 Runoff Coefficients............................................................................................ 5-20
5.52 Time of Concentration....................................................................................... 5-21
5.5.3 Intens ity.............................................................................................................. 5-21
5.6 Bowstring Method (Modified Rational Method)........................................................ 5-23
5.6.1 Design Steps....................................................................................................... 5-24
5.7 Water Budget Method................................................................................................. 5-25
5.7.1 Introducti on........................................................................................................5-25
5.72 Methodology......................................................................................................5-25
5.7.3 Design Steps.......................................................................................................5-27
5.7.4 Curve Number Adjustment................................................................................ 5-28
5.7.5 Climatological Data........................................................................................... 5-28
Appendix SA —Example Calculation: Weighting Curve Numbers.......................................5-31
Appendix SB —Example Calculation: Bowstring Method.................................................... 5-33
Appendix SC —Example Calculation: Water Budget (Preferred Method)............................ 5-37
Appendix SD—Example Calculation: Water Budget(Alternative Method).........................5-41
Appendix SE—Hydrologic Soil Series for Washington State....................................5-45
Chapter 6. Water Quality Treatment Design ...............................................................................6-1
6.1 Introduction...................................................................................................................6-1
6.2 Protection of Aquifer Water Quality.............................................................................6-1
6.2.1 Aquifer Sensitive Area.........................................................................................6-1
6.22 Critical Aquifer Recharge Areas..........................................................................6-3
6.3 UIC Facilities................................................................................................................ 6-5
6.3.1 Limitations on the Use of UIC Facilities.............................................................6-5
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SPOKANE REGIONAL STORMWATER MANUAL
6.3 2 Drywell Registration............................................................................................ 6-6
6.4 Pollutant Generating Impervious Surface Areas...........................................................6-6
6.4.1 High-Use Sites .....................................................................................................6-7
6.42 High-ADT Sites ...................................................................................................6-8
6.4.3 Moderate-Use Sites.............................................................................................. 6-8
6.5 Treatment Goals............................................................................................................ 6-9
6.5.1 Total Suspended Solids (TSS).............................................................................6-9
6.52 Total Petroleum Hydrocarbons (TPH).................................................................6-9
6.53 Metals Treatment...............................................................................................6-10
6.5.4 Phosphorous treatment.......................................................................................6-10
6.6 Applicability ...............................................................................................................6-10
6.6.1 Basic Treatment.................................................................................................6-11
6.62 Oil Control.........................................................................................................6-12
6.63 Metals Treatment...............................................................................................6-12
6.6.4 Phosphorus Treatment .......................................................................................6-13
6.7 Treatment BMPs.........................................................................................................6-14
6.7.1 Bio-infiltration Swales.......................................................................................6-14
6.72 Biofiltration Channels........................................................................................ 6-17
6.73 Vegetated Buffer Strips...................................................................................... 6-19
6.7.4 Oil/Water Separators.......................................................................................... 6-22
6.7.5 Wetlands ............................................................................................................ 6-25
6.8 Emerging Technologies .............................................................................................. 6-26
6.8.1 Background........................................................................................................6-26
6.82 Ecology's Role in Evaluating Emerging Technologies.....................................6-27
6.83 Local Jurisdiction's Role in Evaluating Emerging Technologies...................... 6-27
6.8.4 Testing Protocol.................................................................................................6-28
6.8.5 Assessing Levels of Development of Emerging Technologies .........................6-28
Appendix 6A —Example calculation: Bio Infiltration Swale................................................ 6-31
Appendix 6B —Example calculation: Biofiltration Channel.................................................6-33
Appendix 6C —Example calculation: Vegetated Buffer Strip...............................................6-35
Chapter7. Flow Control ..............................................................................................................7-1
7.1 Introduction...................................................................................................................7-1
7.2 Applicability and Exemptions.......................................................................................7-1
7.2.1 Applicability ........................................................................................................7-1
7.22 Exemptions ..........................................................................................................7-1
7.3 Detention Facilities.......................................................................................................7-2
7.3.1 Introduction..........................................................................................................7-2
7.32 Minimum Requirements ...................................................................................... 7-3
7.4 Outflow Control Structures...........................................................................................7-4
7.4.1 Introduction..........................................................................................................7-4
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SPOKANE REGIONAL STORMWATER MANUAL
7.42 Outflow Control Structure Types.........................................................................7-4
7.4.3 Minimum Requirements ......................................................................................7-8
7.5 Infiltration Facilities....................................................................................................7-10
7.5.1 Introduction........................................................................................................7-10
7.5 2 Minimum Requirements ....................................................................................7-11
7.6 Natural Dispersion......................................................................................................7-13
7.6.1 Introduction........................................................................................................7-13
7.62 Minimum Requirements ....................................................................................7-13
7.7 Evaporation Facilities .................................................................................................7-15
7.7.1 Introduction........................................................................................................7-15
7.72 Minimum Requirements ....................................................................................7-15
7.8 Additional Requirements for All Facilities................................................................. 7-17
7.8.1 General...............................................................................................................7-17
7.82 Setbacks .............................................................................................................7-18
7.8.3 Drawdown Time ................................................................................................ 7-18
7.8.4 Side Slopes.........................................................................................................7-18
7.8.5 Emergency Overflow Spillway.......................................................................... 7-19
7.8.6 Embankments.....................................................................................................7-19
7.8.7 Fencing...............................................................................................................7-20
7.8.8 Planting Requirements.......................................................................................7-21
7.8.9 Landscaping.......................................................................................................7-21
7.8.10 Maintenance.......................................................................................................7-22
7.8.11 Dam Safety.........................................................................................................7-22
7.9 Special Requirements..................................................................................................7-23
7.9.1 Special Drainage Areas...................................................................................... 7-23
7.92 Floodplains.........................................................................................................7-23
7.9.3 Wetlands and Classified Streams.......................................................................7-26
7.9.4 Closed Depressions............................................................................................ 7-27
7.10 Regional Stormwater Facilities...................................................................................7-27
Chapter8. Conveyance................................................................................................................8-1
8.1 Introduction................................................................................................................... 8-1
8.2 Applicability ................................................................................................................. 8-1
8.3 Natural and Constructed Channels................................................................................ 8-1
8.3.1 Channel Analysis ................................................................................................. 8-1
8.32 Minimum Requirements ...................................................................................... 8-3
8.3.3 Channel Design.................................................................................................... 8-5
8.3.4 Preservation of Natural Location of Drainage Systems (NLDS)......................... 8-8
8.4 Culverts....................................................................................................................... 8-11
8.4.1 Culvert Analysis................................................................................................. 8-ll
8.42 Minimum Requirements for Culverts................................................................ 8-ll
8.43 Culvert Design................................................................................................... 8-15
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SPOKANE REGIONAL STORMWATER MANUAL
8.5 Storm Drain Systems ..................................................................................................8-15
8.5.1 Pipe Analysis ..................................................................................................... 8-15
8.52 Minimum Requirements .................................................................................... 8-16
8.5.3 Pipe Design........................................................................................................8-19
8.6 Gutters......................................................................................................................... 8-19
8.6.1 Gutter Analysis .................................................................................................. 8-20
8.62 Gutter Design..................................................................................................... 8-21
8.7 Drainage Inlets............................................................................................................ 8-24
8.7.1 Minimum Requirements .................................................................................... 8-24
8.72 Drainage Inlet Design........................................................................................ 8-26
Appendix 8A —Example Calculation: Non-Flooded Width (Uniform Gutter Section)........ 8-33
Appendix 8B —Example Calculation: Non-Flooded Width (Composite Gutter Section)..... 8-35
Appendix 8C —Example Calculation: Grate Inlet Capacity (Uniform Gutter Section)........ 8-37
Appendix 8D — Example Calculation: Grate Inlet Capacity, Continuous Grade (Composite
GutterSection)............................................................................................................ 8-39
Appendix 8E — Example Calculation: Grate Inlet Capacity, Continuous Grade (Uniform
GutterSection)............................................................................................................ 8-41
Appendix 8F — Example Calculation: Curb Inlet Capaciry, Continuous Grade (Composite
GutterSection)............................................................................................................ 8-43
Appendix 8G—Example Calculation: Combination Inlet Capacity, Sump .......................... 8-45
Appendix 8H—Example Calculation: Curb Inlet Capacity, Sump....................................... 8-47
Chapter 9. Erosion and Sediment Control Design.......................................................................9-1
9.1 Introduction...................................................................................................................9-1
9.2 Applicability .................................................................................................................9-1
9.3 Exemptions ...................................................................................................................9-2
9.4 Erosion and Sediment Control (ESC) Plan...................................................................9-2
9.4.1 Introduction..........................................................................................................9-2
9.42 Minimum Requirements for ESC Plans...............................................................9-3
9.43 Best Management Practices for ESC Plans .........................................................9-3
9.4.4 Modification to ESC Plans.................................................................................9-11
9.5 Additional Information Regarding ESC Plans............................................................ 9-ll
9.5.1 Performance Standards ......................................................................................9-11
9.52 Maintenance Responsibility...............................................................................9-12
9.53 Enforcement and Appeals Process.....................................................................9-12
Appendix 9A —ESC Standard Plan Notes.............................................................................9-13
Chapter10. Source Control........................................................................................................ 10-1
10.1 Introduction................................................................................................................. 10-1
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SPOKANE REGIONAL STORMWATER MANUAL
10.2 Applicability ............................................................................................................... 10-1
Chapter 1 L Maintenance, Tracts, and Easements ..................................................................... 11-1
11.1 Maintenance................................................................................................................ 11-1
11.1.1 Introduction........................................................................................................ 11-1
11.12 Applicability ...................................................................................................... 11-1
11.13 Homeowners' and Property Owners' Associations........................................... 11-2
11.1.4 Operation and Maintenance Manual.................................................................. 11-2
11.1.5 Financial Plan..................................................................................................... 11-3
11.1.6 Maintenance Access Requirements ................................................................... ll-3
11.2 Tracts and Easements.................................................................................................. 11-4
11.2.1 Tracts.................................................................................................................. 11-4
11.22 Easements .......................................................................................................... 11-4
Appendix 11A —Example Calculation: Sinking Fund.......................................................... 11-7
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SPOKANE REGIONAL STORMWATER MANUAL
LIST OF FIGURES
No. Title Page No.
4A-1 Percentage of Fines (%passing the No. 200 Sieve)vs. Hydraulic Conductivity(k).....................4-10
4A-2 Hydraulic Conductivity(k)vs.Normalized Outflow Rate ............................................................4-ll
4G1 Hydraulic Conductivity(k)vs.Normalized Outflow Rate (duplicate of 4A-2).............................4-19
5-1 2-year, 24 hour Isopluvial Map......................................................................................................5-13
5-2 10-year, 24 hour Isopluvial Map....................................................................................................5-14
5-3 25-year, 24 hour Isopluvial Map....................................................................................................5-15
5-4 50-year, 24 hour Isopluvial Map....................................................................................................5-16
5-5 100-year, 24 hour Isopluvial Map .................................................................................................5-17
5-6 Mean Annual Precipitation ............................................................................................................5-18
5-7 Bowstring Method Hydrographs....................................................................................................5-23
SB-1 Bowstring Method Spreadsheet Example......................................................................................5-36
SG1 Evaporative Pond Spreadsheet Example (Preferred Method)........................................................5-39
SD-1 Evaporative Pond Spreadsheet Example (Alternate Method)........................................................5-43
6-1 Aquifer Sensitive Area(ASA) .........................................................................................................6-2
6-2 Spokane County Critical Aquifer Recharge Areas ..........................................................................6-4
6-3 Vegetated Buffer Strip...................................................................................................................6-21
6-4 Typical Vegetated Buffer Strip (details)........................................................................................6-23
7-1 Contracted Weir...............................................................................................................................7-5
7-2 Suppressed Weir...............................................................................................................................7-5
7-3 Broad-Crested Weir.........................................................................................................................7-6
7-4 Sharp-Crested Weir..........................................................................................................................7-6
7-5 Flow Rates vs. Head(riser)..............................................................................................................7-8
7-6 Flow Control Structure Example....................................................................................................7-10
7-7 Separated Evaporative/Detention Facility Cross-Section..............................................................7-16
7-8 Stacked Evaporative/Detention System Cross-Section..................................................................7-17
8-1 Riprap Revetment at Outfall Schematic...........................................................................................8-7
8-2 Non-Flooded Road Width(L)........................................................................................................8-20
8-3 Uniform Gutter Section..................................................................................................................8-22
8-4 Composite Gutter Section..............................................................................................................8-22
8-5 Typical Grate Inlet Cross-Section..................................................................................................8-27
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SPOKANE REGIONAL STORMWATER MANUAL
LIST OF TABLES
No. Title Page No.
2-1 Allowable Discharge Rates............................................................................................................2-15
4-1 Spokane 200 Method Results Summary..........................................................................................4-4
4-2 Example Sub-Level Structure Feasibility Summary........................................................................4-8
4A-1 Outflow Rate Factors of Safety for Spokane 200 Method.............................................................4-12
4B-1 Outflow Rate Factors of Safety for Full-Scale Drywell Test Method............................................4-14
4C-1 Outflow Rate Factors of Safety for Test Pit Method......................................................................4-20
5-1 Runoff Curve Numbers—Antecedent Runoff Condition (ARC) II.................................................5-6
5-2 Curve Number Based on Antecedent Runoff Condition(ARC)......................................................5-7
5-3 Friction Values (n and k) for Use in Computing Time of Concentration......................................5-10
5-4 Suggested Values of Manning's Roughness Coefficient(n) for Channel Flow ............................5-ll
5-5 Runoff Coefficients for the Rational Method(10-year Return Frequency)...................................5-20
5-6 Ground Cover Coefficients............................................................................................................5-22
5-7 Index to Rainfall Coefficients........................................................................................................5-23
5-8 Curve Number Adjustment for Antecedent Runoff Condition(ARC) ..........................................5-28
5-9 Average Monthly Precipitation and Pan Evaporation Values........................................................5-29
SE-1 Hydrologic Soil Series for Washington State ................................................................................5-45
6-1 Bialnfiltration Swale Design Criteria...........................................................................................6-16
7-1 Allowable Discharge Rates..............................................................................................................7-3
7-2 Flow Equations for Various Weir and Ori�ce Types.......................................................................7-7
7-3 Optimal Application of Control Structures......................................................................................7-9
8-1 Permissible Velocities for Channels with Erodible Linings,
Based on Uniform Flow in Continuously Wet, Aged Channels......................................................8-2
8-2 Maximum Ditch or Channel Side Slopes.........................................................................................8-4
8-3 Criteria for Gravel or Sand Filter Blanket Gradation.......................................................................8-8
8-4 Manning's Roughness Coefficients (n) for Culverts......................................................................8-12
8-5 Minimum Culvert Sizes .................................................................................................................8-12
8-6 Manning's Roughness Coefficients (n) for Closed Systems..........................................................8-16
8-7 Non-Flooded Road Width Requirements.......................................................................................8-20
8-8 Manning's Roughness Coefficients (n) for Street&Pavement Gutters........................................8-21
8-9 Allowable Width and Perimeter for Grate Capacity Analysis.......................................................8-27
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SPOKANE REGIONAL STORMWATER MANUAL
GLOSSARY
DEFI NITIONS
All Weather Drivable Surface—Any roadway, driveway, alley or parking lot
surface paved with crushed stone, asphalt, concrete or other pervious or impervious
material in a manner that will support the weight of anticipated vehicular traffic in all
weather conditions and minimize the potential for ruts,potholes or pooling of water.
Antecedent Runoff Condition—The degree of wetness of a watershed or within the
soil at the beginning of a storm.
Aquifer—A geologic stratum containing groundwater that can be withdrawn and
used for human purposes.
Arterial—A road or street primarily for through traffic. A major arterial connects an
interstate highway to cities and counties. A minor arterial connects major arterials to
collectors. A collector connects an arterial to a neighborhood or local access roads. A
local access road connects individual homes to a collector.
Average Daily Traffic—The expected average number of vehicles using a roadway
in a day.
Backwater—An unnaturally high stage in a stream caused by obstruction or
confinement of flow, as by a dam, a bridge or a levee. Its measure is the excess of
unnatural over natural stage, not the difference in stage upstream and downstream
from its cause.
Bank—Lateral boundary of a stream; limits confining water flow.
Base Flood—The flood having a 1% chance of being equaled or exceeded in any one
year.
Basic Requirement—Any of eight stormwater management measures that must be
completed for new development and redevelopment projects that meet the regulatory
threshold, unless exempted in this Manual.
Basin (Drainage Basin)—The portion of the earth's surface upon which falling
precipitation runs off to a common point. Often referred to as a drainage basin.
Bedrock—The more or less solid rock in place on or beneath the surface of the earth.
It may be soft, medium, or hard and have a smooth or irregular surface.
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SPOKANE REGIONAL STORMWATER MANUAL
Berm—A constructed barrier of compacted earth, rock or gravel. In a stormwater
facility, a berm may serve as a vertical divider, typically built up from the bottom.
Best Management Practices—The schedules of activities, prohibitions of practices,
maintenance procedures, and structural and/or managerial practices approved by
Ecology that, when used singly or in combination, prevent or reduce the release of
pollutants and other adverse impacts to waters of Washington State.
Buffer (or Buffer Area or Buffer Zone)—The area adjacent to a critical or sensitive
area established to ensure protection of the critical area by separating incompatible
uses from the critical or sensitive area. Buffer locations and limits are described by
federal, state or local governments.
Capacity—The effective carrying ability of a drainage structure. Generally measured
in cubic feet per second.
Catch Basin—A drainage structure that collects water from the side or through a
grating.
Cation Exchange Capacity—The amount of exchangeable cations that a soil can
absorb at pH 7.0.
Channel—A depression in the earth's surface which conveys water from one
location to another. This may be either a natural faciliry or man made.
Channel Protection—Erosion prevention and stabilization of velociry distribution in
a channel using vegetation,jetties, drops, revetments, or biological communiry.
Check Dam—Small dam constructed in a gully or other small watercourse to
decrease the stream flow velociry, minimize channel scour, and promote deposition of
sediment.
Cleanout—An access opening to a storm drain system. Usually consists of a manhole
shaft, a special chamber or an opening into a shallow culvert or drain.
Clear Zone—An unobstructed, relatively flat area provided beyond the edge of a
traveled roadway for the recovery of errant vehicles.
Common Plan of Development or Sale- "A site where multiple separate and distinct
construction activities may be taking place at different times on different schedules,
but still under a single plan. Examples include phased projects and projects with
multiple filing or lots, even if the separate phases or filing/lots will be constructed
under separate contract or by separate owners (e.g. development where lots are sold
to separate builders); a development that may be phased over multiple years, but is
still under a consistent plan for long term development; and projects in a contiguous
area that may be unrelated but still under the same contract, such as construction of a
building extension and a new parking lot at the same faciliry. If the project is part of
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a common plan of development or sale, the disturbed areas of the entire plan shall be
used in determining permit requirements.
Concentrated Flow—Flowing water that has been accumulated into a single fairly
narrow stream.
Concept Drainage Report—A preliminary drainage report to demonstrate that the
proposed drainage facilities generally can meet the stormwater requirements for
certain land use actions or land development permits. They are needed for sites that
have limiting layers or shallow groundwater or are in a critical area.
Conveyance System—The drainage facilities, both natural and man-made, that
collect, contain, and provide for the flow of surface and stormwater from the highest
points on the land down to a receiving water. The natural elements of the conveyance
system include swales and small drainage courses, streams, rivers, lakes and
wetlands. The man-made elements of the conveyance system include gutters, ditches,
pipes, channels, and most detention facilities.
Critical Area—Any of the following areas and ecosystems: wetlands, areas with a
critical recharging effect on aquifers used for potable water, fish and wildlife habitat
conservation areas, frequently flooded areas, and geologically hazardous areas.
Critical Flow Depth—The depth of water in a conduit at which the maximum flow
takes place, if the conduit is on the critical slope with the water flowing at its critical
velociry and there is an adequate supply of water. The depth of water flowing in an
open channel or a conduit partially filled for which the velocity head equals one-half
the hydraulic mean depth.
Critical Slope—The slope at which the maximum flow will occur at the minimum
velociry.
Critical Velocity—Mean velociry of flow in channel when flow is at critical depth.
Culvert—A conduit for allowing water to pass under a roadway. A culvert differs
from a bridge in size.
Design Frequency—The recurrence interval for hydrologic events used for design.
Design Storm—That storm that generates the amount of runoff that drainage
facilities are designed to handle. This storm is selected for design on the basis of its
probable recurrence.
Detention—The release of stormwater runoff from a site at a slower rate than it is
collected by the stormwater facility system, the difference being held in temporary
storage.
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Detention Facility—An above-ground or below-ground facility, such as a pond or
tank, that temporarily stores stormwater runoff and subsequently releases it at a
slower rate than it is collected by the drainage faciliry system. There is little or no
infiltration of stored stormwater.
Design Deviation—An administrative approval of design elements that do not
conform to or are not explicitly addressed by this Manual.
Development—Any man-made change to improved or unimproved real estate,
including but not limited to buildings or other structures, mining, dredging, filling,
grading, paving, excavation or drilling operations or storage of equipment or
materials located within the area of special flood hazard.
Discharge—The volume of water flowing out of a drainage structure or facility.
Dispersion—Release of surface stormwater runoff from a drainage faciliry system
such that the flow spreads over a wide area, located so as not to allow flow to
concentrate anywhere upstream of a drainage channel with erodible underlying
granular soils.
Ditch—A long narrow excavation dug in the earth for drainage with a top width of
less than 10 feet at design flow.
Diversion—The change in character, location or direction of flow of a natural
drainage course.
Down-Gradient—When used in this manual if may refer to either downstream
(surface) or down-gradient(sub-surface) or both.
Drain—A buried pipe or other conduit (closed drain). A ditch (open drain) for
carrying off surplus surface water or groundwater.
Drainage—(1) The process of removing surplus ground or surface water by artificial
means. (2) The manner in which the waters of an area are removed. (3) The area from
which waters are drained; a drainage basin.
Drainage Basin—The portion of the earth's surface upon which falling precipitation
flows to a common point.
Drainage Submittal—The submittal of documentation including narrative, basin
maps, plans, calculations and other supporting documentation to demonstrate that a
proposed project will adequately treat and dispose of stormwater.
Drywell—A well installed above the water table so that its bottom and sides are
rypically dry except when receiving fluids. Drywells are designed to disperse water
below the land surface.
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Easement—A right to use the land of others. The right may be from the common law
or may be acquired, usually by purchase or condemnation and occasionally by
prescription or inverse condemnation. The right is not exclusive, but subject to rights
of others in the same land, the lesser right being subservient to a prior right which is
dominant. Easements for drainage may give rights to impound, divert, discharge or
concentrate surface flow, extend pipelines, deposit silt, erode, scour, or any other
necessary consequence of a development.
Energy Dissipation—Use of a structure to slow the flow of water and reduce the
erosive forces present in a rapid-flowing body of water.
Engineer—Professional engineer, currently licensed in the State of Washington
Erosion—The wearing away of the land surface by running water, wind, ice, or other
geological agents, including such processes as gravitational creep.
Erosion and Sediment Control Facility—A drainage facility designed to hold
surface and stormwater runoff for a period of time to allow sediment contained in the
runoff to settle out.
Erosion and Sedimentation Control—Any temporary or permanent measures taken
to reduce erosion, control siltation and sedimentation, and ensure that sediment-laden
water does not leave a site.
Existing Condition—The site condition prior to development; not necessarily the
pre-developed condition.
Floodplain- An area determined by the Federal Emergency Management Agency
(FEMA) to have a one percent chance of flooding in any given year.
Floodway—The channel of a river or other watercourse and the adjacent land areas
that must be reserved in order to discharge the base flood without cumulatively
increasing the water surface elevation more than a designated height.
Flow—The movement of water, silt, sand, etc; discharge; total quantity carried by a
stream.
Freeboard—The distance between the normal operating level and the top of the sides
of an open conduit, the crest of a dam, etc., left to allow for wave action, floating
debris or any other condition or emergency, without overtopping the structure.
Freeways Fully controlled and partially controlled limited access highways, located
either inside or outside the urban growth area delineated by a local jurisdiction.
Geotechnical Engineer—Professional engineer, currently licensed in the State of
Washington, specializing in geotechnical engineering.
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Groundwater—Water in a saturated zone or stratum beneath the land surface.
Groundwater recharge—Inflow to a groundwater reservoir or aquifer.
Groundwater table—The free surface of the groundwater, that surface subject to
atmospheric pressure under the ground, generally rising and falling with the season,
the rate of withdrawal, the rate of restoration, and other conditions. It is seldom static.
Head—An available force equivalent to a certain depth of water. This force is the
motivating force in the movement of water. The height of water above any point or
plan or reference. Used also in various compounds, such as energy head, entrance
head, friction head, static head,pressure head, lost head, etc.
High-ADT Roadway—Any road with an average daily traffic (ADT) greater than
30,000 vehicles per day.
High-Use Site—Sites that generate high concentrations of oil due to high traffic
turnover or the frequent transfer of oil and/or other petroleum products.
Hydraulic Grade Line—A line that represents the relative force available due to the
potential energy available. This is a combination of energy due to the height of the
water and the internal pressure. In any open channel, this line corresponds to the
water surface. In a closed conduit, if several openings were placed along the top of
the pipe and an open tube were inserted, a line connecting the water surface in each of
these tubes would represent the hydraulic grade line.
Hydraulic Jump—Transition of flow from a rapid state to a tranquil state; rise in
elevation of liquid surface. Sudden transition from supercritical flow to the
complementary subcritical flow, conserving momentum and dissipating energy.
Hydraulic Radius—The right cross-sectional area of a stream of water divided by
the length of that part of its periphery in contact with its containing conduit; the ratio
of area(A) to wetted perimeter(P): R= A/P
Hydraulically Connected—Impervious areas from which stormwater flows directly
onto another impervious area without traveling over a pervious area. May include
driveway or sidewalk areas adjacent to curbs from which stormwater collects in the
gutter.
Hydrograph—A graph showing stage, flow, velocity or other properties of water
with respect to time.
Hydrologic Soil Groups—A soil classification system defined by the U.S. Soil
Conservation Service in which a soil may be categorized into one of four groups (A,
B, C, or D)based on infiltration rate and other properties.
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Hydrology—The science dealing with the occurrence and movement of water upon
and beneath land areas of the earth. Overlaps and includes portions of other sciences
such as meteorology and geology. The particular branch of Hydrology that the
engineer is generally interested in is surface runoff, which is the result of excessive
precipitation.
Impervious Surface—A hard surface area that either prevents or retards the entry of
water into the soil mantle. Common impervious surfaces include, but are not limited
to, rooftops, walkways, patios, driveways, parking lots, storage areas, concrete or
asphalt paving, gravel roads, packed earthen materials and oiled, macadam or other
surfaces that impede the natural infiltration of stormwater.
Infiltration—The passage of water through the soil surface into the ground.
Initial Abstraction—The sum of all water losses before runoff begins, including
retention in surface depressions, interception by vegetation, evaporation and
infiltration.
Inlet—The portion of a drainage facility through which storm water enters a drainage
system.
Intermittent Channel—A stream or portion of a stream that flows only in direct
response to precipitation. Intermittent streams receive little or no water from springs,
have long-continued supply from melting snow or other sources, and are dry for a
large part of the year.
Invert The bottom of a drainage facility along which the lowest flows pass.
Isopluvial Map—A map with lines representing constant depth of total precipitation
for a given return frequency.
Land-Disturbing Activity—Any activity that results in movement of earth or a
change in the existing soil cover (vegetative and non-vegetative) or topography.
Land-disturbing activities include, but are not limited to clearing, grading, filling, and
excavation. Compaction associated with stabilization of structures and road
construction is also considered a land-disturbing activity. Vegetation maintenance
practices are not considered land-disturbing activiry.
Legally Non-Conforming—A project that was constructed, has been approved for
construction prior to the adoption of these standards, or is being constructed under a
valid permit authorizing the construction.
Level Pool Routing—The basic technique of storage routing used for sizing and
analyzing detention storage and determining water levels for ponding water bodies.
The level pool routing technique is based on the continuity equation: inflow minus
outflow equals change in storage.
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Local Jurisdiction—Any county, city, town or special purpose district having its
own incorporated government for local affairs.
Maintenance—Activities conducted on structures, facilities, and equipment that
involve no expansion or use beyond previously existing use, and result in no
significant adverse hydrologic impact.
Manhole—An entrance to a drainage faciliry for the purpose of inspection and
cleaning. This may consist of a circular manhole shaft, frame and round cover or an
opening into a structure where the top of the structure is at the surface; in this case,
the opening may be round or rectangular.
Manning's Number ("n" Value)—A number used in a mathematical formula to
determine the theoretical flow velocity in a drainage facility. This number varies
according to the roughness of the material through or over which the water is flowing.
Often referred to as a roughness coefficient.
Moderate-Use Sites—Sites that are expected to generate sufficient concentrations of
metals that additional runoff treatment is needed to protect water quality in non-
exempt water bodies.
National Pollutant Discharge Elimination System—A provision of the Clean
Water Act that prohibits point-source discharges of pollutants into waters of the
United States unless a special permit is issued; administered by the Washington
Department of Ecology as the delegated authority in Washington State.
Native Growth Protection Easement—An easement granted for the protection of
native vegetation within a sensitive area or its associated buffer.
New Development—The conversion of undeveloped or pervious surfaces to
impervious surfaces. New development occurs on either vacant land or through
expansion of partially developed sites.
Non-Flooded Road Width—The portion of a road that is not used to carry water
during a storm.
Non-Pollutant Generating Impervious Surfaces (NPGIS)—Impervious surfaces
that are insignificant sources of pollutants in stormwater runoff. Roofs that are subject
only to atmospheric deposition or normal heating, ventilation and air conditioning
vents are considered NPGIS. The following may also be considered NPGIS: paved
bicycle pathways and pedestrian sidewalks that are separated from and not subject to
drainage from roads for motor vehicles, fenced fire lanes, and infrequently used
maintenance access roads.
NRCS Method—The Natural Resources Conservation Service Urban Hydrograph
Method; a single-event hydrologic analysis technique for estimating runoff based on
the curve number method.
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Off-Site Drainage—Runoff that originates outside the site of a development.
Oil/Water Separator—A vault, usually underground, designed to provide a
quiescent environment to separate oil from water.
Open Channel—A drainage course with no restrictive top. It is open to the
atmosphere and may or may not permit surface flow to pass over its edge and into the
channel in an unrestricted manner. In many cases where dikes or berms are
constructed to increase the channel capacity, entrance of surface waters is necessarily
controlled.
Ordinary High Water Mark—The line on the shore marking the normal highest
level achieved during fluctuations in water levels; indicated by physical
characteristics such as a clear, natural line impressed on the bank; shelving; changes
in the character of soil destruction on terrestrial vegetation, or the presence of litter
and debris; or other means that consider the characteristics of the surrounding area.
Orifice—An opening with a closed perimeter and of regular form through which
water flows.
Outlet—The portion of a drainage system through which storm waters exit.
Overflow—Flow exceeding the capacity of a drainage system; or the device or
pathway through which this flow passes in exiting the drainage system.
Overland Flow—Flow of surface waters before reaching a natural water course.
Peak Flow—Maximum momentary stage or discharge of a stream or flood. Design
discharge.
Percolation—The movement of water through soil.
Perennial Stream—A stream reach that does not go dry during a year of normal
precipitation. The elevation of the water table is always above the bottom of the
stream channel during a year of normal precipitation.
Permeable Soils—Soil materials with a sufficiently rapid infiltration rate to greatly
reduce or eliminate surface and stormwater runoff. These soils are generally classified
as hydrologic soil types A and B.
Plat A map or representation of a subdivision showing the division of a tract or
parcel of land into lots, blocks, streets, or other divisions and dedications.
Point Discharge—The release of collected or concentrated surface and stormwater
runoff from a pipe, culvert, or channel.
Pollutant Generating Impervious Surface (PGIS)—Impervious surfaces that are
significant sources of pollutants in stormwater runof£ Such surfaces include those
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SPOKANE REGIONAL STORMWATER MANUAL
that are subject to vehicular use, industrial activities, or storage of erodible or
leachable materials that receive direct rainfall, or run-on or blow-in of rainfall. Metal
roofs are considered to be PGIS unless coated with an inert, non-leachable material.
Roofs that are subject to venting of manufacturing, commercial, or other indoor
pollutants are also considered PGIS. A surface, whether paved or not, shall be
considered PGIS if it is regularly used by motor vehicles. The following are
considered regularly-used surfaces: roads, unvegetated road shoulders, bike lanes
within the traveled lane of a roadway, driveways, parking lots, unfenced fire lanes,
vehicular equipment storage yards, and airport runways.
Precipitation—Rainfall, snow, sleet, fog, hail, dew and frost.
Pre-Developed Condition—The native vegetation and soils that existed at a site
prior to the influence of Euro-American settlement.
Project—Any proposed action to alter or develop a site; or the proposed action of a
permit application or an approval that requires drainage review.
Project Proponent—The person or entity responsible for oversight of a project; may
include the property owner or his sponsor, agent,proj ect manager or engineer.
Rational Method—A means of estimating the amount of stormwater arriving at a
given point. Determined by the equation Q = CIA; where Q = flow in cubic feet per
second, C = runoff coefficient which is a factor based on the imperviousness of the
area upon which the water is falling, I = rainfall intensity (inches per hour) based on
the time of concentration for the given drainage area; A =the drainage area in acres.
Reach—A length of channel that is uniform with respect to discharge, depth, area
and slope.
Redevelopment—The replacement of impervious surfaces on a developed site.
Redevelopment occurs when existing facilities are demolished and rebuilt or
substantially improved through reconstruction.
Regulatory Threshold— The "trigger" for compliance with the Basic Requirements
of this Manual. In Spokane Counry and the City of Spokane Valley, it is defined as
"the addition or replacement of 5,000 square feet or more of impervious surfaces or
the disturbance of one acre or more." In the City of Spokane, the threshold is defined
as "the addition or replacement of any impervious surfaces." The regulatory threshold
applies to the total impervious added or replaced at full build-out. Refer to the
definition of common plan of development to determine whether a project will trigger
the regulatory threshold". All projects proposing underground injection control
facilities must comply with the Basic Requirements, regardless of whether they
trigger the regulatory threshold.
Retention—The process of collecting and holding surface and stormwater runoff
with no surface outflow.
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Riprap—Broken concrete, sacked concrete or rock used for protection against
erosion.
Runoff—The portion of precipitation that contributes to flow in streams or drainage
systems.
Rural Road—A road outside Urban Growth Areas delineated by local jurisdictions.
Scour—Wearing of the bed of a stream by entrainment of alluvium and corrosion of
native rock Also caused by excessive velocities at the outlet of a concentrated stream
of water onto unstable material.
Seasonal Stream—A stream or segment of a stream that normally goes dry during a
year of normal rainfall. Seasonal streams often receive water from springs or long-
continued water supply from melting snow or other sources.
Sedimentation—Gravitational deposit of transported material in flowing or standing
water.
Sheet Flow—Any flow spread out and not confined (e.g. flow across a flat open
field).
Silt—(1) Waterborne sediment carried in suspension or deposited by flowing water,
ranging in diameter from 0.0002 to 0.002 inches. The term is generally confined to
fine earth, sand, or mud, but is sometimes broadened to include all material carried,
including both suspended and bed load. (2) Deposits of waterborne material as in a
reservoir, on a delta, or on floodplains.
Sorption—The physical or chemical binding of pollutants to sediment or organic
particles as a means of pollutant removal.
Special Drainage Areas—Areas that rypically have shallow soils, bedrock near the
surface of the land and soils or geological features that may make long-term
infiltration of stormwater difficult or pose potential problems for adjacent properties.
These areas may also contain steep slopes where infiltration of stormwater may be
difficult and the potential for erosion is high.
Special Drainage Districts—Special Drainage Areas within the City of Spokane.
Special Flood Hazard Areas (SFHA)- the land covered by the floodwaters of the
base flood in the SFHA on the NFIl' maps. The SFHA is the area where the NFIP's
floodplain management regulations must be enforced and the area where the
mandatory purchase of flood insurance applies.
Spillway—A passage for spilling water.
Spring—An issue of water from the earth.
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Stabilization—Measures to inhibit soil erosion, including the use of concrete or
asphalt paving, quarry spalls at access points, ditch lining, pre-manufactured erosion
products, or vegetative cover.
Storage—Detention or retention of water for future flow. Natural storage occurs in
channels and marginal soils; Artificial storage occurs in reservoirs.
Storm—A disturbance of the ordinary, average conditions of the atmosphere which,
unless specifically qualified, may include any or all meteorological disturbances, such
as wind, rain, snow, hail, or thunder.
Storm Drain—Any conveyor of stormwater.
Storm Sewer—A sewer that carries stormwater and surface water, street wash and
other wash waters or drainage, but excludes sewage and industrial wastes. Also called
a storm drain.
Stormwater—That portion of precipitation that does not naturally percolate into the
ground or evaporate, but flows, via overland flow, interflow, pipes and other features
of a stormwater drainage system, into a defined surface water body or constructed
infiltration facility.
Stormwater Management—An all-encompassing process that includes stormwater
volume and rate control and water quality treatment.
Subcritical Flow—Stream flow with velocity below the critical velocity.
Sump—Any low spot that does not permit the escape of water.
Supercritical flow—Flow at velocities higher than the critical velocity..
Surface Runoff—Any movement of water on the earth's surface, whether over the
surface of the ground or through channels.
Surveyor—A professional surveyor currently licensed in the State of Washington
Swale—A shallow drainage conveyance with relatively gentle side slopes, generally
with flow depths less than 1 foot.
Target Soil—The soil deposit or layer into which stormwater is designed to infiltrate.
For example, the soil layer that occurs at the active barrel section of a drywell.
Time of Concentration—Time required for discharge from the most distant point in
a drainage area to reach the point where all flow in the drainage area is concentrated.
Travel time—The estimated time for surface water to flow between two points of
interest.
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Treatment Train—A pollutant treatment scenario in which two stormwater best
management practices (BMPs) are constructed in series in order to capture pollutants
more efficiently. This concept is typically seen when there are very high
concentrations of pollutants, such as oil or phosphorus, for which no one treatment
BMP can adequately remove the pollutant on its own while meeting the remaining
treatment goals of this Manual.
Treatment Zone—The layer of soil in a bio-infiltration swale where water quality
treatment occurs. It consists of sod installed over medium- to well-draining soil at
least 6 inches thick underlain by a subgrade infiltration layer at least 48 inches thick.
Trip End—The expected number of vehicles using a parking area for the proposed
land use. Trip end counts are estimated by using the Trip Generation Manual
published by the Institute of Transportation Engineers
Type A NLDS—Natural drainage systems that may be considered for use as regional
facilities and serve important functions in existing management of stormwater runoff.
Type B NLDS—Natural drainage systems that are generally less prominent, yet are
deemed necessary for managing stormwater in its existing location.
Urban Road—A road located within the urban growth area delineated by local
jurisdictions.
Water Budget—An analysis used in the design of an evaporation pond that uses
average monthly precipitation and pan evaporation values to estimate the net
stormwater runoff volume increase over a 2-year cycle.
Water Surface—The top of water in a lake, channel, reservoir or river
Water Table—The upper surface or top of the saturated portion of the soil or
bedrock layer, indicating the uppermost extent of groundwater.
Weir—A low overflow dam or sill for measuring, diverting or checking flow.
Wetland—An area characterized by saturated or nearly saturated soils most of the
year that forms an interface between terrestrial (land-based) and aquatic
environments. Wetlands include marshes around lakes or ponds and along river or
stream channels.
ABBREVIATIONS AND ACRONYMS
• AASHTO—American Association of State Highway and Transportation
Officials
• ADT—Average Daily Traffic
• APWA—American Public Works Association
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• ARC—Antecedent Runoff Condition
• ASA—Aquifer Sensitive Area
• ASTM—American Society for Testing and Materials
• BFE—Base Flood Elevation
• BMP—Best Management Practice
• BST—Bituminous Surface Treatment
• CARA—Critical Aquifer Recharge Area
• CC&R—Conditions, Covenants and Restrictions
• CEC—Cation Exchange Capacity
• cfs—Cubic Feet per Second
• CMP—Corrugated Metal Pipe
• CN—Curve Number
• DOH—Department of Health
• EPA—Environmental Protection Agency
• ESC—Erosion & Sediment Control
• ETE—Equivalent Trip End
• FEMA—Federal Emergency Management Agency
• FHWA—Federal Highway Administration
• FIRM—Flood Insurance Rate Map
• FS—Factor of Safety
• GPA—Grassed Percolation Area
• GW—Grate Width
• GSC—Geotechnical Site Characterization
• HDPE—High-Density Polyethylene
• HGL—Hydraulic Grade Line
• HOA—Homeowner's Association
• IBC—International Building Code
• IRC—International Residential Code
• NLDS—Natural Location of Drainage Systems
• NOAA National Oceanic and Atmospheric Administration
• NPDES—National Pollutant Discharge Elimination System
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• NPGIS—Non-Pollutant Generating Impervious Surface
• NRCS Natural Resources Conservation Service
• O&M—Operation and Maintenance
• PAM—Polyacrylamide
• PGIS—Pollutant Generating Impervious Surface
• POA—Property Owners Association
• RCW—Revised Code of Washington
• SDA—Special Drainage Areas
• SDD—Special Drainage District
• sf—Square Feet
• TMDL—Total Maximum Daily Load
• TPH—Total Petroleum Hydrocarbons
• TSS—Total Suspended Solids
• UIC—Underground Injection Control
• USBR—United States Bureau of Reclamation
• USGS—United States Geological Survey
• WAC—Washington Administrative Code
• WRCC—Western Region Climate Center
• WSDOT—Washington State Department of Transportation
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REFERENCES
AASHTO Geometric Design of Highways and Streets, Exhibit 2-4, 2004 Edition [Section
11.1.6]
Open Channel Hydraulics, 1959, Ven Te Chow(1959) [Tables 5-4, 8-3, 8-6]
Civil Engineering Reference Manual, 8th Edition [Table 8-2]
Guidance for UIC Wells that Manage Stormwater, Washington State Department of
Ecology [Section 7.4.1]
Engman (1983) [Tables 5-3 and 5-4]
FHWA, Hydraulic Engineering Circular No. 22, Second Edition [Table 8-8, Section
8.62]
Florida Department of Transportation Drainage Manual (1986) [Tables 5-3 and 5-4]
Guidelines for Stormwater Management, Spokane County, February 1998 [Table 8-5]
Handbook of Applied Hydrology, (Chow, Ven Te, 1964) [Section 5.4]
Handbook of Hydraulics, by Brater and King [Section 8.4.3]
Infiltration Rate and Soil Classification Correlation, File No. 0188-082-00, May 28,
2004; prepared by GeoEngineers, Budinger & Associates, Inc., and Cummings
Geotechnology, Inc. [Chapter 4 Appendices]
King County Surface Water Design Manual, 1998. [Figures 6-3 and 6-4]
length of an apron (La) is determined using the following empirical relationships that
were developed for the U.S. Environmental Protection Agency, ASCE (1992) [Section
8.3.2]
NOAA Atlas 2, Volume IX, 1973 [Sections 5.3.4 and 5.53]
Open-Channel Hydraulics by French. [Section 8.43]
WSDOT Standard Plans, July 2001 [Section 8.32]
SCS National Engineering Handbook(1985). [Section 5.32 and Table 5-2]
Section 3-8 of the U.S. Bureau of Reclamation Drainage Manual. [Chapter 4 Appendices]
Apri12008 References
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SPOKANE REGIONAL STORMWATER MANUAL
Soil Conservation Service's Technical Release No. 55 , (210-VI-TR-55, Second Ed., June
1986). [Section 5.3.3 and Table 5-1]
Special Committee on Irrigation Research, American Society of Civil Engineers, 1926.
[Table 8-1]
Standard Plans jointly published by WSDOT and APWA (M21-01) [Section 8.6.1]
Stormwater Management Manual for Eastern Washington [Everywhere]
USGS WRI Report 97-4277, Magnitude and Frequency of Floods in Washington
[Section 7.82]
WSDOT Highway Runoff Manual (2004) [Tables 5-3 and 5-4]
WSDOT Hydraulics Manual, March 2004 [Tables 5-5 and 5-7, Section 8.3.2]
Apri12008 References
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SPOKANE REGIONAL STORMWATER MANUAL
CHAPTER 1 - INTRODUCTION
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Chapter Organization
1.1 Objective And Purpose.................................................................................................... 1-1
1.2 Vision Statement.............................................................................................................. 1-1
1.3 Using The Stormwater Manual........................................................................................ 1-1
1.3.1 Technical Information................................................................................................. 1-2
1.32 Policy Information....................................................................................................... 1-3
1.4 Relevant Programs and Concepts .................................................................................... 1-3
1.4.1 Underground Injection Control Program Compliance................................................ 1-4
1.42 Spokane Valley-Rathdrum Prairie Sole-Source Aquifer............................................. 1-4
1.43 National Pollutant Discharge Elimination System Compliance.................................. 1-5
1.4.4 Pollutant Generating Impervious Surface ................................................................... 1-6
1.5 General Requirements...................................................................................................... 1-6
1.6 Standard of Care .............................................................................................................. 1-7
1.7 Severability...................................................................................................................... 1-8
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SPOKANE REGIONAL STORMWATER MANUAL
1 .1 OBJECTIVE AND PURPOSE
Development projects in urban areas generally result in the replacement of open land,
where rainfall can infiltrate into the soil, with impervious surfaces that prevent
infiltration. This changes the patterns of stormwater runoff, which can lead to flooding
problems—at the project site and on properties downstream—and can affect water
quality, as sediment and pollutants are transported into streams, wetlands, lakes and
groundwater.
The Spokane Regional Stormwater Manual establishes standards for stormwater design
and management to protect water quality, natural drainage systems and down-gradient
properties as urban development occurs. The Manual meets or exceeds applicable criteria
from the Washington State Department of Ecology's Stormwater Management Manual
for Eastern Washington (SWMMEW), available online at
www.ecv.wa._�ov/pro�rams/wc�/stormwater/tech.html, for underground injection and
discharge to surface waters.
The purpose of this Manual is to help communities in the Spokane region to protect water
qualiry, prevent adverse impacts from flooding, and control stormwater runoff to levels
equivalent to those that occurred prior to development. Acceptable stormwater
management should be achieved when the criteria and standards presented in this Manual
are met.
1 .2 VISION STATEMENT
The Spokane Regional Stormwater Manual was developed jointly by Spokane County
and the Cities of Spokane and Spokane Valley, whose vision is to provide a document
with clearly defined stormwater management design and maintenance criteria to serve the
current and future stormwater needs of the Spokane region. The criteria in this document
are meant to enhance and promote future development in a way that reasonably protects
the health, safety, and welfare of current and future property owners, while at the same
time preserving or enhancing the existing natural drainage systems.
1 .3 USING THE STORMWATER MANUAL
This Manual provides engineers, developers and the general public with procedures and
assistance for designing stormwater management facilities associated with land
development, road and drainage projects. It outlines minimum requirements for the
design of stormwater management systems. The Manual is sufficiently comprehensive
that its contents, along with good engineering judgment, will address the myriad of
drainage concerns in the Spokane region.
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SPOKANE REGIONAL STORMWATER MANUAL
The Manual provides essential information for development project proponents (owners
or their agents)in two key areas: technical guidelines and government policies.
1.3.1 TECHNICAL INFORMATION
Technical information in this Manual consists of design criteria and minimum
requirements for use in the analysis and design of specific stormwater management
facilities. The technical information is generally organized to match eight basic
requirements for stormwater management associated with development:
• Chapter 2 generally describes the eight basic requirements.
• Chapter 3 describes requirements for a "drainage submittal" that
verifies compliance with all applicable requirements for a given
project (Basic Requirement No. 1).
• Chapter 4 describes geotechnical requirements (Basic Requirement
No. 2).
• Chapter 5 describes procedures for hydrologic analysis that are to be
used in conjunction with several of the basic requirements.
• Chapter 6 describes requirements for water quality treatment (Basic
Requirement No. 3).
• Chapter 7 describes requirements for flow control (Basic Requirement
No. 4)
• Chapter 8 describes requirements for conveyance systems (Basic
Requirement No. 5).
• Chapter 9 describes requirements for erosion and sediment control
(Basic Requirement No. 6).
• Chapter 10 describes requirements for controlling sources of pollutants
(Basic Requirement No. 7).
• Chapter 11 describes maintenance requirements (Basic Requirement
No. 8).
In general, each chapter describes minimum requirements for compliance with this
Manual and explains design procedures and criteria. Appendices to many of the
chapters give example calculations to demonstrate procedures for facility analysis and
design. Full compliance with this Manual may require exceeding the minimums
presented in the chapters.
The Manual is intended to provide project proponents, regulatory agencies, and others
with technically sound stormwater management practices which are presumed to meet
the stated stormwater objectives of federal, state and local regulations. Project
proponents always have the option of not following the stormwater management
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SPOKANE REGIONAL STORMWATER MANUAL
practices in this Manual. However, if a project proponent chooses not to follow the
practices in the Manual then the project proponent may be required to individually
demonstr°ate that the project will not adversely impact surface waters, groundwater or
neighboring properties by collecting and providing appropriate supporting data to
show that the alternative approach satisfies all relevant stormwater regulations.
It is the intention of this Manual that project proponents who fully comply with all its
requirements will also be in compliance with the requirements of Ecology's
Stormwater Management Manual for Eastern Washington. Some of the requirements
and definitions presented in this Manual are established by Ecology's manual, and
future changes to Ecology's manual may affect these requirements and definitions. It
is the obligation of the project proponent to refer to the most recent version of the
Ecology manual as needed to ensure compliance with its requirements.
1.3.2 POLICY INFORMATION
The policy portions of this manual explain the conditions under which projects are
subject to the requirements of this Manual and outline the procedures that are to be
followed to ensure compliance. Development that affects stormwater management is
subject to requirements laid out in federal, state and local rules and regulations. The
Manual identifies the key sources of these rules and regulations and describes the
steps toward compliance or directs users of the Manual to other resources for
compliance information. Policy information is included throughout the Manual as
appropriate.
This Manual was jointly developed by Spokane County and the Cities of Spokane and
Spokane Valley. It generally applies to development projects in unincorporated
Counry areas or within the limits of the two cities. It is not intended for use outside
Spokane County or in incorporated areas of the County other than Spokane and
Spokane Valley. Developers of projects in other jurisdictions should contact those
jurisdictions for guidance.
1 .4 RELEVANT PROGRAMS AND CONCEPTS
A number of regulatory programs and technical concepts are more broadly relevant to the
goals of this manual than specifically applicable to individual requirements or
procedures. Brief discussions of these topics are presented below to familiarize users of
the Manual with their history and implications.
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SPOKANE REGIONAL STORMWATER MANUAL
1.4.1 UNDERGROUND INJECTION CONTROL PROGRAM
COMPLIANCE
Stormwater can contain contaminants such as oil, grease, pathogens, nitrates,
pesticides, and metals such as cadmium, chromium and lead. When stormwater is
infiltrated into the ground through underground injection facilities such as drywells,
these contaminants can pollute groundwater.
The U.S. Congress created the Underground Injection Control (UIC) Program to
protect underground sources of drinking water from discharges of fluids to the
ground. The UIC Program in the State of Washington is administered by the
Department of Ecology. To implement the program, the Department of Ecology
adopted Washington Administrative Code (WAC) Chapter 173-218—Underground
Injection Control in 1984. The two requirements of the UIC Program are as follows:
• Register UIC wells with the state.
• Ensure that current and future underground sources of groundwater are
not endangered by pollutants in the discharge (non-endangerment
standard).
Pollution of groundwater from stormwater discharges can be prevented by proper
design, siting, operation and maintenance of the UIC well, by the use of treatment
before discharge to the sub-surface, and by reducing the stormwater contact with
potential sources of contamination.
The entire Spokane region is subject to the Department of Ecology's UIC regulations.
1.4.2 SPOKANE VALLEY-RATHDRUM PRAIRIE SOLE-SOURCE
AQUIFER
AquifeN Description and Potential Water Quality Threats
The only source of water for most of the people in Spokane County, Washington and
Kootenai Counry, Idaho is a high quality underground water body called the Spokane
Valley-Rathdrum Prairie Aquifer. This aquifer extends across an area of about 325
square miles and provides drinking water for more than 500,000 people. Most of the
developed areas in the Spokane region and in North Idaho lie directly over the
aquifer.
Unlike many other aquifers, the Spokane Valley-Rathdrum Prairie Aquifer does not
have protective layers of clay or rock to deter infiltration of surface contaminants.
The soil layer above the aquifer is relatively thin in most areas, and fluids readily
infiltrate into the porous sands and gravel that make up the aquifer materials.
Potential contamination is the most important issue that must be addressed to preserve
and maintain the aquifer as a regional drinking water resource. A contaminant on the
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SPOKANE REGIONAL STORMWATER MANUAL
surface may reach the aquifer water table in a matter of hours or days, particularly
contaminants that are dissolved in water that is recharging the aquifer. Contamination
in the aquifer may be cleaned up, or remediated, but the clean-up process is costly
and does not eliminate 100% of the contamination.
The water quality of the aquifer has been tested since 1977 and the monitoring
suggests that human activities on the land surface over the aquifer are deteriorating
the water quality. Contaminants are conveyed to the aquifer by stormwater, septic
tank leachate, fertilizer leachate, leakage from underground storage tanks and other
sources that percolate downward from the surface. Stormwater accounts for about
30% of the pollution reaching the aquifer. Stormwater can collect a large variety of
contaminants as it flows across roads, parking lots, roofs and other impervious
surfaces. Pollutants such as coliform bacteria, nitrates and volatile organic
compounds have been detected in aquifer water samples.
Regul�to�y Steps to Protect the Aquifer
Sole-Source Aquifer Designation: One of the first important steps to protect the
aquifer was taken by the U.S. Environmental Protection Agency (EPA) in 1978 when
it designated the Spokane Valley-Rathdrum Prairie a "Sole-Source Aquifer" under
Section 1424(e) of the federal Safe Drinking Water Act. It was the second aquifer in
the nation to receive this designation.
Aquifer Sensitive Area: Groundwater recharge areas have critical impacts on aquifers
used for potable water, as defined by 365-190-030 (2) WAC. Incorporated areas of
Spokane Counry, including the Cities of Spokane and Spokane Valley, are subject to
regulations governing the Aquifer Sensitive Area (ASA), as described in
Section 6.2.1.
Critical Aquifer Recharge Areas: By Resolution No. 3-0754, Spokane County has
designated Critical Aquifer Recharge Areas (CARA) for the protection of aquifer
water quality. Requirements associated with this designation affect all unincorporated
areas of the County. Section 6.2.2 provides detailed information about the CARA
designation.
1.4.3 NATIONAL POLLUTANT DISCHARGE ELIMINATION
SYSTEM COMPLIANCE
The National Pollutant Discharge Elimination System (NPDES) permit program was
established by the federal Clean Water Act, which is the primary federal law
protecting water quality. The Water Pollution Control Act (Revised Code of
Washington (RCW) Chapter 90.48) is the primary Washington State law protecting
water quality. The Washington Department of Ecology issues and implements
combined permits for point source stormwater and wastewater discharges to waters of
the United States and waters of the state that are designed to satisfy requirements of
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SPOKANE REGIONAL STORMWATER MANUAL
the NPDES and the Water Pollution Control Act. "Waters of the state" means all
lakes, rivers, ponds, streams, inland waters, ground waters, salt waters, and all other
waters and water courses within the jurisdiction of the state of Washington
(Washington Administrative Code (WAC) Chapter 173-216-030(20)).
In December 1999, the U.S. EPA adopted NPDES Phase II stormwater regulations,
identifying municipalities that are subject to NPDES municipal stormwater permitting
requirements. Federal regulations required that Phase II permits be issued by
December 2002 and that designated Phase II communities submit an application for
permit coverage by March 2003. The Department of Ecology issued the Eastern
Washington Phase II Municipal Stormwater Permit on January 17, 2007, effective
February 16, 2007, which applies to Spokane Counry and the Cities of Spokane and
Spokane Valley. The Phase II Permit requires the development of a Stormwater
Management Program (SWMP).
This Manual is intended to be technically equivalent to the Department of Ecology's
Stormwater Management Manual for Eastern Washington in order to meet the
NPDES requirements for both permitted and non-permitted communities in Spokane
County.
1.4.4 POLLUTANT GENERATING IMPERVIOUS SURFACE
Pollutant generating impervious surface (PGIS) areas are significant sources of
pollutants in stormwater runoff. These areas include surfaces subject to vehicular use,
industrial activities, or storage of erodible or leachable materials that receive direct
rainfall. The following are considered PGIS areas: roads, unvegetated road shoulders,
bike lanes within the traveled lane of a roadway, driveways, hydraulically connected
sidewalks, parking lots, some roofs, fire lanes, vehicular equipment storage yards and
airport runways. Please refer to Section 6-4.
1 .5 GENERAL REQUIREMENTS
The owner or project proponent and his agent are responsible for the following:
• Coordinating project consultants
• Providing complete drainage submittals
• Ensuring adherence to:
o The standards and criteria presented in this Manual
o The Administrative Conditions of Approval, if applicable
o Any conditions established by local jurisdiction staff
• General project management.
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SPOKANE REGIONAL STORMWATER MANUAL
The owner or project proponent and his agent are required to obtain acceptance of the
drainage submittal from the local jurisdiction prior to any of the following:
• Final plat approval
• Final short plat approval
• Binding site plan approval
• Issuance of a building permit
• Issuance of a road approach permit
• Any other land use action as defined by code, regulation or resolution of the local
jurisdiction.
The drainage submittal shall be prepared in accordance with this Manual, the applicable
version of the local jurisdiction's design standards, the International Building Code (IBC)
and the International Residential Code (IRC). The submittal shall be prepared by a
professional civil engineer currently licensed in the State of Washington and shall be
submitted to the local jurisdiction for review and acceptance.
The rate and volume of stormwater runoff originating on any proposed land development,
road or area draining to, across or through the project site shall be estimated in
accordance with the criteria presented in this Manual. These estimates shall be the basis
of the drainage report. Unless specifically approved by the local jurisdiction, the peak
rate and volume of stormwater runoff from any proposed land development to any natural
or constructed point of discharge downstream shall not exceed the pre-development peak
rate or volume of runoff. A down-gradient analysis demonstrating that there will be no
expected adverse impacts on downgradient properties will be required. Exceptions with
regard to rate and volume control can be made for regional facilities planned by the local
jurisdiction.
Stormwater runoff from a developed site shall leave the site in the same manner and
location as it did in the pre-developed condition. Flow may not be concentrated onto
down-gradient properties where sheet flow previously existed. Drainage shall not be
diverted and released downstream at points not receiving drainage prior to the proposed
development.
1 .6 STANDAR D OF CAR E
The standards presented in this Manual should be considered the minimum requirements
to be used in the design of stormwater facilities. Due to special site conditions,
environmental constraints, or other applicable laws, stormwater management designs
may frequently need to exceed the minimum requirements. It is incumbent upon the
engineer to use good engineering practice and to be aware of, and implement, new design
practices and procedures that reflect current techniques in stormwater design, providing
sufficient measures to ensure that the drainage facilities function as intended. Good
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SPOKANE REGIONAL STORMWATER MANUAL
engineering practice is defined in these standards as professional and ethical conduct that
meets the current codes and regulations adopted for engineers. The proposed design shall
consider functionality, constructability and maintenance, including the health, safety and
welfare of the public.
1 .7 SEVERABILITY
If any section, sentence, clause or phrase of this Manual should be held invalid or
unconstitutional, the validity or constitutionality thereof shall not affect the validiry or
constitutionality of any other section, sentence, clause or phrase of this Manual.
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SPOKANE REGIONAL STORMWATER MANUAL
CHAPTER 2 - BASIC REQUIREMENTS
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Chapter Organization
2.1 Introduction......................................................................................................................... 1
2.1.1 Regulatory Threshold.........................................................................................1
2.12 New Development .............................................................................................1
2.1.3 Redevelopment .................................................................................................2
2.1.4 Exemptions ........................................................................................................3
2.1.5 Generally Exempt..............................................................................................4
2.1.6 Design Deviation ...............................................................................................5
22 Basic Requirements ............................................................................................................ 6
2.2.1 Basic Requirement No. 1 —Drainage Submittal................................................6
2.22 Basic Requirement No. 2 — Geotechnical Site Characterization .......................9
2.23 Basic Requirement No. 3 —Water Quality Treatment.....................................10
2.2.4 Basic Requirement No. 4 —Flow Control........................................................14
2.2.5 Basic Requirement No. 5 —Natural And Constructed Conveyance Systems..16
2.2.6 Basic Requirement No. 6 —Erosion And Sediment Control ...........................18
2.2.7 Basic Requirement No. 7 — Source Control.....................................................19
2.2.8 Basic Requirement No. 8 — Operation And Maintenance...............................20
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SPOKANE REGIONAL STORMWATER MANUAL
2.1 INTRODUCTION
This chapter introduces the eight Basic Requirements for stormwater management for
new development and redevelopment projects in the Spokane region:
• Basic Requirement No. 1 —Drainage Submittal;
• Basic Requirement No. 2 —Geotechnical Site Characterization;
• Basic Requirement No. 3 —Water Quality Treatment;
• Basic Requirement No. 4 —Flow Control;
• Basic Requirement No. 5 —Natural and Constructed Conveyance Systems;
• Basic Requirement No. 6 —Erosion and Sediment Control;
• Basic Requirement No. 7 — Source Control; and,
• Basic Requirement No. 8 —Operation and Maintenance.
The applicability of these requirements depends on the type, size and location of the
project. It is the responsibility of the project proponent to become familiar with the Basic
Requirements in order to determine when they are applicable.
2.1.1 REGULATORY THRESHOLD
The regulatory threshold is the "trigger" for requiring compliance with the Basic
Requirements of this Manual. This threshold varies from jurisdiction to jurisdiction. In
Spokane County and the City of Spokane Valley, it is defined as "the addition or
replacement of 5,000 square feet or more of impervious surfaces or the disturbance of 1
acre or more." The regulatory threshold applies to the total impervious area replaced or
added at full build-out. Refer to the definition of"common plan of development or sale"
to determine whether your project will trigger the regulatory threshold. In the City of
Spokane, the threshold is defined as "the addition or replacement of any impervious
surfaces."
All projects proposing underground injection control (UIC) facilities must comply with
UIC requirements, regardless of whether they trigger the regulatory threshold.
2.1.2 NEW DEVELOPMENT
New development is the conversion of previously undeveloped or permeable surfaces
to impervious surfaces and managed landscape areas. New development occurs on
vacant land or through expansion of partially developed sites.
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SPOKANE REGIONAL STORMWATER MANUAL
All new development projects, regardless of whether the project meets the regulatory
threshold, shall comply with the following:
• Basic Requirement No. 5 — Natural and Constructed Conveyance
Systems;
• Basic Requirement No. 6 —Erosion and Sediment Control; and,
• Basic Requirement No. 7— Source Control.
All new development projects that meet the regulatory threshold or propose UIC
facilities shall comply with the following:
• Basic Requirement No. 3 —Water Quality Treatment
• Basic Requirement No. 8 — Operation and Maintenance.
All new development projects that meet the regulatory threshold shall comply with
the following:
• Basic Requirement No. 1 — Drainage Submittal, unless exempted per
Sections 2.1.4 or 2.1.5. All projects shall provide for stormwater
management in their design regardless of whether the local jurisdiction
requires a drainage submittal.
• Basic Requirement No. 4 — Flow Control (refer to Section 2.2.4 for
additional information);
Review Section 22.2 to determine if Basic Requirement No. 2 — Geotechnical Site
Characterization is applicable.
The local jurisdiction reserves the right to require compliance with any or all of the
Basic Requirements regardless of the size of the project or the amount of impervious
area added or replaced.
2.1.3 REDEVELOPMENT
Redevelopment is the replacement of impervious surfaces on a developed site.
Redevelopment occurs when existing facilities are demolished and rebuilt or
substantially improved through reconstruction. Rebuilt or reconstructed facilities are
regarded in the same manner as new development (refer to Section 2.12) and shall
generally comply with the Basic Requirements of this Manual, as applicable. On
redeveloped sites where pre-existing facilities remain, the old facilities are not subject
to the requirements of this Manual if they remain hydraulically isolated from the new
facilities. For projects that are implemented in incremental stages, the redevelopment
threshold applies to the total amount of impervious surface replaced at full build-out;
the new development thresholds apply to the total amount of new impervious surfaces
added at full build-out.
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SPOKANE REGIONAL STORMWATER MANUAL
The long-term goal of the redevelopment standard is to reduce stormwater pollution
from existing developed sites, especially when a water quality problem has been
identified or the site is being improved to accommodate a use with a greater potential
to contribute pollution to the receiving waters. More stringent redevelopment
thresholds and requirements may be identified through a water cleanup plan such as a
Total Maximum Daily Load (TMDL) study and allocation.
A project may be granted a variance when site conditions prevent full compliance
with the Basic Requirements; however, every effort should still be made to find
creative ways to meet the intent of the Basic Requirements. Variances will generally
not be granted waiving stormwater requirements for new impervious surfaces. The
local jurisdiction may allow the Basic Requirements to be met for an area with
equivalent flow and pollution characteristics within the same site.
Sites with 100% existing building coverage that are currently connected to a
municipally owned storm sewer or combined sewer must be evaluated on a case-by-
case basis to continue to be connected without treatment; additional local
requirements such as flow restrictors may also be required.
2.1.4 EXEMPTIONS
Projects are exempt from the Basic Requirements when falling under any of the
following categories:
• Commercial agriculture as regulated under Revised Code of
Washington (RCW) Chapter 8434.020, except for the construction of
impervious surfaces related to commercial agriculture;
• Forest practices regulated under Washington Administrative Code
(WAC) Title 222, except for Class IV General Forest Practices that are
conversions from timberland to other uses;
• Oil and gas field activities or operations including construction of
drilling sites, waste management pits, access roads, and transportation
and treatment infrastructure such as pipelines, natural gas treatment
plants, natural gas pipeline compressor stations, and crude oil pumping
stations;
• Actions by a public utility or any other governmental agency to
remove or alleviate an emergency condition, restore utility service, or
reopen a public thoroughfare to traffic;
• Records of survey, boundary (i.e. minor lot line) adjustments, and
property aggregations, unless the action affects drainage tracts or
easements;
• Projects that, when completed, will not have physically disturbed the
land;
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SPOKANE REGIONAL STORMWATER MANUAL
• Road and parking area preservation and maintenance projects such as:
o Pothole and square cut patching;
o Crack sealing;
o Shoulder grading;
o Reshaping or regrading of drainage systems;
o Vegetation maintenance; and,
• Operation and maintenance or repair of existing facilities.
2.1.5 GENERALLY EXEMPT
The practices below are generally exempt from all of the Basic Requirements except
for Basic Requirement No. 5 —Natural and Constructed Conveyance Systems, Basic
Requirement No. 6 — Erosion and Sediment Control, and Basic Requirement No. 7 —
Source Control. However, they may be required to comply with any or all of the
Basic Requirements as determined by the local jurisdiction:
• Projects that do not meet the regulatory threshold as defined in Section
2.1.1 and do not include new UIC facilities;
• Certificates of exemption;
• Single-family residential/duplex building permits without special
conditions (A surface drainage plan and other information may be
required in the City of Spokane; however, a full drainage submittal is
generally not required);
• Temporary use permits, unless the use could cause adverse water
quality or other drainage-related impacts;
• Land-disturbing activities that do not require a permit, unless the
activity could cause adverse water qualiry or other drainage-related
impacts;
• Underground utility projects that replace the ground surface with in-
kind material or materials with similar runoff characteristics;
• Projects to improve motorized or non-motorized user safety that do not
increase the traffic capaciry of a roadway. Certain safety improvement
projects such as sidewalks, bike lanes, bus pullouts and other transit
improvements shall be evaluated on a case-by-case basis to determine
whether additional Basic Requirements apply. A safety project that
increases the traffio-carrying capacity of a roadway is not exempt from
other Basic Requirements;
• Legally non-conforming projects, except those that drain to the new
construction area and drainage improvements;
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SPOKANE REGIONAL STORMWATER MANUAL
• Maintenance projects that do not increase the traffic capacity of a
roadway or parking area, such as:
o Removing and replacing a concrete or asphalt roadway to base course
or subgrade or lower without expanding or improving the impervious
surfaces;
o Repairing a roadway base or subgrade;
o Resurfacing with in-kind material without expanding the area of
coverage;
o Overlaying existing asphalt or concrete pavement with bituminous
surface treatment (BST, commonly referred to as chip seal), asphalt or
concrete without expanding the area of coverage;
o Overlaying existing gravel with BST, asphalt or concrete, or
overlaying BST with asphalt; in either case, without expanding the
area of coverage. This partial exemption only applies if the overlaid
surface continues to drain to the existing facilities or structures and if:
♦ The road traffic surface will be subject to an average daily
traffic (ADT) volume of less than 7,500 on an urban road or
less than 15,000 on a rural road, freeway, or limited access
control highway;
♦ The parking area traffic surface will be subject to less than 40
trip ends per 1,000 square feet of building area or 100 total trip
ends; or,
2.1.6 DESIGN DEVIATION
A design deviation is an administrative approval of design elements that do not
conform to or are not explicitly addressed by this Manual. Contact the local
jurisdiction for specific design deviation procedures.
The requirements of this Manual represent the minimum criteria for the design of
stormwater management systems. Designs that offer a superior alternative to standard
measures, or creative means not yet specified in the standards, are encouraged.
Applicability
The project proponent shall request a design deviation when either of the following
situations apply:
• The project proposes non-standard methods, analysis, design elements
or materials; or,
• The project proposes design elements above maximum criteria or
below the minimum criteria found in this Manual.
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SPOKANE REGIONAL STORMWATER MANUAL
A design deviation will only be considered for review if:
• The design elements proposed do not conflict with or modify a
condition of approval; and,
• The design elements proposed are based on sound engineering
principles and best management practices, and are not inconsistent
with the public interest in stormwater control and environmental
protection.
Submittal
For consideration of a design deviation, the project proponent shall submit a design
deviation request and supporting documentation. Contact the local jurisdiction for a
design deviation form or acceptable alternative. The supporting documentation shall
include sufficient information for the local jurisdiction to make a decision as to the
adequacy of the proposed facility or design. If infiltration is proposed, negative
impacts on down-gradient properties are of concern, or seasonal high groundwater is
suspected, then a geotechnical site characterization shall be submitted as part of the
design deviation package. The design deviation package shall demonstrate that:
• There are special physical circumstances or conditions affecting the
property that may prohibit the application of some of the Basic
Requirements in this Manual;
• Every effort has been made to find alternative ways to meet the
objectives of the Basic Requirements;
• Approving the design deviation will not cause adverse impact on
down-gradient properties,public health or welfare; and,
• Approving the design deviation will not adversely affect the
recommendations of any applicable comprehensive drainage plan.
2.2 BASIC REQUIREMENTS
2.2.1 BASIC REQUIREMENT NO. 1 - DRAINAGE SUBMITTAL
Objective
Projects are expected to demonstrate compliance with all applicable Basic
Requirements through the preparation of a Drainage Submittal. The Drainage
Submittal shall include road and drainage construction plans, a drainage report that
describes the proposed measures to dispose of stormwater, and other supporting
documentation as needed. The contents of the Drainage Submittal will vary with the
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type, size and location of the project, individual site characteristics, and requirements
of the local jurisdiction.
The local jurisdiction reviews the Drainage Submittal for compliance with this
Manual and other applicable standards. Specific requirements for the Drainage
Submittal are discussed in Chapter 3.
Applicability
A Drainage Submittal is generally required for any land-disturbing activity. Land-
disturbing activities are those that result in a change in the existing soil cover (both
vegetative and non-vegetative) or site topography. The sections below summarize the
types of activities that require a Drainage Submittal, as well as those that are exempt.
A drainage submittal is alwavs required for the following types of activities:
• Projects that meet the regulatory threshold as defined in Section 2.1.1
or propose UIC facilities;
• Plats and binding site plans; and,
• Manufactured and mobile home parks.
A drainage submittal is _eg nerallv required for the following types of activities:
• Commercial building permits including institutional and multi-family
residential projects;
• Short plats;
• Change of use permits;
• Conditional use permits;
• Grading permits; and,
• Public or private road proj ects.
The following types of activities are generallv exem�t from the requirement to
prepare a drainage submittal:
• Certificates of exemption;
• Single-family residential/duplex building permits (A surface drainage
plan and other information may be required in the Ciry of Spokane,
however a full drainage submittal is generally not required);
• Temporary use permits, unless the use could cause adverse water
quality impacts or other drainage-related impacts;
• Land-disturbing activities that do not require a permit, unless the
activity could cause adverse water quality impacts or other drainage-
related impacts;
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• Underground utility projects that replace the ground surface with in-
kind material, or materials with similar runoff characteristics;
• Projects to improve motorized and/or non-motorized user safety that
do not increase the traffic capacity of a roadway. Certain safety
improvement projects such as sidewalks, bike lanes, bus pull-outs and
other transit improvements shall be evaluated case-by-case. A safety
improvement project that increases the traffio-carrying capacity is not
exempt;
• Legally non-conforming projects, except those that drain to the new
construction area and drainage improvements;
• Maintenance proj ects that do not increase the traffio-carrying capacity
of a roadway or parking area, such as:
o Removing and replacing a concrete or asphalt roadway to base course
or subgrade or lower without expanding or improving the impervious
surfaces;
o Repairing a roadway base or subgrade;
o Resurfacing with in-kind material without expanding the area of
coverage;
o Overlaying existing asphalt or concrete pavement with BST, asphalt or
concrete without expanding the area of coverage;
o Overlaying existing gravel with BST, asphalt or concrete, or
overlaying BST with asphalt; in either case without expanding the area
of coverage. This partial exemption only applies if the overlaid surface
continues to drain to the existing facilities or structures and if:
♦ The road traffic surface will be subject to an ADT volume of
less than 7,500 on an urban road or less than 15,000 on a rural
road, freeway or limited access control highway; or,
♦ The parking area traffic surface will be subject to less than 40
trip ends per 1,000 square feet of building area or 100 total trip
ends; or,
The following types of activities are exempt from the requirement to prepare a
drainage submittal:
• Commercial agriculture as regulated under RCW Chapter 8434.020,
except for the construction of impervious surfaces related to
commercial agriculture;
• Forest practices regulated under WAC Title 222, except for Class IV
General Forest Practices that are conversions from timberland to other
uses;
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SPOKANE REGIONAL STORMWATER MANUAL
• Oil and gas field activities or operations, including construction of
drilling sites, waste management pits, access roads, and transportation
and treatment infrastructure such as pipelines, natural gas treatment
plants, natural gas pipeline compressor stations and crude oil pumping
stations;
• Actions by a public utility or any other governmental agency to
remove or alleviate an emergency condition, restore utility service, or
reopen a public thoroughfare to traffic;
• Records of survey, boundary line adjustments, and property
aggregations, unless the action affects drainage tracts and easements;
• Operation and maintenance or repair of existing facilities; and,
• Road and parking area preservation/maintenance projects, such as:
o Pothole and square-cut patching;
o Crack sealing;
o Shoulder grading;
o Reshaping or regrading drainage system; or,
o Vegetation maintenance.
2.2.2 BASIC REQUIREMENT N0. 2 - GEOTECHNICAL SITE
CHARACTERIZATION
Objective
A geotechnical site characterization (GSC) is required to demonstrate suitabiliry for
stormwater disposal and to determine sub-level structure construction feasibiliry. A
geotechnical engineer shall perform the study in accordance with the criteria specified
in Chapter 4.
Applicability
A GSC will be required for most projects. The scope and geographic extent of the
investigation may vary depending on the general location and setting of the site, the
characteristics of the target soil deposits, and whether there are known or anticipated
drainage problems in the vicinity of the site.
A GSC is required for:
• Projects proposing infiltration (drywells, detention facilities receiving
credit for pond bottom infiltration, etc.) or non-standard drainage
systems;
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• Projects located in a Special Drainage Area(SDA) or Special Drainage
District (SDD);
• Projects located within or draining to a problem drainage area or study
area as recognized by the local jurisdiction;
• Projects with administrative conditions requiring a GSC; or,
• Projects with proposed sub-level structures, as required by the local
jurisdiction.
In areas where there has been a long-standing record of satisfactory performance of
standard subsurface disposal facilities and no drainage problems are known to exist, the
minimum requirements found in Section 4.3 may be reduced or waived after a formal
written request from the project proponent's engineer has been reviewed and accepted by
the local jurisdiction.
2.2.3 BASIC REQUIREMENT N0. 3 - WATER QUALITY
TREATMENT
Objective
Water qualiry treatment is required to reduce pollutant loads and concentrations in
stormwater and can be achieved using physical, biological, and chemical removal. An
analysis of the proposed land use at the project site is used to determine the pollutants
of concern and the appropriate treatment methods to apply.
The most effective basic treatment best management practices (BMPs) remove about
80% of the total suspended solids contained in the runoff treated and a much smaller
percentage of the dissolved pollutants. Additional treatment to remove oil, metals,
and/or phosphorus from stormwater runoff may be required.
The BMPs described in Chapter 6 are designed to reduce or eliminate certain
pollutants. For discharges to UIC facilities, the selected BMPs must remove or reduce
the target pollutants to levels that will comply with state groundwater quality
standards when the discharge reaches the water table or first comes into contact with
an aquifer(see WAC 173-200). Discharges to surface waters shall comply with WAC
173-201A, Water Quality Standards for Surface Waters of the State of Washington.
The goal of this Manual is for stormwater facilities to treat approximately 90% of the
annual runoff from the pollutant-generating impervious surfaces (PGIS) at a project
site. The total quantity of pollutants removed from the stormwater will vary greatly
from site to site based on precipitation patterns, land use, effectiveness of source
control, and operation and maintenance of the treatment facilities. When required,
treatment facilities shall be designed according to the criteria specified below and in
Chapter 6.
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Design C�iteria
The 6-month NRCS (Natural Resources Conservation Service) Type II 24-hour storm
event is the water quality design storm for both volume-based and flow rate-based
water quality BMPs. Please refer to Chapter 5 for complete design guidance.
Applicability
Any exemptions for this Basic Requirement are superseded by requirements set forth
in any applicable total maximum daily load (TMDL) or other water cleanup plan. At
the time of the writing of this Manual, no TMDLs exist for water bodies in Spokane
County. Contact the local jurisdiction for current information on whether any TMDLs
have been issued.
Basic Treatment A�licabilit� Basic treatment provides removal of total suspended
solids (TSS) and is required for all projects proposing UIC facilities that are:
• Located within the moderate or high susceptibiliry areas of the Critical
Aquifer Recharge Area(CARA);
• Located within Township 26 North Range 43 East (excluding the
delineated low susceptibiliry areas isolated in the northeast corner of
this Township and Range);
• Located within a 1,000-foot radius of Group A and Group B wells
without reported plans;
• Located within a Department of Health approved wellhead protection
area;
• Proposing a moderate-use, high-use or high-ADT site and located in
the low or moderate susceptible areas of the CARA;
• Located within the Aquifer Sensitive Area(ASA)boundaries; or
• Located within the City of Spokane.
Basic treatment is also required for all projects:
• Meeting the regulatory threshold and discharging to waters of the
state, including perennial and seasonal streams, lakes and wetlands;
• Requiring a 401 Water Quality Certification; or,
• Regulated to provide water quality treatment under other rules outside
of Phase II jurisdictions.
Basic treatment is not required for:
• Non-pollutant generating impervious surface (NPGIS) areas unless the
runoff from these areas is hydraulically connected to PGIS areas;
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• Projects that discharge to the subsurface and are located within the low
susceptibility areas of the CARA and are not proposing moderate-use,
high-use, or high-ADT sites; or,
• Projects discharging non-waste fluids from roofs (WAC 173-218) directly
to drywells.
Oil Control A�licability: All projects that meet the regulatory threshold and are
high-use or high-ADT areas are required to provide oil control. High-use sites
generate high concentrations of petroleum hydrocarbons due to high traffic turnover
or the frequent transfer of oil and/or other petroleum products.
High-use sites and high-ADT roadways and parking areas shall treat runoff from the
high-use portion of the site prior to discharge or infiltration. For high-use sites located
within a larger project area, only the impervious area associated with the high-use site
is subject to oil control treatment, as long as the flow from that area is separated;
otherwise the treatment controls shall be sized for the entire area.
Non-high-use sites and non-high ADT sites are exempt from oil treatment
requirements.
Metals Treatment A�licabilitv: Metals treatment is required for all projects that are
moderate- or high-use sites, and for sites that discharge to a surface water or UIC
facility and meet any of the following definitions:
• Industrial sites as defined by the EPA (40 CFR 12226(b)(14)) with
benchmark monitoring requirements for metals;
• Industrial sites that handle, store, produce, or dispose of inetallic
products or other materials, particularly those containing arsenic,
cadmium, chromium, copper, lead, mercury, nickel or zinc;
• High-use or high-ADT roadways or parking areas;
• Urban roads with expected ADT greater than 7,500;
• Rural roads or freeways with expected ADT greater than 15,000;
• Commercial or industrial sites with an equivalent trip end (ETE) count
equal to or greater than 40 vehicles per 1,000 square feet of gross
building area;
• Parking lots with 100 ETE or more;
• Public on-street parking in commercial and industrial zones;
• Highway rest areas;
• Runoff from metal roofs not coated with an inert, non-leachable
material; or
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• Discharge to a surface water of the state that has been identified
through a TMDL or other water clean-up plan as requiring metals
removal.
Stormwater runoff is exempt from metals treatment requirements in the following
situations, unless a specific water quality problem has been identified:
• Discharges to non-fish bearing streams;
• Subsurface discharges, unless identified as hydraulically connected to
surface waters of the State; the Spokane Valley Rathdrum Prairie
Aquifer is hydraulically connected to a surface water of the State;
• Restricted residential and employee-only parking areas, unless subject
to through traffic;
• Preservation/maintenance projects and some improvement or safety
enhancement projects that do not increase motorized vehicular
capacities; and,
• Discharges to some Category 4 wetlands; contact the Washington
Department of Ecology for additional information
Phosphorus Treatment A�licabilitv: Phosphorus treatment is required where it has
been determined by the federal, state, or local government that a water body is
sensitive to phosphorus and that a reduction in phosphorus from new development
and redevelopment is necessary to achieve the water qualiry standard to protect its
beneficial uses. Where it is deemed necessary, a strategy will be adopted to achieve
the reduction in phosphorus. The strategy will be based on knowledge of the sources
of phosphorus and the effectiveness of the proposed methods of removing
phosphorus.
Phosphorus treatment may be required for water bodies reported under Section 305(b)
of the Clean Water Act and for those listed in Washington State's Non-point Source
Assessment required under Section 319(a) of the Clean Water Act.
The Spokane River has been designated as not supporting beneficial uses due to
phosphorus, and phosphorus treatment may be required.
Projects that do not propose to discharge to a water body sensitive to phosphorus are
exempt from phosphorus treatment requirements.
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2.2.4 BASIC REQUIREMENT N0. 4 - FLOW CONTROL
Objective
Flow control facilities are necessary to protect stream morphology and habitat and to
mitigate potential adverse impacts on down-gradient properties and floodplains due to
the increase in stormwater runoff caused by land development.
Unless specifically approved by the local jurisdiction, the peak rate and volume of
stormwater runoff from any proposed land development to any natural or constructed
point of discharge downstream shall not exceed the pre-development peak rate or
volume of runof£ A down-gradient analysis demonstrating that there will be no
expected adverse impacts on downgradient properties will be required (refer to
Section 3.4.5 for down-gradient analysis criteria). Local jurisdictions reserve the right
to deny a request for increased stormwater flows or to condition any approval at their
sole discretion.
Exceptions with regard to rate and volume control can be made for regional facilities
planned by a local jurisdiction.
When site conditions allow, infiltration is the preferred method of flow control for
urban runoff. All projects are encouraged to infiltrate stormwater runoff on site to the
greatest extent possible if such infiltration will not have adverse impacts on down-
gradient properties or improvements. Flow control facilities shall be designed and
constructed according to the criteria in Chapters 5 and 7.
Design Criteria
The NRCS Type IA 24-hour storm event is the design storm for all flow control
facilities that use a surface discharge or a combined surface and subsurface system.
Flow control facilities that use only infiltration into the subsurface may use either the
NRCS Type IA or Type II 24-hour storm event.
Infiltration Facilities: For projects proposing infiltration, the facilities shall be
designed based on the 10-year design storm event. The facility shall be designed to
bypass storm events that exceed the 10-year design storm frequency and shall provide
an overIlow path, wherever possible, with the capacity to convey the 100-year storm
event. The overflow path shall drain toward the natural discharge point of the
contributing basin, such that the overflow route or termination of stormwater does not
adversely impact down-gradient properties or structures.
Detention Facilities: For projects proposing to detain and release stormwater runoff,
the facilities shall be designed such that the release rate does not exceed the pre-
developed conditions for a range of storm events. The analysis of multiple design
storms is needed to control and attenuate both low and high flow storm events.
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The total post-developed discharge rate leaving the site (including bypass flow) shall
be limited to the pre-development rates listed in Table 2-1. Bypass flow is the runoff
that leaves the site without being conveyed through the drainage system.
TABLE 2-1
ALLOWABLE DISCHARGE RATES
Design F requency Post-Developed
(24-hour storm) Discharge Rate�
2-year < 2-year pre-developed
25-year < 25-year pre-developed
100-year� (Emergency Overflow) Overflow route only
�Post-developed flow is equal to the release from detention facility plus the bypass flow
�The emergency overflow shall direct the 100-year post-developed flow safely toward the
downstream conveyance system
Evaporation Facilities: For projects proposing to evaporate runoff as the means of
stormwater disposal, the facilities shall be designed to control the mean annual
precipitation. Design shall meet the criteria described in Section 7.7.2.
Applicc�bility
All projects that meet the regulatory threshold shall comply with this Basic
Requirement.
Projects are exempt from flow control if they discharge to any of the following:
• The Spokane River or other exempt water bodies, which are defined in
the Stormwater Management Manual for Eastern Washington as fifth-
order or greater stream channels, as determined from a 1:150,000 scale
map;
• A river or stream that is fifth-order or greater as determined from a
1:24,000 scale map;
• A river or stream that is fourth-order or greater as determined from a
1:100,000 scale map;
• A stream that flows only during runoff-producing events. These
streams are defined as those that do not discharge via surface flow to a
non-exempt surface water following receipt of the 2-year, NRCS
Type lA, 24-hour rainfall event. In addition, for the stream to be
exempt, it shall be able to carry the runoff from an average snowmelt
event, but shall not have a period of base flow during a year of normal
precipitation;
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• A lake or reservoir with a contributing watershed areas greater than
100 square miles;
• A reservoir with outlet controls that are operated for varying
discharges to the downstream reaches as for hydropower, flood
control, irrigation or drinking water supplies (discharges to
uncontrolled flow-through impoundments are not exempt).
In order to be exempted the discharge shall meet all of the following requirements:
• The project area must be drained by a conveyance system that consists
entirely of manmade conveyance elements (i.e. pipes, ditches, outfall
protection); and,
• The conveyance system must extend to the ordinary high water mark
line of the receiving water, or (in order to avoid construction activities
in sensitive areas) flows are properly dispersed before reaching the
buffer zone of the sensitive or critical area; and,
• Any erodible elements of the conveyance system for the project area
must be adequately stabilized to prevent erosion; and,
• Surface water from the project area must not be diverted from or
increased to an existing wetland, stream, or near-shore habitat
sufficient to cause a significant adverse impact. Adverse impacts are
expected from uncontrolled flows causing a significant increase or
decrease in the 1.5- to 2-year peak flow rate.
Maps shall be standard U.S. Geological Survey (USGS) maps or geographic
information system (GIS) data sets derived from USGS base maps.
Floodplains
Projects proposed in or around identified Areas of Special Flood Hazard shall
conform to National Flood Insurance Program and the flood ordinance of the local
jurisdiction. Refer to Section 7.9.2 for specific requirements. Projects discharging to
the Spokane River or other exempt water bodies shall comply with floodplain
requirements.
2.2.5 BASIC REQUIREMENT N0. 5 - NATURAL AND
CONSTRUCTED CONVEYANCE SYSTEMS
Objective
A conveyance system includes all natural or constructed components that collect
stormwater runoff and convey it away from structures in a manner that adequately
drains sites and roadways, minimizing the potential for flooding and erosion.
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Engineered conveyance elements for proposed projects shall be analyzed, designed,
and constructed to provide a minimum level of protection against damage to property
and improvements from uncontrolled or diverted flows, flooding and erosion.
Projects shall be designed to protect certain natural drainage features including
floodplains, drainageways, and natural depressions that store water or allow it to
infiltrate into the ground. These features are collectively referred to as the "natural
location of drainage systems" (NLDS). Preserving the NLDS will help ensure that
stormwater runoff can continue to be conveyed and disposed of at its natural location.
Preservation also increases the opportunity to use the predominant systems as
regional stormwater facilities. Refer to Chapter 8 for more information on NLDS.
Stormwater runoff shall be discharged in the same manner and at the same location as
in the pre-developed condition, unless otherwise specifically accepted by the local
jurisdiction. Stormwater runoff shall not be concentrated onto down-gradient
properties where sheet flow previously existed and shall not be diverted to points not
receiving stormwater runoff prior to development.
Applicc�bility
All projects shall comply with this Basic Requirement regardless of whether they
meet the regulatory threshold.
Design Crite�ia
Natural and Constructed Channels: Constructed and natural channels shall be
designed with sufficient capacity to convey, at a minimum, the depth associated with
the 50-year design storm event peak flow rate plus an additional 30%, assuming
developed conditions for on-site tributary areas and existing conditions for any off-
site tributary areas. Refer to Chapter 8 for additional criteria.
The design shall provide bypass for storm events that exceed the above criteria and
shall provide an overflow path with capacity to convey the 100-year storm event,
wherever possible. The overflow path shall drain toward the natural discharge point
of the contributing basin, such that the overflow route or termination of stormwater
does not adversely impact down-gradient properties or structures.
Culverts: New culverts shall be designed with sufficient capacity to convey the 50-
year design storm event assuming developed conditions for the on-site basin and
existing conditions for the off-site basin.
New culverts shall be designed with sufficient capacity to meet the headwater and
tailwater requirements in Chapter 8.
Gutters: Gutter flows in roadways shall allow for the passing of vehicular traffic
during the 10-year design storm event by providing non-flooded zones. For paved
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SPOKANE REGIONAL STORMWATER MANUAL
roadways, the non-flooded width requirement varies with the road classification. The
design shall meet the criteria specified in Chapter 8.
Storm Drain Svstems and Inlets: The Rational Method and the 10-year design storm
shall be used to size the conveyance system regardless of the method used to size the
disposal facility.
Enclosed systems may surcharge or overtop drainage structures for storm events that
exceed the drainage facility design storm, as long as an overflow path is provided,
wherever possible. The overflow path shall be capable of conveying the 100-year
storm event and should either drain toward the natural discharge point of the
contributing basin (preferred) or away from adjacent buildings, residences, etc. so as
to avoid adverse impacts due to flooding.
2.2.6 BASIC REQUIREMENT N0. 6 - EROSION AND SEDIMENT
CONTROL
Objective
During the construction phase, sediment-laden runoff can enter newly constructed or
existing drainage facilities, thus reducing their infiltration or treatment capacity and
their lifetime of operation, or increasing maintenance costs.
Controlling erosion and preventing sediment and other pollutants from leaving the
project site during the construction phase is achievable through implementation and
selection of BMPs that are appropriate both to the site and to the season during which
construction takes place.
The objectives of the erosion and sediment control (ESC)Plan are to:
• Protect existing or proposed stormwater management infrastructure;
• Minimize the impacts of erosion, sedimentation and increased runoff
caused by land-disturbing activities on private property, public roads
and rights-of-way, and water bodies;
• Protect the health, safety and welfare of the general public (this
objective shall not be construed to establish any duties to protect or
benefit any particular person or class of persons); and,
• Protect water quality.
Applicability
Land-disturbing activities are activities that result in a change in existing soil cover
(vegetative or non-vegetative) or site topography. Land-disturbing activities include,
but are not limited to, demolition, construction, clearing and grubbing, grading and
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SPOKANE REGIONAL STORMWATER MANUAL
logging. An ESC plan may not be required for all of these situations; however that
does not relieve the proponent from the responsibility of controlling erosion and
sediment during construction nor the liability for damage claims associated with
adverse impacts on off-site properties.
The following land-disturbing activities require an ESC plan:
• Major land-disturbing activities involving 1 acre or more of disturbed
area; or,
• Minor land-disturbing activities, such as grading, involving less than 1
acre of disturbed area but requiring a permit by the local jurisdiction.
An ESC plan is rypically not required for the projects listed in Section 2.1.4.
An ESC plan, when required, shall be submitted with either the road and drainage
plans or the permit application, prior to any land-disturbing activity. Clearing and
grading activities for developments will be permitted only if conducted pursuant to an
accepted site development plan that establishes permitted areas of clearing, grading,
cutting, and filling. When establishing these permitted clearing and grading areas,
consideration shall be given to minimizing removal of existing trees and minimizing
disturbance and compaction of native soils except as needed for building purposes.
These permitted clearing and grading areas and any other areas with a preservation
requirement, such as critical or sensitive areas, buffers, native growth protection
easement areas or tree retention areas, shall be delineated on the site plans and
development site plan. ESC plans are only required to address the area of land that is
subject to the land-disturbing activiry for which a permit is being requested and the
area of land that will serve as the stockpile or staging area for materials.
All ESC plans shall adhere to the minimum requirements specified in Chapter 9 of
this Manual. Examples and descriptions of the BMPs referenced in this Manual can
be found in the most current version of the Stormwater Management Manual for
Eastern Washington (SMMEW) available online at
www.ecv.wa.,�ov/pro,�rams/wq/stormwater/tech.html.
2.2.7 BASIC REQUIREMENT N0. 7 - SOURCE CONTROL
Objective
The intent of source control BMPs is to prevent pollutants from coming into contact
with stormwater, thereby reducing the likelihood that pollutants will enter
groundwater and violate water quality standards. Source control BMPs are a cost-
effective means of reducing pollutant loading and concentrations in stormwater and
should be a first consideration in all projects.
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SPOKANE REGIONAL STORMWATER MANUAL
Applicability
All projects, unless exempted in Section 2.1.4, shall comply with this Basic
Requirement. Project proponents are required to implement applicable source controls
through the use of BMPs as specified in Chapter 8 of the Stormwater Management
Manual for Eastern Washington.
A project proponent is not relieved from the responsibility of preventing pollutant
release from coming in contact with stormwater, whether or not the project exceeds
the regulatory threshold.
2.2.8 BASIC REQUIREMENT N0. 8 - OPERATION AND
MAINTENANCE
Objective
To ensure that stormwater control facilities are adequately maintained and properly
operated, documentation describing the applicable preventive maintenance and
recommended maintenance schedule shall be prepared and provided to the entity
responsible for maintaining the stormwater system.
For drainage ponds and other drainage facilities outside of the public road right of
way, the project proponent shall provide the financial means and arrangements for the
perpetual maintenance of the drainage facilities.
Proponents shall operate and maintain the facilities in accordance with an operation
and maintenance plan that meets the criteria specified in Chapter 1l. The operation
and maintenance plan shall also include applicable source control BMPs, as described
in Chapter 10.
Applicability
All projects that meet the regulatory threshold and that propose drainage facilities or
structures shall comply with this Basic Requirement. All projects that propose UIC
facilities also must comply with the operation and maintenance requirements,
regardless of whether they exceed the regulatory threshold.
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SPOKANE REGIONAL STORMWATER MANUAL
CHAPTER 3 - DRAINAGE SUBMITTAL
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Chapter Organization
3.1 Introduction......................................................................................................................3-1
3.2 Applicability ....................................................................................................................3-1
3.2.1 Required...................................................................................................................3-1
3.22 Generally Required..................................................................................................3-1
3.2.3 Generally Exempt....................................................................................................3-2
3.2.4 Exempt.....................................................................................................................3-3
3.3 Concept Drainage Report.................................................................................................3-4
3.3.1 In tro du c ti on..............................................................................................................3-4
3.32 Applicability ............................................................................................................3-4
3.3.3 Exemptions ..............................................................................................................3-5
3.3.4 Scope........................................................................................................................3-5
3.4 Drainage Report...............................................................................................................3-7
3.4.1 Introducti on..............................................................................................................3-7
3.42 Narrative ..................................................................................................................3-7
3.4.3 Figures......................................................................................................................3-9
3.4.4 Calculations............................................................................................................3-10
3.4.5 Down-Gradient Analysis .......................................................................................3-11
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3.5 Road and Drainage Plans...............................................................................................3-13
3.5.1 Introduction............................................................................................................3-13
3.52 Minimum Plan Elements........................................................................................3-13
3.5.3 Revisions After Plan Acceptance...........................................................................3-14
3.6 Other Submittal Elements..............................................................................................3-14
Appendix 3A —Example Tables for Drainage Report...............................................................3-17
Appendix 3B —Standard Notes for Road and Drainage Plans ..................................................3-19
Appendix 3C —Lot Plan Criteria and Example.........................................................................3-21
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3.1 INTRODUCTION
This chapter describes the contents of a Drainage Submittal and provides a framework for
preparing the submittal in order to promote consistency throughout the Spokane region.
Specific best management practices (BMPs), design methods and standards to be used are
contained in Chapters 4 through 11. Properly drafted construction engineering plans and
supporting documents will help facilitate the operation and maintenance of the proposed
system long after its review and acceptance.
The Drainage Submittal is a comprehensive report containing all of the technical
information and analysis necessary for regulatory agencies to evaluate a proposed new
development or redevelopment project for compliance with stormwater regulations. At a
minimum, the Drainage Submittal shall include Construction Plans, Erosion and
Sediment Control Plans, and Drainage Calculations. Other supporting documentation
shall be submitted as needed. Contents of the Drainage Submittal will vary with the rype
and size of the project, individual site characteristics, and special requirements of the
local jurisdictions.
State law requires that engineering work for the Drainage Submittal be performed by or
under the direction of a professional engineer currently licensed in the state of
W ashington.
3.2 APPLICABILITY
A Drainage Submittal is generally required for any land-disturbing activity. Land-
disturbing activities are those activities that result in a change in the existing soil cover
(both vegetative and non-vegetative) or site topography. The following sections
summarize the activities that require a Drainage Submittal as well as those that are
exempt.
3.2.1 REQUIRED
• Projects that meet the regulatory threshold as defined in Section 2.1.1 or
propose UIC facilities;
• Plats and binding site plans; and,
• Manufactured and mobile home parks.
3.2.2 GENERALLY REQUIRED
• Commercial building permits including institutional and multi-family
residential projects;
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• Short plats;
• Change of use permits;
• Conditional use permits;
• Grading permits; and,
• Public or private road proj ects.
3.2.3 GENERALLY EXEMPT
• Certificates of exemption;
• Single-family residential/duplex building permits (A surface drainage plan
and other information may be required in the Ciry of Spokane, however a
full drainage submittal is generally not required;
• Temporary use permits, unless the use could cause adverse water quality
impacts or other drainage-related impacts;
• Land-disturbing activities that do not require a permit, unless the activity
could cause adverse water quality impacts or other drainage-related
impacts;
• Underground utility projects that replace the ground surface with in-kind
material, or materials with similar runoff characteristics;
• Projects to improve motorized and/or non-motorized user safety that do
not increase the traffic capacity of a roadway. Certain safety improvement
projects such as sidewalks, bike lanes, bus pull-outs and other transit
improvements shall be evaluated case-by-case. A safety improvement
project that increases the traffic-carrying capacity is not exempt;
• Legally non-conforming projects, except those that drain to the new
construction area and drainage improvements;
• Maintenance projects that do not increase the traffio-carrying capacity of a
roadway or parking area, such as:
o Removing and replacing a concrete or asphalt roadway to base course
or subgrade or lower without expanding or improving the impervious
surfaces;
o Repairing a roadway base or subgrade;
o Resurfacing with in-kind material without expanding the area of
coverage;
o Overlaying existing asphalt or concrete pavement with BST, asphalt or
concrete without expanding the area of coverage;
o Overlaying existing gravel with BST, asphalt or concrete, or
overlaying BST with asphalt; in either case, without expanding the
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area of coverage. This partial exemption only applies if the overlaid
surface continues to drain to the existing facilities or structures and if:
♦ The road traffic surface will be subject to an ADT volume of
less than 7,500 on an urban road or less than 15,000 on a rural
road, freeway, or limited access control highway; or,
♦ The parking area traffic surface will be subject to less than 40
trip ends per 1,000 square feet of building area or 100 total trip
ends; or,
3.2.4 EXEMPT
• Commercial agriculture as regulated under Revised Code of Washington
(RCW) Chapter 8434.020, except for the construction of impervious
surfaces related to commercial agriculture;
• Forest practices regulated under Washington Administrative Code (WAC)
Title 222, except for Class N General Forest Practices that are
conversions from timberland to other uses;
• Oil and gas field activities or operations, including construction of drilling
sites, waste management pits, access roads, and transportation and
treatment infrastructure such as pipelines, natural gas treatment plants,
natural gas pipeline compressor stations and crude oil pumping stations;
• Actions by a public utility or any other governmental agency to remove or
alleviate an emergency condition, restore utility service, or reopen a public
thoroughfare to traffic;
• Records of survey, boundary line adjustments, and property aggregations,
unless the action affects drainage tracts and easements;
• Operation and maintenance or repair of existing facilities; and,
• Road and parking area preservation/maintenance projects, such as:
o Pothole and square-cut patching;
o Crack sealing;
o Shoulder grading;
o Reshaping or regrading drainage system; or,
o Vegetation maintenance.
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3.3 CONCEPT DRAINAGE REPORT
3.3.1 INTRODUCTION
Concept Drainage Reports are used by staff of the local jurisdiction to preliminarily
assess the drainage requirements on certain land-use actions and land development
permits. The purpose of the Concept Drainage Report is to demonstrate that the
proposed drainage facilities can meet the intent of this Manual and are feasible with
respect to design, construction, and maintenance. Its contents are similar to those of
the Drainage Report described in Section 3.4, though for many items the Concept
Drainage Report does not require as much detail as the Drainage Report.
Preparation of a Concept Drainage Report is an initial step in the Drainage Submittal
process, and acceptance of a Concept Drainage Report does not imply that the
concept proposed is accepted as the final design. Acceptance only implies that the
project proponent (or his agent) has demonstrated that stormwater disposal is feasible.
It does not relieve the project proponent from a geotechnical site characterization
(refer to Chapter 4), a down-gradient analysis, or changes to the design that may be
necessary in order to meet the criteria and standards presented in this Manual.
3.3.2 APPLICABILITY
The need for a Concept Drainage Report varies depending upon the nature, scope and
complexity of the proposed project and the existing drainage conditions. Due to the
number of variables involved, this Manual does not include an exhaustive listing of
all scenarios that may require a Concept Drainage Report. Concept Drainage Reports
will be required for any of the following situations, and the local jurisdiction has the
authority to require a Concept Drainage Report for any other project.
• A Certificate of Concurrency is required per the City of Spokane
Municipal Code "Concurrency Certification;"
• The project lies within or drains to critical areas, as designated by the local
jurisdiction;
• The project lies within or drains to an official 100-Year Flood Zone as
mapped by the Federal Emergency Management Agency (FEMA) (See
Section 7.9.2);
• The project lies within or drains to a Special Drainage Area (SDA) or
study area as recognized by the local jurisdiction (See Section 7.9.1);
• The project lies within or drains to an area identified as having drainage
problems;
• The project lies within or drains to an area identified as having
floodplains;
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• The project is especially large, phased, or master-planned and may require
interim facilities;
• The project involves significant off-site drainage and relies upon a
predominant drainageway;
• The project site has features that can be classified as Natural Locations of
Drainage Systems (NLDS) (refer to 82.4 for definition);
• The project has the potential to impact existing or future regional
stormwater facilities;
• The project proposes non-standard stormwater treatment BMPs; and,
• The project proposes conventional subsurface disposal systems in areas
that are typically not conducive to subsurface disposal (such as where
shallow groundwater or other limiting layers may be present).
3.3.3 EXEMPTIONS
In unincorporated Spokane County and the Ciry of Spokane Valley, standard drainage
systems proposed in deep free-draining soils are generally exempt from the Concept
Drainage Report requirement. This exemption only applies to sites that do not involve
significant off-site drainage issues and are not located in or near a known drainage
problem area, floodplain or critical area.
3.3.4 S C O P E
The Concept Drainage Report shall demonstrate that the existing or proposed
drainage infrastructure is adequate to control the increase in runoff due to the
proposed project by meeting the minimum requirements of this Manual.
A detailed design is not required at this stage because the exact nature of the proposal
may not be certain. However, the Concept Drainage Report shall provide sufficient
information and analysis to demonstrate that adequate infrastructure can be provided.
Due to varying design parameters, design challenges, and potential solutions, the
level of requirements can change from site to site. For example, if an infiltration
facility is proposed in an area known to have shallow groundwater or bedrock, a
geotechnical site characterization would be required at the Concept Drainage Report
stage to support the use of infiltration. But a concept drainage report proposing an
evaporation facility in the same area would not typically require geotechnical work at
this stage.
Any alternatives that will be considered by the project proponent during final design
shall be included in the Concept Drainage Report. For projects that require a public
hearing, all alternatives shall be presented for review by staff and the public prior to
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the hearing. If the concept does not include any other alternatives, it will be assumed
that the accepted concept is the final design concept.
The accepted Concept Drainage Report shall be implemented in the final construction
plans. New or significantly altered conceptual elements on the final plans are subject
to reconsideration or denial.
The Concept Drainage Report shall include the following elements:
• Narrative: The narrative shall generally follow Section 3.42 and shall
describe all proposed methods and alternatives for stormwater treatment
and disposal, as well as provide sufficient information, supporting
technical data, assumptions, design criteria, and drainage calculations to
demonstrate that the proposed stormwater system will meet the
requirements of this Manual. If phasing is anticipated, an explanation of
how the drainage system will be phased and constructed shall also be
included;
• Schematic: The schematic plan of the proposed stormwater system shall
show the approximate size and location of all drainage components;
• Basin Map: The drainage basin maps shall generally follow the
requirements for maps presented in Section 3.4.3, but may be less detailed;
• Geotechnical Information: If a non-standard disposal system or infiltration
is proposed, then sufficient site characterization work shall be completed
in accordance with Chapter 4 to demonstrate that the proposed facilities
will function as intended;
• Drainage Features: The report shall generally follow the requirements for
pre-development basin information presented in Section 3.4.2, but may be
less detailed;
• Critical Areas: The report shall generally follow the requirements for
critical areas presented in Section 3.42, but may be less detailed;
• Per�etual Maintenance of Facilities: The report shall generally follow the
requirements for Perpetual Maintenance of Facilities presented in Section
3.42, but may be less detailed;
• Offsite Easements: The report shall follow the requirements for off-site
easements presented in Section 3.4.2; and,
• Re�ional Facilities: The report shall follow the requirements for regional
facilities presented in Section 3.4.2.
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3.4 DRAINAGE REPORT
3.4.1 INTRODUCTION
The purpose of the Drainage Report is to identify drainage impacts resulting from
land development activities and determine the improvements necessary to control the
increase in stormwater runoff and to treat the pollutants that can adversely impact
water quality.
A Drainage Submittal package is required after a formal decision on a land use action
has been made and the conditions of approval have been accepted. If a formal
decision is not required for the project, the Drainage Submittal package is submitted
as part of the project permit application.
The Drainage Report shall be inclusive, clear, legible, and reproducible. An
uninvolved third parry shall be able to review the Drainage Submittal and determine
whether all applicable standards in this Manual have been met.
The basic elements of a Drainage Report are summarized in the following sections.
3.4.2 NARRATIVE
The drainage report narrative shall include the following elements:
• Project Description: The project description shall include information
about the size of the project, the number of lots proposed, the project
location (including Section, Township and Range), and background
information relevant to drainage design, including topography, surface
soils, surface and vegetative conditions, etc.;
• Geotechnical Information: This part of the narrative shall summarize the
geotechnical site characterization (GSC) for the project including
recommended outflow rates for drywells, infiltration rates and on-site soil
descriptions;
• Pre-Development Basin Information: This information shall summarize
the pre-development drainage patterns for all basins contributing flow to,
on, through, and from the site. This section shall include all assumptions
and justifications used to determine curve numbers and/or runoff
coefficients used in the analysis, including a table that presents existing
impervious and pervious areas as shown in the example in Appendix 3A.
The narrative shall identify and discuss all existing on-site and/or off-site
drainage facilities, natural or constructed, including but not limited to
NLDS, conveyance systems, and any other special features on or near the
proj ect;
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• Post-Develo�ment Basin Information: This information shall summarize
all assumptions used to determine the characteristics of the post-developed
basins, such as the size of roofs and driveways, and the curve numbers
and/or runoff coefficients used in the analysis (refer to Appendix 3A for
an example table). In addition, a table shall be included that summarizes
the impervious and pervious areas for each subbasin, as shown in
Appendix 3A;
• Critical Areas: If the local jurisdiction's Critical Areas Ordinance requires
a Critical Areas evaluation for the project proposal, any required
mitigation measures shall be incorporated into the proposed drainage
facility design and addressed in the Drainage Report and Road and
Drainage Plans;
• Down-Gradient Analvsis: This analysis shall identify and discuss the
probable impacts down-gradient of the project site. Refer to Section 3.4.5
for additional requirements;
• Methodolo�v: The hydraulic methods and storm events used in sizing the
drainage facilities, including the BMPs proposed for the project, shall be
discussed;
• Water Qualitv Treatment: A discussion of treatment requirements, based
on the criteria in Chapter 6, shall be included;
• Results: The results of the calculations and a description of the proposed
stormwater facilities shall be included. When applicable, a table
comparing the pre-developed and post-developed conditions including
rates and volumes shall also be included. A table shall be provided when
applicable, summarizing the maximum water elevation of the facilities for
the design storms, outflow structure information, the size of facilities
"required" by the calculations, and the size of the facilities "provided" in
the proposed design (refer to Appendix 3A for an example table);
• Operational Characteristics: Sufficient information shall be provided about
the operation of the stormwater system so that an uninvolved third party
can read the report and understand how the proposed system will function
under various conditions.
• Per�etual Maintenance of Facilities: A discussion shall be included of the
provisions set forth to operate and maintain the drainage facilities. The
project proponent's mechanism for funding the operation and maintenance
for stormwater facilities, including sinking fund calculations, shall be
included (refer to Chapter 11 for operation and maintenance requirements
and Chapter 10 for source control requirements);
• Off-Site Easements: The anticipated location of any off-site easements
shall be identified either on the basin map or in a separate schematic. Off-
site easements will be required for proposed stormwater conveyance or
disposal facilities outside the project boundaries. These easements shall be
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obtained and recorded prior to the acceptance of the final Drainage
Submittal (refer to Chapter 11 for more information); and,
• Re�ional Facilities: A discussion of any expected future impacts on or
connections to existing or proposed regional facilities shall be included
(refer to Section 7.10).
3.4.3 FIGURES
Basin Map
The drainage report shall include a basin map. In most cases, both a pre-developed
and post-developed basin map shall be provided. The minimum elements required
include the following:
• Vicinity map,project boundaries, and section, township and range;
• Basin limits:
o Basin limits shall include on-site, off-site, and bypass areas
contributing runoff to or from the project;
o In all cases, the engineer shall field-verify the basin limits, including
any contributing off-site areas, and shall describe how the basin limits
were determined.
o Drainage basins shall be clearly labeled and correlated with the
calculations;
• Time of concentration routes, with each segment clearly labeled and
correlated with the calculations;
• Labeled topographic contours:
o Contours shall extend beyond the project or drainage basin boundaries
to the extent necessary to confirm basin limits used in the calculations.
o For commercial projects, spot elevations may be acceptable in lieu of
contours on post-developed basin maps.
o Projects in an urban area shall use a maximum contour spacing of
2 feet.
o At the discretion of the local jurisdiction, projects outside an urban
area, such as a large lot subdivision, may use the best available
topographic information; this may involve contours on a scale larger
than the 2-foot minimum.
• Any NLDS including, but not limited to, natural or constructed drainage
features, wetlands, creeks, streams, seasonal drainageways, closed
depressions, ditches, culverts, storm drain systems and drywells;
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• Floodplain limits, as defined by FEMA or other studies;
• Any geologically hazardous areas;
• Footprint of proposed drainage facilities such as ponds, infiltration
facilities,pipes, and ditches;
• North arrow and scale;
• Existing and proposed easements; and,
• Adjacent streets.
Other Figures
• Soils map;
• Site photos;
• Any graphs, charts, nomographs, maps, or figures used in the design,
when applicable; and,
• If infiltration is proposed, a geotechnical site characterization is typically
required (refer to Chapter 4). As part of that study, a geologic cross-
section of the stormwater disposal area drawn to scale shall be included.
The proposed stormwater disposal facilities shall be superimposed on the
cross-section. All relevant geologic units shall be clearly identified
including the target disposal layer and limiting layers.
3.4.4 CALCULATIONS
Calculations shall be presented in a logical format and provide sufficient information
to allow an uninvolved third party to reproduce the results. All assumptions, input and
output data, and variables listed in computer printouts and hand calculations shall be
clearly identified. Basins and design storm events shall be clearly identified on all
calculations.
The Drainage Report shall incorporate all calculations used to determine the size of
the facilities. Typical calculations include, but are not limited to:
• Hydrologic/hydraulic calculations including pre- and post-developed peak
rate and volume calculations, routing calculations, design information for
outflow structures, orifice information, a pond volume rating table or pond
volume calculations, etc.;
• Time of concentration calculations;
• Curve number(CN) or runoff coefficient (C)
• Water budget calculations;
• Water quality treatment calculations;
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• Inlet capacity and bypass calculations;
• Detention/retention storage capacities;
• Calculations for ditches and natural channels;
• Culvert and pipe calculations;
• Non-flooded width calculations; and,
• Energy dissipation calculations.
Refer to Chapters 5 through 8 for additional information regarding the above
calculations.
3.4.5 DOWN-GRADIENT ANALYSIS
The purpose of a Down-Gradient Analysis is to inventory natural and constructed
down-gradient drainage features and to identify and evaluate adverse down-gradient
impacts that could result from the proposed project. Common adverse impacts of land
development include erosion, flooding, slope failures, changed runoff patterns and
reduced groundwater recharge (to springs, streams, wetlands and wells, etc.).
Proposed drainage facilities are to be designed to mitigate adverse impacts identified
in the Down-Gradient Analysis.
A Down-Gradient Analysis is required for all projects, unless waived by the local
jurisdiction. The level of detail required will vary depending on the location and
complexiry of the project. Project sites that have well-draining soils, no identified
drainage problems nearby and no features that rely on groundwater recharge in the
vicinity may require only a minimal level of effort to meet this requirement.
Conversely, if the project is located in an SDA or a known problem drainage area, as
determined by the local jurisdiction, if non-standard disposal systems are proposed, or
if land features of concern have been identified down-gradient of the project site, then
the level of analysis shall match the complexity of the site. Typically, the analysis
should extend a quarter of a mile down gradient and may be limited in scope by lack
of access to adjacent properties.
At a minimum, this analysis shall include:
• A visual inspection of the site and down-gradient area by the engineer that
extends to the location where adverse impacts are anticipated to be
negligible;
• A site map that clearly identifies the project boundaries, study area
boundaries, down-gradient flow path, and any existing or potential areas
that have been identified as problematic;
• A written summary addressing the following items:
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o Existing or potential off-site drainage problems that may be
aggravated by the proj ect;
o The condition and capacity of the conveyance route including all
existing and proposed elements, potential backwater conditions on
open channels, constrictions or low capacity zones, surcharging of
enclosed systems, or localized flooding;
o The presence of existing natural or constructed land features that are
dependent upon pre-developed surface or subsurface drainage patterns;
o Potential changes to groundwater characteristics that may negatively
impact sub-level structures, foundations, or surface areas due to an
increased amount or increased frequency or duration of groundwater
intrusion;
o Existing or potential erosive conditions such as scour or unstable
slopes on-site or down-gradient of the project; and,
o Flood hazard areas identified on FEMA maps.
If there are existing or potential off-site drainage problems down-gradient of the
project, it shall be demonstrated that the proposed stormwater disposal system has
been designed to meet all of the following conditions:
• The stormwater runoff leaves the site in the same manner as that of the
pre-developed condition;
• Reduced or increased groundwater recharge has been considered with
respect to potential adverse impacts on downgradient features; and
• The proposed design does not aggravate an existing drainage problem or
create a new drainage problem.
If down-gradient surface release at a rate or volume greater than the pre-developed
condition is proposed, then the following items shall also be addressed in detail:
• Potential adverse impacts on natural or constructed drainage channels due
to an increase in stormwater rate, velocity and flow duration; and
• Potential adverse impacts on undeveloped down-gradient properties that,
if developed in the future, could be adversely impacted.
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3.5 ROAD AND DRAINAGE PLANS
3.5.1 INTRODUCTION
Construction drawings shall be submitted for review by the local jurisdiction. The
submittal and acceptance process shall be in accordance with the local jurisdiction's
standards and specifications
All plan sheets for every submittal shall be signed and dated by the project proponent,
or his authorized agent, and all plan sheets shall be stamped and signed by the project
engineer. Road and drainage plans shall include the local jurisdiction's standard notes
for construction (refer to Appendix 3B).
Once the Drainage Submittal has been accepted by the local jurisdiction, a set of
Road and Drainage plans shall be submitted on good qualiry, reproducible Mylar
media. The Mylar set shall be stamped and signed by the engineer and signed and
dated by the project proponent or his authorized agent.
3.5.2 MINIMUM PLAN ELEMENTS
The road and drainage plans shall provide enough detail for a third party to construct
the proposed facilities per the engineer's design. At a minimum, the plans shall meet
the criteria of the local jurisdiction's standards and specifications, and provide the
following information:
• Flow line and/or spot elevations, slopes, lengths, and cross-sections of
ditches;
• Rim elevations of inlet grates, drywells, and other structures;
• A profile of the stormwater conveyance system including pipes, culverts,
ditches and connections, where applicable. The profile shall include the
sizes, material types, lengths and invert elevations of all conveyance
elements.
• For lateral pipe connections to storm drain lines in existing right of way
(i.e. from a catch basin to a drywell, a main line stormwater system, a
pond or a swale), fixed invert elevations are preferred but not required.
The minimum depth from finish grade to pipe invert and the minimum
pipe slope necessary to satisfy the freeboard and self-cleaning velocity
requirements shall be provided. If necessary, invert elevations may be
adjusted during construction to avoid potential conflicts with existing
utilities in the right of way;
• Where drainage infrastructure such as roadside swales or parallel
conveyance ditches or channels may interfere with driveway locations,
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driveway locations shall be fixed as part of the road and drainage plans
and shown on the lot plans;
• Record drawing information, including invert elevations of any existing
drainage system elements that will be used in the new design;
• Construction details drawn to scale or a referenced standard drawing for
all structures. Standard details shall be referenced, not reproduced, on the
construction drawings;
• Drainage easements with all survey information shown and a recording
number if applicable;
• Grading plan for drainage ponds and swales. The grading plan shall
include existing and proposed one foot contours and catch points. A cross-
section of each pond or swale shall be provided in the plans, showing
pond/swale bottom elevation, maximum water surface elevation for the
design storm(s), inlet and outlet elevations, berm elevation and slopes,
landscaping and vegetation requirements, compaction requirements and
keyway location and dimensions;
• Each drainage pond/swale corner, pipe inlet or outlet, pipe system angle
point, ditch, and drainage structure, shall be horizontally defined with
respect to property corners, street stationing, or a coordinate system; and,
• Material gradation, thickness, and dimensions of riprap pads.
3.5.3 REVISIONS AFTER PLAN ACCEPTANCE
When changes to the design are necessary, acceptance of any proposed plan changes
shall be obtained in writing from the local jurisdiction. The proposed revisions shall
be stamped and signed by an engineer and submitted to the local jurisdiction for
review and acceptance prior to construction. The submittal shall include:
• A brief description of the proposed changes and the purpose for the
change;
• Substitute pages of the originally accepted construction plans that include
the proposed changes; and,
• Calculations and supporting documentation for the proposed change
demonstrating that the proposed modified design is at least equivalent to
the originally accepted design.
3.6 OTHER SUBMITTAL ELEMENTS
As determined by the local jurisdiction, the following items shall be included as part
of the Drainage Submittal:
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• Acknowledgement (acceptable in the form of a letter) of inspection
responsibilities. Contact the local jurisdiction for specific project
inspection requirements;
• A geotechnical site characterization, which may also include a sub-level
structure feasibility analysis, pavement analysis, pavement subgrade
sampling, down-gradient analysis, etc. (refer to Chapter 4);
• An erosion and sediment control plan (refer to Chapter 9);
• A draft copy of the Conditions, Covenants and Restrictions (CC&Rs) for
the homeowners' association in charge of operating and maintaining the
drainage facilities (refer to Chapter 11);
• An operations and maintenance manual (refer to Chapter 11);
• A financial plan (refer to Chapter 11);
• On-site and/or off-site easement documentation (refer to Chapter 11);
• Lot Plans (refer to criteria and example in Appendix 3C).
Note that lot plans are not required when the only drainage facilities for a
given project are located within the public right of way. However, when a
lot plan is required, and facilities are located both on private property in an
easement and in the public right of way, the facilities within the right of
way shall be identified and dimensioned as noted.
• UIC registration forms (refer to Section 6.32).
• Documentation that applicant has contacted Ecology regarding the need
for a NPDES Construction Stormwater Permit
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APPENDIX 3A - EXAMPLE TABLES FOR
DRAI NAGE R E PORT
BASIN INFORMATION SUMMARY TABLE
Pr�Development Post-Development
Condition Condition
Total Site acres 15 15
Pollutant Generating Impervious n/a 4.2
Surface acres
Surface Cover n/a roads & driveways
NRCS Soil Type n/a Type B
CN Value n/a 98
Hydraulically Unconnected 11 2.4
Im ervious Surfaces acres
Surface Cover dirt road roofs & sidewalks
NRCS Soil Type Type B Type B
CN Value 85 98
Pervious Surfaces acres 13.9 8.4
Com osite CN 52 59
Surface Cover#1 woods, good woods, good
Sub-Area#1 (acres) 3.9 0.6
NRCS Soil Type#1 Type A Type A
CN Value #1 30 30
Surface Cover#2 brush, fair open space &lawn area, good
Sub-Area#2 (acres) 3 6
NRCS Soil Type #2 Type B Type B
CN Value #2 56 61
Surface Cover#3 herbaceous, good herbaceous, good
Sub-Area#3 (acres) 7 1.8
NRCS Soil Type#3 Type A Type A
CN Value #3 62 62
NRCS—Natural Resources Conservation Service
Apri12008 Appendix 3A—Example Tables for Drainage Report
3-17
SPOKANE REGIONAL STORMWATER MANUAL
TREATMENT AND FLOW CONTROL REQUIREMENT SUMMARY TABLE
10-Year Storm Total Volume
Treatment Volume (cubic feet) Event Required Provided
Re uired Provided Volume cubic feet cubic feet
Basin A 135 150 75 300
Basin B 110 120 95 240
Basin C 255 280 155 560
POLLUTANT-GENERATING IMPERVIOUS SURFACE SUMMARY TABLE
Total PGIS
Basin Grass Roof Roofs Driveway Driveway Pavement Sidewalk Area
Area s s Number s Number Area s s s s
Basin A 37,575 5,000 10 15,000 10 5,000 12,000 575 17,000
Basin B 16,750 3,500 5 7,500 4 2,000 3,500 250 5,500
Basin C 36,075 4,500 10 15,000 8 4,000 12,000 575 16,000
Apri12008 Appendix 3A—Example Tables for Drainage Report
3-18
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 3B - STANDARD NOTES FOR ROAD
AND DRAINAGE PLANS
1. All work and materials shall be in conformance with the local jurisdiction road
standards (insert applicable road standards name), 20XX (insert applicable year per
land-action conditions of approval or the most current version for municipal road
proj ects).
2. Prior to site construction, the Contractor is responsible for locating underground
utilities. Call the underground utility location service at 1-800-424-5555 before you
dig.
3. Locations of existing utilities shown in the plans are approximate. The Contractor
shall be responsible for locating all underground utilities. Any conflicting utilities
shall be relocated prior to construction of road and drainage facilities.
4. The Contractor is required to have a complete set of the accepted road and drainage
plans on the job site whenever construction is in progress.
5. If the Contractor discovers any discrepancies between the plans and existing
conditions encountered, the contractor shall immediately notify the design engineer.
6. The Contractor should take precautions to protect the infiltration capacity of
stormwater facilities (e.g., line the facility with filter fabric, over-excavate upon
completion of the infrastructure, etc.)
Supplemental notes used when applicable
7. For any curb grades less than 0.8% (0.008 ft/ft), a Professional Land Surveyor
currently licensed in the State of Washington shall verify that the curb forms are at
the grades noted on the accepted plans, prior to placement of concrete. The
Contractor is responsible for arranging and coordinating work with the Surveyor.
8. The Contractor shall employ a Professional Land Surveyor currently licensed in the
State of Washington to verify that the cross-gutter forms are at the correct plane grade
prior to concrete placement. The cross-gutters shall be constructed prior to paving,
and the pavement shall then match the edge of concrete gutter.
9. For construction of drywells, install filter fabric (Amoco 4545 or approved
equivalent)between the washed drain rock and the native soils.
10.Bio-infiltration ponds/swales shall have a maximum treatment design depth (from
pond/swale bottom to elevation of drywell grate or first overflow/outflow
mechanism) of 6 inches. Either organic matter content or Cation Exchange Capacity
(CEC) testing shall be completed in order to substantiate the treatment soil
composition. The tests shall be performed on composite samples taken from the
treatment soil layer from the constructed pond bottom. A composite sample consists
of well-mixed soil obtained from at least four cores, to a depth of at least 6 inches,
randomly distributed over the pond bottom test area. Stockpile samples from on-site
or a material supplier can be tested for informational purposes to determine initial
Apri12008 Appendix 3B—Standard Notes for Road and Drainage Plans
3-19
SPOKANE REGIONAL STORMWATER MANUAL
suitabiliry and possible soil amendments, but will not be accepted in-lieu of in-place
testing. A minimum of one test shall be performed for each bio-infiltration
pond/swale 1,500 square feet or less, with one additional test for each additional
2,000 square feet of pond/swale bottom, or fraction thereo£ "One test" is equal to
four core samples taken as described above. Testing results shall be submitted as part
of the Construction Certification Submittal required for release of surety posted on
proj ect.
11.Concrete aprons are required at the inlet into any swale or pond. The finish grade of
the swale/pond side slope, where the concrete inlet apron ends, shall be a minimum of
2 inches below the finished elevation of the concrete curb apron extension. The
intention is to allow stormwater runoff to enter the swale/pond unobstructed, without
backing up into the street and gutter due to sod overgrowth at the inlet.
12.Unlined pond and bioinfiltration swale bottoms are expected to infiltrate via the pond
floor, and therefore, shall not be heavily compacted; equipment traffic shall be
minimized on the pond bottoms. The facility subgrade shall be a medium- to well-
draining material, with a minimum thickness of 48 inches and a minimum infiltration
rate of 0.15 in/hr. The faciliry shall drain within 72 hours of a storm event. If the pond
also serves as a water qualiry treatment faciliry, the treatment zone (sod and 6 inches
of treatment soil) shall be a medium- to well-draining material, with a minimum
infiltration rate of 0.25-0.50 in/hr.; silty loam or loamy soils are presumed to have an
infiltrative rate that falls within this range. Scarify the finish grade of the pond bottom
prior to hydroseeding/sodding. Testing that verifies subgrade minimum infiltration
rate is required by the local jurisdiction prior to construction certification to ensure
adequate drainage. Infiltrative testing of the treatment zone is only required if soils
other than silty loam or loamy soils are proposed.
13.If, during final inspection, it is found that the constructed pond or swale does not
conform to the accepted design, the system shall be reconstructed so that it does
comply.
Refer to Appendix 9A for Erosion and Sediment Control Standard Notes.
Apri12008 Appendix 3B—Standard Notes for Road and Drainage Plans
3-20
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 3C - LOT PLAN CRITERIA AND
EXAMPLE
Lot plans, stamped and signed by a Professional Engineer, shall be prepared for residential
lots containing any of the following elements:
• Drainage facilities in easements on the lot;
• Drainage facilities located in the public right of way or private road tract which are in
front of or adj acent to the lot;
• Floodplain encroachment;
• An easement or tract is located on the lot, including but not limited to sewer, domestic
water, access, sight distance, NLDS (refer to Section 83.4), aviation,petroleum or utility.
A separate lot plan* shall be prepared for each lot meeting the above conditions and shall
contain the following elements or adhere to the following criteria:
• The lot plan and attached section sheets shall be on letter or legal size paper and shall be
signed and stamped by an engineer;
• The lot plan shall include a north arrow, scale, lot address (if available), name of adjacent
street(s); curb sidewalk and/or edge of asphalt, lot dimensions, lot number, block number
and the name of the sub-division;
• All easements shall be labeled and dimensioned with respect to property lines or corners;
• Elevations and dimensions of the pond bottom area shall be clearly shown if applicable;
• Invert or rim elevations for any curb drops, sidewalk inlets, grates, and drywells shall be
clearly shown;
• The locations of any required ditches shall be shown, including ditch flow line grade and
riprap if required;
• The final/fixed location of driveways shall be shown, including any information about the
culvert (material, diameter, length, invert elevation(s), depth of cover, etc.);
• A standard detail for all structures shall be called out;
• Locations of all utilities shall be shown (City of Spokane only);
• Text placed inside the lot lines should be limited to lot line dimensions with all remaining
text placed outside of the lot lines for clarity; and,
• The scale of the drawing shall be such that each lot is depicted as large as possible while
still including the curb line(s) of the adjacent street(s).
Apri12008 Appendix 3C—Lot Plan Ciiteria and Example
3-21
SPOKANE REGIONAL STORMWATER MANUAL
* A "typical" lot plan will be considered representative for more than one lot only if the
drainage facility geometry is consistent with regard to length, width, depth and grade, subject
to local jurisdiction approval.
Lot plans shall be accompanied by a dimensioned cross-sectional drawing through the
drainage faciliry. Cross-sections shall be drawn to scale and depict the following items as
applicable:
• Properry and right of way lines;
• Easements;
• Curb and sidewalk;
• Edge of asphalt;
• Swale side slopes;
• Total depth of swale or ditch;
• Rim elevation of the drywell;
• Spot elevations for normal gutter line, the required 2-inch drop to finish grade below the
concrete apron, and the finish grade of the swale bottom;
• Grass, seeding or vegetation requirements; and,
• Any other applicable information which would further assist in achieving the proper
construction.
Example plan and cross-sectional drawings are shown on the following pages.
Note that lot plans are not required when the only drainage facilities for a given project are
located within the public right of way. However, when a lot plan is required, and facilities
are located both on private property in an easement and in the public right of way, the
facilities within the right of way shall be identified and dimensioned as noted above for
informational purpose.
Apri12008 Appendix 3C—Lot Plan Ciiteria and Example
3-22
SPOKANE REGIONAL STORMWATER MANUAL
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Apri12008 Appendix 3C—Lot Plan Ciiteria and Example
3-23
SPOKANE REGIONAL STORMWATER MANUAL
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Apri12008 Appendix 3C—Lot Plan Ciiteria and Example
3-24
SPOKANE REGIONAL STORMWATER MANUAL
CHAPTER 4 - GEOTECHNICAL SITE
CHARACTERIZATION
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Chapter Organization
4.1 Introduction......................................................................................................................4-1
4.2 Applicability ....................................................................................................................4-1
4.3 Geotechnical Site Characterization Report......................................................................4-2
4.3.1 Field and Laboratory Testing...................................................................................4-4
4.32 Sub-Level Structure Feasibility ...............................................................................4-6
Appendix 4A —Spokane 200 Method..........................................................................................4-9
Appendix 4B —Full-Scale Drywell Test Method......................................................................4-13
Appendix 4C —Test Pit Method................................................................................................4-17
Appendix 4D —Single-Ring Infiltrometer Test Method............................................................4-21
Appendix 4E—Swale Flood Test ..............................................................................................4-23
Appendix 4F —Pond Flood Test................................................................................................4-25
Apri12008 Chapter 4—Geotechnical Site Characterization
4-i
SPOKANE REGIONAL STORMWATER MANUAL
4.1 INTRODUCTION
This chapter outlines the minimum requirements for a geotechnical site characterization
(GSC), which is used in developing recommendations for stormwater disposal and
determining the feasibiliry of constructing sub-level structures. A qualified geotechnical
engineer (a professional engineer currently licensed in the State of Washington with
geotechnical engineering as a specialry) is required to perform the GSC. Hydrogeologists
and engineering geologists may prepare geotechnical site characterization studies,
excluding structural, foundation and pavement design. The following geotechnical
studies, if required, can be performed at the same time as the GSC:
• Geohazard analysis;
• Pavement subgrade evaluation;
• Down-gradient analysis (refer to Section 3.4.5);
• Evaporative or detention pond liner recommendations, including potential
groundwater effects on impervious pond liners (mounding, uplift, etc.);
• Embankment recommendations for proposed disposal facilities that would impound
stormwater(refer to Section 7.8.6 for embankment criteria).
• Recommendations for all cut and fill slopes.
Contact the local jurisdiction for specific requirements with regard to geohazardous areas
and road surfacing. The requirements of this chapter are in addition to any field or
laboratory testing that may be required, or recommended by the project engineer, with
regard to footings, foundations, utility work, etc.
In areas other than Special Drainage Areas (SDAs) and known drainage problem areas,
the geotechnical engineer may make recommendations on the feasibility of sub-level
structures based on the information available from the initial site investigation.
4.2 APPLICABILITY
A GSC will be required for most projects. The scope and geographic extent of the
investigation may vary depending on the general location and setting of the site, the
characteristics of the target soil layer, and whether there are known or anticipated
drainage problems in the vicinity of the site.
A GSC is required for:
• Projects proposing infiltration (drywells, detention facilities receiving credit for pond
bottom infiltration, etc.) or non-standard drainage systems;
Apri12008 Chapter 4—Geotechnical Site Characterization
4-1
SPOKANE REGIONAL STORMWATER MANUAL
• Projects located in a Special Drainage Area (SDA) or a Special Drainage District
(SDD);
• Projects located within or draining to a drainage problem or study area as recognized
by the local jurisdiction;
• Projects with administrative conditions requiring a GSC; or,
• Projects with proposed sub-level structures, as required by the local jurisdiction.
In areas where there has been a long-standing record of satisfactory performance of
standard subsurface disposal facilities and no drainage problems are known to exist, the
minimum requirements found in Section 4.3 may be reduced or waived after a formal
written request from the project proponent's engineer has been reviewed and accepted by
the local jurisdiction.
4.3 GEOTECHNICAL SITE CHARACTERIZATION REPORT
The following are minimum requirements for the GSC:
• The Studv shall include:
o A surface reconnaissance of the site and adjacent properties to assess
potential impacts from the proposed stormwater system and to verify that
the conditions are consistent with the mapped information. Typically, the
evaluation should extend a quarter of a mile down gradient. Where access
to adjacent properties is unavailable, the project proponent shall rely upon
the best known information for the area, supplemented with information
available from the local jurisdiction, including any existing geotechnical
engineering reports or studies for sites in the vicinity;
o A review of available geologic, topographic, soils, and Critical Areas
maps to identify any site conditions that could impact the use of storm
drainage systems or the construction of sub-level structures. This review
shall include all available previous geotechnical engineering reports or
studies for sites in the vicinity; and,
o An evaluation of the potential impacts of groundwater on the proposed
storm drainage facilities and roadways, when a seasonally high
groundwater table is suspected.
• The Re�ort Narrative shall include:
o A brief project description including size, number of lots proposed,project
location (section, township and range), and background information
relevant for drainage design;
o A discussion of the study investigations;
o A description of the soil units on the site and in the viciniry of the site;
Apri12008 Chapter 4—Geotechnical Site Characterization
4-2
SPOKANE REGIONAL STORMWATER MANUAL
o A description of the site including surface, soil, and groundwater
conditions, etc; and,
o Conclusions and recommendations
• The Site Plan shall include:
o Project boundaries (including all existing and proposed property lines);
o Labeled topographic contours, extending beyond the project and drainage
basin. Projects in an urban area shall use a maximum contour spacing of
2 feet. At the discretion of the local jurisdiction, projects outside an urban
area, such as a large lot subdivision, may use the best available
topographic information, which may involve contours with spacing of
more than 2 feet. In either case, the engineer shall field verify the basin
limits;
o Location of the soil units identified;
o Location of significant structures, properties or geologic features (such as
basalt outcroppings, etc.) on site and in the project vicinity;
o Location of existing natural or constructed drainage features on site and in
the proj ect vicinity; and,
o Location of proposed site infrastructure including roadways and drainage
features such as ponds, drywells, etc.
• Test Method Documentation shall include:
o A map with the location of all subsurface field explorations and any in-
place field tests;
o A description of any difficulties encountered during excavation and
testing;
o A description of the equipment used to perform the field explorations or
tests. When applicable, describe the type of fabric lining and gravel
backfill used;
o Logs of subsurface explorations which shall identify the depth to
groundwater, the presence of any limiting layers and the target soil layer;
include test pit or excavation dimensions, with photographs, where
applicable;
o Report test data in a format that includes time of day, flow meter readings,
incremental flow rates, observed head levels, water depths and total flow
volumes in the drywell, test pit or infiltrometer;
o A description of the condition of any existing facilities being tested, noting
any silt build-up, water level, connections to other structures (including
distance to inverts of any interconnecting pipes), measured depths and
dimensions, etc.;
Apri12008 Chapter 4—Geotechnical Site Characterization
4-3
SPOKANE REGIONAL STORMWATER MANUAL
o Results of field and laboratory testing conducted, including the grain size
analysis represented both graphically and in tabular format (refer to
Section 4.3.1). For the Spokane 200 method, results shall also be
summarized using the formatting shown in Table 4-l.;
o A report on the normalized outflow rates for drywells, and the actual and
proposed design outflow rates for test pits;
o Results of the sub-level structure feasibility study (refer to Section 43.2)
and a summary of the down-gradient analysis (refer to Section 3.4.5), as
applicable;
o A geologic cross-section of the stormwater disposal area drawn to scale,
with the proposed stormwater disposal facilities superimposed on the
cross-section. All relevant geologic units shall be clearly identified
including the target disposal layer and limiting layers; and,
o For the Spokane 200 Method, results summarized using the formatting
shown in Table 4-1, as well as the sieve analysis data, presented both
graphically and in tabular format.
TABLE 4-1
SPOKANE 200 METHOD RESULTS SUMMARY
Normalized
Drywell Factor
Exploration Sample USCS Percent Hydraulic Outflow of
I D Depth C lassification F ines C onductivity R ate Safety
TP #2 4.0' SP-SM 6.3% 0.011 cm/sec 0.032 cf/ft 2.0
TP #2 10.5' GP 4.5% 0.035 cm/sec 0.090 cf/ft 1.3
4.3.1 FIELD AND LABORATORY TESTING
The subsurface exploration, testing, and associated engineering evaluations are
necessary to identify permeable soils and to determine the thickness, extent, and
variability of the soils. This information is necessary to properly design stormwater
disposal facilities.
Field explorations and laboratory testing shall be conducted under the direct
supervision of a geotechnical engineer.
Test Methods
Soil infiltration and drywell outflow rates shall be determined using one or more of
the following methods:
Apri12008 Chapter 4—Geotechnical Site Characterization
4-4
SPOKANE REGIONAL STORMWATER MANUAL
• The Spokane 200 Method (refer to Appendix 4A) uses "percent fines"
(i.e. soil gradation data) to estimate drywell outflow rates for design
purposes. This test method is an option provided to the geotechnical
engineer to initially assess the suitability of the on-site soils for
subsurface stormwater disposal in drywells;
• The full-scale drywell test (refer to Appendix 4B) uses field data to
determine the actual outflow rates of a drywell. This test method is
required for all existing drywells to verify the condition and capacity
of the structure;
• The test pit method (refer to Appendix 4C) uses field data to estimate
the outflow rates of drywells and other subsurface disposal facilities. A
geotechnical engineer may elect to use this test method to further
verify the design outflow rates used in the drainage design when soil
gradations indicate marginal outflow rates, as determined by the
Spolcane County 200 Method. Also, this test method may be used for
estimating outflow rates for non-standard subsurface disposal systems
(infiltration galleries, under-drain systems, etc);
• The single-ring infiltrometer test (refer to Appendix 4D) or pond flood
test (refer to Appendix 4F) can be used to verify pond drawdown times
as required in Section 7.83, as well as the infiltration rates of the
subgrade and treatment zone of a water quality facility as discussed in
Section 6.7.L The single-ring infiltrometer test uses field data to
determine the hydraulic conductivity of surficial soils;
• The swale flood test (refer to Appendix 4E) uses field data to verify
swale drawdown times and functionality; and,
• Additional or alternate test methods, upon approval from the local
jurisdiction.
Minimum Requirements
The following minimum requirements, when applicable, shall be met for field
explorations and laboratory testing when subsurface disposal is proposed:
• Test borings and/or test pits shall be located within the footprint of
proposed stormwater disposal facilities;
• For each facility, a minimum of one subsurface exploration shall be
performed for up to 1200 square feet of disposal area. Another
subsurface exploration shall be performed for each additional 15,000
square feet, or fraction thereof, of disposal area. For a linear roadside
swale, a minimum of one subsurface exploration shall be performed
every 500 feet, staggered on both sides of the road, unless site
conditions or test results indicate that additional explorations are
necessary. Subsurface explorations and sampling shall be conducted
Apri12008 Chapter 4—Geotechnical Site Characterization
4-5
SPOKANE REGIONAL STORMWATER MANUAL
according to applicable standards of the American Society for Testing
and Materials (ASTM);
• Unless otherwise recommended by the geotechnical engineer,
subsurface explorations shall extend to a depth of H plus 5 feet below
the stormwater faciliry, where H is equivalent to the maximum head of
water within the facility. For example, for a double depth drywell with
a maximum head of 10 feet, the minimum required depth of
exploration below the drywell is 15 feet, or 25 feet below the proposed
rim of drywell; and,
• When the Spokane 200 Method is used, a minimum of two "percent
fines" tests shall be performed per subsurface exploration. Tests
should be performed on samples taken at varying depths below the
ground surface, within the target soil layer, in order to adequately
characterize the proposed disposal site soils. Laboratory testing shall
be conducted according to applicable ASTM standards.
Post-Const�uction Testing
Newly constructed drywells may require a full-scale drywell test prior to project
certification. Swales may also require a flood test or infiltrometer test prior to project
certification, bond release or issuing a Certificate of Occupancy. Contact the local
jurisdiction for additional information.
4.3.2 SUB-LEVEL STRUCTURE FEASIBILITY
If sub-level structure construction is being considered, a sub-level structure feasibility
study is required. In the City of Spokane Valley, contact the Building Department for
basement restriction information. The sub-level structure feasibility study shall
include the following, at a minimum:
• A layout of the site showing lot lines and lot and block numbers;
• Identification by lot and block number of sites where sub-level
structure construction is feasible. Provide recommendations with
details of construction (i.e. maximum below grade floor elevations,
minimum drainage system requirements, and any site specific
recommendations). Recommendations shall be coordinated with the
International Building Code (IBC) and International Residential Code
(�C);
• Discussion of the effects of hydrostatic pressure that may lead to
basement flooding and recommendations as to the effectiveness of
waterproofing;
• If infiltration is proposed as a method for stormwater disposal,
discussion of any potential adverse impacts on proposed sub-level
Apri12008 Chapter 4—Geotechnical Site Characterization
4-6
SPOKANE REGIONAL STORMWATER MANUAL
structures, taking into consideration the contribution of imported water
(due to lawn watering, etc.); and,
• Identification of locations where sub-level structure construction is not
feasible.
In-lieu of conducting a sub-level structure feasibility study, the owner may elect to
prohibit sub-level structure construction throughout the entire plat. If a potential
buyer would like to construct a sub-level structure, then a site specific geotechnical
evaluation shall be performed by a geotechnical engineer for the individual lot prior
to a building permit being issued. Language regarding sub-level structure restrictions,
as provided by the local jurisdiction, shall be placed or referenced on the face of the
plat.
Recommendations shall be summarized and provided electronically in Microsoft
Excel,per the format found in Table 4-2.
Apri12008 Chapter 4—Geotechnical Site Characterization
4-7
SPOKANE REGIONAL STORMWATER MANUAL
TABLE 4-2
EXAMPLE SUB-LEVEL STRUCTURE FEASIBILITY SUMMARY
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m J �„ � ,� � o ,� � o J J Summary of Geotechnical Recommendations3
Block Lots yes Maximum C=15 feet Based upon the clean nature of the soils at the sub-
1 1-8 allowable level elevations and the depth to groundwater,
GrW=25 feet
footing drains are not required. However,below-
B=30 feet grade walls shall be well reinforced to reducing
cracking and thoroughly damp-proofed with a water-
resistant bituminous emulsion or modified cement
base coating. Back�ll material shall consist of only
clean granular material which is free of fine-grained
soils, organic material, debris and large rocks.
Block Lots yes 4 feet GrW=13 feet Below-grade walls shall be well reinforced to reduce
2 1-3 cracking and waterproofed with a membrane (per
IRC)which is lapped and sealed from the top of the
footing to the finished grade. An under slab
waterproof inembrane (per IBC)which is lapped and
sealed shall be integrated with the wall membrane.
Backfill material shall consist of only clean granular
material which is free of fine-grained soils, organic
material, debris and large rocks. Walls and footings
shall have a drain system with cleanouts, emptying a
minimum of 15 feet in a down-slope direction away
from structures. Precautions shall be taken not to
excavate a closed depression over rock or clay that is
intended to dispose of sump water from a foundation
drain system.
Block Lots no n/a GrW=3 feet Due to the very shallow presence of groundwater,
2 4-g B=7 feet sub-level structures are not recommended on these
lots. If a crawl space is proposed, a drain system with
cleanouts shall be provided that empties a minimum
of 15 feet in a down-slope direction away from
structures.Precautions shall be taken not to excavate
a closed depression over rock or clay that is intended
to dispose of sump water from foundation drain
system.
1 Maximum depth measured from original pre-construction/pre-grading ground surface elevation ar existing ground
surface,whichever provides a greater distance between the lowest floor elevation and the limiting layer.
�GrW=groundwater,B=bedrock or basalt,C=clayey-silty soils
3 Refer to the Geotechnical Report for this project for further information,which may include construction details
that support these recommendations.
Apri12008 Chapter 4—Geotechnical Site Characterization
4-8
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 4A - SPOKANE 200 METHOD
PURPOSE AND APPL ICABIL lTY
The Spokane 200 Method estimates the normalized outflow rates for drywells and the
hydraulic conductivity of a soil using the results of laboratory soil gradation tests.
The geotechnical engineer may use this method to initially assess the suitability of site
soils for stormwater disposal. A full-scale drywell test, as deemed necessary by the local
jurisdiction, may be required prior to final project certification in order to verify design
outflow rates determined by this method.
PR OCEDUR E
The following procedure for the Spokane 200 Method is taken from Infiltration Rate and
Soil Classification Correlation (File No. 0188-082-00, May 28, 2004; prepared by
GeoEngineers, Budinger& Associates, Inc., and Cummings Geotechnology, Inc.)
l. Determine the percent of fines (percent passing the #200 sieve) for the target soil
layer, based on a minimum of 2 gradation tests.
2. Estimate the hydraulic conductivity (k) using the percent fines value and the best-fit
line on Figure 4A-1. Enter the figure on the x-axis with the percent fines value from
Step l. Extend a vertical line from that value until the Best Fit Line is intersected.
Drawing a horizontal line from that point to the y-axis will yield the hydraulic
conductivity. Alternatively the equation K = 0.6392 F'-�8796 may be used, where K is
the hydraulic conductivity and F is the percent fines.
3. Determine the normalized outflow rate for a drywell using the estimated hydraulic
conductivity from Step 2, and the best-fit line-drywell data on Figure 4A-2. Enter the
figure on the x-axis with the estimated hydraulic conductivity (k) from Step 2. Extend
a vertical line from that value until the Best-Fit Line-Drywell Data is intersected.
Drawing a horizontal line from that point to the y-axis will yield the normalized
outflow rate. Alternatively the drywell equation qQ =1.5582 Ko.sbo� may be used,
where K is the hydraulic conductiviry and qA is the normalized outflow rate.
4. Determine the actual outflow rate for a drywell (qA) by multiplying the normalized
outflow rate by the maximum design drywell head (6 feet for a single-depth drywell
and 10 feet for a double-depth drywell).
5. Determine the design outflow rates for a drywell (q�) by applying the appropriate
factor of safety (FS) from Table 4A-l. q� _�A
FS
Apri12008 Appendix 4A—Spokane 200 Method
4-9
SPOKANE REGIONAL STORMWATER MANUAL
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Figure 4A-1 —Percentage of Fines (% passing the No. 200 Sieve)
vs. Hydraulic Conductivity (k)
Apri12008 Appendix 4A—Spokane 200 Method
4-10
SPOKANE REGIONAL STORMWATER MANUAL
� � r
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Figure 4A-2 —Hydraulic Conductivity (k)vs. Normalized Outflow Rate
Apri12008 Appendix 4A—Spokane 200 Method
4-11
SPOKANE REGIONAL STORMWATER MANUAL
TABLE 4A-1
OUTFLOW RATE FACTORS OF SAFETY
FOR SPOKANE 200 METHOD
Percent Finer than U.S. No. 200 Minimum Factor of Safety
Sieve
0 to 5% 1.3
>5 to 6% 1.5
>6 to 8% 2.0
>8 to 10% 2.3
>10 to 12% 2.7
>12% not suitable for drywell disposal
The factors of safety listed in Table 4A-1 are based on optimal separation between the
bottom of a drywell and a limiting layer (bedrock, groundwater, clay lens, etc.). The
optimal separation between the bottom of the drywell and the limiting layer shall
equal the maximum head (H) in the drywell, which is 6 feet for single depth drywells
and 10 feet for double depth drywells.
When the distance between the bottom of the drywell and the limiting layer is less
than the above requirements, the factor of safety from Table 4A-1 shall be increased
by 0.1 for each foot of separation less than H. The separation shall not be less than 4
feet between the bottom of the drywell and the limiting layer. For a pond or swale
with no infiltrative structure, the separation shall be a minimum of 4.5 ft below the
pond bottom, to allow for a 6-inch treatment zone and 48 inches of subgrade
infiltrative soil. The local jurisdiction reserves the authority to increase the required
depth to the limiting layer should there be evidence that the subgrade will be
negatively impacted by the limiting layer such as groundwater.
The factors of safety shown in Table 4A-1 are minimums. The geotechnical engineer
may recommend a factor of safety greater than those shown based on site specific
conditions.
For infiltration facilities other than drywells, a geotechnical engineer shall make a
conservative recommendation for design outflow rates.
6. Hydraulic conductivities estimated using Figure 4A-1 may be used to estimate design
outflow rates for infiltration facilities other than drywells.
Apri12008 Appendix 4A—Spokane 200 Method
4-12
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 4B - FULL -SCALE DRYWELL TEST
METHOD
PURPOSE AND APPL ICABIL lTY
The full-scale drywell test method determines the normalized outflow rates for drywells.
This testing is required for drywells if requested by the local jurisdiction.
PR OCEDUR E
1. Install the drywell per the local jurisdiction's standard plans, specifications and
applicable construction guidelines.
2. Inspect the drywell and take photographs, if necessary.
3. Before beginning the test, field check the accuracy of the flow meter by filling up a
suitable container of known volume, such as a calibrated 55-gallon barrel.
4. Introduce clean water into the drywelL Monitor flow using an in-line flow meter.
5. Raise the water level in the drywell until it reaches the top of the active barrel section.
In the case of drywells interconnected by pipes, raise the water level to the invert
elevation of the connecting pipe, or use an expandable pipe plug to seal the
connecting pipe.
6. Monitor and record the flow rate required to maintain the constant head level in the
drywell at appropriate intervals. In no case shall the interval exceed 10 minutes in
length.
7. Maintain the water level in the drywell, by adjusting the flow rate, for a minimum of
2 hours or until a stabilized flow rate has been achieved, whichever is longer. Test
time begins after the water level in the drywell has reached the top of the active barrel
section, or the invert elevation of any interconnecting pipes. The flow rate is
considered stable when the water level in the drywell is maintained and the
incremental flow rate does not vary by more than 10%.
8. Upon completion of the constant head period, discontinue flow and monitor and
record the water level in the drywell at intervals of no longer than 5 minutes, for a 30-
minute time period. This time may need to be extended depending upon the soil
performance.
CA L CUL A TIONS
1. Calculate the normalized outflow rate (qA)
Apri12008 Appendix 4B—Full-Scale Drywell Test Method
4-13
SPOKANE REGIONAL STORMWATER MANUAL
R'A = ���*H�
Where: Q = stabilized flow rate observed near the end of the
constant-head portion of the test, in cubic feet per
second(cfs);
H = level of water within the drywell (feet); and,
H� = maximum design drywell head (6 feet for single-
depth, 10 feet for double-depth).
2. Determine the design outflow rates for a drywell (q�). Apply the appropriate factor of
safety (FS) from Table 4B-1. When sieve analysis data is unavailable a FS of 2.5
shall be used.
qA
qD — FS
TABLE 4B-1
OUTFLOW RATE FACTORS OF SAFETY
FOR FULL-SCALE DRYWELL TEST METHOD
Percent Finer than U.S. No. 200 Minimum Factor of Safety
S i eve�
0 to 5% l.l
>5 to 6% 1.3
>6 to 8% 1.8
>8 to 10% 2.1
>10 to 12% 2.5
>12% not suitable for drywell disposal
� When no sieve analysis data are available, a factor of safety of 2.5 shall be applied to field-
determined outflow rate.
The factors of safety listed in Table 4B-1 are based on optimal separation between the
bottom of a drywell and a limiting layer (bedrock, groundwater, clay lens, etc.). The
optimal separation between the bottom of the drywell and the limiting layer shall
equal the maximum head (H) in the drywell, which is 6 feet for single-depth drywells
and 10 feet for double-depth drywells.
Apri12008 Appendix 4B—Full-Scale Drywell Test Method
4-14
SPOKANE REGIONAL STORMWATER MANUAL
When the distance between the bottom of the drywell and the limiting layer is less
than the above requirements, the factor of safery shall be increased by 0.1 for each
foot of separation less than H. The separation shall not be less than 4 feet between the
bottom of the drywell and the limiting layer. For a pond or swale with no infiltrative
structure, the separation shall be a minimum of 4.5 ft below the pond bottom, to allow
for a 6-inch treatment zone and 48 inches of subgrade infiltrative soil. The local
jurisdiction reserves the authority to increase the required depth to the limiting layer
should there be evidence that the subgrade will be negatively impacted by the limiting
layer such as groundwater.
The factors of safety shown in Table 4B-1 are minimums. The geotechnical engineer
may recommend a factor of safety greater than those shown based on site specific
conditions.
Apri12008 Appendix 4B—Full-Scale Drywell Test Method
4-15
SPOKANE REGIONAL STORMWATER MANUAL
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Apri12008 Appendix 4B—Full-Scale Drywell Test Method
4-16
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 4C - TEST PIT METHOD
PURPOSE AND APPL ICABIL lTY
This test method is used to estimate drywell outflow rates using test pits. This test method
may also be used for analyzing non-standard subsurface disposal systems (infiltration
galleries, under-drain systems, etc). However, more complex calculations that are not
presented in this Manual may be required to correlate the results and substantiate the
recommended infiltration rates.
PR OCEDUR E
l. Excavate a test pit to an elevation such that the walls and the bottom of the test pit
expose the target soil layer being tested. As much as practical, excavate the pit to
neat-line dimensions, and keep it free of surface slough, organics, and other
deleterious material.
2. Measure and record the dimensions (length, width, depth) of the test pit. Include
photographs of the test pit.
3. Line the walls and bottom of the pit with a highly porous, non woven, geotextile
fabric. Install a vertical, PVC observation pipe in the pit. Then backfill the pit with
clean, uniform, pervious, fine gravel; or clean, uniform, pervious, open graded coarse
gravel. The omission of the PVC observation pipe and pervious gravel backfill is an
allowable practice if the test pit walls will not slough when water is introduced.
4. Introduce clean water into the test pit. Monitor flow using an in-line flow meter.
Before beginning the test, field check the accuracy of the flow meter by filling up a
suitable container of known volume, such as a calibrated 55-gallon barrel.
5. Raise the water level in the pit until a level consistent with the operating head
anticipated in the proposed drainage structure is achieved.
6. Adjust the flow rate as needed to maintain a constant head level in the pit. Monitor
and record the flow rate required to maintain the constant head level at appropriate
intervals. In no case shall the interval exceed 10 minutes in length.
7. Maintain the water level in the pit, by adjusting the flow rate, for a minimum of two
hours or until a stabilized flow rate has been achieved, whichever is longer. Test time
begins after the water level in the pit has reached the operating level of the proposed
structure. The flow rate is considered stable when the water level in the pit is
maintained and the incremental flow rate does not vary by more than 10%.
8. Upon completion of the constant head period, discontinue flow, then monitor and
record the water level in the test pit at intervals of no longer than five minutes, for a
Apri12008 Appendix 4C—Test Pit Method
4-17
SPOKANE REGIONAL STORMWATER MANUAL
30-minute falling head time period. This time may need to be extended depending
upon the soil performance.
CA L CUL A TIONS
1. Calculate the normalized outflow rate of the test pit (qN):
Q (cfs/foot)
qN — H
Where: Q = stabilized flow rate observed near the end of the
constant-head portion of the test (cfs); and,
H = level of water in the test pit (feet).
2. Estimate the hydraulic conductivity of the soil using Figure 4C-1, from Step 1, and
the Best Fit Line to Test Pit data. Determine the normalized outflow rate of the
drywell (qN�). Enter into Figure 4C-1 on the y-axis with the normalized outflow rate
(qN) calculated in Step l. Extend a horizontal line from that value until the Best-Fit
Line to Test-Pit Data is intersected. From that point, draw a vertical line to the Best-
Fit Line to Drywell Data. The normalized outflow rate of the drywell may then be
obtained by drawing a horizontal line from that point on the Best-Fit Line to Drywell
Data back to the y-axis. Alternatively the test pit equation qN� = 0.9242 Ko.9646 may
be used, where K is the hydraulic conductivity and qN�is the normalized outflow rate
of the test pit. Then the K can be inserted into drywell equation qN� =1.5582 Ko.sbo�
where K is the hydraulic conductivity and qND is now the normalized outflow xof the
drywell.
3. Calculate the actual outflow rate(qA):
qA = qN� '�H� (cfs)
Where: H� = maximum design drywell head (6 feet for
single-depth, 10 feet for double-depth).
4. Determine the design outflow rates for a drywell (q�). Apply the appropriate factor of
safety (FS) from Table 4C-l. When sieve analysis data is unavailable, a FS of 2.5
shall be used.
�A (cfs)
qD — FS
For infiltration facilities other than drywells, a geotechnical engineer shall make a
conservative recommendation for design outflow rates. The above calculations shall
be considered when determining the design outflow rates.
Apri12008 Appendix 4C—Test Pit Method
4-18
SPOKANE REGIONAL STORMWATER MANUAL
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Figure 4C-1 —Hydraulic Conductivity (k)vs. Normalized Outflow Rate
Apri12008 Appendix 4C—Test Pit Method
4-19
SPOKANE REGIONAL STORMWATER MANUAL
TABLE 4C-1
OUTFLOW RATE FACTORS OF SAFETY
FOR TEST PIT METHOD
Percent Finer than U.S. No. 200 Minimum Factor of Safety
Sieve'
0 to 5% 1.1
>5 to 6% 1.3
>6 to 8% 1.8
>8 to 10% 2.1
>10 to 12% 2.5
>12% not suitable for drywell disposal
� When no sieve analysis data are available, a factar of safety of 2.5 shall be applied to field-
determined outflow rate.
The factors of safety listed in Table 4G1 are based on optimal separation between the
bottom of a drywell and a limiting layer (bedrock, groundwater, clay lens, etc.). The
optimal separation between the bottom of the drywell and the limiting layer shall
equal the maximum head (H) in the drywell, which is 6 feet for single-depth drywells
and 10 feet for double-depth drywells.
When the distance between the bottom of the drywell and the limiting layer is less
than the above requirements, the factor of safety from Table 4C-1 shall be increased
by 0.1 for each foot of separation less than H. The separation shall not be less than 4
feet between the bottom of the drywell and the limiting layer. For a pond or swale
with no infiltrative structure, the separation shall be a minimum of 4.5 ft below the
pond bottom, to allow for the 6-inch treatment zone and 48 inches of subgrade
infiltrative soil. The local jurisdiction reserves the authority to increase depth to the
limiting layer should there be evidence that the subgrade will be negatively impacted
by the limiting layer such as groundwater.
The factors of safety shown in Table 4G1 are minimums. The geotechnical engineer
may recommend a factor of safety greater than those shown based on site specific
conditions.
Apri12008 Appendix 4C—Test Pit Method
4-20
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 4D - SINGLE -RING INFILTROMETER
TEST METHOD
PUR POSE
The single-ring infiltrometer test method is applicable for estimating infiltration and
permeability rates of surficial soils to verify drawdown times in bio-infiltration swales
and detention ponds.
PR OCEDUR E
l. Drive, jack, or hand advance a short section of steel or PVC pipe, at least 20 inches
long and with a minimum inside diameter of 12 inches and a beveled leading edge,
(referred to as a "ring" in this test method) into the soil surface to a depth of about 8
inches, leaving approximately 12 inches of pipe exposed above the ground surface. If
after installation the surface of the soil surrounding the wall of the ring shows signs of
excessive disturbance such as extensive cracking or heaving, reset the ring at another
location using methods that will minimize the disturbance. If the surface of the soil is
only slightly disturbed, tamp the soil surrounding the inside and outside wall of the
ring until it is as firm as it was prior to disturbance.
2. Introduce clean water into the ring. Use some form of splash guard such as a sheet of
thin aluminum or a diffuser apparatus such as a highly porous, non woven, geotextile
fabric to prevent erosion at the surface of the soil during filling and testing. Monitor
flow using an in-line flow meter. Before beginning the test, field check the accuracy
of the flow meter by filling up a suitable container of known volume, such as a
5-gallon bucket or a 55-gallon barrel.
3. Raise the water level in the ring until a head level of at least 6 inches above the soil
surface is achieved.
4. Monitor and record the flow rate required to maintain the constant head level at
appropriate intervals. In no case shall the interval exceed 10 minutes in length.
5. Maintain the water level in the ring, by adjusting the flow rate, for a minimum of 2
hours or until a stabilized flow rate has been achieved, whichever is longer. Test time
begins after the water level in the ring has reached 6 inches above the soil surface.
The flow rate is considered stable when the water level in the ring is maintained and
the incremental flow rate does not vary by more than 10%.
6. Upon completion of the constant-head period, discontinue flow, and monitor and
record the water level in the ring at intervals of no longer than 5 minutes, for a 30-
minute period.
Apri12008 Appendix 4D—Single-Ring Infiltrometer Test Method
4-21
SPOKANE REGIONAL STORMWATER MANUAL
7. One single-ring infiltrometer test shall be performed for every 2,500 square feet of
bio-infiltration swale/pond bottom area or detention pond bottom area, with a
minimum of one per swale or pond or as required by the local jurisdiction.
CA L CUL A TIONS
l. Calculate the surface infiltration rate (�
I= � (feet/second)
Where: Q = stabilized flow rate observed near the end of the
constant-head portion of the test (cfs); and,
A = area of soil inside the ring (square feet).
2. Compute the permeabiliry rate(K)
K = ��*�� (feet/second)
� �
Where: L = depth of soil contained within the ring (inches);
A = area of soil inside the ring (square feet); and,
H = constant level of water within the ring, measured
from the base of the ring to the free water surface
(inches).
Apri12008 Appendix 4D—Single-Ring Infiltrometer Test Method
4-22
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 4E - SWALE FLOODTEST
PUR POSE
The swale flood test verifies the path of flow into a swale and the drawdown time of a
bio-infiltration swale. The flood test shall be conducted, when required, after the swale
has been constructed and the vegetation has been established (i.e. is not in danger of
being washed out when water is introduced into the swale).
PR OCEDUR E
l. Introduce clean water into the swale by directing the water (via hose from a hydrant
or other clean water source) along the curb and gutter upstream of the swale inlet.
2. Raise the water level in the swale until it reaches 6 inches in depth (typically to the
rim of the drywell or catch basin). Discontinue flow and note the time; this is the
beginning of the flood test.
3. If the swale is draining rapidly, the progress is observed, and when the swale is
empty, the time is documented, and the flood test has ended.
4. If the swale is not draining, measure the depth of water currently in the swale,
documenting the time, and return to the swale site at a later time in order to verify that
the swale has completely drained within 72 hours.
NOTE: Contact the local jurisdiction for specific requirements for this Test Method.
Apri12008 Appendix 4E—Swale Flood Test
4-23
SPOKANE REGIONAL STORMWATER MANUAL
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Apri12008 Appendix 4E—Swale Flood Test
4-24
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 4F - POND FLOOD TEST
PUR POSE
The Pond Flood Test Method verifies drawdown time of a stormwater disposal facility,
such as a detention pond. The pond flood test shall be conducted, when required, after the
pond has been constructed, and after vegetation has been established (i.e. is not in danger
of being washed out when water is introduced into the pond).
PR OCEDUR E
l. Introduce clean water into the pond. Use some form of splash-guard or diffuser
device to prevent surface erosion of the pond.
2. Raise the water level in the pond until it reaches operational depth (i.e. to the invert
elevation of the first outlet device(culvert, orifice, weir, etc.)). Discontinue flow.
3. Document the time and measure the depth of water in the pond; this is the beginning
of the pond flood test.
4. The pond's abiliry to drain is observed. If the pond appears to be emptying rapidly, as
soon as the pond is empry, the time is documented, and the flood test has ended.
5. If the pond is not draining, or is draining very slowly, measure the depth of water
currently in the pond, documenting the time, and return to the pond site at a later time
in order to verify that the pond has completely drained within 72 hours.
NOTE: Contact the local jurisdiction for specific requirements for this Test Method.
Some ponds will be large enough that a pond flood test may not be the most efficient
method of determining drawdown time or infiltrative ability. Consideration may need to
be given to other types of infiltrative test methods, such as the single-ring infiltrometer
test. If the pond flood test is pursued for larger ponds, the local water purveyor must be
contacted so that water service is not disrupted.
Apri12008 Appendix 4F—Pond Flood Test
4-25
SPOKANE REGIONAL STORMWATER MANUAL
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SPOKANE REGIONAL STORMWATER MANUAL
CHAPTER 5 - HYDROLOGIC ANALYSIS
AND DESIGN
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Chapter Organization
5.1 Introduction......................................................................................................................5-1
5.2 Hydrologic Analysis Methods .........................................................................................5-1
5.3 Curve Number Method....................................................................................................5-2
5.3.1 Introducti on..............................................................................................................5-2
5.32 Curve Number Method Theory................................................................................5-2
5.3.3 Limitations...............................................................................................................5-3
5.3.4 Design Steps.............................................................................................................5-3
5.4 Level Pool Routing Method...........................................................................................5-19
5.5 Rational Method.............................................................................................................5-19
5.5.1 Runoff Coefficients................................................................................................5-20
5.52 Time of Concentration...........................................................................................5-21
5.5.3 Intens ity..................................................................................................................5-21
5.6 Bowstring Method (Modified Rational Method)...........................................................5-23
5.6.1 Design Steps...........................................................................................................5-24
5.7 Water Budget Method....................................................................................................5-25
5.7.1 In tro du c ti on............................................................................................................5-2 5
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5.72 Methodology..........................................................................................................5-25
5.7.3 Design Steps...........................................................................................................5-27
5.7.4 Curve Number Adjustment....................................................................................5-28
5.7.5 Climatological Data...............................................................................................5-28
Appendix SA —Example Calculation: Weighting Curve Numbers...........................................5-31
Appendix SB —Example Calculation: Bowstring Method........................................................5-33
Appendix SC —Example Calculation: Water Budget(Preferred Method)................................5-37
Appendix SD —Example Calculation: Water Budget(Alternative Method).............................5-41
Appendix SE - Hydrologic soil series for Washington State.....................................................5-45
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5.1 INTRODUCTION
This chapter provides the tools for estimating peak flow rates and volumes for sizing
conveyance, treatment, and flow control facilities. Standard flow control facilities are
detention and retention facilities, drywells, and evaporation ponds. Flow control facilities
are necessary to mitigate potential adverse impacts on down-gradient properties due to
the increase in stormwater runoff caused by land development.
Unless specifically accepted by the local jurisdiction, the peak rate and volume of
stormwater runoff from any proposed land development to any natural or constructed
point of discharge downstream shall not exceed the pre-development peak rate or volume
of runoff. A down-gradient analysis demonstrating that there will be no expected adverse
impacts on downgradient properties will be required (refer to Section 3.4.5 for down-
gradient analysis criteria). Exceptions with regard to rate and volume control can be made
for regional facilities planned by the local jurisdiction.
Stormwater runoff from a developed site shall leave the site in the same manner and
location as in the pre-developed condition. Flow may not be concentrated onto down-
gradient properties where sheet flow previously existed. Drainage shall not be diverted
from a proposed development and released downstream at points not receiving
stormwater runoff prior to the proposed development.
Non-standard systems shall be evaluated individually by the local jurisdiction and shall
require a geotechnical site characterization report, a down-gradient analysis, and any
additional information deemed necessary by the local jurisdiction. Refer to Section 2.1.6
for variance procedures.
All engineering work for hydrologic analysis and design shall be performed by, or under
the direction of, a professional engineer currently licensed in the State of Washington.
5.2 HYDROLOGIC ANALYSIS METHODS
The following methods shall be used for the design of flow control and conveyance
systems:
• The Curve Number (CN) Method can be used to estimate peak flow rates and
volumes; the most commonly used Curve Number Method in the Spokane Region is
the Natural Resources Conservation Service Urban Hydrograph Method (NRCS
Method); an acceptable but seldom-used alternative method is the Santa Barbara
Urban Hydrograph Method;
• The Level Pool Routing Method can be used to route hydrographs;
• The Rational Method can be used to estimate peak runoff rates;
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• The Modified Rational Method (Bowstring Method) can be used to estimate peak
flow rates and detention volumes; and,
• The Water Budget Method can be used to size evaporation facilities.
5.3 CURVE NUMBER METHOD
5.3.1 INTRODUCTION
Single-event hydrograph methods based on the curve number equation can be used in
combination with a routing technique to size detention facilities. These methods are
used to develop hydrographs to estimate the peak flow rate and volumes for a specific
design storm.
5.3.2 CURVE NUMBER METHODTHEORY
This section presents a general description of this methodology, for additional
information refer to the National Engineering Handbook (1985). The amount of
runoff from a site calculated using the Curve Number Method depends on the
precipitation at the site and the natural storage capaciry of the soil. The curve number
equation and the NRCS rainfall excess equation are shown in Equations 5-1 and 5-2:
1000
S = �N —10 (5-1)
Where: S = maximum storage volume of water on and within
the soil (inches);
CN = curve number(dimensionless);
Q _ �P—0.25�� (5-2)
�P+0.85�
Q = OforP< 0.25
Where: S = maximum storage volume of water on and within
the soil (inches);
Q = runoff(inches);
P = precipitation (inches); and,
0.25 = initial abstraction; the fractional amount estimated
as intercepted, evaporated and/or absorbed by the
soil (inches).
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5.3.3 LIMITATIONS
Another method approved by the local jurisdiction shall be used when:
• The calculated depth of runoff is less than 0.5 inch;
• The value (P-02S)is a negative number; or
• The weighted CN is less than 40.
For additional limitations, see the Soil Conservation Service's Technical Release
No. 55 (Publication 210-VI-TR-55, Second Ed., June 1986).
Local jurisdictions reserve the authority to limit discharge to public facilities.
Regardless of the methodology used, if a given method yields a runoff volume or rate
that is inconsistent with the physical site characteristics, the engineer will be required
to provide additional supporting documentation.
5.3.4 DESIGN STEPS
The following steps are based on the assumption that the engineer uses a sofnvare
package that utilizes the Curve Number Method for hydrologic computations and the
level pool method for reservoir routing (refer to Section 5.4). If hand calculations are
proposed, the engineer can consult currently available technical publications for
additional information.
l. Determine the pre-developed and post-developed drainage basin boundaries and
identify pervious and impervious areas as described in the Basin Areas subsection
below;
2. Determine the hydrologic soil group classifications, as described in the
Hydrologic Soil Group Classification subsection below, and correlate to the
drainage basin boundaries;
3. Identify the appropriate land uses within the delineated basins and select CN
values for each of the pre-developed and post-developed basins, as described in
the Curve Numbers subsection below;
4. Determine the time of concentration for both pre-developed and post-developed
conditions, as described in the Time of Concentration subsection below;
5. Compute the surface area or volume at incremental stages (heights) of the
drainage facility, beginning at the bottom of the anticipated drainage facility to an
elevation at least 1 foot above the overflow;
6. Create basin links for combining and/or routing basin hydrographs to the
proposed facility. Links may have routing elements, such as pipes or channels;
7. Determine the precipitation for the required design storms specified in Chapter 2.
Precipitation maps for the design storms are provided in the Precipitation Maps
subsection below;
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8. Set the routing and hydrograph time increments in the computer software program
to six-minutes or less;
9. Determine the required NRCS Storm Distribution, as described in the Design
Storm Distribution subsection below, and select it in the software program;
10. Input the geometry of the anticipated outflow structures (i.e. weirs, orifices, etc.);
11. Input the elevation and storage volume relationship;
12. Compute peak rates and volumes for the pre-developed basins and determine the
allowable release rates per the design criteria specified in Chapter 2;
13. Compute the hydrographs of the post-developed basins, combine and route the
hydrographs to the drainage facility and route the inflow hydrograph through the
facility;
14. Verify that the release from the site does not exceed the allowable release rate (or
volume, when required), as determined in step 12. Modify the pond geometry and
outflow structure input data if the results indicate that the allowable thresholds are
exceeded.
Basin Areas
The basin modeling must reflect the actual runoff characteristics as closely as
possible and be consistent with the assumptions within the model used. The
impervious and pervious areas must be estimated from best available plans,
topography, or aerial photography, and verified by field reconnaissance.
Hyd�ologic Soil Group Classification
The NRCS has classified over 4,000 soil types into the following four soils groups:
• Grou� A soils have high infiltration rates, even when thoroughly wetted,
and consist chiefly of deep, well-to-excessively drained sands or gravels.
These soils have a high rate of water transmission (greater than 030
inches/hour) and low runoff potential.
• Group B soils have moderate infiltration rates when thoroughly wetted,
and consist chiefly of moderately fine to moderately coarse textures.
These soils have a moderate rate of water transmission (0.15 to 0.30
inches/hour) and moderately low runoff potential.
• Group C soils have slow infiltration rates when thoroughly wetted, and
consist chiefly of soils with a layer that impedes downward movement of
water, or soils with moderately fine to fine textures. These soils have a
slow rate of transmission (0.05 to 0.15 inches/hour) and moderately high
runoff potential.
• Grou� D soils have very slow infiltration rates when thoroughly wetted,
and consist chiefly of clay soils with a high swelling potential, soils with a
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permanent high water table, soils with a hardpan or clay layer at or near
the surface, and shallow soils over nearly impervious materials. These
soils have a very slow rate of transmission (0-0.05 inches/hour) and high
runoff potential.
Refer to the current Spokane County NRCS maps for the hydrologic soil group
classification for soils common to the Spokane Region.
Curve Numbers
Curve numbers (CNs) indicate the runoff potential of a watershed. The higher the CN
value, the higher the potential for runoff. The CN takes into consideration the
hydrologic soil group, land use, and cover.
Table 5-1 lists CN values for agricultural, suburban and urban land use
classifications. These values are for Antecedent Runoff Condition (ARC) II, which is
defined below. See NRCS Publication 210-VI-TR-55 for additional CN values.
Wei�htin� Curve Numbers: Basins often include areas with differing curve numbers
based on their soils, land use and cover. Overall CNs for these basins are determined
by weighting the CN for each area based on the size of the area. For an example of
weighting CNs refer to Appendix SA.
In most cases, if areas in the same basin have CN values that differ by more than
20 points, separate hydrographs shall be generated for each and the hydrographs shall
be combined. As an exception to this rule, separate hydrographs are not required for
unconnected impervious areas. Unconnected impervious areas are defined as those
that discharge over a pervious area in the form of sheet flow, such as a tennis court in
the middle of a lawn or runoff from roofs flowing over lawn. Unconnected
impervious areas can be weighted with pervious areas.
Connected impervious areas shall not be weighted with pervious areas. Connected
impervious areas can include driveways and sidewalks that are adjacent to (i.e.
hydraulically connected to) a pollution generating impervious roadway and discharge
directly into a drainage system without first traversing an area of pervious ground.
Basin configurations shall be consistent with surface runoff patterns. For example, the
roof and lawn areas of residential neighborhoods can be combined and considered
one basin when the roof runoff travels through lawns before getting to the streets or
drainage system. The driveway and adjacent sidewalk areas must be combined with
the street areas, if they are hydraulically connected and would be considered a
separate basin. The impervious and pervious hydrographs shall then be linked with or
without a routing element, such as a pipe or a channel.
Antecedent Runoff Condition — Curve Number Adjustment: The moisture condition
in a soil prior to a storm event is referred to as the antecedent runoff condition (ARC).
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TABLE 5-1
RUNOFF CURVE NUMBERS
ANTECEDENT RUNOFF CONDITION ARC II
Group Group Group Group
Cover e and h drolo ic condition A Soils B Soils C Soils D Soils
Open Space(lawns,parks,golf courses,cemeteries,landscaping,etc.):1
Poor condition(grass cover<50%of the area) 68 79 86 89
Fair condition(grass cover on 50%to 75%of the area) 49 69 79 84
Good condition(grass cover on>75%of the area) 39 61 74 80
Impervious Areas:
Open water bodies:lakes,wetlands,ponds eta 100 100 100 100
Paved parking lots,roofs,driveways,eta(excluding right of way) 98 98 98 98
Porous pavers and permeable interlocking concrete(assumed as 85%impervious and 15%lawn):
Fair lawn condition(weighted average CNs) 91 94 96 97
Gravel 76 85 89 91
Dirt 72 82 87 89
Pasture,Grassland,or Range-Continuous Forage for Grazing:
Poor condition(ground cover<50%or heavily grazed with no mulch). 68 79 86 89
Fair condition(ground cover 50%to 75%and not heavily grazed) 49 69 79 84
Good condition(ground cover>75%and lightly or only occasionally grazed) 39 61 74 80
Cultivated Agricultural Lands:
Row Crops(good)e.g.corn,sugar beets,soy beans 64 75 82 85
Small Grain(good)e.g.wheat,barley,flax 60 72 80 84
Meadow(continuous rass, rotected fi�om razin and enerall mowed for ha ) 30 58 71 78
Brush(brush-weed-grass mixture with brush the major element):
Poor(<50%ground cover) 48 67 77 83
Fair(50%to 75%ground cover) 35 56 70 77
Good(>75%ground cover)� 30 48 65 73
Woods-grass combination(orchard or tree farm)3:
Poor 57 73 82 86
Fair 43 65 76 82
Good 32 58 72 79
Woods:
Poor(Forest litter,small trees,and brush are destroyed by heavy grazing or regular burning) 45 66 77 83
Fair(Woods are grazed but not burned,and some forest litter covers the soil) 36 60 73 79
Good(Woods are protected fi•om grazing,and litter and brush adequately cover the soil) 30 55 70 77
Herbaceous(mixture of grass,weeds,and low-growing brush,with brush the minor element)4:
Poor(<30%ground cover) 80 87 93
Fair(30%to 70%ground cover) 71 81 89
Good(>70% round cover) 62 74 85
Sagebrush with Grass Understory4:
Poor(<30%ground cover) 67 80 85
Fair(30%to 70%ground cover) 51 63 70
Good(>70%ground cover) 35 47 55
� Composite CNs may be computed far other combinations of open space cover type.
�Actual curve number is less than 30;use CN=30 for runoff computations.
3 CNs shown were computed far areas with 50%woods and 50%grass(pasture)cover. Other combinations of
conditions may be computed from the CNs for woods and pasture.
4 Curve numbers have not been developed for group A soils.
For a more detailed and complete description of land use curve numbers refer to Chapter 2 of the Soil
Conservarion Service's Technical Release No. 55 (Publication 210-VI-TR-55, Second Ed.,June 1986).
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The NRCS developed three antecedent runoff conditions:
• ARC I(Dry Condition): soils are dry but surface cracks are not evident.
• ARC II(Average Condition): soils are not dry or saturated. The CN values
listed in Table 5-1 are applicable under this condition and do not account
for snowmelt or runoff on frozen ground conditions.
• ARC III (Wet Condition): soils are saturated or near saturation due to
heavy rainfall or light rainfall and low temperatures within the last 5 days.
The design of detention or infiltration ponds shall be based on ARC II. When ARC III
applies, such as when designing evaporation facilities or modeling the winter months,
Table 5-2 shall be used to adjust the CN values.
TABLE 5-2
CURVE NUMBER BASED ON ANTECEDENT RUNOFF CONDITION ARC
CN ARC II CN ARC I CN ARC III CN ARC II CN ARC I CN ARC III
100 100 100 76 58 89
99 97 100 75 57 88
98 94 99 74 55 88
97 91 99 73 54 87
96 89 99 72 53 86
95 87 98 71 52 86
94 85 98 70 51 85
93 83 98 69 50 84
92 81 97 68 48 84
91 80 97 67 47 83
90 78 96 66 46 82
89 76 96 65 45 82
88 75 95 64 44 81
87 73 95 63 43 80
86 72 94 62 42 79
85 70 94 61 41 78
84 68 93 60 40 78
83 67 93 59 39 78
82 66 92 58 38 76
81 64 92 57 37 75
80 63 91 56 36 75
79 62 91 55 35 74
78 60 90 54 34 73
77 59 89 50 31 70
Curve number conversions far different ARC are for the case of initial abstraction(Ia)=0.2 S.Initial
abstraction represents all water losses before runoff begins(water retained in surface depressions,water
intercepted by vegetation, evaporation,infiltration, etc.)
Source:U.S. Soil Conservation Service National Engineering Handbook Table 10.1.
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Time of ConcentNation
Time of concentration is affected by the way stormwater moves through a watershed.
Stormwater can move in the form of sheet flow, shallow concentrated flow, open
channel flow, or some combination of these. The rype of flow should be verified by
field inspection.
The time of concentration for rainfall shall be computed for all overland flow,
ditches, channels, gutters, culverts, and pipe systems. When using the Curve Number
Method, the time of concentration for the various surfaces and conveyances shall be
computed using the procedures presented in this section. These procedures are based
on the methods described in the Soil Conservation Service's Technical Release
No. 55.
Travel time (Tt) is the time it takes stormwater runoff to travel from one location to
another in a watershed. Time of concentration (T�) is the time for stormwater runoff
to travel from the hydraulically most distant point to the point of discharge of a
watershed. T� is computed by adding all the travel times for consecutive components
of the drainage conveyance system as given by the following equation:
T� = Ttl + Tt2 + ... T� (5-3)
Where: T� = time of concentration (minutes);
n = number of flow segments; and,
Tt = travel time (minutes) is the ratio of flow length to
flow velocity given by:
Tt = 6� (5-4)
where: L = flow length (feet);
V = average velocity (feet/second); and,
60 = conversion factor(seconds to minutes).
T� influences the shape and peak of the runoff hydrograph. Urbanization usually
decreases T�, thereby increasing the peak discharge. But T� can be increased as a
result of ponding behind small or inadequate drainage facilities including storm drain
inlets and road culverts, or reduction of land slope through grading. T� shall not be
less than 5 minutes.
Sheet Flow: Sheet flow is flow over plane surfaces and shall not be used over
distances exceeding 100 feet. Use Manning's kinematic solution to directly compute
Tt:
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0.42�nsL�0�8 (5-5)
�t �PZ�0.5����0.4
Where: Tt = travel time (minutes);
ns = Manning's effective roughness coefficient for sheet
flow (use Table 5-3);
L = flow length (feet);
Pz = 2-year, 24-hour rainfall (inches), (use Figure 5-1);
So = slope of hydraulic grade line (land slope, feet/foot).
The friction value (ns) is used to calculate sheet flow. The friction value is Manning's
effective roughness coefficient modified to take into consideration the effect of
raindrop impact, drag over the plane surface, obstacles such as litter, depressions,
crop ridges and rocks, and erosion and transportation of sediment. The ns values are
for very shallow flow depths of about 0.1 foot and are only used for travel lengths up
to 100 feet. Table 5-3 gives Manning's ns values for sheet flow for various surface
conditions.
Shallow Concentrated Flow: After 100 feet, sheet flow is assumed to have developed
into shallow concentrated flow. The travel time (Tt) for the shallow concentrated flow
segment can be computed using Equation 5-4. The average velocity for shallow
concentrated flow is calculated using the following equation:
V = k So (5-6)
Where: V = velocity (feet/second);
k = ks or k�, time of concentration velocity factor
(feet/second); and,
So = slope of flow path (feet/foot).
Table 5-3 provides "k" for various land covers and channel characteristics with
assumptions made for hydraulic radius using the following rearrangement of
Manning's equation:
1.49R��3
k = (5-7)
n
Where: R = hydraulic radius; and,
n = Manning's roughness coefficient for open channel
flow (Table 5-3 or 5-4).
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TABLE 5-3
FRICTION VALUES (n and k) FOR USE
IN COMPUTING TIME OF CONCENTRATION
Sheet Flowl ns
Bare sand 0.010
Smooth surfaces (concrete, asphalt, gravel, or bare hard soil) 0.011
Asphalt and gravel 0.012
Fallow fields of loose soil surface (no vegetal residue) 0.05
Cultivated soil with crop residue(slope < 020 feet/foot) 0.06
Cultivated soil with crop residue(slope> 020 feet/foot) 0.17
Short prairie grass and lawns 0.15
Dense grass 0.24
Bermuda grass 0.41
Range, natural 0.13
Woods or forest, poor cover 0.40
Woods or forest, good cover 0.80
Shallow, Concentrated Flow ks
Forest with heavy ground litter and meadows (n = 0.10) 3
Brushy ground with some trees (n =0.06) 5
Fallow or minimum tillage cultivation (n= 0.04) 8
High grass (n = 0.035) 9
Short grass,pasture and lawns (n = 0.030) 11
Newly-bare ground (n = 0.025) 13
Paved and gravel areas (n = 0.012) 27
Channel Flow Intermittent, R= 0.2 k�
Forested swale with heavy ground litter(n=0.10) 5
Forested drainage course/ravine with defined channel bed 10
(n=0.050)
Rock-lined waterway (n=0.035) 15
Grassed waterway (n=0.030) 17
Earth-lined waterway (n=0.025) 20
Corrugated metal pipe (n=0.024) 21
Concrete pipe (n=0.012) 42
Other waterways and pipes 0.508/n
Channel Flow Continuous Stream, R=0.4 k�
Meandering stream with some pools (n=0.040) 20
Rock-lined stream (n=0.035) 23
Grassed stream (n=0.030) 27
Other streams, man-made channels and pipe 0.807/n
1 These values were determined specifically for sheet flow conditions and are not appropriate for
conventional open channel flow calculations.
Source: WSDOT Highway RunoffManual(2004)Table 4B-5;Engman(1983); and the Florida
Department of Transportation Drainage Manual(1986).
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TABLE 5-4
SUGGESTED VALUES OF MANNING'S ROUGHNESS COEFFICIENT "n"
FOR CHANNEL FLOW
Type of Channel and Description "n"1 Type of Channel and Description "n"�
A. C O N ST R U C T E D C H A N N E L S 7. Veiy weedy reaches, deep pools, or
a. Earth, straight and uniform floodways with heavy stand of timber 0.100
and underbrush
1. Clean,recently completed 0.018
2. Gravel,uniform selection,clean 0.025 b. Moz�ntain streams, no vegetation tn channel, banks
3. With short grass,few weeds 0.027 usually steep, trees and bi^z�sh along banks submerged at
b. Earth, winding and sluggish high stages
L No vegetation 0.025 1. Bottom: gravel, cobbles and few
2. Grass,some weeds 0.030
boulders 0.040
3. Dense weeds or aquatic plants in deep 0.035 2. Bottom:cobbles with large boulders 0.050
channels
4. Earth bottom and rubble sides 0.030 B-2 Floodplains
5. Stony bottom and weedy banks 0.035 a. Pasture, no br�ush
6. Cobble bottom and clean sides 0.040 1. Shart grass 0.030
c. Rock laned 2. High grass 0.035
1. Smooth and uniform 0.035 b. Cxrltavated areas
2. Jagged and irregular 0.040 L No crop 0.030
d. Channels not maintained, weeds and bvzrsh uncut 2. Mature row crops 0.035
1. Dense weeds,high as flow depth 0.080 3. Mature field crops 0.040
2. Clean bottom biush on sides 0.050 c. Brzish
3. Same,highest stage of flow 0.070 1. Scattered brush,heavy weeds 0.050
4. Dense biush,high stage 0.100 2. Light brush and hees 0.060
B. N A T U R A L ST R E A M S 3. Medium to dense brush 0.070
B-1 Minor streams (top width at flood stage < 100 4. Heavy,dense biush 0.100
a. Streams on plaan d. Trees
1. Clean,straight,full stage,no iifts or 1. Dense willows,straight 0.150
deep pools 0.030
2. Cleared land with tree stumps, no
2. Same as No. 1, but more stones and
sprouts 0.040
weeds 0.035
3. Same as No. 2, but with heavy
3. Clean,winding,some pools and shoals
growth of sprouts 0.060
0.040
4. Heavy stand of timber, a few down
4. Same as No.3,but some weeds 0.045 trees, little undergrowth, flood stage 0.100
below branches
5. Same as No.4,but more stones 0.050
5. Same as above, but with flood stage 0.120
6. Sluggish reaches,weedy deep pools 0.070 reaching branches
�The"n"values presented in this table are the"Normal"values as presented in Chow(1959). Far an extensive
range and far additional values refer to Chow(1959)
Source: W SDOT Hyway Runoff Manual(2004)Table 4B-6;Engman(1983)and the Florida Department of
Transportation Drainage Manual(1986).
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O�en Channel Flow: Open channels are assumed to exist where channels are visible
on aerial photographs, where streams appear on United States Geological Survey
(USGS) quadrangle sheets, or where topographic information indicates the presence
of a channel. The k� values from Table 5-3 used in Equation 5-6 or water surface
profile information can be used to estimate average flow velocity. Average flow
velocity is usually determined for bank-full conditions. After average velocity is
computed the travel time (Tt) for the channel segment can be computed using
Equation 5-4.
P�ecipit�tion Mczps
The following isopluvial maps for the Spokane Region were generated with computer
software by Spokane County from rain data collected from National Oceanic and
Atmospheric Administration (NOAA) Atlas 2, Volume IX, 1973. The numbers
shown on the isopluvial curves represent inches of precipitation.
Design Storm Distributions
These methods require the selection of, or the input of, a rainfall distribution and the
precipitation associated with a design storm. The following storm distributions shall
be used:
• The NRCS Type II 24-hour storm for sizing water quality treatment
facilities; or,
• The NRCS Type IA 24-hour storm for sizing flow control facilities.
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REGtONAL STQRM WATER MANIJAL
2�HDUR IS�PLUVlALS R42E F243E R44E R45E
. i �
�_ I' . I .
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5-14
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5-15
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5-16
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Apri12008 Chapter 5-Hydrologic Analysis and Design
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SPOKANE REGIONAL STORMWATER MANUAL
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Figure 5-6—Mean Annual Precipitation
Apri12008 Chapter 5—Hydrologic Analysis and Design
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SPOKANE REGIONAL STORMWATER MANUAL
5.4 LEVEL POOL ROUTING METHOD
This section presents a general description of the methodology for routing a hydrograph
through an existing retention/detention facility or closed depression, and for sizing a new
retention/detention facility using hydrograph analysis. The "level pool routing" technique
presented here is one of the simplest and most commonly used hydrograph routing
methods. This method is described in Handbook of Applied Hydrology (Chow, Ven Te,
1964) and elsewhere, and is based upon the continuity equation:
Inflow- Outflow= Change in storage
CI�+Iz _O�+Oz� _�S =Sz-S� (5-8)
2 2 4t
Where: I = inflow at time 1 and time 2;
O = outflow at time 1 and time 2;
S = storage at time 1 and time 2; and,
4t = time interval, time 2 —time 1.
The time interval, 4t, must be consistent with the time interval used in developing the
inflow hydrograph. The 4t variable can be eliminated by dividing it into the storage
variables to obtain the following rearranged equation:
I� +Iz +25� —O� = Oz +2Sz (5-9)
If the time interval, 4t, is in minutes, the units of storage (S) are in cubic feet per minute,
which can be converted to cubic feet per second by multiplying by 1 minute/60 seconds.
The terms on the left-hand side of the equation are known from the inflow hydrograph
and from the storage and outflow values of the previous time step. The unknowns 02 and
S2 can be solved interactively from the given stage-storage and stage-discharge curves.
5.5 RATIONAL METHOD
The rational method is used to predict peak flows for small drainage areas. The rational
method can be used for the design of conveyance, flow control, and subsurface
infiltration facilities. The greatest accuracy is obtained for areas smaller than 100 acres
and for developed conditions with large impervious areas. The peak flow rate is
calculated using the following equation:
QP = CIA (5-10)
Where: QP = peak flow rate (cfs);
C = runoff coefficient(dimensionless units);
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5-19
SPOKANE REGIONAL STORMWATER MANUAL
I = rainfall intensity (inches/hour) (refer to
Section 5.5.3); and,
A = drainage area(acres).
5.5.1 RUNOFF COEFFICIENTS
Table 5-5 provides runoff coefficients for the 10-year storm frequency. Steeply
sloped areas and less frequent, higher intensity storms require the use of higher
coefficients because infiltration and other losses have a proportionally smaller effect
on runof£ Generally, runoff coefficients should be increased by 10% when designing
for a 25-year frequency; by 20% for a 50-year frequency; and by 25% for a 100-year
frequency. Runoff coefficients should not be increased above 0.95.
TABLE 5-5
RUNOFF COEFFICIENTS FOR THE RATIONAL METHOD
10-YEAR RETURN FREQUENCY
R unoff Coefficient (C)
Flat Rolling (2% - Hilly (>10%)
T pe of C over (<2%) 10%)
Pavement and Roofs 0.90 0.90 0.90
Earth Shoulders 0.50 0.50 0.50
Drives and Walks 0.90 0.90 0.90
Gravel Pavement 0.50 0.55 0.60
Lawns, Sandy Soil 0.10 0.15 0.20
Lawns, Heavy Soil 0.17 0.22 0.35
Grass Shoulders 0.25 0.25 0.25
Side Slopes, Earth 0.60 0.60 0.60
Side Slopes, Turf 0.30 0.30 0.30
Median Areas, Turf 0.25 0.30 0.30
Cultivated Land, Clay and 0.50 0.55 0.60
Loam
Cultivated Land, Sand and 0.25 0.30 0.35
Gravel
Woodland and Forest 0.10 0.15 0.20
Meadow and Pasture Land 0.25 0.30 0.35
Source: WSDOT Aydraulics Manual,
March 2004
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SPOKANE REGIONAL STORMWATER MANUAL
5.5.2 TIME OF CONCENTRATION
The travel time, the time required for flow to move through a flow segment, shall be
computed for each flow segment. The time of concentration is equal to the sum of the
travel times for all flow segments. The procedure described below was developed by
the NRCS. It is sensitive to slope, type of ground cover, and the size of channel. The
time of concentration can be calculated as follows:
T� _ �� (5-11)
T�= Ttl + Tt2 + �-� + Ttn (5-12)
Where: Tt = travel time of flow segment (minutes);
T� = time of concentration (minutes);
L = length of segment (feet);
K = ground cover coefficient, Table 5-6 (feet/minute);
S = slope of segment(feet/foot); and,
n = number of flow segments.
The time of concentration shall not be less than 5 minutes. For a few drainage areas,
the time of concentration that produces the largest amount of runoff is less than the
time of concentration for the entire basin. This can occur when two or more basins
have dramatically different types of cover. The most common case would be a large
paved area together with a long narrow strip of natural area. In this case, the engineer
shall check the runoff produced by the paved area alone to determine if this scenario
would cause a greater peak runoff rate than the peak runoff rate produced when both
land segments are contributing flow. The scenario that produces the greatest runoff
shall be used, even if the entire basin is not contributing flow to this runoff.
5.5.3 INTENSITY
The equation for calculating rainfall intensity is:
I =�� (5-13)
c
Where: m = coefficient of rainfall intensity, Table 5-7;
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SPOKANE REGIONAL STORMWATER MANUAL
n = coefficient of rainfall intensity, Table 5-7;
I = rainfall intensity (inches/hour); and,
T� = time of concentration (minutes).
TABLE 5-6
GROUND COVER COEFFICIENTS
K
T e of Cover feet/minute
Forest with heavy ground cover 150
Minimum tillage cultivation 280
Short pasture grass or lawn 420
Nearly bare ground 600
Small roadside ditch w/grass 900
Paved area 1,200
Gutter flow:
4 inches deep 1,500
6 inches deep 2,400
8 inches deep 3,100
Storm Sewers:
12 inch diameter 3,000
18 inch diameter 3,900
24 inch diameter 4,700
Open Channel Flow(n = .040):
12 inches deep 1,100
Narrow Channel (w/d=1):
2 feet deep 1,800
4 feet deep 2,800
Open Channel Flow(n = .040):
1 foot deep 2,000
Wide Channel (w/d=9):
2 feet deep 3,100
4 feet deep 5,000
Source: WSDOT Hydraulics Manual,March 2004
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SPOKANE REGIONAL STORMWATER MANUAL
TABLE 5-7
INDEX TO RAINFALL COEFFICIENTS
2-year 10-year 25-year 50-year 100-year
E vent E vent E vent E vent E vent
m m m � m m m � m n
3.47 0.556 6.98 0.609 9.09 0.626 10.68 0.635 12.33 0.643
Source: WSDOT Hydraulics Manual,March 2004
The rainfall intensity (� coefficients (m and n) have been determined for Spokane for
the 2-, 10-, 25-, 50-, and 100-year storm events. These coefficients were developed
from NOAA Atlas 2 and are shown in Table 5-7.
5.6 BOWSTRING METHOD (MODIFIED RATIONAL
METHOD)
This method is used to estimate storage requirements for a given design storm using a
series of hydrographs for different storm durations (t).
Depending on the relative magnitude of the time of concentration (T�) and the storm
duration, the shape of the hydrograph generated with this method varies from triangular
to trapezoidal (see Figure 5-7).
t � T t = T t � T
� � t �
Q�, C��, C�p
t 1.67t T� 1.67T� T� 1.67T�
�
Figure 5-7—Bowstring Method Hydrographs
The recession period(TR) of the hydrograph is given by Equation 5-14.
Apri12008 Chapter 5—Hydrologic Analysis and Design
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SPOKANE REGIONAL STORMWATER MANUAL
TR =1.67TP �5-14�
Where: TP = T�, when t> T�, or,
TP =t, when t< T�.
The volume (V)under the hydrograph at a given time (t)is given by:
V(t) =1.34QPt for t< T� (triangular hydrograph) (5-15)
V(t) = QPt+034QPT� for t> T�(trapezoidal hydrograph) (5-16)
With these equations, the base of the triangular hydrograph is equal to 2.67t. For the
trapezoidal hydrograph, the time base is t + 1.67T�. The top width of the trapezoid is
equal to t—T�. With this method, the hydrograph for each storm duration is overlaid with
the outflow hydrograph. The outflow hydrograph is given by the following equation:
Vor,r�t) = Qor,rt (5-17)
The critical storm duration is the storm duration that results in the maximum required
detention storage.
5.6.1 DESIGN STEPS
Under certain circumstances as allowed by the local jurisdiction the Bowstring
Method can be used for detention design with following procedure:
l. Compute the peak flow rate (QP) for t = T� using Equation 5-10 for the pre-
developed condition. If the project proposes to release runoff off site, this is the
maximum peak flow rate that shall be released.
2. Compute QP for t= T�using Equation 5-10 for the post-developed condition.
3. Compute intensities (I), peak flow rates (QP), and volumes (V) for various times
(e.g., t= 5, 10, 25...minutes)using Equations 5-10, 5-13, 5-15, and 5-16.
4. Determine the allowable release rate (QouT), which is limited to either the pre-
developed peak flow rate or the allowable infiltration rate through drywells as
determined by Section 4.3.1.
5. Calculate the outflow volume (VouT)using Equation 5-17.
6. The required storage is obtained as the maximum difference between inflow and
outflow volumes by the tabular methods. The tabular method is illustrated in the
example given in Appendix SB.
Although credit is not given for infiltration through the pond bottom for ponds and
swales, they shall comply with the criteria in Section 7.8.3.
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SPOKANE REGIONAL STORMWATER MANUAL
5.7 WATER BUDGET METHOD
5.7.1 INTRODUCTION
A water budget analysis is required for the design of an evaporative pond. The
analysis utilizes average monthly precipitation and pan evaporation values to estimate
the net stormwater runoff volume increase during a two year cycle. The precipitation
values are adjusted to account for the changes in precipitation over the Spokane
Region. The water budget analysis is conducted for a two-year cycle to account for
seasonal variations in precipitation, pan evaporation and antecedent runoff conditions
and to verify that equilibrium is reached.
Equilibrium is reached when the analysis confirms that the required pond size does
not increase in the second year of the cycle.
5.7.2 METHODOLOGY
The water budget analysis is performed utilizing the following relationships:
UsTO�GE(X) =UIN(X)—VouT(X) +VsTOxaG� (x -1) (5-18)
UPON�=max[VsTOxAGE(x)] (5-19)
Where: x = any given month;
VIN = water volume entering the evaporative pond in a
given month. Stormwater runoff volume is
calculated using the NRCS runoff Equations 5-1
and 5-2;
vouT = stormwater volume leaving the evaporative pond in
a given month (i.e. pan evaporation, surface
release);
UsTO�GE = storage volume necessary for a given month; and,
VPON� = storage volume necessary to reach equilibrium in a
2-year cycle.
The analysis is repeated until the maximum storage volume in the second year is
equal to or less than the maximum storage volume in the first year.
The cycle shall start in October, the month that yields the greatest net storage volume
for the year.
Water loss through evaporation from overland surface areas is not considered in the
water budget due to the wide variation in evaporation rates that occur over these types
Apri12008 Chapter 5—Hydrologic Analysis and Design
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SPOKANE REGIONAL STORMWATER MANUAL
of surfaces. Depressional storage is the only reduction that can be considered in this
analysis. This reduction may be considered if closed depressions are present on site in
the pre-developed condition and are proposed to remain as an existing topographical
feature, set aside for drainage purposes. Vegetal and minor topographical abstraction
and interception are already accounted for in the NRCS curve numbers.
Depending on the site conditions, evaporative systems shall be designed using the
Preferred or Alternative method design criteria described below.
The Prefer�ed Method
The Preferred Method is used to size evaporation facilities that store the increase in
stormwater runoff volume (after evaporation losses). Refer to Figures 7-7 and 7-8 for
schematics of how this design is implemented.
The water budget analysis must demonstrate that the volume of runoff leaving the site
over a 2-year cycle is less than or equal to the pre-developed volume for the cycle. If
the facility has a surface release, the rate of release from the facility shall meet the
detention design criteria (Section 7.32). If site conditions permit, the pre-developed
volume could be infiltrated when a defined release point is not present on site.
If the evaporative system is designed in combination with a surface discharge, then
Equations 5-18 through 5-21 shall be used:
VALL � UPRE �S-ZO�
QALL � QPRE �5-21�
Where: VALL = the total volume released from the site in two year
cycle (not including pan evaporation or infiltration);
VPRE = the total pre-developed volume of runoff for two
year cycle;
QPRE = the pre-developed rate for the contributing basin;
and,
QALL = the release rate from the faciliry.
The Alte�native Method
The Alternative Method is used to size evaporation facilities that store the total post-
developed runoff volume (less evaporative losses) or full containment evaporative
systems. The Alternative Method is used when the project site does not have a
defined discharge point or when site conditions are not conducive to infiltration of the
pre-developed volume.
The facility shall be sized to store the volume per Equations 5-18, 5-19, and 5-22:
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SPOKANE REGIONAL STORMWATER MANUAL
UsTO�GE�o) —VIN��) - VouT�o) �5-22)
Where: o = first month of the two year cycle.
The facility shall include a factor of safery on the maximum depth of 1.2. The extra
capacity provides emergency storage in the event that above average total annual
precipitation is experienced. A full containment evaporative pond is required when
there is no discharge point or site conditions prohibit the use of infiltration. These
conditions may include little infiltrative capacity in the soil, existing high
groundwater, or potential for adverse impacts on adjacent or down-gradient properties
from additional stormwater being injected into the subsurface.
5.7.3 DESIGN STEPS
The following steps outline how to use the spreadsheets that have been developed for
this method (check with the local jurisdiction for the most current spreadsheets for a
proposed project). Example calculations are presented in Appendices SC and SD.
l. Determine the drainage basin boundaries that contribute to the evaporative pond
and the land surface characteristics (i.e. grass, pavement, roof area, sidewalk,
woods, etc.) for the post-developed conditions. These parameters also need to be
determined for the pre-developed conditions when using the Preferred Method;
2. Determine the ARC II CN values for the pervious and impervious surfaces using
Table 5-1 and weight the CN values per Section 5.3.4.
3. Determine the associated ARC III CN values per Table 5-2. Input the ARC II and
ARC III CN values;
4. Input the impervious basin and total basin size, in acres;
5. Input the mean annual precipitation, in inches,per Figure 5-6;
6. Input the proposed pond side slopes;
7. Input an assumed pond depth, for the Preferred Method only, based upon depth to
limiting layer or desired depth. Pond depth is calculated automatically for the
Alternative Method based upon the necessary surface area (projected from pond
bottom area) and the required volume necessary to store/evaporate;
8. Assume a value for the pond bottom area and input that value, in square feet, into
the pond bottom area cell of the spreadsheet;
9. The pond bottom perimeter is calculated as a square for simplicity; should the
actual perimeter be known (or general shape), this can be inserted in place of the
calculated field; however, each time the pond bottom is changed during the
iterative process, the pond bottom perimeter needs to be adjusted; and,
10. Vary the pond bottom area(up or down)until:
o The "Amount Spilled" is less than or equal to the "Total Annual Pre-
Developed Volume"for the Preferred Method; or,
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SPOKANE REGIONAL STORMWATER MANUAL
o The month in which the "Total Volume Stored" in Pond (STORAGE
column) shows a decrease from year one to year two for the
Alternative Method.
11. Note that for the Preferred Method, these steps only satisfy the requirement to
control volume to the pre-developed condition. In order to satisfy the requirement
to control flow rates to the pre-developed condition, Section 7.3 must be utilized
to design the detention portion of the drainage faciliry.
5.7.4 CURVE NUMBER ADJUSTMENT
The antecedent runoff condition (ARC) needs to be considered during the months of
the year when the ground may be saturated or frozen. The CNs shall be adjusted as
indicated in Table 5-8 and Table 5-2.
TABLE 5-8
CURVE NUMBER ADJUSTMENT FOR
ANTECEDENT RUNOFF CONDITION ARC
Antecedent Runoff
Month Condition (ARC) CurveNumber
April through October Normal (ARC = I� See Table 5-2
November and March Wet (ARC = II� See Table 5-2
December, January & February n/a 95
The following should be noted when choosing CN values:
• For impervious surfaces such as roads, sidewalks and driveways, the ARC
II CN is typically 98, and the correlating ARC III CN is 99. From
December through February, the assumption is that if the CN of 98 goes
up to 99 during the wet months, it will not revert to 98 during frozen
ground conditions; and,
• During December through February, the CN for pervious surfaces is 95
regardless of the ARC II or III CNs; this is meant to approximate runoff
from pervious surfaces during snowpack buildup and snowmelt.
5.7.5 CLIMATOLOGICAL DATA
Average monthly precipitation rates were obtained from the Western Regional
Climate Center (WRCC), based on records from January 1, 1890 to December 31,
2005. This information is found in Table 5-9 and is updated quarterly at the following
web site:
Apri12008 Chapter 5—Hydrologic Analysis and Design
5-28
SPOKANE REGIONAL STORMWATER MANUAL
• http://www.wrcc.dri.edu/cgi-bin/c1iRECtM.p1?waspok.
The monthly pan evaporation values were also obtained from the WRCC and are
current for the period from 1889 through 2002. In Washington State many pan
evaporation stations do not take readings during winter. A "0.00" total indicates that
no measurement was taken. Some totals are computed from meteorological
measurements using a form of the Penman equation. The rates in this table were
obtained using this method according to the WRCC website:
http:wrcc.dri.edu/htmlfiles/westevap.final.html.
TABLE 5-9
AVERAGE MONTHLY PRECIPITATION
AND PAN EVAPORATION VALUES
Month /Data Precipitation (in) Pan Evaporation (in)
January 1.97 0.61
February 1.54 1.11
March 1.39 2.28
April 1.11 4.45
May 1.42 6.69
June 1.20 8.14
July 0.5 5 10.70
August 0.63 9.42
September 0.80 5.90
October 1.17 2.58
November 2.08 0.92
December 2.20 0.51
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SPOKANE REGIONAL STORMWATER MANUAL
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Apri12008 Chapter 5—Hydrologic Analysis and Design
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SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 5A - EXAMPLE CALCULATION :
WEIGHTING CURVE NUMBERS
Gl VEN
• The existing site is approximately 10-acres, consisting of Type B soils. Existing
surface vegetative conditions include short grass and weeds.
• Post-developed site conditions consist of:
0 38 — 10,000 square foot lots;
0 1,500-square-foot homes with 500-square-foot driveways;
0 1.30 acres of road impervious areas; and,
o No sidewalks are proposed.
CA L CUL A TIONS
l. Use Table 5-1 to find the CNs for the lawn areas and the roofs, driveways, and
streets:
CN= 61 for lawns (good condition)—Type B soils
CN= 98 for streets, driveways, and roofs (impervious areas)
2. Compute the CN for the impervious basin. The connected impervious areas are
driveways and streets. No weighting is required because the CNs values for the
impervious areas are the same.
Total driveway area = (38 driveways)(500 square feet/driveway)
= 19,000 square feet= 0.44 acres
Total connected impervious area= 0.44 +130 = 1.74 acres
CN FOR THE IMPERVIOUS BAS1N= 98
3. Compute the CN for the pervious basin. Although the roof area is impervious, it can
be weighted with the lawn area because the two are considered homogeneous; i.e. the
roofs are not hydraulically connected to the roads or driveways.
Total roof area = (38 houses)(1500 square feet/house)
= 57,000 square feet= 1.31 acres
Totallawn area = Total site—total impervious area
= 10—1.74—131 = 6.95•acres
Total pervious basin= 131+6.95 = 826•acres
Apri12008 Appendix SB—Example Calculation:Bowstring Method
5-31
SPOKANE REGIONAL STORMWATER MANUAL
Weighted CN for pervious basin: 6.95(61)+1.31(98) _ 66.87 � 67
826
CN FOR THE PERVIOUS BAS1N= 67
Apri12008 Appendix SB—Example Calculation:Bowstring Method
5-32
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 5B - EXAMPLE CALCULATION :
BOWSTRING METHOD
Gl VEN
• The existing site is approximately 5-acres, consisting of sandy soils. Existing surface
vegetative conditions include short grass and weeds.
• Post-developed site conditions consist of:
0 20 — 10,000 square foot lots;
0 1,500-square-foot homes with 500-square-foot driveways;
0 0.50 acres of road impervious area; and,
o Topographic relief ranges 2 to 5%.
• When the site is developed, the longest time of concentration will consist of:
0 100 feet of overland flow @ 1%;
0 300 feet of gutter flow @ 1%; and,
0 300 feet of pipe flow @ 2%.
• The proponent proposes a pond with drywells for stormwater runoff disposal.
• Field samples were collected at the proposed location of the drywells. Using the
Spokane County 200 Method, it was estimated that the drywell outflow rate is 1 cfs.
CA L CUL A TIONS
1. Determine the weighted Runoff Coefficient (C) for the post-developed condition:
From Table 5-5:
C = 0.15 pervious areas —lawns (sandy soils, rolling terrain)
C = 0.90 impervious areas —streets, driveways, and sidewalks
Total roof area = 20(1,500 square feet)
= 30,000 square feet.
= 0.69 acres
Total driveway area = 20(500 square feet)
= 10,000 square feet
= 0.23 acres
Total impervious area= 0.69 + 0.23 + 0.5 = 1.42 acres
Total permeable area: 5 — 1.42 acres = 3.58 acres
Apri12008 Appendix SB—Example Calculation:Bowstring Method
5-33
SPOKANE REGIONAL STORMWATER MANUAL
we�gnted c: = 3.ss�o.ls��1.42�0.90� _ 0.36
2. Determine the time of concentration (T�).
Ground Cover Coefficient(K): (use Table 5-6)
Flow Segment Travel Time (Tt): T� = K� (Equation 5-11)
FLOW LENGTH SLOPE K Tt
SEGMENT (feet) (feet/foot) (feet/minute) (minutes)
Overland Flow 100 0.01 420 2.38
Gutter Flow 300 0.01 1500 2.00
Pipe Flow 300 0.02 3000 0.71
Total Time of Concentration 5.09 min
3. Determine the intensity using Equation 5-13.
m
I =—
T�
From Table 5-7; m and n are 6.98 and 0.609,
respectively, for the 10-year storm.
I _ 6.9809 = 2 59 inches/hour
5.09
4. Determine the peak flow rate t= T�using Equation 5-10.
QP = CIA= 036(2.59 inches/hour)(5 acres) = 4.66 cfs
5. Compute the volume for t= T�using Equation 5-15.
V(t) = 1.34QPt =1.34(4.66 cfs)(5.09 min)(60 sec/min)=1,907 cubic feet
6. Determine the allowable release rate (QouT)�
QouT = 1.0 cfs (Given)
7. Compute the outflow volume (VouT) for t= T�using Equation 5-17.
Apri12008 Appendix SB—Example Calculation:Bowstring Method
5-34
SPOKANE REGIONAL STORMWATER MANUAL
Vor-r = Qor,�rt = (1.0 cfs)(5.09 min)(60 sec/min) = 305 cubic feet
8. Compute intensities (�, peak flow rates (QP), and inflow and outflow volumes (V,
VoUT) for various times (i.e. t = 5, 10, 25... minutes) using Equations 5-10, 5-13,
5-15, 5-16 and 5-17. This is simply done in a spreadsheet program, as shown in the
sample spreadsheet on Figure SB-1
9. The required storage is obtained as the maximum difference between inflow and
outflow volumes (see spreadsheet, Figure SB-1).
Apri12008 Appendix SB—Example Calculation:Bowstring Method
5-35
SPOKANE REGIONAL STORMWATER MANUAL
BOWSTRING METHOD(TEN YEAR STORM DESIGN) PROJECT: PROJECT NAME
PROJECT NO: PROJECT NUMBER
BASIN: BASIN NAME
NUMBER OF DRYWELLS PROPOSED DESIGNER: DESIGNER NAME
0 Single(type A) 1 Double(type B) DATE: 03/14/06
Time of Conc. (min) 5.09
Area(Acres) 5.00
Weighted"C" 0.36
Volume Provided 1650 storm: 1603
Outflow(cfs) 1.0 Drywell
Area'C 1.80
#1 #2 #3 #4 #5 #6 #7
Time Time Intensity Q dev. V in V out Storage
(min.) (sec.) (in./hr.) (cfs) (cu.ft.) (cu.ft.) (cu.ft.)
(#1'60) (A'C'#3) (Outf.'#2) (#5-#6)
5.09 305.40 2.59 4.66 1909 305.40 1603
5 300 2.62 4.71 1895 300 1595
10 600 1.72 3.09 2176 600 1576
15 900 1.34 2.41 2424 900 1524
20 1200 1.13 2.03 2643 1200 1443
25 1500 0.98 1.77 2838 1500 1338
30 1800 0.88 1.58 3014 1800 1214
35 2100 0.80 1.44 3177 2100 1077
40 2400 0.74 1.33 3327 2400 927
45 2700 0.69 1.24 3468 2700 768
50 3000 0.64 1.16 3600 3000 600
55 3300 0.61 1.09 3726 3300 426
60 3600 0.58 1.04 3845 3600 245
65 3900 0.55 0.99 3959 3900 59
70 4200 0.53 0.95 4067 4200 -133
75 4500 0.50 0.91 4172 4500 -328
80 4800 0.48 0.87 4272 4800 -528
85 5100 0.47 0.84 4370 5100 -730
90 5400 0.45 0.81 4463 5400 -937
95 5700 0.44 0.78 4554 5700 -1146
100 6000 0.42 0.76 4642 6000 -1358
DRYWELL REQUIREMENTS- 10 YEAR DESIGN STORM
Maximum storage by Bowstring = required: 1603 cu.ft.
provided: 1650 cu.ft. oK!
Number and type of drywells required= 0 SINGLE
1 DOUBLE
Figure 5B-1 -Bowstring Method Spreadsheet Example
Apri12008 Appendix SB-Example Calculation:Bowstring Method
5-36
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 5C - EXAMPLE CALCULATION :
WATER BUDGET (PREFERRED METHOD)
Gl VEN
• The project is located in Section 31, Township 26 N, Range 42 E
• Pre-developed Site Conditions
o Woods and grass combination, good condition CN= 58
• Post-developed Site Conditions
o Total basin= 14 acres
o Impervious basin CN= 98, 1.75 acres
o Pervious basin CN= 67 (includes roofs and lawns), 8.00 acres
o Remaining open area= to be used as open space or drainage
o Open Space CN = 61
o Pond Area CN = 98
CA L CUL A TIONS
Spreadsheets referred to in these calculations are available from the local jurisdiction for
a given project. Figure SG1 shows a sample spreadsheet.
l. Determine the ARC II CN values for the pervious and impervious surfaces. Refer to
Appendix SA for an example.
2. Determine the associated ARC III CN values per Table 5-2. Input the ARC II and
ARC III CN values into the spreadsheet.
ARC II CN ARC III CN
58 76
61 78
67 83
98 99
3. Input the impervious basin and total basin size, in acres, into the spreadsheet:
Total impervious area= 1.75 acres
Total pervious area= 8.0 acres
Total basin area = 14.0 ac (includes pond areas & open areas)
4. Input the mean annual precipitation, in inches:
Apri12008 Appendix SC—Example Calculation:Evaporation(Prefeired Method)
5-37
SPOKANE REGIONAL STORMWATER MANUAL
Mean annual precipitation= 17.0 inches (Figure 5-6)
5. Input the proposed pond side slopes into the spreadsheet:
Assume: 3:1 for side slopes
6. Input the proposed pond depth into the spreadsheet pond depth:
Assume: 1.5 feet for maximum surface water elevation
7. Input the pond bottom area in square feet:
Assume: 10% of the total developed area
Pond bottom area= 0.10*�9.75acres�*(43,560squarefeet/acre)
=42,471 square feet
8. Adjust the pond bottom area up and down until the "Amount Spilled" is less than or
equal to the"Total Annual Pre-developed Volume" for the Preferred method.
RESULTS OF THE EVAPORATIVE CELL
The pond bottom area required is 35,700 square feet
The depth of the evaporative cell is 1.5 feet *12 = 1.8 feet
(1.2 is the factor of safery; refer to Section 7.7.2)
This is the size of the first cell of a separated (two-cell) system, or the
lower portion of a stacked(one-cell) system.
9. Begin sizing the detention cell facility by determining the peak flow rates for the pre-
developed basin and post-developed basins using the design steps outlined in
Section 5.3.4 for the 2 and 25-year, 24 hour storm.
o If sizing a separated system (two separate cells), the detention cell is sized per
the steps outlined in Section 5.3.4 and placed downstream of the evaporative
cell. The overflow from the evaporative cell is placed at or above the required
evaporative depth of 1.8 feet.
o If sizing a stacked system (one cell), the detention portion of the cell is placed
on top of the evaporative portion. Thus, the detention cell "bottom" and
outflow structure has to be placed at or above the maximum surface water
elevation of the evaporative system (including the factor of safety). The
detention portion is designed per the criteria specified in Section 73 and shall
have a 1 foot freeboard above the maximum water surface elevation.
Apri12008 Appendix SC—Example Calculation:Evaporation(Prefeired Method)
5-38
SPOKANE REGIONAL STORMWATER MANUAL
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Figure 5C-1 —Evaporative Pond Spreadsheet Example (Preferred Method)
Apri12008 Appendix SC—Example Calculation:Evaporation(Prefeired Method)
5-39
SPOKANE REGIONAL STORMWATER MANUAL
THIS PAGE IS INTENTIONALLY LEFT BLANK
Apri12008 Appendix SC—Example Calculation:Evaporation(Prefeired Method)
5-40
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 5D - EXAMPLE CALCULATION :
WATER BUDGET (ALTERNATIVE METHOD)
Gl VEN
• The project is located in Section 31, Township 26 N, Range 42 E
• Pre-Developed Site Conditions
o Woods and grass combination, good condition CN= 58
• Post Developed Site Conditions
o Total basin= 14 acres
o Impervious basin CN= 98, 1.75 acres
o Pervious basin CN= 67 (includes roofs and lawns), 8.00 acres
o Remaining open area=to be used as open space or drainage
o Open Space CN = 61
o Pond Area CN = 98
CA L CUL A TIONS
Spreadsheets referred to in these calculations are available from the local jurisdiction for
a given project. Figure SD-1 shows a sample spreadsheet.
l. Determine the ARC II CN values for the pervious and impervious surfaces. Refer to
Appendix SA for an example.
2. Determine the associated ARC III CN values per Table 5-2. Input the ARC II and
ARC III CN values into the spreadsheet.
ARC II CN ARC III CN
61 78
67 83
98 99
3. Input the impervious basin and total basin size, in acres, into the spreadsheet;
Total impervious area= 1.75 ac
Total pervious area= 8.0 ac
Total basin area = 14.0 ac (includes pond areas & open areas)
4. Input the mean annual precipitation, in inches.
Mean annual precipitation = 17.0 inches (Figure 5-6)
Apri12008 Appendix SD—Example Calculations:Evaporation(Alt.Method)
5-41
SPOKANE REGIONAL STORMWATER MANUAL
5. Input the proposed pond side slopes into the spreadsheet.
Use: 3:1 for side slopes
6. Input the pond bottom area in square feet
Assume 30% of the total area developed.
Pond bottom area = 030*�9.75acres�*(43,560squarefeet/acre)
= 127,413 square feet
The pond bottom perimeter is calculated as a square for simplicity; should the actual
perimeter be known (or general shape), this can be inserted in place of the calculated
field. Note that each time the pond bottom is changed during the iterative process, the
pond bottom perimeter needs to be adjusted.
7. Adjust the pond bottom area up and down until the month in which the "Total
Volume Stored" in the pond (STORAGE column) is the largest and shows a decrease
from Year 1 to Year 2 of the water budget cycle.
o The month with the largest volume requirements is March in this example
o The pond bottom area required is 137,000 square feet
o The depth of the evaporative cell is 2.21 feet
o Apply the factor of safety to the depth:
2.21 feet x 12 =2.66 feet
o Add the freeboard to determine total pond depth:
2.66 feet+ 1.0 foot= 3.66 feet
Apri12008 Appendix SD—Example Calculations:Evaporation(Alt.Method)
5-42
SPOKANE REGIONAL STORMWATER MANUAL
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Figure 5D-1 —Evaporative Pond Spreadsheet Example (Alternate Method)
Apri12008 Appendix SD—Example Calculations:Evaporation(Alt.Method)
5-43
SPOKANE REGIONAL STORMWATER MANUAL
THIS PAGE IS INTENTIONALLY LEFT BLANK
Apri12008 Appendix SD—Example Calculations:Evaporation(Alt.Method)
5-44
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 5E - HYDROLOGIC SOIL SERIES FOR
WASH I NGTON STATE
Soil Type Hydrologic Soil Group Soil Type Hydrologic Soil Group
A new C Dimal D
Ahl B Dra oon C
Aits C Du ont D
Alderwood C Earlmont C
Arents,Aldeitivood B Ed ewick C
Arents,Everett B Eld B
Ashoe B Eloika B
Athena B Elwell B
Baldhill B Emdent D
Barneston C Es uatzel B
Baum ard B Everett A
Beausite B Everson D
Belfast C Freeman C
Bellin ham D Galvin D
Bellin ham variant C Garfield C
Bernhill B Garrison B
Boistfort B Getchell A
Bon A Giles B
Bonner B Glem•ose B
Bow D Godfi•e D
Brickel C Green Bluff B
Brid eson D Greenwater A
Briscot D Grove C
Buckle C Ha en B
Bunker B Hardest B
Ca e C Haistine C
Caldwell C Hartnit C
Carlsbor A Hesseltine B
Case D Hoh B
Cassolar C Hoko C
Cathcart B Hoods art C
Cedonia B Hoo dal C
Centralia B Ho us A
Chehalis B Huel A
Chene B Indianola A
Chesaw A Jonas B
Cinebar B Jum e B
Clallam C Kalaloch C
Cla ton B Ka owsin C/D
Coastal beaches variable Katula C
Cocolalla D Kilchis C
Colter C Kitsa C
Custer D Klaus C
Custer,Drained C Klone B
Dabob C Konner D
Apri12008 Appendix SE—Hydrologic Soil Series
5-45
SPOKANE REGIONAL STORMWATER MANUAL
Soil Type Hydrologic Soil Group Soil Type Hydrologic Soil Group
Dear ton C Lakesol B
Del hi D Laketon C
Dick A Lance B
Larkin B Poulsbo C
Latah D Prather C
Lates C Puget D
Lebam B Pu allu B
Lummi D Queets B
L nnwood A Quilcene C
L stan• B Ra nar B
Mal C Rainier C
Manley B Raught B
Marble A Reardan C
Mashel B Reed D
Ma town C Reed,Drained or Protected C
McKenna D Renton D
McMuira D Re ublic B
Melbourne B Riverwash variable
Menzel B Rober C
Mixed Alluvial vaiiable Salal C
Molson B Salkum B
Mondovi B Sammamish D
Moscow C San Juan A
Mukilteo GD Scamman D
Naff B Schneider B
Narcisse C Schumacher B
Nar ar A Seattle D
National B Sekiu D
Neilton A Semiahmoo D
Newber B Shalcar D
Nez Perce C Shano B
Nis uall B Shelton C
Nooksack C Si C
Noima C/D Sinclair C
O art C Ski o a D
Olete C Sk komish B
Olomount C Snaho ish B
Ol m ic B Snohomish D
Orcas D Snow B
Oridia D Solduc B
Oitin D Solleks C
Oso C S ana D
Ovall C S anawa A/B
Palouse B S ei le B
Pastik C S okane C
Peone D S rin dale A
Pheene C Sulsavar B
Phelan D Sultan C
Phoebe B Sultan variant B
Pilchuck C Sumas C
Potchub C Swantown D
Tacoma D Vailton B
Apri12008 Appendix SE—Hydrologic Soil Series
5-46
SPOKANE REGIONAL STORMWATER MANUAL
Soil Type Hydrologic Soil Group Soil Type Hydrologic Soil Group
Tanwax D Vassar B
Tanwax,Drained C Verlot C
Tealwhit D Wa ato D
Tekoa C Warden B
Tenino C Wethey C
Tisch D Whidbey C
Tokul C Wilkeson B
Townsend C Winston A
Triton D Wolfeson C
Tukwila D Woodinville B
Tukey C Yelm C
Uhlig B Zynbar B
Urbana C
Notes:
Hydrologic Soil Group Classifications, as defined by the Soil Conservation Service:
A=(Low runoffpotential) Soils having low runoffpotential and high infiltration rates, even when
thoroughly wetted. They consist chiefl of deep,well- to excessively drained sands or gravels, and
have ahigh rate of water transmission �reater�an 030 in/hr).
B =(Moderately low runoffp otential) Soils having moderate infiltration rates when thoroughly wetted
and consistin��g chiefly of moderately deep to dee , moderately well-to well-drained soiIs,with
moderately fi�ne to moderately coarse textures. �hese soils have a moderate rate of water
transmission (015-0.3 in/hr).
C=(Moderately high runoff potential) Soils having low infiltration rates when thoroughly wetted and
consisting chie�ly of soils with a layer that impedes downward movement of water and soils with
moderately fine to fine textures. �l'hese soils have a low rate of water transmission (0.05-015 in/hr).
D=(High runoff otential) Soils having high runoff potential. They have very low infiltration rates
when thoroug�ly wetted and consist chiefly of clay soils with a high swelhng potential; soils with a
permanent high water table; soils with a hard an or clay layer at or near the surface; and shallow soils
over nearly impervious material. These soils�ave a very low rate of water transmission (0-0.05 in/hr).
* =From SCS, TR-55, Second Edition,June 1986 E�ibit A-1. Revisions made from SCS, Soil
Interpretat�on Record,Form#5, September 19$8 and various county soil surveys.
This information can also be found online at � websoilsurvev.nres.usda.�ov/a�/WebSoilSurvev.as�x
Apri12008 Appendix SE—Hydrologic Soil Series
5-47
SPOKANE REGIONAL STORMWATER MANUAL
THIS PAGE IS INTENTIONALLY LEFT BLANK
Apri12008 Appendix SE—Hydrologic Soil Series
5-48
SPOKANE REGIONAL STORMWATER MANUAL
CHAPTER 6 - WATER QUALITY
TREATMENT DESIGN
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Chapter Organization
6.1 Introduction......................................................................................................................6-1
6.2 Protection of Aquifer Water Quality................................................................................6-1
6.2.1 Aquifer Sensitive Area.............................................................................................6-1
6.22 Critical Aquifer Recharge Areas..............................................................................6-3
6.3 UIC Facilities...................................................................................................................6-5
6.3.1 Limitations on the Use of UIC Facilities.................................................................6-5
6.32 Drywell Registration................................................................................................6-6
6.4 Pollutant Generating Impervious Surface Areas..............................................................6-6
6.4.1 High-Use Sites .........................................................................................................6-7
6.42 High-ADT Sites .......................................................................................................6-8
6.4.3 Moderate-Use Sites..................................................................................................6-8
6.5 Treatment Goals...............................................................................................................6-9
6.5.1 Total Suspended Solids (TSS).................................................................................6-9
6.52 Total Petroleum Hydrocarbons (TPH).....................................................................6-9
6.5.3 Metals Treatment...................................................................................................6-10
6.5.4 Phosphorous treatment...........................................................................................6-10
Apri12008 Chapter 6—Water Quality Treatment Design
6-i
SPOKANE REGIONAL STORMWATER MANUAL
6.6 Applicability ..................................................................................................................6-10
6.6.1 Basic Treatment.....................................................................................................6-11
6.62 Oil Control.............................................................................................................6-12
6.6.3 Metals Treatment...................................................................................................6-12
6.6.4 Phosphorus Treatment ...........................................................................................6-13
6.7 Treatment BMPs............................................................................................................6-14
6.7.1 Bio-infiltration Swales...........................................................................................6-14
6.72 Biofiltration Channels............................................................................................6-17
6.73 Vegetated Buffer Strips..........................................................................................6-19
6.7.4 Oil/Water Separators..............................................................................................6-22
6.7.5 Wetlands ................................................................................................................6-25
6.8 Emerging Technologies .................................................................................................6-26
6.8.1 Background............................................................................................................6-26
6.82 Ecology's Role in Evaluating Emerging Technologies.........................................6-27
6.8.3 Local Jurisdiction's Role in Evaluating Emerging Technologies..........................6-27
6.8.4 Testing Protocol.....................................................................................................6-28
6.8.5 Assessing Levels of Development of Emerging Technologies .............................6-28
Appendix 6A —Example calculation: Bio-Infiltration Swale....................................................6-31
Appendix 6B —Example calculation: Biofiltration Channel.....................................................6-33
Appendix 6C —Example calculation: Vegetated Buffer Strip...................................................6-35
Apri12008 Chapter 6—Water Quality Treatment Design
6-ii
SPOKANE REGIONAL STORMWATER MANUAL
6.1 INTRODUCTION
Water qualiry treatment facilities are designed to remove pollutants contained in
stormwater runof£ The pollutants of concern include sand, silt, and other suspended
solids; metals such as copper, lead and zinc; nutrients such as nitrogen and phosphorus;
certain bacteria and viruses; and organics such as petroleum hydrocarbons and pesticides.
Methods of pollutant removal include sedimentation/settling, filtration, plant uptake, ion
exchange, adsorption, and organic and inorganic decomposition. Floatable pollutants
such as oil and debris can be removed with separator structures.
Many treatment facilities, if designed correctly, can function as both a water quality
treatment faciliry and a flow control facility. This chapter describes design criteria for
water quality treatment and Chapter 7 provides design criteria for flow control.
All engineering work for water quality treatment facilities shall be performed by, or
under the direction of, a professional engineer currently licensed in the State of
W ashington.
6.2 PROTECTION OF AQUIFER WATER QUALITY
The Spokane Valley-Rathdrum Prairie aquifer extends across an area of about 325 square
miles and provides drinking water for more than 500,000 people. Most of the developed
areas in the Spokane region and in North Idaho lie directly over the aquifer. The aquifer
is designated by the U.S. EPA as a "sole source aquifer" because it is the only feasible
source of drinking water available to the local communiry. The following sections
describe state and local measures adopted to protect the quality of water in the aquifer.
6.2.1 AQUIFER SENSITIVE AREA
Groundwater recharge areas have critical impacts on aquifers used for potable water,
as defined by WAC 365-190-030 (2). The City of Spokane is subject to regulations
governing the Aquifer Sensitive Area (ASA), delineated in Figure 6-l. Under these
regulations, any project within the City of Spokane is required to provide treatment
for all stormwater runoff from pollution generating impervious surfaces. In urban
areas, bio-infiltration swales are the expected BMP for providing basic treatment.
Apri12008 Chapter 6—Water Quality Treatment Design
6-1
SPOKANE REGIONAL STORMWATER MANUAL
Pertd Oreilfe County
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Apri12008 Chapter 6—Water Quality Treatment Design
6-2
SPOKANE REGIONAL STORMWATER MANUAL
6.2.2 CRITICAL AQUIFER RECHARGE AREAS
Spokane County and the City of Spokane Valley have designated the Critical Aquifer
Recharge Areas (CARA) shown in Figure 6-2; this figure is also available on the
Spokane County website:
• http://www.spokanecounry.org/BP/GISmaps/CARA/caral7.pdf
CARAs have prevailing geologic conditions associated with infiltration rates that
create a high potential for contamination of groundwater resources, and they
contribute significantly to the replenishment of groundwater.
The CARA resolution adopted by Spokane County requires that special consideration
be given to stormwater runoff from areas with commercial and industrial
development where chemical spills are more likely to occur. Specific potential
problem areas include outdoor loading docks, fueling stations, and those activities
involving toxic and hazardous materials handling.
For all land development in unincorporated areas of Spokane County and the Ciry of
Spokane Valley, the requirements of this regulation supersede those that govern the
ASA. The incorporated areas of Spokane County, including the City of Spokane, are
still subject to the requirements of the ASA. In addition, the entire Spokane region is
subject to the Department of Ecology's Underground Injection Control (UIC)
regulations (refer to Section 1.4.1).
Aquifer recharge areas are rated as having a high, moderate, or low susceptibility for
contamination based on a scientific analysis of soils, hydraulic conductivity, annual
rainfall, depth to the aquifer, vadose zone, and wellhead protection information.
Wellhead protection areas approved by the Department of Health (DOH) and areas
within a 1,000-foot radius of Group A and B wells without reported plans are treated
as high susceptibility areas. Due to the numerous well-head capture zones found in
Township 26 North, Range 43 East, and the difficulry associated with determining the
exact "on-the-ground" locations of these protected zones on the CARA map, projects
located within T26N, R43E are also subject to the requirements of high susceptibility
areas (with the exception of the low susceptibiliry area located in the northeast corner
of T26N, R43E).
Apri12008 Chapter 6—Water Quality Treatment Design
6-3
SPOKANE REGIONAL STORMWATER MANUAL
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Figure 6-2 — Spokane County Critical Aquifer Recharge Areas
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SPOKANE REGIONAL STORMWATER MANUAL
6.3 UIC FACILITIES
A UIC facility is a constructed subsurface infiltration system consisting of an assemblage
of perforated pipes, drain tiles, or similar mechanisms intended to infiltrate fluids into the
ground or a dug hole that is deeper than the largest surface dimension (WAC 173-218-
030). Subsurface infiltration systems include drywells,pipe or French drains, drain fields,
and similar devices that are designed to discharge stormwater directly into the ground.
The following are not UIC facilities:
• Storm drain pipe systems that collect stormwater runoff and convey it to a disposal or
treatment facility;
• Surface infiltration basins and flow dispersion stormwater infiltration facilities;
• Infiltration trenches designed without perforated pipe or a similar mechanism; or
• Storm drain components that contain perforated pipes, drain tiles, or other similar
mechanism designed and intended to convey water directly or indirectly to a surface
water body.
For discharge to UIC facilities, site BMPs must be chosen that will remove or reduce
target pollutants to levels that comply with state groundwater qualiry standards when the
discharge reaches the water table or first comes into contact with an aquifer (see WAC
173-200). Discharges to surface waters shall comply with WAC 173-201A, Water
Quality Standards for Surface Waters of the State of Washington. Ecology's Guidance
for UIC Wells that Manage Stormwater provides additional information.
6.3.1 LIMITATIONS ON THE USE OF UIC FACILITIES
Some water qualiry treatment BMPs presented in this chapter are not considered
protective of groundwater for certain land uses. Because of the potential to
contaminate groundwater, stormwater runoff shall not be discharged directly to UIC
facilities from areas used for any of the following activities:
• Vehicle maintenance, repair and servicing;
• Commercial or fleet vehicle washing;
• Airport de-icing;
• Storage of treated lumber;
• Storage or handling of hazardous materials;
• Generation, storage, transfer, treatment or disposal of hazardous wastes;
• Handling of radioactive materials;
• Recycling (unless limited to glass products); and,
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• Industrial or commercial areas without management plans for proper
storage and spill prevention, control, and containment appropriate to the
types of materials handled at the facility (refer to Ecology's Stormwater
Management Manual for Eastern Washington for more information on
stormwater pollution prevention plans and source control).
Stormwater runoff from these areas shall be handled on site with a closed-loop
system or discharged to the sanitary sewer if allowed by the local jurisdiction.
Stormwater from any portions of a site that do not contact the areas listed, such as
roofs and parking areas, may be discharged to drywells, assuming that pre-treatment
has been provided in accordance with the requirements presented in this chapter.
6.3.2 DRYWELL REGISTRATION
Drywells shall be registered with Ecology. The registration provides Ecology
information to determine if a drywell is rule-authorized. Chapter 173-218 WAC lists
the submittal requirements. Registration forms can be found on Ecology's UIC
Program website.
A UIC facility may be rule-authorized when both of the following requirements are
met:
• A registration form is submitted to Ecology; and,
• Discharge from the UIC facility does not contaminate groundwater.
Residential UIC facilities used for roof runoff or basement flood control
automatically meet the non-endangerment standard and are considered rule-
authorized; they are thus exempt from registering.
The project proponent should begin the registration process during the design phase
and submit the completed paperwork prior to first use of the UIC faciliry.
6.4 POLLUTANT GENERATING IMPERVIOUS SURFACE
AREAS
Pollutant generating impervious surface (PGIS) areas are significant sources of pollutants
in stormwater runof£ These areas include surfaces subject to vehicular use, industrial
activities, or storage of erodible or leachable materials that receive direct rainfall. A
surface, whether paved or not, shall be considered a PGIS area if it is regularly used by
motor vehicles. The following are considered PGIS areas: roads, unvegetated road
shoulders, bike lanes within the traveled lane of a roadway, driveways, hydraulically
connected sidewallcs, parking lots, some roofs, fire lanes, vehicular equipment storage
yards and airport runways.
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Commercial roof runoff shall be pre-treated prior discharge to a UIC facility unless the
project proponent, or his authorized agent, demonstrates that the runoff from the roof is
not a waste fluid under WAC 173-218. Stormwater runoff is considered a waste fluid
when discharged from the following:
• Metal roofs, unless coated with an inert, non-leachable material; or,
• Roofs subject to venting or manufacturing, commercial or other indoor pollutants
(such as restaurants where oils or other solid particles are expelled due to cooking,
processing, etc.); or,
• Asphalt-based roofs; or,
• Any roof area having electrical or mechanical equipment that is not hydraulically
isolated from the remainder of the roof.
Some PGIS areas have additional requirements, as described in the following sections.
6.4.1 HIGH-USE SITES
High-use sites generate high concentrations of oil due to high traffic or the frequent
transfer of petroleum products. High-use sites are land uses where sufficient
quantities of free oil are likely to be present.
The following high-use sites require oil separator technology:
• A commercial or industrial site storing or transferring petroleum, not
including locations where heating fuel is routinely delivered to end users;
• A commercial or industrial site subject to use, storage, or maintenance of a
fleet of 25 or more vehicles that are over 10 tons gross weight;
• Fueling stations and facilities;
• Maintenance and repair facilities for vehicles, aircraft, construction
equipment, railroad equipment or industrial machinery and equipment;
• Railroad yards, and,
• High-density road intersections with expected ADT count equal to or
greater than 25,000 on the main roadway and equal to or greater than
15,000 on any intersecting roadway.
For the above sites, oil separator technology is defined as removing the oil from the
stormwater inflow in a step separate from any other pollutant removal via BMPs such
as a coalescing plate or baffle-type oil control mechanism. This typically involves a
"treatment train" of two BMPs in series in order to meet the treatment goals of
pollutants other than oil.
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The following high-use sites are subject to oil control, but are only required to
employ sorptive technologies (such as swales) that physically or chemically bind the
pollutants to sediment organic particles:
• A commercial or industrial site with an expected trip end count equal to or
greater than 100 vehicles per 1,000 square feet of gross building area;
• A parking lot with an expected trip end count equal to or greater than
300 vehicles;
• Commercial on-street parking areas located on streets with an expected
total ADT count equal or greater than 7,500; and,
• Outdoor storage yards and other sites subject to frequent use or storage of
forklifts or other hydraulic equipment.
6.4.2 HIGH-ADT SITES
The following are high-ADT sites and require oil separator technology:
• Non-employee parking areas of commercial or industrial sites with trip
end counts greater than 100 vehicles per 1,000 square feet gross building
area or greater than 300 total trip ends, and,
• Any road or parking area with an expected ADT count equal to or greater
than 30,000 (assumes a straight stretch of road, where intersecting ADTs
are low).
For the above sites, oil separator technology is defined as removing the oil from the
stormwater inflow in a step separate from any other pollutant removal via BMPs such
as a coalescing plate or baffle-type oil control mechanism. This typically involves a
"treatment train" of two BMPs in series in order to meet the treatment goals of
pollutants other than oil.
6.4.3 MODERATE-USE SITES
Moderate-use sites are defined as moderate-ADT roadways and parking areas;
primary access points for high-density residential apartments; most intersections
controlled by traffic signals; and transit center stops. The following land uses are
moderate-use sites:
• Urban roads with expected ADT between 7,500 and 30,000;
• Rural roads or freeways with expected ADT between 15,000 and 30,000;
and,
• Parking areas with 40 to 100 trip ends per 1,000 square feet of gross
building area, or between 100 and 300 trip ends.
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6.5 TREATMENT GOALS
The goal for water quality treatment facilities is to treat approximately 90% of the annual
runoff volume generated at a project site. Facilities that are designed according to the
criteria set forth in this chapter should also capture and treat nearly all of the runoff from
first flush events (heavy rainfall after a dry period). In urban areas, bio-infiltration swales
are the expected BMP for providing basic treatment. The following subsections describe
the key pollutants of concern.
6.5.1 TOTAL SUSPE NDE D SOL I DS (TSS)
Basic treatment facilities presented in this chapter are intended to achieve 80%
removal of suspended solids, including solid components of inetals, for flows with
TSS concentrations ranging from 100 mg/L to 200 mg/L. The following BMPs have
been found to provide a significant removal process for TSS:
• Bio-infiltration swales;
• Biofiltration channels;
• Vegetated buffer strips;
• Evaporation ponds.
6.5.2 TOTAL PETROLEUM HYDROCARBONS (TPH)
The oil control facilities presented in this chapter are intended to achieve the goal of
removing any visible sheen and reducing the TPH concentration to a maximum of
10 mg/L for a 24-hour average and a maximum of 15 mg/L for a discrete sample. The
following BMPs provide removal of TPH:
• Significant removal for high-use and high-ADT sites:
o Bio-infiltration swales;
o Oil/water separators (coalescing plate and baffle type);
o Vegetated buffer strips (for High-ADT sites only); and,
o Evaporation ponds designed using the Alternative Method (refer to
Section 5.72)
• Significant removal for all sites except high-ADT sites:
o Oil/water separators (spill control rype).
• Lesser removal (this BMP shall not be used for high-use or high-ADT
sites unless preceded by an oil/water separator):
o Biofiltration channels.
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6.5.3 METALS TREATMENT
Metals treatment facilities presented in this chapter are intended to achieve
approximately 50% removal of dissolved metals. The following BMPs have been
found to provide removal for metals:
• Significant removal:
o Bio-infiltration swales; and,
o Evaporation ponds designed using the Alternative Method (refer to
Section 5.72)
• Lesser removal (this BMP shall not be used for high-use or high-ADT
sites without being preceded by another treatment BMP)
o Biofiltration channels.
6.5.4 PHOSPHOROUSTREATMENT
The phosphorus treatment facilities are intended to achieve a goal of 50% total
phosphorus removal for a range of influent concentrations from 0.1 to 0.5 mg/L of
total phosphorus. Bio-infiltration swales are the only BMP presented here that have
been found to meet this removal goal for phosphorus. The following BMPs have been
found to provide a lesser removal of phosphorus and shall only be used for
phosphorus removal in combination with some other basic treatment BMP:
o Biofiltration channels;
o Vegetated buffer strips; and,
o Evaporation ponds designed using the Alternative Method (refer to
Section 5.72)
6.6 APPLICABILITY
The exemptions listed in the sections below are superseded by requirements set forth in
any applicable Total Maximum Daily Load (TMDL) or other water cleanup plan. At the
time of the writing of this Manual, no TMDLs exist for water bodies in Spokane County.
Contact the local jurisdiction for current information on whether any TMDLs have been
issued.
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6.6.1 BASIC TREATMENT
Applicability
Basic treatment provides removal of total suspended solids (TSS) and is required for
all projects proposing UIC facilities that are:
• Located within the moderate or high susceptibility areas of the CARA;
• Located within Township 26 North Range 43 East (excluding the
delineated low susceptibiliry areas isolated in the northeast corner of this
Township and Range);
• Located within a 1,000-foot radius of Group A and Group B wells without
reported plans;
• Located within a DOH-approved wellhead protection area;
• Proposing a moderate-use, high-use or high ADT site and located in the
low or moderate susceptible areas of the CARA;
• Located within the ASA boundaries; or
• Located within the City of Spokane.
Basic treatment is also required for all projects:
• Meeting the regulatory threshold and discharging to waters of the state,
including perennial and seasonal streams, lakes and wetlands;
• Requiring a 401 Water Quality Certification; or,
• Regulated to provide water quality treatment under other rules outside of
Phase II jurisdictions.
Exemptions
Basic treatment is not required for:
• Non-pollutant generating impervious surface (NPGIS) areas unless the
runoff from these areas is hydraulically connected to PGIS areas;
• Projects that discharge to the subsurface and are located within the low
susceptibility areas of the CARA and are not proposing moderate-use,
high-use, or high-ADT sites; or,
• Projects discharging non-waste fluids from roofs (WAC 173-218) directly
to drywells.
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6.6.2 OIL CONTROL
Applicability
All projects that meet the regulatory threshold and are high-use or high-ADT areas
are required to provide oil control. High-use sites generate high concentrations of
petroleum hydrocarbons due to high traffic turnover or the frequent transfer of oil
and/or other petroleum products.
High-use sites and high-ADT roadways and parking areas shall treat runoff from the
high-use portion of the site prior to discharge or infiltration. For high-use sites located
within a larger project area, only the impervious area associated with the high-use site
is subject to oil control treatment, as long as the flow from that area is separated;
otherwise the treatment controls shall be sized for the entire area.
Refer to Section 6.7.4 for information used to determine the appropriate oil control
technology.
Exemptions
Non-high-use sites and non-high ADT sites are exempt from oil treatment
requirements.
6.6.3 METALS TREATMENT
Applicability
Metals treatment is required for all projects that are moderate- or high-use sites, and
for sites that discharge to a surface water or UIC facility and meet any of the
following definitions:
• Industrial sites as defined by the EPA (40 CFR 12226(b)(14)) with
benchmark monitoring requirements for metals;
• Industrial sites that handle, store, produce, or dispose of inetallic products
or other materials, particularly those containing arsenic, cadmium,
chromium, copper, lead, mercury, nickel or zinc;
• High-use or high-ADT roadways or parking areas;
• Urban roads with expected ADT greater than 7,500;
• Rural roads or freeways with expected ADT greater than 15,000;
• Commercial or industrial sites with an equivalent trip end (ETE) count
equal to or greater than 40 vehicles per 1,000 square feet of gross building
area;
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• Parking lots with 100 ETE or more;
• Public on-street parking areas in commercial and industrial zones;
• Highway rest areas;
• Runoff from metal roofs not coated with an inert, non-leachable material;
or,
• Discharge to a surface water of the state that has been identified through a
TMDL or other water clean-up plan as requiring metals removal.
Exemptions
Stormwater runoff is exempt from metals treatment requirements in the following
situations, unless a specific water qualiry problem has been identified:
• Discharges to non-fish bearing streams;
• Subsurface discharges, unless identified as hydraulically connected to
surface waters of the State. The Spokane Valley — Rathdrum Prairie
Aquifer is hydraulically connected to a surface water of the state;
• Restricted residential and employee-only parking areas, unless subject
to through traffic;
• Preservation/maintenance projects and some improvement or safety
enhancement projects that do not increase motorized vehicular
capacities; and,
• Discharges to some Category 4 wetlands; contact the Washington
Department of Ecology for additional information.
6.6.4 PHOSPHORUSTREATMENT
Applicability
Phosphorus treatment is required where it has been determined by the federal, state,
or local government that a water body is sensitive to phosphorus and that a reduction
in phosphorus from new development and redevelopment is necessary to achieve the
water quality standard to protect its beneficial uses. Where it is deemed necessary, a
strategy will be adopted to achieve the reduction in phosphorus. The strategy will be
based on knowledge of the sources of phosphorus and the effectiveness of the
proposed methods of removing phosphorus. At the time of the writing of this manual,
no TMDLs exist for any water body in Spokane County. Contact the local jurisdiction
to determine if any have been issued that may affect a project design with regard to
phosphorus treatment.
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Phosphorus treatment may be required for water bodies reported under Section 305(b)
of the Clean Water Act, and for those listed in Washington State's Nonpoint Source
Assessment required under Section 319(a) of the Clean Water Act.
The Spokane River has been designated as not supporting beneficial uses due to
phosphorus, and phosphorus treatment may be required.
Exemptions
Projects that do not propose to discharge to a water body sensitive to phosphorus are
exempt from phosphorus treatment requirements.
6.7 TREATMENT BMPS
Infiltration-based swales and ponds, filtration-based vegetated buffer strips and channels,
and evaporative ponds can all be effective in treating stormwater runof£ In most cases,
soil properties must be appropriate to achieve effective treatment without adversely
impacting groundwater resources. The location and depth to bedrock, water table, or
impermeable layers, and the proximiry to wells, foundations, septic system drain-fields,
and unstable slopes can preclude the use of infiltration. If a lined treatment facility is
proposed, the soil properties are less important, as the treatment is meant to occur via
containment, plant uptake, and evaporation of the pollutants within the area of the facility
that does not drain.
Oil/water separators (OWS) can be used to physically separate petroleum products from
stormwater. An OWS does not, however, meet the other treatment goals set forth in this
Manual, so it may have to be used in combination with another water qualiry treatment
BMP, depending upon the expected pollutants.
This section provides design information and minimum requirements for all treatment
BMPs identified in this Manual except evaporation ponds; evaporation ponds are
addressed in Chapter 7. Inspection and maintenance standards for treatment BMPs and
information about tracts and easements are found in Chapter 11. Selection, establishment
and maintenance criteria for treatment facility vegetated cover are discussed in Chapter 7.
6.7.1 BIO-INFILTRATION SWALES
Bio-infiltration swales (formerly referred to as grassed percolation areas (GPAs) or
`208' swales) combine plant material and soil to remove stormwater pollutants by
both physical and chemical (ionic bonding, decomposition, plant root uptake, etc.)
means via filtration and percolation into the ground. Bio-infiltration swales are sized
to treat the volume equivalent of the 6-month NRCS Type II 24-hour water quality
design storm. If the bio-infiltration facility is designed to function as a flow control
facility as well as a water quality treatment facility, it shall also accommodate the
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flow control design storm event (refer to Section 22.4). If a bio-infiltration facility
will also be used as a detention facility, refer to Section 73.2 for additional
information.
Bio-Infilt�ation Swale Design
Bio-infiltration swales shall be sized using either Equation 6-la or 6-lb. These
equations estimate the volume required to treat stormwater runoff and were
developed using the Alternate Hydrograph Method found in the Stormwater
Management Manual for Eastern Washington.
V =1133AP�53 (6-la)
V =1815AP�53 (6-lb)
Where: V = volume of bio-infiltration swale (cubic feet);
A = hydraulically connected impervious area to be
treated (acres); and,
P = precipitation amount for the 6-month NRCS Type II
24 hour water quality design storm.
P shall be 1 inch for the all of the Spokane region, therefore the above equations can
be simplified as follows:
V = 1133A (6-lc)
V = 1815A (6-ld)
Equations 6-la and 6-lc can only be used when the following requirements are met,
otherwise, Equations 6-lb and 6-ld shall be used:
• The subgrade soils have less than 12% fines; and,
• The subgrade soils have an infiltration rate greater than 0.15 in/hr.
Appendix 6A provides an example calculation for bioinfiltration swales.
Bio Infiltrntion Swnle Minimum Requirements
Bio-infiltration facilities shall meet the minimum requirements for limiting layers,
setbacks, slopes, embankments, planting, and general requirements specified in
Sections 7.5.2 and 7.8. In addition, the design of bio-infiltration swales shall conform
to the requirements described below.
Treatment Desi_gn De�th and Soil Criteria: Bio-infiltration swales shall fully contain
the design treatment volume with a maximum treatment design depth (from the swale
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bottom to the elevation of the drywell grate or the first overflow or outflow
mechanism) of 6 inches.
Organic matter content or cation exchange capacity (CEC) testing must be completed
in order to substantiate the treatment soil composition. The tests shall be performed
on composite samples taken from the treatment soil layer from the constructed pond
bottom. A composite sample consists of well-mixed soil obtained from at least four
cores, to a depth of at least 6 inches, randomly distributed over the pond bottom test
area. A minimum of one test shall be performed for each bio-infiltration swale of
1,500 square feet or less, with one additional test for each additional 2,000 square feet
of swale bottom or fraction thereo£ "One test" is equal to four core samples taken
uniformly over the percolation area. The soils will be considered suitable if the
minimum criteria for CEC or soil organic matter content are met. Testing results shall
be submitted as part of the construction certification process prior to the release of
surety posted on project(contact the local jurisdiction for specific requirements).
Unless recommended otherwise by a geotechnical engineer, bio-infiltration swales
shall be constructed with a treatment zone of inedium- to well-draining soil (tested for
infiltrative and treatment criteria) at least 12 inches thick, underlain by a subgrade
infiltrative layer at least 48 inches thick All soils, including amended native soils,
shall meet the infiltrative rate criteria indicated in Table 6-1.
TABLE 6-1
BIO-INFILTRATION SWALE DESIGN CRITERIA
Criteria Design Requirement
Treatment Zone Infiltration Rate Between 0.25 and 0.50 inches/hour
(vegetated cover and treatment layer)i
Subgrade Infiltration Rate��3 At least 0.15 inches/hour and facility must
completely drain within 72 hours
Average Cation Exchange Capacity At least 15 milliequivalents/100 grams
Organic Matter Content At least 2%by weight
�Sand and coarser soils are not suitable to be used as top soils when treatment is required.
�An infiltration test(for example, a single-ring infiltrometer test)demonstrating the facility's
conformance to the infiltration rate criteria may be required prior to construction certification.
3 The 48-inch layer of infiltrative subgrade soils must meet the geotechnical recommendations as per
the requirements found in Chapter 4.
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Unless otherwise approved by the local jurisdiction, the treatment zone shall be
planted with sod or dryland grass. Trees and large shrubs may be planted in the
treatment zone provided they do not inhibit the growth of the grass. Contact the local
jurisdiction for additional information. In all cases the plant materials shall meet the
requirements of Section 7.8.9.
Inlets and Overflow: Curb inlets discharging into bio-infiltration swales shall be per
the criteria specified in Chapter 8. A minimum separation of 3 inches shall be
maintained between the flow line in the gutter(at the curb drop) or swale inlet and the
top of the drywell grate. In addition, a 2-inch drop to the finish grade (of the swale
side slope or swale bottom) below the concrete apron shall be provided to inhibit
vegetation overgrowth and ensure positive flow into the swale.
A bypass or overflow structure to a flow control facility must be provided unless the
treatment facility is able to accommodate the flow control design storm event as well
as the water quality design storm event. Swales shall not be designed to overflow to a
street unless approved by the local jurisdiction.
Construction and Inspection: In order to reduce the potential for over-compaction of
the swale bottom, construction equipment and vehicles shall be kept off the treatment
facility. Unless waived by the local jurisdiction, an infiltration test (for example, a
single-ring infiltrometer test) demonstrating the facility's conformance to the
infiltrative rate criteria is required prior to construction certification. The treatment
facility must have vegetation established prior to passing final inspection. In addition,
if during final inspection, it is found that the constructed bio-infiltration swale does
not conform to the accepted design, the system shall be reconstructed so that it does
comply.
6.7.2 BIOFILTRATION CHANNELS
Biofiltration is the simultaneous process of filtration, particle settling, adsorption, and
biological uptake of pollutants in stormwater that occurs when runoff flows over and
through vegetated areas. A biofiltration channel is a sloped, vegetated channel or
ditch that both conveys and treats stormwater runof£ It does not provide flow control
but can convey runoff to facilities designed for that purpose.
Biofilt�ation Ch�nnel Design
The following procedure shall be followed when designing biofiltration channels:
1. Determine the peak flow rate using the 6-month NRCS Type II 24-hour storm
which shall be 1 inch for the all of the Spokane region. The methods for
calculating peak flow rates are found in Chapter 5
2. Determine the bottom width of the ditch using equation 6-2 or 6-3.
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Q _ 1.486AR�S� � _ �
62
n
Where: Q = flow (cfs);
A = cross-sectional area(square feet);
R = hydraulic radius (feet); and,
S = longitudinal slope of strip (feet/foot); slope criteria
are given in the minimum geometry requirements in
the following subsection; and,
n = Manning's roughness coefficient; Use n = 0.30 for
sod (or channels that will be mowed regularly) and
higher values such as n = 0.20 for natural (less
dense)vegetation such as meadow or pasture.
For a trapezoidal channel with shallow flow, the hydraulic radius can be
approximated to the depth of flow. Using this assumption, the following can
be used to solve for the required width:
( n
1.486 Q
B = ���� —Zy (6-3)
Where: B = bottom width of the strip (feet);
n = Manning's roughness coefficient
y = depth of flow(feet); (3 inches maximum for dryland
grass and 4 inches maximum for sod);
S = longitudinal slope of strip (feet/foot); slope criteria
are given in the minimum geometry requirements in
the following subsection;
Z = side slope of the strip in the form Z:l; and,
Q = flow (cfs).
3. Calculate the cross-sectional area of flow for the given channel;
4. Calculate the flow velocity. If the velocity is less than 1 foot/second, proceed
to Step 5. Otherwise, change the channel dimensions and/or slope and return
to Step 3; and,
5. Calculate the length of the channel and verify that the residence time is at least
9 minutes. The minimum channel length is 200 feet unless the width is
increased per the minimum geometry requirements in the following
subsection.
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SPOKANE REGIONAL STORMWATER MANUAL
Commercially available software is most commonly used to compute many of the
parameters associated with the sizing of a biofiltration channel. Appendix 6B
provides an example calculation for biofiltration channels.
Biofilt�ation Channel Minimum Requi�ements
Biofiltration channels shall meet the minimum requirements found in Section 7.8, as
well as the following geometry requirements:
• The biofiltration channel shall have a length of 200 feet. If a length of 200
feet is not possible, the width of the biofiltration channel must be
increased so that the treatment area is the same as or more than it would be
if a 200 foot length had been used. The length shall not be reduced such
that the minimum residence time and/or maximum flow depth criteria are
violated. The length shall in no case be less than 100 feet.
• The maximum bottom width is 10 feet and the minimum width is 1 foot. If
the calculated bottom width exceeds 10 feet, parallel biofiltration channels
shall be used in conjunction with a device that splits the flow and directs
an equal amount to each channel.
• The ideal cross-section is a trapezoid with side slopes no steeper than 3:1.
However, a rectangular shape can be proposed if there are topographical
constraints or other construction concerns.
• Typically, the depth of flow shall not exceed 4 inches during the design
storm. The depth of flow is 4 inches for sod and 3 inches for dryland
grasses.
• The channel slope shall be at least 1% and no greater than 5%. Slopes of
2% to 4% provide the best performance. When slopes less than 2% are
used, an under-drain is required. A 6-inch-diameter perforated pipe shall
be installed in a trench lined with filter fabric and filled with 5/8-inch-
minus round rocks. The pipe shall be placed at least 12 inches below the
biofiltration channel bed and the bed shall incorporate topsoil that has a
proportionately high sand content.
• The flow velocity shall not exceed 1 foot/second and the design shall
provide for a 9 minute residence time.
6.7.3 VEGETATED BUFFER STRIPS
A vegetated buffer strip is a facility that is designed to provide stormwater quality
treatment of conventional pollutants, but generally does not provide stormwater flow
control.
Vegetated buffer strips are primarily used in rural areas adjacent to and parallel to
paved areas such as parking lots or driveways, and along rural roadways where sheet
Apri12008 Chapter 6—Water Quality Treatment Design
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SPOKANE REGIONAL STORMWATER MANUAL
flow from the paved area will pass through the buffer strip before entering a
conveyance system or a flow control facility or being dispersed into areas where it
can be infiltrated or evaporated.
Vegetated buffer strips are used to intercept overland sheet flow runoff from adjacent
impervious areas. They slow runoff velocities, filter out sediment and other
pollutants, and provide infiltration into underlying soils. One challenge associated
with vegetated buffer strips is the difficulty in maintaining sheet flow. Concentrated
flows can short circuit the buffer strips which can then contribute to eroded rills or
flow channels across the strips. This results in little or no treatment of stormwater
runoff.
This BMP is acceptable for use on any rural project that meets the following general
criteria:
• The flow from the roadway must enter the buffer strip as sheet flow. Thus,
the vegetated buffer strips must not receive concentrated flow discharges.
• A maximum flow path (paved width) of 30 feet can contribute to a buffer
strip designed via this method (vegetated buffer strips should rypically not
be proposed for super-elevated roads, unless the 30 foot width is adhered
to);
• Buffer strips may be used where the roadway ADT is less than 30,000;
• The longitudinal slope of the contributing impervious drainage area
(parallel to the edge of the buffer area) shall be 5% or less;
• The lateral slope of the contributing drainage area perpendicular to the
pavement edge (typically referred to as the cross-slope of the road)shall be
2% or less.
Vegetated buffer strips shall be constructed after other portions of the project are
completed.
Vegetczted Buffe�St�ip Design
This procedure is based on the narrow area filter strips presented in the 1998 King
County Surface Water Design Manual. The sizing of the buffer strip is based on the
length of the flow path draining to the buffer strip and the longitudinal slope of the
buffer strip itself (parallel to the flow path). The following design steps shall be
followed:
l. Determine the flow path length draining to the buffer strip. Normally this is
the width of the paved area draining to the strip, but if the site is sloped, the
flow path may be longer. For crowned roads, the flow path is the distance
from the crown to the edge of pavement;
Apri12008 Chapter 6—Water Quality Treatment Design
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SPOKANE REGIONAL STORMWATER MANUAL
2. Determine the average lateral or cross slope of the buffer strip: Calculate the
cross slope of the buffer strip (parallel to the flow path), averaged over the
total width of the buffer strip. If the slope is less than 2%, use 2% for sizing
purposes. The maximum cross slope allowed is 6:1 horizontal to vertical or
17%; and,
3. Determine the required width of the buffer strip: Use Figure 6-3 to size the
buffer strip. To use the figure, find the curve representing the appropriate
width of the flow path (interpolate between curves as necessary). Find the
point along the curve where the design longitudinal or cross slope of the
buffer strip is directly below and read the buffer strip width to the left on the
y-axis. The buffer strip must be designed to provide this minimum width (W)
along the entire stretch of pavement draining to it.
Appendix 6C provides an example calculation for vegetated buffer strips.
20.D
�
^
15.4
�
� Flowpath
� = 3� feet �
�
.,�
''�r 1 fl.Q
c. 2� feet
,..t
�.
�
�
�
� 1 Ll feet
...
G`" �,a
i
fl.Q
0°lc 5%a 14%v I 5°la 2[7�1'0
Filter SErip SIape
Source: King County Surface Water Design Manual, 1998.
Figure 6-3 —Vegetated Buffer Strip
Apri12008 Chapter 6—Water Quality Treatment Design
6-21
SPOKANE REGIONAL STORMWATER MANUAL
Vegetated Buffer StNip Minimum Requi�ements
Vegetated buffer strips shall meet the minimum requirements for planting, and
general requirements specified in Sections 7.5.2 and 7.8. In addition, the design of
buffer strips shall conform to the following requirements (see Figure 6-4):
• Geometry:
o The minimum required buffer strip width is: 4 feet for a 10-foot flow
path; 4.5 feet for a 25 foot flow path; and 5.5 feet for a 30-foot flow
path. Flow path is the width of the paved surface discharging to the
buffer strip.
o The cross-slope of the buffer strip shall be no steeper than 6:1.
o Along roadways, buffer strips shall be placed at least 1 foot, and
preferably 3 to 4 feet, from the edge of pavement, to accommodate a
vegetation free zone.
• Energy Dissipation:
o A gravel-filled trench shall be installed between the pavement surface
and the buffer strip to maintain sheet flow. This area serves as a flow
spreader and shall consist of a trench filled with crushed aggregate
(WSDOT Crushed Aggregate Base Course or WSDOT Crushed
Aggregate Top Course).
o The gravel filled trench shall be a minimum of 12 inches deep and 18
inches wide.
6.7.4 OILM/ATER SEPARATORS
Three types of OWS are included in this Manual:
• Coalescing plate types (gravity mechanism for separation),
• Baffle types,
• Spill control separators, such as T's or elbows located inside a catch basin.
OWSs are only effective in achieving oil and TPH removal at the required levels
when regular maintenance is provided. Without proper sludge, oil and sediment
removal, there is a high potential for clogging which can impair the long-term
efficiency of the separator.
Apri12008 Chapter 6—Water Quality Treatment Design
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SPOKANE REGIONAL STORMWATER MANUAL
A
a a o a a
a
° a CONTRIBUTING DRAINAGE AREA o ° a a
o a o a a a a ° o
a o a a a
a a z � ° � o
0 �'Ja a � a J �J
° ~ 0 ° � ° a � X ¢ � waa ° 30 FT MAX
a U � ° o a Q �
� � 5%IVIAX LONGITUDINAL SLOPE a a — � H � O
� O ° a a ° c� ¢ UcJi�
90 o a a � a
a ° aa ° a
a� ° ao a
a a a � o o a a
Da Da Da �a 18 IN MINIMUM
� � � � � � � � � � � � � � � � �
� � � � CRUSHED SURFACING � � � �
� � � � � �" � � � � � LENGTH��L��
� � � � � � � � � � � � � � � � SEE 6.3.3.2
� � � � � � � � � � � � � � � GEOMETRY FOR
� .i. .i. .i. .i. .i. .i. .i. .i. .i. .i. .i. .i. .i. .i. .i. .i. � LENGTH CRITERIA�
`VEGETATED BUFFER STRIP �
� � � � � � � � � � � � � � � �
A
I► WIDTH"W' —
PLAN VIEW
NTS
CRUSHED SURFACE
PAVEMENT SURFACE TO DAYLIGHT
2%MAX --► �
D q"r3�� 2 IN COMPOST TILLED INTO
� � ���� ��� � � ���� � ����� 6 IN OF NATIVE SOIL,MIN.
CRUSHED SURFACE %��������,,�'�,,��,? COLLECTOR DITCH
�����
PER PAVEMENT DESIGN ����T����,�„�„�„�,,, M'�s•
T����?�?v�,�. 1 (IF APPLICABLE)
v����� ���������������
�;%;U��v,���yv';,�
���?.�
SECTION A-A
NTS
ADAPTED FROM KING COUNTY
SURFACE WATER DESIGN
MANUAL 1998
Figure 6-4 —Typical Vegetated Buffer Strip (details)
Oil/Wate�Separator Design
Detailed design information for coalescing plate and baffle type OWS can be found in
Section 5.10.7 of the Stormwater Management Manual for Eastern Washington.
Design information for spill control separators is presented in the minimum
requirements subsection below.
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SPOKANE REGIONAL STORMWATER MANUAL
Oil/Water Separator Minimum Requirements
The following design criteria are applicable to all rypes of oil control BMPS:
• Only impervious conveyances shall be used for oil-contaminated
stormwater; and,
• Oil/water separators shall not be used for the removal of dissolved or
emulsified oils such as coolants, soluble lubricants, glycols, and alcohols.
The following are design criteria applicable to spill control separators:
• "T" or elbow separators in a catch basin are not allowed as an oil control
device for high-ADT sites unless used in series with another water quality
treatment facility;
• Oil control shall occur prior to dispersal into or through a water quality
treatment facility;
• If an oil/water separator is applicable, it shall comply with the local
jurisdiction's standard plan.
The following design criteria are applicable to both coalescing plate and baffle type
oil/water separators:
• If practical, determine expected oil/grease (or TPH) and TSS
concentrations, lowest temperature, pH, empirical oil rise rates in the
runoff, oil viscosity and specific gravity of the oil;
• Locate the separator off-line and bypass flows in excess of the water
quality design flow rate;
• Follow industry standards such that the separator has a forebay, separator
section, and afterbay;
• Design the surface area of the forebay at 20 square feet per 10,000 square
feet of area draining to the separator;
• The length of the forebay shall be one-third to one-half the length of the
entire separator;
• Include roughing screens for the forebay to remove debris (screen
openings should be about 3/4 inch);
• Include a submerged inlet pipe with a turned-down elbow in the forebay at
least two feet from the bottom; the outlet pipe shall be a "T" sized to pass
the design peak flow and placed at least 12 inches below the water
surface;
• Size the separator bay for the water quality design flow rate;
• Include a shutoff inechanism at the separator outlet pipe; and,
• Use absorbents and/or skimmers in the afterbay as needed.
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SPOKANE REGIONAL STORMWATER MANUAL
The following are additional design criteria applicable to baffle type oil/water
separators:
• Oil retaining baffles (top baffles) shall be located at least a quarter of the
total separator length from the outlet and shall extend down at least 50%
of the water height and at least 1 foot from the separator bottom; and,
• Baffle height to water depth ratios shall be 0.85 for top baffles and 0.15
for bottom baffles.
6.7.5 W E T L A N D S
Some wetlands can be considered for use in stormwater treatment, if approved by
Ecology. The following criteria are applicable for wetlands that discharge to surface
waters of the State, which are generally defined as wetlands with a defined outlet.
Typically, a wetland must meet one of the following criteria in order to be considered
for use as a stormwater treatment facility:
• It is a Category 4 wetland according to the Eastern Washington Wetland
Rating System; or,
• It is a Category 3 wetland according to the Eastern Washington Wetland
Rating System and the wetland has been previously disturbed by human
activity such as agriculture, fill areas, ditches or the wetland is dominated
by introduced or invasive weedy plant species as identified in the rating
analysis.
In addition, the wetland must meet the criteria for hydrologic modification of a
wetland. Hydrologic modification of a wetland for the purpose of stormwater
management means that the wetland will receive a greater total volume of surface
runoff following development than it receives in the current condition. A Category 3
or 4 wetland can only be considered for hydrologic modification if both of the
following are demonstrated:
• There is good evidence that the natural hydrologic regime of the wetland
can be restored by augmenting its water supply with excess stormwater
runoff; or the wetland is under imminent threat exclusive of stormwater
management and could receive greater protection if acquired for a
stormwater management project rather than left in existing ownership;
and,
• The runoff is from the same natural drainage basin; the wetland lies in the
natural routing of the runoff; and the site plan allows runoff discharge at
the natural location. Exceptions may be made for regional facilities
planned by a local jurisdiction, but the wetland should receive water from
sites in the same watershed.
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SPOKANE REGIONAL STORMWATER MANUAL
Hydrologic modification is not allowed for wetlands classified as Category 1 or 2
under the Eastern Washington Wetland Rating System unless the project proponent
demonstrates to Ecology that the stormwater disposal methods outlined in this
Manual are not possible at the site, and that other options would result in more
damage to the wetland by limiting base flow.
Basic treatment (TSS removal) is not required prior to discharge to a Category 4
wetland, but it is required prior to discharge to a Category 3 wetland. A Category 3
wetland that meets the above requirements may be used to meet metals treatment
requirements. Oil control is required for all high-use sites discharging to a wetland.
Contact Ecology for specific treatment requirements with regard to any other wetland
category or pollutant of concern.
Mitigation is usually required for the impact of hydrologic modification on a wetland.
Appropriate measures include expansion, enhancement and/or preservation of a
buffer around the wetland.
For wetlands that are isolated (i.e. are not hydraulically connected to a surface water
of the State via an outlet from the wetland), the project proponent shall contact
Ecology for further information with regard to using such a wetland for stormwater
management.
6.8 EMERGING TECHNOLOGIES
Emerging technologies are new technologies that have not been evaluated using approved
protocols, but for which preliminary data indicate that they may provide a desirable level
of stormwater pollutant removal.
6.8.1 BACKGROUND
During recent years, new technologies have been under development to meet the
needs of urban stormwater pollutant control. However, because no standardized
statewide procedure for evaluating these technologies was available, local
jurisdictions and commercial entities have had to decide individually as to the
appropriateness of their use. This has resulted in a wide range of differences in the
criteria for accepting new technologies.
Some emerging technologies have already been installed in Washington state as parts
of treatment trains or as stand-alone systems for specific applications. In some
instances, emerging technologies can be used for retrofits and/or where land is
unavailable for larger treatment systems.
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SPOKANE REGIONAL STORMWATER MANUAL
6.8.2 ECOLOGY'S ROLE IN EVALUATING EMERGING
TECHNOLOGIES
Ecology has developed a new technology evaluation program, which is described
briefly in this section. The program is based on reviewing engineering reports on the
performance of new technologies and reporting the results at Ecology's website. The
program includes:
• A Technical Review Committee (TRC) including representatives from
local governments in eastern and western Washington that acts in an
advisory capacity to provide recommendations to Ecology on the level of
development of each technology.
• A website with brief descriptions of each new technology, TRC
recommendations, and Ecology's determinations of the levels of
development of each technology. Ecology's main website address can be
accessed at http://www.ecy.wa.gov/.
6.8.3 LOCAL JURISDICTION'S ROLE IN EVALUATING EMERGING
TECHNOLOGIES
Local jurisdictions reserve the right to deny the use of any emerging technology even
if it has been approved by Ecology. Local jurisdictions shall consider the following as
they make decisions regarding the use of new stormwater technologies in their
jurisdictions:
• Remember the _goal: The goal of any stormwater management program or
BMP is to treat and release stormwater in a manner that does not harm
beneficial uses. Compliance with other water quality standards is one
measure of determining whether beneficial uses will be harmed. Emerging
technologies proposed for use in the Spokane area shall be compatible
with use over a sole-source aquifer.
• Exercise reasonable caution: An emerging technology shall not be
considered for use for new development sites unless there are strong
supporting data indicating that its performance is expected to be
reasonably equivalent to the BMPs already approved by Ecology. Local
jurisdictions can refer to Ecology's website to obtain the latest
performance verification of an emerging technology.
• Conduct a monitorin� pro�ram: Identify an acceptable monitoring
protocol to apply to those emerging technologies that have not yet been
verified for limited or full-scale use at Ecology's website.
• Review Treatment Goals: Refer to Section 6.5 for acceptable performance
objectives.
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SPOKANE REGIONAL STORMWATER MANUAL
• Maintenance: Some emerging technologies may not be approved for use in
public roads due to maintenance concerns. Use of emerging technologies
in private roads and tracts may require the formation of a Homeowners'
Association to provide perpetual maintenance of the drainage facilities.
Contact the local jurisdiction for additional information.
To achieve the goals of the Clean Water Act and the Endangered Species Act, local
jurisdictions may find it necessary to retrofit existing stormwater systems. In these
situations, the use of any BMPs that make substantial progress toward these goals is a
step forward and is encouraged by Ecology.
6.8.4 TESTING PROTOCOL
To properly evaluate new technologies, performance data must be obtained using an
industry accepted protocol. A test protocol has been developed which serves to
standardize the test conditions. Sampling criteria, site and technology information,
QA/QC, target pollutants, and evaluation report content are specified in the protocol.
6.8.5 ASSESSING LEVELS OF DEVELOPMENT OF EMERGING
TECHNOLOGIES
Ecology has received several submittals from vendors to approve their technologies
for statewide applications. Moreover, it is evident that some technologies have been
under development for many years and have improved considerably during that time.
To assess and classify levels of developments, Ecology is proposing to use the criteria
below. These criteria can also be found on Ecology's website. Emerging technologies
shall be used only within the application criteria and performance limits listed at
Ecology's website.
Pilot Use Level Desi�nation: For emerging technologies with limited performance
data, the pilot use level designation (PULD) allows limited use to enable field testing.
PULDs may be given based solely on laboratory performance data. Ecology will
limit the number of installations to five during the pilot use level period.
Local governments may allow PULD technologies to be installed provided the
proponent agree(s) to conduct additional field testing based on the TAPE at all
installations to obtain a general use level designation (GULD). Proponents must
conduct field testing at a minimum of one site in the Pacific Northwest to obtain a
GULD.
Local governments covered by a municipal stormwater NPDES permit must notify
Ecology in writing when a PULD technology is proposed. Ecology encourages other
jurisdictions to notify Ecology headquarters when a PULD technology is proposed.
Ecology also encourages all local governments to require proponents to provide a
Apri12008 Chapter 6—Water Quality Treatment Design
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SPOKANE REGIONAL STORMWATER MANUAL
performance guarantee stating that PULD facilities will be upgraded as necessary, to the
maximum extent practical, to meet Ecology performance goals.
General Use Level Designation: The general use level designation (GULD) confers a
general acceptance for treatment device. Technologies with a GULD may be used
anywhere in Washington, subject to Ecology conditions. Ecology plans to include
GULD technologies in future stormwater manual updates.
Conditional Use Level Desi�nation: The TRC established the conditional use level
designation (CULD) for emerging technologies that have a considerable amount of
performance data but the data were not collected per the TAPE protocol. The TRC may
recommend a CULD based on field data collected by a protocol that is reasonably
consistent but does not necessarily fully meet the TAPE protocol. The field data must
meet the statistical goals set out in the TAPE guidelines (Appendix D). Laboratory data
may be used to supplement field data. Conditional use level designations apply for a
specified time period only. During this time period, the vendor must complete all field
testing and submit a TER to Ecology and the TRC. Proponents must complete field
testing at a minimum of one site in the Pacific Northwest to obtain a general use level
designation.
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Apri12008 Chapter 6—Water Quality Treatment Design
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APPENDIX 6A - EXAMPLE CALCULATION :
BIO-I NFI LTRATION SWAL E
Gl VEN
• The existing site is approximately 5 acres, consisting of sandy soils. Existing surface
vegetative conditions include short grass and weeds.
• The subgrade soil has 10% fines and an infiltration rate of 0.10 inches per hour.
• Post developed site conditions consist of:
0 20 — 10,000-square foot lots
0 1,500-square-foot homes with 500-square-foot driveways
0 0.50 acres of road impervious areas
CA L CUL A TIONS
1. Determine the total PGIS for the site.
Road PGIS: (0.50 acres)(43,560 square feet/acre) = 21,780 square feet
Driveway PGIS: (500 square feet)(20 driveways) = 10,000 square feet
Total PGIS: 10,000 square feet+ 21,780 square feet=31,780 square feet
= 0.73 acres
2. Determine the required treatment volume, using Equation 6-l.
V =1815A (Equation 6-ld)
V=(1815)(0.73)=1,325 cubic feet
3. Determine the geometry of the bio-infiltration facility
Use a treatment depth of 6 inches
Pond Bottom Area Required* = 1,325•cf* 12•in = 2,648•sq.ft.
6•in lft
* For this example, there are no space constraints and side slope volume
has been ignored.
Provide an infiltrative facility with a 2,650 square foot pond bottom area
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Apri12008 Appendix 6A—Example Calculation:Bialnfiltration Swale
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APPENDIX 6B - EXAMPLE CALCULATION :
BIOFI LTRATION CHANNE L
Gl VEN
• Weighted pervious CN= 67
• Pervious area= 425 acres
• Impervious CN= 98
• Impervious area= 0.75 acres
• PGIS = 31,720 square feet
• 6-month, 24 hour storm precipitation= 1.0 inch
• Time of concentration= 5 minutes
• n = 02
CA L CUL A TIONS
l. Determine the peak flow rate using the 6-month NRCS Type II 24-hour storm and the
methods described in Chapter 5.
6-month precipitation= 1 inch
Using a computer program, the peak flow rate is estimated to be 0.8 cfs.
2. Determine the bottom width of the ditch using Equation 6-3.
Assume: A trapezoidal channel with 3:1 side slopes;
3 inches for flow depth; and,
3%longitudinal slope for biofiltration channel.
n
1.486 Q
B = —Zy
��3��
�.2
(0.8cfs)
B = 1.4s� _ �3� Iz�n = s.6ft
3in ft � �t
12 i� 0.03 ft
t
Apri12008 Appendix 6B—Example Calculation:Biofiltration Channel
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SPOKANE REGIONAL STORMWATER MANUAL
3. Calculate the cross-sectional area of flow for the given channel and verify that the
flow velociry is less than or equal to 1 foot/second;
Area= y (B+ Zy) = 3•in 5.6•ft+3•ft 3•in = 1.6•sq.ft.
12 Z�t 12 Z�t
Velocity= Q= 0.80•cfs = 0.5•ft/sec <_ 1 OK
A 1.6•sq.ft
4. Calculate the length of the channel to meet the 9 minute residence time. The
minimum channel length is 200 feet unless the width is increased as described in the
minimum requirements in Section 6.7.2.
L = (V)(t� _ �o.sftse���9m�n��6o�se�� = z�o ft
1 min
If the site cannot accommodate the required channel length, the width can be
increased. Steps 3 and 4 should be repeated until the channel geometry best fits the
existing site conditions.
The proposed channel geometry design is as follows:
o Trapezoidal shape with 3:1 side slopes
0 3% longitudinal slope
o Channel bottom width is 5.6 feet
o Minimum channellength is 270 feet
Apri12008 Appendix 6B—Example Calculation:Biofiltration Channel
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SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 6C - EXAMPLE CALCULATION :
VEGETATED BUFFER STRIP
Gl VEN
• A rypical crowned road with a 30-foot-wide half road
• Average lateral road cross slope=2%
• Average longitudinal slope=4%
• The land adjacent to the road (where the buffer strip will be located) slopes away at
an average slope = 5%
CA L CUL A TIONS
1. Determine the flow path length draining to the buffer strip.
Flow path is 30 feet
2. Determine the average lateral or cross slope of the buffer strip:
Calculate the cross slope of the buffer strip (parallel to the flow path),
averaged over the total width of the buffer strip.
Use 5%
3. Determine required width of the buffer strip using Figure 6-3 to size the buffer strip.
From Figure 6-3: buffer strip width = 9 feet
9 feet > 5.5 feet (the minimum width for buffer strips
with a 30-foot flow path) OK
Provide an 18-inch-wide, 1-foot-deep gravel filled trench
Apri12008 Appendix 6C—Example Calculation:Vegetated Buffer Strip
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Apri12008 Appendix 6C—Example Calculation:Vegetated Buffer Strip
6-36
SPOKANE REGIONAL STORMWATER MANUAL
CHAPTER 7 - FLOW CONTROL
.ti�
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SPOKANE REGIONAL STORMWATER MANUAL
7.7 Evaporation Facilities ....................................................................................................7-15
7.7.1 Introduction............................................................................................................7-15
7.72 Minimum Requirements ........................................................................................7-15
7.8 Additional Requirements for All Facilities....................................................................7-17
7.8.1 General...................................................................................................................7-17
7.82 Setbacks .................................................................................................................7-18
7.8.3 Drawdown Time ....................................................................................................7-18
7.8.4 Side Slopes.............................................................................................................7-18
7.8.5 Emergency Overflow Spillway..............................................................................7-19
7.8.6 Embankments.........................................................................................................7-19
7.8.7 Fencing...................................................................................................................7-20
7.8.8 Planting Requirements...........................................................................................7-21
7.8.9 Landscaping...........................................................................................................7-21
7.8.10 Maintenance...........................................................................................................7-22
7.8.11 Dam Safety.............................................................................................................7-22
7.9 Special Requirements.....................................................................................................7-23
7.9.1 Special Drainage Areas..........................................................................................7-23
7.92 Floodplains.............................................................................................................7-23
7.9.3 Wetlands and Classified Streams...........................................................................7-26
7.9.4 Closed Depressions................................................................................................7-27
7.10 Regional Stormwater Facilities......................................................................................7-27
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7.1 INTRODUCTION
This chapter outlines the minimum requirements for sizing flow control facilities.
Standard flow control facilities are detention/retention ponds, drywells, and evaporation
ponds. Any other faciliry is considered a non-standard system, and shall be evaluated
individually by the local jurisdiction. Flow control facilities are necessary to mitigate
potential adverse impacts on down-gradient properties due to the increase in stormwater
runoff caused by land development.
Unless specifically approved by the local jurisdiction, the peak rate and volume of
stormwater runoff from any proposed land development to any natural or constructed
point of discharge downstream shall not exceed the pre-development peak rate or volume
of runoff. A down-gradient analysis demonstrating that there will be no expected adverse
impacts on downgradient properties will be required (refer to Section 3.4.5 for down-
gradient analysis criteria). Exceptions with regard to rate and volume control can be made
for regional facilities planned by the local jurisdiction.
All engineering work for flow control facilities shall be performed by, or under the
direction of, a professional engineer currently licensed in the State of Washington.
Refer to Chapter 11 for maintenance requirements.
7.2 APPLICABILITY AND EXEMPTIONS
7.2.1 APPLICABILITY
All projects that meet the regulatory threshold shall comply with this Basic
Requirement.
7.2.2 EXEMPTIONS
Projects are exempt from flow control if they discharge to any of the following:
• The Spokane River or other exempt water bodies, which are defined in the
Stormwater Management Manual for Eastern Washington as fifth-order or
greater stream channels, as determined from a 1:150,000 scale map;
• A river or stream that is fifth-order or greater as determined from a
1:24,000 scale map;
• A river or stream that is fourth-order or greater as determined from a
1:100,000 scale map;
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• A stream that flows only during runoff-producing events. These streams
are defined as those that do not discharge via surface flow to a non-exempt
surface water following receipt of the 2-year, NRCS Type lA, 24 hour
rainfall event. In addition, for the stream to be exempt, it shall be able to
carry the runoff from an average snowmelt event, but shall not have a
period of base flow during a year of normal precipitation;
• A lake or reservoir with a contributing watershed areas greater than 100
square miles;
• A reservoir with outlet controls that are operated for varying discharges to
the down-gradient reaches as for hydropower, flood control, irrigation or
drinking water supplies (discharges to uncontrolled flow-through
impoundments are not exempt).
In order to be exempted the discharge shall meet all of the following requirements:
• The project area must be drained by a conveyance system that consists
entirely of manmade conveyance elements (i.e. pipes, ditches, outfall
protection); and,
• The conveyance system must extend to the ordinary high water mark line
of the receiving water, or (in order to avoid construction activities in
sensitive areas) flows are properly dispersed before reaching the buffer
zone of the sensitive or critical area; and,
• Any erodible elements of the conveyance system for the project area must
be adequately stabilized to prevent erosion; and,
• Surface water from the project area must not be diverted from or increased
to an existing wetland, stream, or near-shore habitat sufficient to cause a
significant adverse impact. Adverse impacts are expected from
uncontrolled flows causing a significant increase or decrease in the 1.5- to
2-year peak flow rate.
Maps shall be standard U.S. Geological Survey (USGS) maps or geographic
information system (GIS) data sets derived from USGS base maps.
7.3 DETENTION FACILITIES
7.3.1 INTRODUCTION
A detention system is a storage facility that has a surface discharge. Detention ponds,
vaults and underground storage tanks are all considered to be detention facilities.
Refer to the Stormwater Management Manual for Eastern Washington for design
criteria for vaults and underground storage tanks.
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A detention facility is intended to control peak stormwater runoff rates, and if
designed per the criteria in this chapter, does not control volume. If the subgrade soils
meet the drawdown criteria specified in Section 7.8.3, the engineer may choose to
propose a system that uses infiltration in conjunction with a detention pond as a
means to control volume. Otherwise, the engineer can use evaporation to control
volume, in conjunction with a detention pond (refer to Section 7.7.2).
7.3.2 MINIMUM REQUIREMENTS
The following minimum requirements shall be met. Additional requirements are
specified in Section 7.8.
Design Criteria
Detention facilities shall be designed such that the release rate does not exceed the
pre-developed conditions for multiple storm events. The analysis of multiple design
storms is needed to control and attenuate both low and high flow storm events.
The total post-developed discharge rate leaving the site (including bypass flow) shall
be limited to the pre-development rates outlined in Table 7-l. Bypass flow is the
runoff that leaves the site without being conveyed through the drainage system.
TABLE 7-1
ALLOWABLE DISCHARGE RATES
Design F requency (24 hr storm) Post-Developed Discharge R ate'
2-year <2-year pre-developed
25-year <25-year pre-developed
100-year2 (Emergency Overflow) Overflow route only
�Post-developed flow is equal to the release from detention facility plus the bypass flow.
�The emergency overflow shall direct the 100-year post-developed flow safely towards the
downstream conveyance system
Detention systems that store any stormwater below the first overflow shall adhere to
the subgrade infiltrative criteria specified in Table 6-1. Unless waived by the local
jurisdiction, the subgrade infiltration rate shall be verified through testing.
If the detention facility is also proposed to function as a water quality treatment
facility, the following criteria must be met:
• The first orifice or outlet from the facility must be placed 6 inches above
the pond bottom; and,
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• The treatment zone shall meet the requirements specified in Table 6-1 and
be verified through testing, unless waived by the local jurisdiction.
The NRCS Type lA 24 hour storm event is the design storm to be used for all flow
control facilities that use a surface discharge; for flow control facilities that involve
infiltration into the subsurface, the NRCS Type II 24 hour storm event can be used for
design.
A wetland may also be considered for use as a flow control facility, if approved by
Ecology. Refer to Section 7.9.3 for additional information.
Setbacks
When a detention faciliry is proposed upslope of developed property or at the top of a
slope inclined 10% or greater, down-gradient impacts shall be evaluated and the
minimum setback from the slope must be greater than or equal to the height of the
slope. The distance between the outlet structure and the inlet into the detention
facility shall be maximized.
Release Point
Stormwater runoff from a developed site shall leave the site in the same manner and
location as it did in the pre-developed condition. Therefore, a detention system may
be used only when a well-defined natural drainage course is present prior to
development.
7.4 OUTFLOW CONTROL STRUCTURES
7.4.1 INTRODUCTION
Control structures are weirs, orifices, culverts, or manholes with a restrictor device
that is used for controlling outflow from a facility to meet a desired standard. This
section presents a general overview of flow control structures. For additional
information, the engineer should consult a hydraulics reference.
7.4.2 OUTFLOW CONTROL STRUCTURE TYPES
WeiNS and Orifices
Weirs and orifices are partial obstructions in an open channel or in a detention facility
at the point of discharge, typically used to limit and measure flow rates. Weirs have
openings with no top, referred to as a notch, through which the water flows when its
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surface elevation is above the bottom of the opening (the weir invert). An orifice is
typically a circular opening cut into the structure obstructing the stream. The
following are common features of weirs:
• Weir Length: The weir length is the length of the notch in the direction
perpendicular to the flow:
o Contracted weirs (see Figure 7-1) have weir lengths less than the
channel width or pond wall, and the falling liquid sheet (called the
nappe) decreases in width as it falls.
o Su�ressed weirs (see Figure 7-2) extend the full channel width.
� � � , � �
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i
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�
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Figure 7-1 —Contracted Weir
APPROACH SECTION
� -
TOP VIEW - WEIR CREST
Figure 7-2 —Suppressed Weir
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• Weir Crest: The weir crest is the surface of the weir invert in the direction
of the flow:
o Broad-crested weirs (see Figure 7-3) have a crest that extends
horizontally in the direction of flow far enough that the flow leaves the
weir in essentially a horizontal direction. A weir is broad-crested if the
length of the crest in the direction of flow is greater than half of the
head(H).
o Shar�-crested weirs (see Figure 7-4) have a narrow crest with a sharp
upstream edge so that water flows clear of the crest. The weir invert or
top of the crest should be set above the pond bottom a height of at least
twice the maximum head,preferably more.
HEAD
�_ _ _ �NAPPE
_ �
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��
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//// // //�//� � �� CREST �
I I� � ��///��//� �` THICKNESS
APPROACH CHANNEL �� �������� ��
Figure 7-3 —Broad-Crested Weir
HEAD - - - - - �NAPPE
- - � - --���
- - -� - \\
� %/i � � �
ii /j ���
/ / /
APPROACH CHANNEL�i /, � CREST
, � i �
� �i i
Figure 7-4 — Sharp-Crested Weir
• Weir Notch Shape: The following are the common shapes of weir
openings, as viewed looking in the direction of the flow:
o V-notch: The opening has two sloped sides coming together at a point
at the bottom.
o Rectangular: The opening has two vertical sides and a horizontal
invert.
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o Trapezoidal (Cipoletti): The opening has a horizontal invert and two
sloped sides.
Table 7-2 provides equations and coefficients for calculating flow through the most
common types of weirs and orifices used for flow control.
TABLE 7-2
FLOW EQUATIONS FOR VARIOUS WEIR AND ORIFICE TYPES
Weir/OrificeType Equation C
Sharp Crested V-notch s
i Q = C�Tan��FI� 0.60
weir
Broad Crested Suppressed Q = CLH� 0.33
Rectangular weir
Rectangular Sharp Crested
W eirs i: �
Q = C�L—02H�H � 327+0.40 y
Contracted
Suppressed Q = CLH 3�
Sharp Crested Cipoletti
(Trapezoidal)i Q = CLH� 3.367
Side slopes are 1:4
wo ad Crested Trapezoidal Q = C(2g)����3 LH� +g�Tan,��H�� 0.60
Orifice Q = CA 2gH 0.62
1 The weir inverts should be set above the pond bottom a height of at least twice the maximum head.
Q=flow(cfs); C=coefficient of discharge;A=area of orifice(square feet);H=hydraulic head(feet);
g=gravity (32.2 feet/second�); 8 =angle of side slopes(degrees); Y=starage depth(feet);L=weir length
or opening(feet)
Risers
A riser typically consists of a circular pipe or box inlet with its opening oriented
parallel to the water surface. A riser operates under three hydraulic flow regimes in
this order as the water surface elevation rises: weir, orifice, and full barrel. Full barrel
flow occurs when the downstream conduit is undersized with respect to the riser
capaciry and when the water surface elevation rises high enough.
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SPOKANE REGIONAL STORMWATER MANUAL
Figure 7-5 can be used to determine the head (in feet) above a riser of given diameter
and for a given flow (usually the 100-year peak flow for developed conditions). For
additional information, consult a hydraulics reference.
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4-� H EAD IH FE ET {m�$$U���d lrom cresi o4 rlser� ��
{��.�,��_T39 gH91°
'�orui�.=3_782 6�Fi%';
0 :n Cis.D #n d H In trrt
$loyc thb��a�o4�ar� 7F1't�3r.mriilc• Yran.sYtlan
Figure 7-5 —Flow Rates vs. Head (riser)
7.4.3 MINIMUM REQUIREMENTS
Outflow control structures shall meet the following requirements:
• A weir used as a flow control structure shall be made of non-erosive
material that is resistant to alteration or vandalism, such as reinforced
concrete or metal with a non-corrosive surface. An emergency overflow
weir can be made of soil with revetment;
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• The inverts for sharp-crested weirs should be set above the pond bottom a
height of at least twice the maximum head;
• The crest length for broad-crested weirs should be at least 3 times the
maximum head and preferably 4 times the maximum head, or more;
• Runoff shall enter the detention facility through a conveyance system
separate from the control and outflow conveyance system. The distance
between the inlet and outlet shall be maximized to reduce sediment from
accumulating in the outflow structure;
• Flow control structures discharging from a high use site to a drainage
facility shall include an oil control BMP that meets the requirements
outlined in Chapter 6;
• Control structures shall be selected taking into consideration the expected
hydraulic heads. Table 7-3 presents typical control structures and their
applicability.
TABLE 7-3
OPTIMAL APPLICATION OF CONTROL STRUCTURES
Control Structure Pond Head
outlet Pipe very Low
V-Notch Weir Low
Slotted Weir Moderate
Multi-Stage Orifice High
• Circular orifices shall be 3 inches in diameter or greater. Slotted weirs can
be used in lieu of smaller orifices to reduce the occurrence of plugging;
• The top of manhole/catch basin grates used for control structures shall be
placed 2 inches above the finish grade when located in earth or gravel
locations.
Figure 7-6 shows a typical flow control structure.
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MANHOLE LID(OR GRATE IF APPLICABLE)
SHALL BE PLACED DIRECTLY OVER LADDER
"„–�-I 1-1 I 1-1 I 1 1 I 1=1 I 1=1 I 1-1 I 1-1 I- -
_ _ _ _ _ _ _ – –
=— ____
=
OVERFLOW PIPE AND OUTLET PIPE
LL MUST HAVE CAPACITY EQUAL TO OR
�z GREATER THAN COMBINED INLET
o�
STANDARD MANHOLE LADDER STEPS OVERFLOW ELEVATION
CHAIN-200#CAPACITY SLACK WHEN–
GATE IS DOWN.N/ELD TO MANHOLE.
STRAP TO MANHOLE WALL
OUTFLOW ORIFICE LOCATION, ---_ —_
FOR EXAMPLE --== –=–
___- __–
OVERFLOW PIPE OUTLET PIPE
_ _ _ _ _
_ _ _ =IIII �
-�_ _= F
– - �
N
WATERTIGHT CLEAN OUT PLATE
WITH MINIMUM 8 GAUGE SLIDE z
�
�
N
48 IN DIAMETER MANHOLE
NOTES: RESTRICTOR PLATE ORIFICE CUT
ALL METAL PARTS AND SURFACES MUST BE MADE OF CORROSION RESISTANT MATERIAL IN PLATE 3 IN MIN.DIAMETER
OR GALVANIZED. COMPLETE CORROSION PROTECTION MUST BE ASSURED.
TOP OF MANHOLE OR GRATE SHALL BE PLACED 2 IN ABOVE FINISH GRADE UNiEN LOCATED
IN EARTH OR GRAVEL.
Figure 7-6—Flow Control Structure Example
7.5 INFILTRATION FACILITIES
7.5.1 INTRODUCTION
An infiltration facility is used for disposing of stormwater runoff into the subsurface
and can be used for flow control provided that:
• The discharge is uncontaminated or properly treated so that it does not
violate water quality criteria per Chapter 6. For additional information
regarding discharges to drywells, refer to Ecology's Guidance for UIC
Wells that Manage Stormwater;
• The Geotechnical Site Characterization demonstrates the suitability of the
soil for subsurface disposal;
• The down-gradient analysis indicates that adverse impacts are not
anticipated; and,
• The discharge does not violate UIC regulations.
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Drywells are considered standard infiltration facilities. The engineer shall consider
the impact of infiltration on groundwater elevations both on site and on down-
gradient properties.
For discharges to UIC facilities, the best management practices chosen for the site
must remove or reduce the target pollutants to levels that will comply with state
groundwater quality standards when the discharge reaches the water table or first
comes into contact with an aquifer (see WAC 173-200). Pre-treatment is required
prior to discharging to a UIC facility (refer to Chapter 6 for additional information).
Discharges to surface waters shall comply with WAC 173-201A, Water Quality
Standards for Surface Waters of the State of Washington. Refer to Chapter 6 for BMP
selection.
7.5.2 MINIMUM REQUIREMENTS
In addition to the requirements specified in Section 7.8, infiltration facilities shall
meet the minimum requirements described below.
Swale Sizing
The methodology for sizing swales is in Section 5.6.
Location
Drywells shall be spaced at least 30 feet center-to-center or twice the depth of the
drywell, whichever is greater.
If the site has the potential for contaminated or unstable soil, then these conditions
shall be investigated and appropriate mitigating measures taken before designing
infiltration facilities in theses areas.
Infiltration facilities shall not be placed on or above a landslide hazard area or slopes
greater than 15 percent without evaluation by a geotechnical engineer and
jurisdictional approval.
Outflow Rates
Outflow rates shall be determined using the field methods presented in Section 4.3.1.
The active barrel of the drywell shall be installed within the target soil layer. Target
soils with more than 12% fines (percent passing the No. 200 sieve) are not suitable
for drywells.
The proponent may assume a maximum outflow rate of 03 cfs and 1.0 cfs for Type A
and Type B drywells, respectively, if all of the following conditions are met:
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• The drywells are located within the NRCS Garrison or Springdale soil
groups. A full-scale drywell test may still be required;
• The soils are verified by a qualified professionaL Field verification should
include classifying the target layer soils, obtaining soil gradation data and
confirming that the site soils are consistent with the design outflow rates.
• There is no history of drainage problems in the vicinity of the drywell
location;
• The anticipated rise in the elevation of the local groundwater table
resulting from the disposal faciliry will not significantly impact adjacent
properties or structures; and,
• The local jurisdiction does not have concerns regarding the soil's abiliry to
drain.
Limiting Layer
The optimal separation between the bottom of the drywell and the limiting layer
(bedrock, groundwater, clay lens, etc.) shall equal the maximum drywell head, which
is 6 feet for single-depth drywells and 10 feet for double-depth drywells. The limiting
layer separation can be reduced to 4 feet when the factors of safety specified in
Appendix 4A are applied.
For a pond or swale with no infiltrative structure, the separation shall be a minimum
of 4.5 feet below the pond bottom, to account for the 6-inch treatment zone and 48
inches of subgrade infiltrative soil.
The local jurisdiction reserves the authority to increase the required depth to the
limiting layer should there be evidence that the functionality of the faciliry will be
negatively impacted.
Setback
Drywells shall be placed with the following setback distances:
• At least 100 feet from water wells;
• At least 200 feet from springs used for drinking water supplies,
• At least 100 feet from septic tanks or drainfields;
• At least 100 feet up-slope and 20 feet down-slope from building
foundations, unless a reduction is geotechnically justified;
• At least 20 feet from a Native Growth Protection Easement; and,
• Per the geotechnical engineer's recommendations when located up-slope
from a structure or behind the top of a slope inclined in excess of 15
percent.
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7.6 NATURAL DISPERSION
7.6.1 INTRODUCTION
Natural dispersion attempts to minimize the hydrologic changes created by new
impervious surfaces by restoring the natural drainage patterns of sheet flow and
infiltration. There are three types of natural dispersion:
• Concentrated Flow Dis�ersion: Used for steep driveways or other small
pavement areas, concentrated flow dispersion uses berms or drains to
direct runoff from the paved area to a vegetated pervious area (the
"dispersal area") that slows entry of the runoff into a conveyance system,
allows for some infiltration and provides some water quality enhancement.
• Sheet Flow Dis�ersion: Used for flat or moderately sloped paved or
cleared areas, sheet flow dispersion consists of a vegetated buffer zone
through which sheet flow from the pavement passes, providing flow
attenuation and treatment.
• Full Dispersion: Use for larger areas of new residential or commercial
development, full dispersion is the preservation of native vegetation on
some portion of the site to allow dispersion of runoff from roofs,
driveways and roads within the site.
Natural dispersion can be used for impervious or pervious surfaces that are graded to
avoid concentrating flows. This flow control method shall only be considered for use
on rural projects, including linear roadway projects, large lot subdivision, short plat
roads, driveways, sport courts, parking lots and roofs that are not guttered. This flow
control method is not intended for use prior to discharge to a lake, stream or water
body.
7.6.2 MINIMUM REQUIREMENTS
This section describes a sheet-flow dispersion technique; concentrated flow
dispersion is not allowed in the Spokane Region at this time.
In addition to the requirements specified in Section 7.8, as applicable, the following
minimum requirements shall be met:
• The dispersal area shall be well-vegetated;
• A vegetated dispersal width of 10 feet must be provided for every 20 feet
of width of impervious surface that drains to the dispersal area, with
10 feet the minimum width in all cases. An additional 0.25 feet of
vegetated dispersal width shall be provided for each additional foot of
impervious surface;
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• A vegetated dispersal width of 1 foot must be provided for every 6 feet of
disturbed pervious area(i.e. bare soil and non-native landscaping);
• Natural dispersion areas (perpendicular to the impervious area) shall have
a slope no steeper than 14% (7H:1 V). If this criterion cannot be met due to
site constraints, the dispersal width must be increased 1.5 feet for each
percent increase in slope above 14%, and in no case shall the slope exceed
20%;
• The average longitudinal slope (roughly parallel to the road or diagonally
away from the road) of the dispersal area shall be no more than 15%;
• The longitudinal slope of the contributing impervious or pervious drainage
area(parallel to the edge of the dispersal area) shall be 5% or less;
• The lateral slope of the contributing impervious or pervious drainage area
(perpendicular to the dispersal area, typically the road cross-slope) shall be
4.5% or less;
• The sheet flow path leading to the natural dispersal area shall not be
longer than 75 feet for impervious areas or 150 feet for pervious areas;
• The longitudinal length of the dispersal area shall be equivalent to or
greater than the longitudinal length of impervious area that is contributing
the sheet flow;
• A 2-foot-wide transition zone (to discourage channeling) shall be provided
between the edge of the impervious surface and the vegetated dispersal
area, or under the eaves of a roof that has not been guttered. This may be
an extension of the sub-grade material (crushed rock), modular pavement,
or drain rock;
• The dispersal area shall have a minimum infiltration rate of 4 inches per
hour;
• Clearing and grubbing of native dispersal area shall be minimized in order
to help maintain the existing root systems that are vital to the success of
natural dispersion;
• The area around the dispersal zones shall not be compacted;
• For sites with septic systems, the dispersal area must be downgradient of
the drain field primary and reserve areas. This requirement may be waived
by the local jurisdiction if the site topography clearly indicates that flow is
prohibited from intersecting the drain field;
• The dispersal area shall be located down-gradient from building sites;
• The dispersal area shall be clearly identified on all construction plans,
including grading plans, so that the area is not cleared, grubbed or
compacted, and shall be clearly delineated on the site;
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• Native vegetation and existing trees should not be removed from the
natural growth retention areas except where required to meet sight
distance, clear-zone or other traffic-related requirements, or if the
vegetation is diseased;
• Dispersal is not allowed across, over or toward a landslide or
geohazardous area; and,
• The dispersal area shall be preserved within the road right of way, a
separate dedicated tract or an easement in order to ensure that treatment
and flow control are not interrupted.
7.7 EVAPORATION FACILITIES
7.7.1 INTRODUCTION
Evaporation systems are used to collect and dispose of stormwater runoff when soils
are not conducive to infiltration, shallow groundwater is present, or there is the
potential for negative impacts due to post-developed stormwater runoff being injected
into the subsurface.
The locally developed spreadsheet described in Section 5.7.3 is the most common
tool used to perform evaporative pond capacity calculations.
7.7.2 MINIMUM REQUIREMENTS
Liner
Geosynthetic or natural liners may be required to limit infiltration in areas where
there is the potential for down-gradient impacts or where the water table may
adversely impact the pond via seepage or mounding. The liner shall be a product
suitable for stormwater storage and installed per the geotechnical engineer's or
manufacturer's recommendation.
When an evaporative pond is proposed, a geotechnical engineer shall provide
evaluation of the following:
• Liner materials and installation;
• The potential for groundwater seepage into the pond from the surrounding
area;
• The potential for any down-gradient adverse impacts due to the injection
of developed stormwater volume into the subsurface; and,
• The potential for groundwater mounding or uplift for a lined pond.
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Based upon the information in these evaluations, the geotechnical engineer shall
make recommendations regarding the following, if applicable:
• Liner materials and installation; and,
• Any proposed mitigation measures.
Pond Geometry
Evaporative systems designed with the Preferred Method (refer to Section 5.7.2) shall
have an evaporation volume separate from the detention volume that provides
attenuation of peak flows. Depending on the site conditions and limitations, the
proponent can provide separated cells or stacked cells to satisfy this requirement.
Separated svstem: This type of facility has one evaporation cell (upstream cell)
followed by a detention or infiltration cell (downstream cell). The storage volume and
design depth of the evaporation cell is determined by a water budget analysis as
described in Section 5.7. A factor of safety of at least 1.2 is applied to the required
evaporative volume or design depth. The invert of the overflow to the detention or
infiltration faciliry is placed at or above the maximum surface water elevation of the
evaporative system (including the factor of safety).
The downstream cell is designed per the criteria for detention facilities (refer to
Section 7.3) or infiltration facilities (refer to Section 7.5). In order to allow a point
discharge from a detention facility, it must be established that there is an existing,
well-defined natural drainage course. A 1-foot freeboard above the maximum surface
elevation of the detention or infiltration cell is required. Figure 7-7 shows a typical
cross-section of a separated system.
1 FT FREEBOARD
----
EVAPOR4TIVE CELL
EXISTING NATUR4L
DETENTION OR DR4INAGE COURSE
INFILTR4TION
CELL
OUTFLOW
STRUCTURE
Figure 7-7 —Separated Evaporative/Detention Facility Cross-Section
A stacked svstem: This type of facility has the evaporative cell (lower cell) below the
detention cell (upper cell). The storage volume and design depth of the evaporation
cell are determined by a water budget analysis as described in Section 5.7. A factor of
safety of at least 12 is applied to the required evaporative volume or design depth.
Once the dimensions of the lower cell are determined, the upper cell is placed on top
of the lower cell. Thus, the detention cell "bottom" and outflow structure must be
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placed at or above the maximum surface water elevation of the evaporative system
(including the factor of safety).
The detention cell is designed per the criteria specified in Section 73. In order to
allow a point discharge from a detention facility, it must be established that there is an
existing, well-defined natural drainage course. A 1-foot freeboard above the
maximum surface elevation of the detention or infiltration cell is required. Figure 7-8
shows a typical cross-section of a stacked system.
1 FT
1F FREEBOARD
DETENTION CELL EXISTING NATUR4L
DR4INAGE COURSE
EVAPOR4TIVE CELL
OUTFALL
STRUCTURE
Figure 7-8 —Stacked Evaporative/Detention System Cross-Section
T�eatment
Evaporative systems designed with the Alternative Method are not subject to water
quality treatment requirements. Evaporative systems designed with the Preferred
Method are required to provide water quality treatment per the goals, applicability
and criteria specified in Chapter 6.
7.8 ADDITIONAL REQUIREMENTS FOR ALL FACILITIES
The following minimum requirements shall be met for all flow control facilities:
7.8.1 G E N E R A L
The design of flow control facilities shall adhere to the following:
• Pond bottoms shall be located a minimum of 0.5 feet below the outlet to
provide sediment storage; and,
• In general, all pond bottoms shall be flat. Roadside swales are considered
flat if the swale bottom slope is 1% or less. When calculating treatment
volume, the designer can assume a flat bottom for swale/pond bottom
slopes up to 1%. Note that if treatment volume versus area is the
methodology used, the volume may be calculated assuming a flat bottom
even if the roadside swale bottom has a slope (maximum of 1%).
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However, for the calculation of stormwater disposal volume, the grade of
the roadside swale bottom shall factor into the geometry used to size the
facility. The drainage facility bottom shall slope away from the pond inlet
and toward the control structure at 1% for a maximum distance of 20 feet.
• Drainage facilities shall be located within the right of way, within a border
easement parallel to the road or within a drainage tract. In unincorporated
Spokane County, drainage facilities may also be located in a drainage
easement on private property (refer to Chapter 11 for specific
information).
7.8.2 SETBACKS
Setbacks for any pond, swale or ditch (measured from the maximum design operating
depth) shall be at least 30 feet when located up-gradient or 10 feet when located
down-gradient from septic tanks or drainfields.
Pond overflow structures shall be located a minimum of 10 feet from any structure or
property line. The toe of the berm or top of bank shall be a minimum of 5 feet from
any structure or property line.
7.8.3 DRAWDOWN TIME
Detention and infiltration facilities shall have a minimum subgrade infiltration rate of
0.15 inches/hour and drain completely within 72 hours after a storm event.
7.8.4 SIDE SLOPES
Pond side slopes shall meet one of the following requirements:
• Interior side slopes shall not be steeper than 3:1 (horizontal to vertical);
• Interior side slopes may be increased to a maximum of 2:1 (horizontal to
vertical) if the surrounding grade creates a cut or fill with no greater depth
than 1.0 foot;
• Exterior side slopes shall not be steeper than 2:1 (horizontal to vertical)
unless analyzed for stability by a geotechnical engineer.
• Pond walls may be vertical retaining walls,provided that:
o A fence is provided along the top of the wall for walls 2.5 feet or
taller;
o A 4-foot-wide access ramp to the pond bottom is provided, with slopes
less than 4:1 (horizontal to vertical); and,
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o The design is stamped by an engineer with structural expertise if the
wall is surcharged or if it is 4 feet or more in height. A separate
building permit may be required by the local jurisdiction if the wall
height exceeds 4 feet.
7.8.5 EMERGENCY OVERFLOW SPILLWAY
An emergency overflow spillway shall be provided, whenever reasonable, to bypass
the 100-year developed peak flow toward the downstream conveyance system in the
event of plugged orifices or high flows that exceed the design storm.
Emergency overflow spillways shall be provided for detention ponds with constructed
berms 2 feet or more in height and for ponds located on grades in excess of 5%.
Emergency overflow spillways shall be analyzed as broad crested trapezoidal weirs
and comply with the following requirements:
• The spillway shall have the capacity to pass the 100 year-developed peak
flow with a 30% freeboard;
• The full width of the spillway shall be armored with riprap and extend
downstream to where emergency overflows enter the conveyance system;
• If the detention facility is located on an embankment, the overflow
spillway shall be armored to a minimum of 10 feet beyond the toe of the
embankment; and;
• The overflow path shall be identified on the construction plans and
easements shall be provided as necessary.
Engineers may choose to design the detention pond multi-stage outflow structure with
an emergency bypass that can route the 100-year storm through the structure and out
of the pond directly into the conveyance channel. However, due to the high potential
for sedimentation and plugged orifices within these structures, an emergency
overflow spillway shall still be provided in order to reduce the potential for a pond
berm breach for detention ponds that require an emergency overflow spillway.
7.8.6 EMBANKMENTS
The height of an embankment is measured from the top of the berm to the catch point
of the native soil at the lowest elevation. Embankments shall meet the following
minimum requirements:
• Embankments 4 feet or more in height shall be constructed as
recommended by a geotechnical engineer. Depending upon the site,
geotechnical recommendations may be necessary for lesser embankment
heights;
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• Embankments shall be constructed on native consolidated soil, free of
loose surface soil materials, fill, roots, and other organic debris or as
recommended by the geotechnical engineer;
• Energy dissipation and erosion control shall be provided to stabilize the
berm and its overflow;
• The embankment compaction shall produce a dense, low permeability
engineered fill that can tolerate post-construction settlements with minimal
cracking. The embankment fill shall be placed on a stable subgrade and
compacted to a minimum of 95% of the Modified Proctor Densiry (ASTM
Procedure D1557);
• Anti-seepage filter-drain diaphragms shall be considered on all outflow
pipes and are required on outflow pipes when design water depths are 8
feet or greater;
• Embankments must be constructed by excavating a key. The key width
shall equal 50 percent of the berm base width, and the key depth shall
equal 50 percent of the berm height; and,
• The berm top width shall be a minimum of 4 feet.
7.8.7 FENCING
Fencing or other barriers may be required to protect the health, welfare and safety of
the public. In general, fencing is required for the following:
• Drainage facilities with the first overflow at 2 or more feet above the pond
bottom;
• Drainage facilities with retaining walls 2.5 feet high or taller;
• Drainage facilities located at, or adjacent to, schools, nursing homes, day-
cares, or similar facilities; and,
• Evaporation Ponds.
Fencing is not required for a typical bio-infiltration swale. However, the local
jurisdiction reserves the authority to require a fence along any swale or pond should
there be a concern for safety.
At the discretion of the local jurisdiction, if a pond is proposed as an amenity (i.e.
enhancements to the disposal facility are proposed, such as rocks, boulders,
waterfalls, fountains, creative landscaping or plant materials), the design will be
reviewed on a case-by-case basis, such that the fencing requirements may be reduced
or waived.
At the discretion of the local jurisdiction, marking fences, terraces, shallower side-
slopes, egress bars, etc. may be allowed instead of fencing.
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The minimum fencing requirements are as follows:
• The fencing shall be at least 4 feet tall unless otherwise specified by the
local jurisdiction, and provide visual access; and,
• Gates are to be provided where drainage facilities are fenced. The gates
shall be a minimum of 12 feet wide and have locks.
The City of Spokane Valley reserves the authoriry to waive any and all fencing in
commercial areas, as reviewed and accepted on a case-by-case basis by City staff.
7.8.8 PLANTING REQUIREMENTS
Exposed earth on the pond bottom and interior side slopes shall be sodded, seeded or
vegetated in a timely manner, taking into account the current season. Unless a dryland
grass or other drought tolerant plant material is proposed, irrigation shall be provided.
All remaining areas of the tract or easement shall be sodded or planted with dryland
grass or landscaped.
7.8.9 LANDSCAPING
Where space and circumstances allow, the landscaping scheme and common use
areas should be integrated with the open drainage features and into the overall
stormwater plan. Plants other than turf grass have characteristics that can provide
additional stormwater management benefits such as enhanced evapotranspiration and
improved soil-holding capabilities.
However, in all cases the landscaping and other uses must be subservient to the
primary stormwater needs and functions. Landscaping that does not conflict with the
collection, conveyance, treatment, storage, and disposal of stormwater is encouraged.
The following general principles should guide the landscaping and selection of plants
in conjunction with stormwater facilities:
• Supplemental landscaping areas should be grouped into irregular islands
and borders outside of the immediate stormwater facilities and not
uniformly dispersed throughout them. The constructed stormwater features
should be irregular and curved in shape to look more natural. Avoid
straight lines and regular shapes where and when possible;
• Trees and shrubs shall not be planted on pond liners due to potential
leakage from root penetration;
• Trees and shrubs shall not be planted near drainage appurtenances such as
outlet control structures, manholes, catch basins, inlets, storm drain lines,
and underground disposal structures such as drywells or drain-fields. The
minimum spacing between the tree or shrub and the drainage structure
shall be equal to the crown diameter of the mature plant;
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• Trees and shrubs shall not be planted within the treatment, storage, and
conveyance zones of swales, ponds, and open channels, unless treatment
and storage calculations take into account the mature tree size and allow
runoff to reach the drainage facilities;
• Self-limiting plants shall be used, not spreading or self-seeding types.
• Full-size forest trees and trees with aggressive root systems should not be
used except where space and circumstances allow. Deciduous trees with
heavy shade and leaf-fall should also be avoided to allow the survival of
the surrounding grass areas and not plug drainage facilities. Evergreens
and smaller ornamental trees are normally better suited to urban
conditions;
• Shrubs should be upright in form and groundcovers should have neat
growth patterns to assist in their maintenance and that of the surrounding
grass areas; and,
• The plant selection needs to consider the native soil conditions and altered
moisture conditions created by the stormwater facilities. The plants need
to be adaptable to the changes in site conditions. Plants that are self-
sufficient and self-limiting, do not require year-round irrigation and
require minimal care are encouraged.
7.8.10 MAINTENANCE
Maintenance is of primary importance for drainage facilities to operate as designed.
The requirements of Chapter 11 shall be met as applicable.
7.8.11 DAM SAFETY
Detention facilities that can impound 10 acre-feet (435,600 cubic feet) or more with
the water level at the embankment crest are subject to the state's dam safety
requirements, even if water storage is intermittent and infrequent (WAC 173-175-
020(1)). The principal safety concern is for the downstream population at risk if the
dam should breach and allow an uncontrolled stormwater release. Peak flows from
dam failures are typically much larger than the 100-year flows which these ponds are
typically designed to accommodate.
Dam safety considerations generally apply only to the volume of water stored above
natural ground level. Per the definition of dam height in WAC 173-175-030, natural
ground elevation is measured from the downstream toe of the dam. If a trench is cut
through natural ground to install an outlet pipe for a spillway or low-level drain, the
natural ground elevation is measured from the base of the trench where the natural
ground remains undisturbed.
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Ecology's Dam Safety Office is available to provide written guidance documents and
technical assistance for owners and engineers to address dam safety requirements. If
the pond exceeds the volume criteria for dam safety, Ecology shall be contacted early
in the facilities planning process.
7.9 SPECIAL REQUIREMENTS
7.9.1 SPECIAL DRAINAGE AREAS
Special Drainage Areas (SDAs) are designated areas with shallow soils, bedrock near
the surface of the land, and soils or geological features that may make long-term
infiltration of stormwater difficult or areas where infiltration may pose potential
problems for on-site or adjacent properties. These areas may also contain steep slopes
where infiltration of water and dispersion of water into the soils may be difficult or
delayed, creating drainage problems such as erosion. Known areas of flooding or
areas that historically have had drainage or high groundwater problems (mapped or
unmapped) are also SDAs.
SDAs in the Ciry of Spokane are described in SMC 17D.060 "Stormwater Facilities."
Additional requirements for development in these areas are included in this
ordinance.
Spokane County has mapped several SDAs. Among the mapped SDAs are portions of
the Glenrose/Central Park Watershed, the North Spokane Stormwater Planning Area
and the West Plains Stormwater Planning Areas. The Spokane County Stormwater
Utility Section maintains and updates these maps. At the discretion of the local
jurisdiction, an area can be designated as an SDA if it is determined that development
may have adverse impacts on existing or future down-gradient or adjacent properties.
Unless specifically approved by the local jurisdiction, the peak rate and volume of
stormwater runoff from any proposed land development to any natural or constructed
point of discharge downstream shall not exceed the pre-development peak rate or
volume of runoff. A down-gradient analysis demonstrating that there will be no
expected adverse impacts on downgradient properties will be required. Exceptions
with regard to rate and volume control can be made for regional facilities planned by
the local jurisdiction.
7.9.2 FLOODPLAINS
In the City of Spokane and the City of Spokane Valley, floodplain requirements are
administered by the planning department. Check with the local jurisdiction for more
information and specific requirements.
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When any property is developed in and around identified Areas of Special Flood
Hazard (100-year floodplains) all work must conform to the requirements of the
National Flood Insurance Program and the flood ordinance of the local jurisdiction.
This section summarizes the general requirements for projects located within a
floodplain. Specific requirements and additional information can be obtained from the
local jurisdiction.
Land-actions located within a floodplain (A and B Flood Zones only) shall conform
with the following requirements:
• A Floodplain Development permit shall be obtained from the local
jurisdiction before any development (including structures, manufactured
homes, bridges, culverts, grading, excavation or fill) is undertaken,
constructed, located, extended, connected or altered on any properry that is
partly or entirely located in a floodplain;
• The 100-year Base Flood Elevation (BFE) shall not increase at any point
by more than 1.0 foot within Unnumbered A Zones and B Zones; increase
in other designated flood hazard areas (numbered A zones and floodways)
may be further restricted.
• Projects proposing any increases in BFEs or in the way floodwaters enter
and exit the property may require approval from the impacted property
owners.
• Disposal of increases in stormwater runoff may not be allowed in an
identified 100-year floodplain.
• The lowest floor (including basement floor) shall be elevated to a
minimum of 1.0 foot above the BFE. Flood Insurance Rate Maps (FIIZMs)
provide the BFEs for some flood zones. Development in areas without
established BFEs may be inspected by the local jurisdiction. When it is not
evident that the proposed building will be outside the flood zone or if a
subdivision is proposed, a flood study may be required to establish the
100-year BFE and delineate the 100-year floodplain;
• Commercial, industrial, or other nonresidential buildings may be
floodproofed to 1 foot above the BFE in lieu of elevating the lowest floor
elevation to a minimum of 1.0 foot above the BFE. Floodproofing
techniques shall be certified by an engineer or architect licensed in the
State of Washington;
• Residential emergency access and egress shall be provided for the 100-
year event;
• The plat dedication of all subdivision proposals associated with
floodplains shall contain language prescribed by the local jurisdiction.
A floodplain study is required when development impacts floodplains or may impact
floodplains in an unnumbered A Flood Zone or when BFEs have not been
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established. Disturbance to the floodplain may include filling, excavating, etc. The
floodplain study shall meet the following requirements:
• The 100-year peak flows and volumes shall be determined for each basin.
The engineer shall review FEMA studies, previously accepted floodplain
studies, USGS studies and gage data, or watershed plans for already
established 100-year flows. If 100-year flows are not available from other
sources acceptable to the local jurisdiction, the engineer shall calculate the
required flow by comparison with similar watersheds where flows have
been determined or the use of regression equations (see USGS Water
Resources Investigations Report 97-4277, Magnitude and Frequency of
Floods in Washington or the most current version), or by running a
hydraulic model per the requirements of this Manual. Contact the local
jurisdiction for guidance on the appropriate storm type and duration to use.
• The study shall include all relevant calculations for determining the 100-
year flow. The study shall be presented in a rational format so as to allow
a reviewer to reproduce the same results; a basin map showing the site
boundary and the limits of the watershed contributing to the floodplain
shall be included. Topographic contours shall extend beyond the
floodplain's watershed boundary, as needed, to confirm the basin limits.
The basin map shall meet the requirements of Section 3.43;
• In determining the BFE, the study shall use field-surveyed cross-sections
of the floodplain in the project area. The cross-sections shall extend
offsite, as necessary, to delineate the floodplain in the area of the proposal.
FEMA-generated cross-sections may be available for use, but these shall
be supplemented with field-surveyed cross-sections for the specific site;
• The BFE shall be determined and the floodplain shall be delineated for the
pre-developed and post-developed conditions. The BFE shall be tabulated
by station in order to estimate any change to the BFE and delineate
modifications to the floodplain. The analysis shall calculate the pre-
developed and post-developed BFEs as follows:
o To the nearest 1/10 of a foot in unnumbered A and B zones;
o To the nearest 1/100 of a foot in numbered A zones; and,
o To the nearest 1/1,000 of a foot (as required by FEMA) in floodway
areas.
• Floodplain analysis maps shall be prepared for the pre-developed and
post-developed conditions and shall meet the following requirements:
o The maps shall show the BFEs on-site to the nearest 1/lOth of a foot
and clearly delineate the 100-year floodplain;
o Topographic contours shall be clearly marked, a bench mark shall be
identified for the topographic work and the details of the bench mark
shall be discussed;
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o Maps shall clearly show no violations to the requirements of the local
jurisdiction's Flood Ordinance;
o All lots and development, a north arrow, and a scale bar shall be
shown on the map; and,
o The map must be stamped and signed by an engineer.
In unincorporated Spokane County, plats, short plats and commercial project
floodplain requirements shall be coordinated during the pre-design meeting and
submitted with the Drainage Submittal. For single-family residential projects, the
engineer shall work directly with the Environmental Programs section of the
Engineering Department as soon as possible in the planning process.
7.9.3 WETLANDS AND CLASSIFIED STREAMS
Wetlands and classified streams are regulated by the Department of Ecology, the
Department of Fish and Wildlife and the local jurisdiction's critical areas ordinance.
Classified streams are those identified and classified under the Washington
Department of Natural Resources' water typing system. This section provides criteria
for using a wetland for stormwater treatment or disposal. The engineer shall
coordinate with the local building and planning department for further requirements.
The term wetland encompasses a variety of aquatic habitats including swamps,
marshes, bogs or floodplains. Wetlands have a natural supply of water, from flooding
rivers, streams, natural drainage channels, connections to groundwater, or a perched
shallow groundwater table, and are typically inundated with water for a portion of the
year. Wetlands are often vegetated with aspen, cattails, cottonwoods, willows, reed
grasses and other aquatic plants.
Sites with a wetland or a classified stream often feature other Natural Location of
Drainage Systems as well. In addition to the requirements in Section 8.3, the
following are required for sites with a wetland or classified stream:
• A qualified wetland biologist shall categorize the wetland, according to
the local jurisdiction's critical areas ordinance and Ecology's Wetland
Rating System for Eastern Washington, and delineate the wetland
boundaries and buffer areas. More information can be found at:
http://www.ecy.wa.gov/programs/sea/wetlan.html;
• The proponent shall submit to the local jurisdiction a Mitigation Plan,
accepted by the Department of Ecology, if the wetland is to be disturbed
due to construction activity or if any natural source of recharge to the
wetland will be eliminated or altered;
• A Hydraulic Permit shall be obtained when work is proposed within the
normal high-water level of classified streams. Site alterations within the
buffers of regulated streams are generally limited to essential access and
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utiliry needs or restoration plans as reviewed and accepted by the State
Department of Fish and Wildlife and under the local jurisdiction's critical
areas ordinance; and,
• The local planning department and state and federal agencies shall be
contacted for permitting and buffer requirements, etc.
Requirements for hydrologic modification of a wetland for stormwater treatment or
disposal are presented in Section 6.7.5.
7.9.4 CLOSED DEPRESSIONS
Closed depressions are natural low areas that hold a fixed volume of surface water.
Depending upon soil characteristics, a closed depression may or may not accumulate
surface water during wet periods of the year. Some closed depressions may be
classified as wetlands. If so, the engineer shall comply with the wetland criteria
specified in this chapter and in Chapter 6. Analysis of closed depressions shall
include the following at a minimum:
• Identification of the location of the closed depression on the pre-developed
basin map;
• A routing analysis of the drainage basins contributing to the closed
depression to estimate the peak flow rates and volumes leaving the site in
the pre-developed condition;
• An estimation of the storage capacity of the closed depression for the 100-
year storm event;
If the closed depression will be filled in, a facility shall be provided that has the
capacity to store the 100-year volume that was historically intercepted by the closed
depression. This is in addition to the drainage facilities required for flow control and
treatment due to the increase in stormwater runoff. The construction plans shall
include a grading plan of any closed depression areas to be filled in. The grading plan
shall show both existing and finish grade contours. The plans shall also specify
compaction and fill material requirements.
7.10 REGIONAL STORMWATER FACILITIES
Regional stormwater facilities are grass-lined ditches, natural drainageways, ponds, pipes
and various other means of conveying, treating and disposing of stormwater runoff that
serve as the "backbone" of a system to which smaller drainage elements can be
connected. Most regional facilities serve more than a single development within a given
contributing drainage basin. Regional facilities have the potential to lessen flooding in
existing drainage problem areas and to provide new development with an alternative to
on-site stormwater disposal.
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If regional facilities consist of pipes or other non-infiltrative conveyance facilities, they
have the potential to significantly increase stormwater runoff and contaminants going
into selected discharge areas. The location of such discharges, and pretreatment levels,
must be carefully considered to avoid adverse impacts on water resources.
Regional facilities may reduce a communiry's long term costs for stormwater
management because they can free up buildable land for development and can be less
expensive to build, operate, and maintain than multiple individual facilities. The local
jurisdiction may assume responsibiliry, or form a partnership, for the design,
construction, operation and maintenance of regional facilities.
Studies are currently being performed and completed for several planned regional
facilities in the Spokane region. In addition, local jurisdictions have begun mapping
natural stormwater features that will need to be incorporated into future regional
stormwater systems. Due to this recent progress, developments in the near future may be
allowed to discharge stormwater into regional systems. As regional facilities come "on-
line," the requirements for on-site treatment and detention may vary from the basic
requirements in this manual. Close coordination with the local jurisdiction will be
required in order to determine the location and timing of any planned regional system,
and to learn the specific design criteria for on-site stormwater facilities that may
discharge into the system.
All projects shall be reviewed for the presence of natural drainageways, and a
determination will be made as to their significance with regard to preservation of natural
conveyance and potential use as part of a regional system.
When a local jurisdiction assumes the responsibility for any or all portions of the design,
construction, operation, and maintenance of the drainage facilities, project proponents
shall be required to contribute a pro-rated share of the cost (via system development
charges or other related fees) based on the estimated cost of improvements the project
proponent would otherwise have been required to install. The proponent shall supply the
information to justify the estimated costs of the foregone individual improvements.
While opportunities may be available for private developments to use public regional
stormwater facilities to accommodate runoff, local jurisdictions reserve the authority to
limit or restrict discharge to public facilities.
Spokane County has completed Stormwater Management Plans for Chester Creek and the
Glenrose, Central Park, North Spokane and West Plains Stormwater Planning Areas. The
Ciry of Spokane has completed a City Stormwater Management Plan and the City of
Spokane Valley may also identify needed regional stormwater facilities in the near future.
Project proponents shall coordinate with the appropriate local jurisdiction early in the
project proposal process if the project is in an area for which natural drainage features
with potential regional significance have been identified where regional facilities have
been proposed, or where capital improvement plans have been adopted.
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CHAPTER 8 - CONVEYANCE
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Chapter Organization
8.1 Introduction......................................................................................................................8-1
8.2 Applicability ....................................................................................................................8-1
8.3 Natural and Constructed Channels...................................................................................8-1
8.3.1 Channel Analysis .....................................................................................................8-1
8.32 Minimum Requirements ..........................................................................................8-3
8.3.3 Channel Design........................................................................................................8-5
8.3.4 Preservation of Natural Location of Drainage Systems (NLDS).............................8-8
8.4 Culverts..........................................................................................................................8-11
8.4.1 Culvert Analysis.....................................................................................................8-11
8.42 Minimum Requirements for Culverts....................................................................8-11
8.43 Culvert Design.......................................................................................................8-15
8.5 Storm Drain Systems .....................................................................................................8-15
8.5.1 Pipe Analysis .........................................................................................................8-15
8.52 Minimum Requirements ........................................................................................8-16
8.5.3 Pipe Design............................................................................................................8-19
8.6 Gutters............................................................................................................................8-19
8.6.1 Gutter Analysis ......................................................................................................8-20
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8.62 Gutter Design.........................................................................................................8-21
8.7 Drainage Inlets...............................................................................................................8-24
8.7.1 Minimum Requirements ........................................................................................8-24
8.72 Drainage Inlet Design............................................................................................8-26
Appendix 8A —Example Calculation: Non-Flooded Width (Uniform Gutter Section)............8-33
Appendix 8B —Example Calculation: Non-Flooded Width (Composite Gutter Section).........8-35
Appendix 8C —Example Calculation: Grate Inlet Capacity (Uniform Gutter Section)............8-37
Appendix 8D —Example Calculation: Grate Inlet Capaciry, Continuous Grade (Composite Gutter
Section)......................................................................................................................................8-39
Appendix 8E — Example Calculation: Grate Inlet Capacity, Continuous Grade (Uniform Gutter
Section)......................................................................................................................................8-41
Appendix 8F —Example Calculation: Curb Inlet Capacity, Continuous Grade (Composite Gutter
Section)......................................................................................................................................8-43
Appendix 8G—Example Calculation: Combination Inlet Capacity, Sump ..............................8-45
Appendix 8H—Example Calculation: Curb Inlet Capacity, Sump ...........................................8-47
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8.1 INTRODUCTION
A conveyance system includes all natural or constructed components of a storm drain
system that collect stormwater runoff and convey it away from structures, minimizing the
potential for flooding and erosion.
Conveyance facilities consist of curbs and gutters, inlets, storm drains, catch basins,
channels, ditches, pipes and culverts. The placement and hydraulic capacities of storm
drain structures and conveyance systems shall consider the potential for damage to
adjacent properties and minimize flooding within traveled roadways. The conveyance
system shall also provide discharge capacity sufficient to convey the design flow at
velocities that are self-cleaning without being destructive to the conveyance facilities.
These objectives are achieved by designing all conveyance facilities using the design
storm event specified for the given facility and by adhering to requirements such as
minimum velocity, freeboard, cover, etc.
A properly designed conveyance system maximizes hydraulic efficiency by using the
proper material, slope and size. Constructed conveyance systems should emulate natural,
pre-developed conditions to the maximum extent feasible. Field-verified defined natural
drainageways must be preserved and protected; filling them in and building on top of
them is not an acceptable practice. In addition, some drainageways may be required for
regional use(refer to Section 83.4 for criteria).
Inflow and discharge from the system shall occur at the natural drainage points in the
same manner as the pre-developed condition as determined by topography and existing
drainage patterns.
8.2 APPLICABILITY
All projects shall comply with this Basic Requirement regardless of whether the project
they meet the regulatory threshold.
8.3 NATURAL AND CONSTRUCTED CHANNELS
8.3.1 CHANNEL ANALYSIS
A channel analysis shall be performed for all constructed channels proposed for a
project and for all field-verified existing natural drainageways/channels present on-
site (refer to Section 83.4 for details). The following requirements apply to the
Drainage Report and the road and drainage plans, when applicable:
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• Complete channel calculations shall be provided, indicating the design
peak flow rates and assumptions, such as channel shape, slope and
Manning's coefficient(see Table 5-4);
• Calculations, including the velocity, capacity, and Froude number
shall be provided for each distinct channel segment whenever the
geometry of the channel changes (i.e. if the slope, shape or roughness
changes significantly);
• The centerline and direction of flow for all constructed drainage
ditches or natural channels within the project limits are to be clearly
shown on the construction plans and basin map. For all proposed
channels, locating information shall be provided at all angle points;
• Calculations shall support the riprap area, thickness, riprap size and
gradation, and filter blanket reinforcement for all channel protection,
which shall be provided when permissible velocities are exceeded (see
Table 8-1). This information shall be included in the plans;
TABLE 8-1
PERMISSIBLE VELOCITIES FOR CHANNELS WITH ERODIBLE LININGS,
BASED ON UNIFORM FLOW IN CONTINUOUSLY WET,AGED CHANNELS
Maximum Permissible
Velocities (feet/second)
Soil TypeOf Lining Clear Water Carrying Water Carrying
(Earth; NoVegetation) Water FineSilts Sand & Gravel
Fine sand(non-colloidal) 1.5 2.5 1.5
Sandy loam (non-colloidal) 1.7 2.5 2.0
Silt loam (non-colloidal) 2.0 3.0 2.0
Ordinary firm loam 2.5 3.5 2.2
Volcanic ash 2.5 3.5 2.0
Fine gravel 2.5 5.0 3.7
Stiff clay (very colloidal) 3.7 5.0 3.0
Graded, loam to cobbles (non-colloidal) 3.7 5.0 5.0
Graded, silt to cobbles (colloidal) 4.0 5.5 5.0
Alluvial silts (non-colloidal) 2.0 3.5 2.0
Alluvial silts (colloidal) 3.7 5.0 3.0
Coarse gravel (non-colloidal) 4.0 6.0 6.5
Cobbles and shingles 5.0 5.5 6.5
Shales and hard pans 6.0 6.0 5.0
Source: Special Committee on Irrigation Research,American Society of Civil Engineers, 1926.
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• The Froude number shall be checked near the beginning and near the
end of a channel that has significant grade changes to determine if a
hydraulic jump occurs (as indicated by the Froude number changing
from <1 to >1, or vice versa). Since it is difficult to correlate the
location of a hydraulic jump to the actual location in the field, the
engineer shall propose evenly spaced riprap berms, check dams, or
other protective measures to ensure that the jump does not erode the
conveyance facility;
• When geosynthetics are used for channel protection, the plans shall
clearly specify fabric type, placement, and anchoring requirements.
Installation shall be per the manufacturer's recommendation; and,
• Plans for grass-lined channels shall specify seed mixture and irrigation
requirements, as applicable.
8.3.2 MINIMUM REQUIREMENTS
Slope
Minimum grades for constructed channels shall be as follows:
• 1.0%for asphalt concrete; and,
• 0.5%for cement concrete, graded earth or close-cropped grass.
Side Slopes
Ditch cross-sections may be V-shaped or trapezoidal. However, V-ditches are not
recommended in easily erodible soils or where problems establishing vegetation are
anticipated.
The side slope of roadside ditches shall conform to the requirements for clear zone of
the local jurisdiction and WSDOT design standards.
No ditches or channels shall have side slopes that exceed the natural angle of repose
for a given material or per Table 8-2.
Location
Constructed channels shall not be placed within or between residential lots. Ditches
and channels shall be located within a drainage tract or within a border easement.
Ditches or channels may be allowed to traverse through lots in large-lot subdivisions
(lots of 1 acre or more) and consideration may be given to placement within an
easement versus a tract. The local jurisdiction will review these proposals on a case-
by-case basis.
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TABLE 8-2
MAXIMUM DITCH OR CHANNEL SIDE SLOPES
Typeof Channel SideSlope (Horizontal: Vertical)
Firm rock �/4:1 to Vertical
Concrete-lined stiff clay �/z:l
Fissured rock 1/2:1
Firm earth with stone lining 1�/z:l
Firm earth, large channels 1�/z:l
Firm earth, small channels 2:1
Loose, sandy earth 2:1
Sandy,porous loam 3:1
Source: Civil Engineering Reference Manual, 8th Edition
Depth
The minimum depth of open channels shall be 1.3 times the flow depth or 1 foot;
whichever is greater.
Velocity
Table 8-1 lists the maximum permissible mean channel velocities for various types of
soil and ground cover. If inean channel velocities exceed these values, channel
protection is required (refer to Section 83.3). In addition, the following criteria shall
apply:
• Where only sparse vegetative cover can be established or maintained,
velocities should not exceed 3 feet/second;
• Where the vegetation is established by seeding, velocities in the range
of 3 to 4 feet/second are permitted;
• Where dense sod can be developed quickly or where the normal flow
in the channel can be diverted until a vegetative cover is established,
velocities of 4 to 5 feet/second are permitted; and,
• On well established sod of good quality, velocities in the range of 5 to
6 feet/second are permitted.
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8.3.3 CHANNEL DESIGN
Channel Capacity
Open channels shall be sized using the following variation of Manning's formula.
Q = VA — 1.486 A R�3 S�� (8-1)
n
Where: Q = rate of flow (cfs);
V = mean velocity in channel (feet/second);
A = cross-sectional area of flow in the channel (square
feet);
R = hydraulic radius (feet); where R=A/P, and
P =wetted perimeter(feet)
S = channel slope (feet/foot);
n = Manning's roughness coefficient (Table 5-4); and,
Note: Manning's equation will give a reliable estimate of velocity only if the
discharge, channel cross-section, roughness, and slope are constant over a sufficient
distance to establish uniform flow conditions. Uniform flow conditions seldom, if
ever, occur in nature because channel sections change from point to point. For
practical purposes, however, Manning's equation can be applied to most open
channel flow problems by making judicious assumptions.
Energy Dissipc�tion Design
An energy dissipater is useful in reducing excess velociry, as a means of preventing
erosion below an outfall or spillway. Common rypes of energy dissipaters for small
hydraulic works are: hydraulic jumps, stilling wells, riprap outfall pads, and gabion
weirs.
Channel Protection
Channel velocities shall be analyzed at the following locations, and if they are found
to be erosive, channel protection shall be provided:
• At the top of a watershed, at the point where the stormwater runoff
becomes concentrated into a natural or constructed channel;
• At all changes in channel configuration (grade, side slopes, depth,
shape, etc.), if an erosive velocity is determined at a change in channel
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configuration, the velocity shall be evaluated up the channel until the
point at which the velociry is determined not to be erosive; and,
• At periodic locations along the entire channelized route.
A material shall be selected that has revetment and armoring capabilities, and the
channel shall be analyzed using the Manning's "n" value for that material to
determine if the material will reduce the velocity in the channel. In some cases,
vegetative cover (natural grasses, etc.) may provide excellent protection without
changing the flow characteristics and should be evaluated. If the calculations reveal
that common materials such as riprap are not adequate, stronger protection such as
gabions and/or stilling pools may be necessary.
RipNap PNOtection at Outlets
If the velocity at a channel or culvert outlet exceeds the maximum permissible
velocity for the soil or channel lining, channel protection is required. The protection
usually consists of a reach between the outlet and the stable downstream channel
lined with an erosion-resistant material such as riprap.
The ability of riprap revetment to resist erosion is related to the size, shape and
weight of the stones. Most riprap-lined channels require either a gravel filter blanket
or filter fabric under the riprap.
Riprap material shall be blocky in shape rather than elongated. The riprap stone shall
have sharp, angular, clean edges. Riprap stone shall be reasonably well-graded.
Apron Dimensions: The length of an apron (La) is determined using the following
empirical relationships that were developed for the U.S. Environmental Protection
Agency (ASCE, 1992):
L� _ �gQ +�7Do� for TW < �° (8-2)
0
Or
LQ = ��z +�7Do� for TW >_ �° (8-3)
0
Where: Do = maximum inside culvert width (feet);
Q = pipe discharge (cfs); and,
TW = tailwater depth (feet).
When there is no well-defined channel downstream of the apron, the width, W, of the
apron outlet as shown in Figure 8-1, shall be calculated using Equation 8-4 or 8-5:
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�� � I
°1 3Do
�
I
La
I
W
Figure 8-1 —Riprap Revetment at Outfall Schematic
W = 3Do +0.4LQ for TW >_�° (8-4)
W = 3Do +LQ for TW <�° (8-5)
When there is a well-defined channel downstream of the apron, the bottom width of
the apron should be at least equal to the bottom width of the channel and the lining
should extend at least 1 foot above the tailwater elevation.
The width of the apron at a culvert outlet should be at least 3 times the culvert width.
A�ron Materials: The median stone diameter, Dso is determined from the following
equation:
� _ �.�ZQ4/3 g-6
50 �,7j7T(DO\ � �
�� l 1
Where: DSO = the diameter of rock, for which 50% of the particles
are finer.
The riprap should be reasonably well graded, within the following gradation
parameters:
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I.ZJ � �max � I.J� al1U �15 = �.J� al1U �min = �.ZJ
Dso Dso Dso
Where: DmaX = the maximum particle size;
Dm�n = the minimum particle size; and,
Dis = the diameter of rock, for which 15% of the particles
are finer.
Minimum Thickness: The minimum thickness of the riprap layer shall be 12 inches,
DmaX or 1.SDso, whichever is greater.
Filter Blanket: A filter fabric blanket under the riprap is normally needed. If a gravel
or sand filter blanket is used, then it shall conform to the gradation parameters listed
in Table 8-3.
TABLE 8-3
CRITERIA FOR GRAVEL OR
SAND FILTER BLANKET GRADATION
Primary Criterion D15 < Sdgs
SCI15 <D15 <4�CI15
Recommended Secondary Criteria
DSp�CI50< $�
Guidelines far Stormwater Management, Spokane County,February 1998
The size of the filter blanket material is designated dXX, the size of the riprap is
designated DXX, and the size of the subgrade is designated d'XX. The thickness of each
filter blanket should be one-half that of the riprap layer. If it is found that D15/d'gs< 2
then no filter blanket is needed. Where very large riprap is used, it is sometimes
necessary to use two filter blanket layers between the sub-grade and the riprap.
8.3.4 PRESERVATION OF NATURAL LOCATION OF DRAINAGE
SY ST E M S (N L DS)
New development shall be designed to protect certain natural drainage features that
convey or store water or allow it to infiltrate into the ground in its natural location,
including drainageways, floodplains (Section 7.9.2), wetlands and streams (including
classified streams) (Section 7.9.3), and natural closed depressions (Section 7.9.4).
These features are collectively referred to as the Natural Location of Drainage
Systems (NLDS). Preserving the NLDS will help ensure that stormwater runoff can
continue to be conveyed and disposed of at its natural location. Preservation will also
increase the ability to use the predominant systems as regional stormwater facilities.
A regional stormwater faciliry is typically defined as a system designed and built by a
local jurisdiction to receive an agreed-upon rate and volume of stormwater from a
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SPOKANE REGIONAL STORMWATER MANUAL
defined contributing drainage area, but it can also refer to a private system that serves
multiple developments.
Projects located within the City of Spokane shall refer to the City of Spokane's
Stormwater Ordinance for specific requirements with regard to the Natural Location
of Drainage Systems that may differ from the information found in this section.
Definitions
Some of the drainageways that need to be evaluated for preservation purposes or for
potential use as part of a regional facility have been mapped. These drainageways are
generally defined as Type A and Type B:
• Tvpe A drainageways are predominant systems that are considered a
significant part of a larger existing natural conveyance system.
• Tvpe B drainageways are systems that are generally less prominent,
but are deemed to perform important functions in the existing
management of stormwater runoff and may be necessary for managing
stormwater as part of a larger regional or natural system.
Because every site is unique, the local jurisdiction shall make interpretations, as
necessary, based on site visits and technical information as to the exact location and
type of drainageways or any NLDS on a project site. The local jurisdiction may also
require the project proponent to provide engineering information to assist in this
determination.
The maps denoting these drainageways are not definitive; a computer program was
used to generate the contours and identify the drainageways. The Type A/B
designations are not concrete labels nor are they all inclusive. The maps are only one
tool that may be used to identify existing natural drainageways; field verification will
typically be required to fully identify the existence of a drainageway and its
significance with regard to a natural conveyance system. The Spokane County
Stormwater Utility Section maintains maps of drainageways identified within the
Spokane County Stormwater Service Areas. The criteria for analysis and preservation
of all other NLDS (floodplains, wetlands, closed depressions and wetlands/streams)
are covered in Chapter 7.
Protection
No cuts or fills shall be allowed in predominant drainageways except for
perpendicular driveway or road crossings with engineering plans showing
appropriately sized culverts or bridges. Predominant drainageways shall be preserved
for stormwater conveyance in their existing location and state, and shall also be
considered for use as regional facilities.
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Less prominent drainageways in a non-residential development and in a residential
development containing lots 1 acre or smaller may be realigned within the
development provided that the drainageway will enter and exit the site at the pre-
developed location and that discharge will occur in the same manner as prior to
development.
Realignment of a less prominent drainageway shall be defined as still following the
"basic" flow path of the original drainageway. An acceptable example would be if the
drainageway is proposed to be realigned such that it will follow a new road within the
proposed development, and will be left in its existing state or utilized as part of the
project's on-site stormwater system.
Stormwater leaving the site in the same manner shall be defined as replicating the
way the stormwater left the site in its existing condition. If the drainageway is
preserved in its existing location and is left undisturbed, this goal should be met. If
the local jurisdiction accepts the proposal to allow a less predominant drainageway to
be routed through the site via a pipe, the following additional criteria shall be met:
• Where the less prominent drainageway enters the site, the design shall
ensure that the entire drainageway is "captured" as it enters the site;
i.e. the surrounding property shall not be regraded to "neck-down" the
drainageway so that it fits into a drainage easement or tract or structure
intended to capture and reroute the off-site stormwater runoff.
• Where the less prominent drainageway exits the site, the design shall
ensure that the stormwater leaves the pipe, pond or structure a
significant distance from the edge of the adjacent property so that by
the time the stormwater reaches the property boundary, its dispersal
shall mimic that of the pre-developed condition.
Since some of the less prominent drainageways may also be useful for managing
regional stormwater, if identified as a significant drainageway (i.e. necessary
conveyance for flood control, or being considered as a connection to a planned
regional facility or conveyance route), then the drainageway may be subject to the
same limitations and criteria as a predominant drainageway.
The size of the tract or easement containing the drainageway shall be determined
based on an analysis of the existing and proposed stormwater flows directed to these
drainage systems and any access and maintenance requirements found in this Manual.
This analysis shall be performed as per the criteria found in Basic Requirement No. 5,
Section 2.2.5.
All new development containing lots that are 1 acre or smaller shall be required to set
aside the drainageway as open space in a separate tract. For new development
containing lots that are greater than 1 acre, the drainageway may be set aside in either
a tract or an easement.
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All projects shall be reviewed for the presence of any NLDS and a determination will
be made as to their significance with respect to preservation for continued natural
conveyance and for potential use as part of a regional system.
8.4 CULVERTS
A culvert is a short pipe used to convey flow under a roadway or embankment. A culvert
shall convey flow without causing damaging backwater flow constriction, or excessive
outlet velocities. Factors to be taken into consideration in culvert design include design
flows, the culvert's hydraulic performance, the economy of alternative pipe materials and
sizes, horizontal and vertical alignment, and environmental concerns.
8.4.1 CULVERT ANALY515
When applicable, the following items shall be included in the Drainage Report, or on
road and drainage plans:
• Complete culvert calculations that state the design peak flow rates,
velocities at the inlet and outlet, flow control type, and design
information for the culvert such as size, slope, length, material type,
and Manning's coefficient(refer to Table 8-4);
• Headwater depths and water surface elevations for the design flow
rate;
• Roadway cross-section and roadway profile;
• Location information for each of the culvert inverts and invert
elevations;
• Type of end treatment (wingwall, flared end sections, etc); and,
• Wall thickness.
8.4.2 MINIMUM REQUIREMENTS FOR CULVERTS
Peak Flow Rate
Culverts shall be sized to handle the design peak flow rates calculated using the
methods described in Chapter 5 and the design criteria specified in Chapter 2.
To avoid saturation of the road base, culverts shall be designed such that the water
surface elevation for the design storm event does not exceed the elevation of the base
course of the roadway.
Culverts shall be designed to withstand the 100-year storm event without damage.
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TABLE 8-4
MANNING'S ROUGHNESS COEFFICIENT (n)
FOR CULVERTS
Material Type n
Concrete pipe 0.013
Ductile iron 0.013
HDPE(only allowed in private roads) 0.013
CMP 0.024
HDPE=high-density polyethylene; CMP=corrugated metal pipe;PVC=polyvinyl
chloride
Allowable Headwater Elevation
Headwater is the depth of water at the culvert entrance at a given design flow.
Headwater depth is measured from the invert of the culvert to the water surface.
Culverts shall be designed to carry the design runoff with a headwater depth less than
2 times the culvert diameter for culverts 18 inches or less in diameter, and less than
1.5 times the culvert diameter for culverts more than 18 inches in diameter.
Velocity and Slope
To avoid silting, the minimum velocity of flow through culverts shall be 4 feet/second
and the minimum slope shall be 0.5%.
Diameter
Table 8-S lists required minimum culvert diameters.
TABLE 8-5
MINIMUM CULVERT SIZES
Culvert Location Minimum Size (inches)
Under public roads 18
Under private roads 12
Under driveways/approaches 12
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Material and Anchoring
Corrugated metal pipe, ductile iron, or concrete boxes can be used for all culverts.
High-density polyethylene (HDPE) is only allowed in private roads. For grades
greater than or equal to 20%, anchors are required unless calculations or the
manufacturer's recommendations show that they are not necessary.
Placement/Alignment
Generally, culverts shall be placed on the same alignment and grade as the
drainageway. Consideration should also be given to changes of conditions over time
by using design measures such as:
• Cambering or crowning under high tapered fill zones;
• Raising intakes slightly above the flow line to allow for sedimentation;
• Using cantilevered outfalls away from road banks to allow for toe
erosion; and,
• Using drop inlets or manholes to reduce exit velocities on steep terrain.
Angle Points
The slope of a culvert shall remain constant throughout the entire length of the
culvert. However, in situations where existing roadways are to be widened, it may be
necessary to extend an existing culvert at a different slope; the location where the
slope changes is referred to as the angle point. The change in slope tends to create a
location in the culvert that catches debris and sediment. If an extension of a culvert is
to be placed at a different grade than the existing culvert, a manhole shall be provided
at the angle point to facilitate culvert maintenance.
Outfalls
Outfalls shall conform to the requirements of all federal, state, and local regulations.
Erosion control shall be provided at the culvert outfall. Refer to Section 8.3.3 for
additional information regarding outfall protection.
Culvert Debris and Safety
The engineer shall evaluate the site to determine whether debris protection shall be
provided for culverts. Debris protection shall be provided in areas where heavy debris
flow is a concern, for example, in densely wooded areas. Methods for protecting
culverts from debris problems include: upsizing the culvert and installing debris
deflectors, trash racks or debris basins. Section 3.4.8 of the WSDOT Hydraulic
Manual has additional information on debris protection.
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Safety bars to prevent unauthorized individuals from entering the culvert shall be
provided for culverts with a diameter greater than 36 inch (see WSDOT standard
drawings).
When a trash rack is proposed, the effects of plugging shall be evaluated.
Consideration should be given to the potential degree of damage to the roadway and
adjacent property, potential hazard and inconvenience to the public, and the number
of users of the roadway.
Structural Design
The WSDOT Hydraulics Manual, Tables 8-11.1 through 8-11.18, shows the
maximum cover for different pipe materials and sizes.
For culverts under roadways, the amount of cover over the culvert is defined as the
distance from the top of the pipe to the bottom of the pavement. It does not include
asphalt or concrete paving above the base. The minimum amount of cover is 2 feet for
culverts, unless proposing ductile iron pipe. The minimum cover for ductile iron pipe
is 1 foot.
The minimum cover for culverts under private driveways is 1 foot from the top of the
pipe to the finish grade of the drivable surface. Driveway culverts shall be a
minimum of 12" CMP or ductile iron pipe.
If the depth of cover is shallow (less than 1 foot) and truck wheel loads are present, it
will be necessary to propose a design to prevent structural damage to the pipe or to
implement the manufacturer's recommendations. Also, extreme fill heights (20 feet or
greater) may cause structural damage to pipes and will require a special design or
adherence to the manufacturer's recommendations.
End T�eatments
The type of end treatment used on a culvert depends on many interrelated and often
conflicting considerations:
• Projectin_g Ends is a treatment in which the culvert is simply allowed
to protrude out of the embankment. This is the simplest and most
economical. There are several disadvantages such as susceptibility to
flotation and erosion, safery when projecting into a roadway clear zone
(an area beyond the traveled roadway provided for recovery of errant
vehicles), and aesthetic concerns;
• Beveled End Sections consist of cutting the end of the culvert at an
angle to match the embankment slope surrounding the culvert. Beveled
ends should be considered for culverts 6 feet in diameter or less.
Structural problems may be encountered for larger culverts not
reinforced with a headwall or slope collar;
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• Flared End Sections are manufactured culvert ends that provide a
simple transition from culvert to a drainage way. Flared end sections
are typically only used on circular pipe or pipe arches. This end
treatment is typically the most feasible option in pipes up to 48 inches
in diameter. Safety concerns generally prohibit their use in the clear
zone for all but the smallest diameters;
• Headwalls are concrete frames poured around a beveled or projecting
culvert. They provide structural support and eliminate the tendency for
buoyancy. They are considered feasible for metal culverts that range
from 6 to 10 feet in diameter. For larger diameters, a slope collar is
recommended. A slope collar is a reinforced concrete ring that
surrounds the exposed culvert end; or,
• Win�walls and A�rons are intended for use on reinforced concrete box
culverts. Their purpose is to retain and protect the embankment, and
provide a smooth transition between the culvert and the channel.
8.4.3 CULVERT DESIGN
Culvert analysis is typically performed using commercially available computer
software. If hand calculations are proposed, example calculations can be found in
several technical publications and open channel hydraulics manuals.
8.5 STORM DRAIN SYSTEMS
A storm drain system is a network of pipes that convey surface drainage from catch
basins or other surface inlets, through manholes, to an outfall.
The design of storm drain systems shall take into consideration runoff rates, pipe flow
capaciry, hydraulic grade line, soil characteristics, pipe strength, potential construction
problems, and potential impacts on down-gradient properties.
8.5.1 PIPE ANALYSIS
The following items shall be included in the Drainage Report, or on road and
drainage plans:
• A basin map showing on-site and off-site basins contributing runoff to
each inlet, which includes a plan view of the location of the
conveyance system;
• Complete pipe calculations that state the design peak flow rates and
design information for each pipe run, such as size, slope, length,
material type, and Manning's coefficient (see Table 8-6);
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• Velocities at design flow for each pipe run;
• The hydraulic grade line at each inlet, angle point, and outlet; and,
TABLE 8-6
MANNING'S ROUGHNESS COEFFICIENTS (n)
FOR CLOSED SYSTEMS
Material Type n
Concrete pipe 0.013
Ductile iron 0.013
HDPEi 0.013
PVC (only allowed in closed system) 0.013
1 Contact the local jurisdiction far additional requirements when
using HDPE pipe.
For lateral pipe connections to storm drain lines in existing rights-of-way (i.e. from a
catch basin to a drywell, a main line stormwater system, a pond or a swale), fixed
invert elevations are preferred but not required. The minimum depth from finish
grade to pipe invert and the minimum pipe slope necessary to satisfy the freeboard
and self-cleaning velociry requirements shall be provided. If necessary, invert
elevations may be adjusted during construction to avoid potential conflicts with
existing utilities in the right of way.
8.5.2 MINIMUM REQUIREMENTS
Peak Flow Rate
Closed pipe systems shall be sized to handle the design peak flow rates. These peak
rates can be calculated using the methods described in Chapter 5 and the design
criteria specified in Chapter 2.
Hyd�aulic Grn�le Line
The hydraulic grade line (HGL) represents the free water surface elevation of the
flow traveling through a storm drain system. Pipes in closed systems will be sized by
calculating the HGL in each catch basin or manhole. A minimum of 0.5 feet of
freeboard shall be provided between the HGL in a catch basin or manhole and the top
of grate or cover.
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SPOKANE REGIONAL STORMWATER MANUAL
Pipe Velocities and Slope
In Spokane County and the Ciry of Spokane Valley pipe systems shall be designed to
have a self-cleaning velociry of 2.5 feet/second at design flow. In the City of
Spokane, pipe systems shall be designed to have a self-cleaning velocity of 3
feet/second or greater calculated under full flow conditions even if the pipe is only
flowing partially full during the design storm.
Pipe velocities should not be excessively high since high flow velocities (approaching
and above 10 feet/second) cause abrasion of the pipes. When the design velocities are
10 feet/second or greater, manufacturer's recommendations demonstrating that the
pipe material can sustain the proposed velocities shall be provided.
When the grade of a storm pipe is greater than or equal to 20%, then pipe anchors are
required at the joints, at a minimum, unless calculations and manufacturer's
recommendations demonstrate that pipe anchors are not needed. Pipe anchor
locations are to be defined on the plans, and a pipe anchor detail shall be referenced
or provided.
Pipe material shall meet the WSDOT standards for storm sewer pipe. All pipe
segments shall be pressure tested, according to WSDOT testing procedures and
standards
Pipe Diameter and Length
The minimum pipe diameter shall be 12 inches, except that single pipe segments less
than 50 feet long may be 8 inches in diameter. The maximum length of pipe between
junctions shall be no greater than 300 feet. No pipe segment shall have a diameter
smaller than the upstream segments.
Placement and Alignment
No storm drain pipe in a drainage easement shall have its centerline closer than 5 feet
to a private rear or side property line. A storm drain located under a road shall be
placed in accordance with the local jurisdiction's requirements or standard plans.
If it is anticipated that a storm drain system may be expanded in the future, provisions
for the expansion shall be incorporated into the current design.
Outfalls
Pipe outfalls shall be placed on the same alignment and grade as the drainage way.
Outfalls shall conform to the requirements of all federal, state, and local regulations.
Erosion control is required at the storm system outfalls. Refer to Section 8.3.3 for
additional information regarding outfall protection.
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SPOKANE REGIONAL STORMWATER MANUAL
Storm DNain Debris �nd Safety
The engineer shall evaluate the site to determine whether debris protection shall be
provided for storm drain systems. Debris protection shall be provided in areas where
heavy debris flow is a concern, for example, in densely wooded areas. Methods for
protecting storm drain systems from debris problems include debris deflectors, trash
racks and debris basins. The WSDOT Hydraulic Manual has additional information
on debris protection.
For enclosed storm drain systems in urban locations, safety bars shall be provided for
outfalls with a diameter 18 inches or greater, in order to prevent unauthorized
individuals from entering the storm drain system. Outfalls within a fenced area are
not required to have safery bars. The clear space between bars shall be 4 inches
maximum.
Structural Design
The WSDOT Hydraulics Manual, Tables 8-11.1 through 8-11.18, shows the
maximum cover for different pipe materials and sizes.
In unincorporated Spokane Counry and the City of Spokane Valley, the amount of
cover over the pipe is defined as the distance from the top of the pipe to the bottom of
the pavement. It does not include asphalt or concrete paving above the base. The
minimum amount of cover is 2 feet, unless proposing ductile iron. The minimum
cover for ductile iron pipe is 1 foot.
In the City of Spokane, cover is measured from the top of pipe to the top of the
pavement. The minimum amount of cover is 3 feet, unless proposing ductile iron. The
minimum cover for ductile iron pipe is 1 foot.
If the depth of cover is shallow (less than 1 foot) and truck wheel loads are present, it
will be necessary to propose a design to prevent structural damage to the pipe or to
implement manufacturer's recommendations. Extreme fill heights (20 feet or greater)
may also cause structural damage to pipes and will thus require a special design or
adherence to the manufacturer's recommendations.
Inve�ts at Junctions
Whenever two pipes of the same size meet at a junction, the downstream pipe shall be
placed with its invert 0.1 feet below the upstream pipe invert. When two different
sizes of pipes are joined, pipe crowns shall be placed at the same elevation. The
exception to this rule is at drop manholes. Exceptions may be allowed by the local
jurisdiction when topographic conditions will significantly impact the depth of the
disposal location.
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SPOKANE REGIONAL STORMWATER MANUAL
Combined Systems
Combined sanitary and stormwater sewer systems are prohibited.
8.5.3 PIPE DESIGN
To analyze the conveyance capacity of a closed pipe system, the following general
steps may be followed when steady flow conditions exist, or conditions can be
accurately approximated assuming steady flow conditions:
1. Estimate the size of the pipes assuming a uniform flow condition, using
Equation 8-l. Refer to Table 8-6 for Manning's coefficient values.
2. For the pipe sizes chosen, determine uniform and critical flow depth;
3. Determine if upstream (accelerated) flow conditions or downstream (retarded)
flow conditions exist. Subcritical flow occurs when downstream conditions
control, supercritical flow occurs when upstream conditions control.
Determine what flow regime will occur by comparing uniform flow depth,
critical flow depth, and initial flow depth. Identify hydraulic jump locations,
and where any other discontinuity of flow depth will occur.
4. Conduct a more detailed analysis by computing the hydraulic grade line. The
direct step method or standard step method is often used to calculate the
hydraulic grade line. For supercritical flow, begin at the upstream end and
compute flow sections in consecutive order heading downstream. For sub-
critical flow, begin at the downstream end and compute flow sections in
consecutive order heading upstream.
The analysis of closed pipe systems is typically done using commercially available
computer software packages. If hand calculations are proposed, example calculations
can be found in several technical publications on open channel hydraulics, such as:
"Handbook of Hydraulics", by Brater and King; and "Open-Channel Hydraulics" by
French.
8.6 GUTTERS
A gutter is a section of pavement adjacent to a roadway that conveys water during a
storm runoff event. Gutter flow calculations are necessary to establish the spread of water
onto the shoulder, parking lane, or travel lane. Roadways shall have an adequate non-
flooded width to allow for the passing of vehicular traffic during the design storm event.
The non-flooded width (L) is shown in Figure 8-2 and the minimum non-flooded widths
for various road classifications are outlined in Table 8-7.
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L
STORMWATER RUNOFF
GUTTER FLOW� �CURB
L SIDEWALK
Figure 8-2 —Non-Flooded Road Width (L)
TABLE 8-7
NON-FLOODED ROAD WIDTH REQUIREMENTS
R oad C lassification Non-F looded W idth (L)
Private Road 12 feet
Local Access 12 feet
Collector Arterial, 2 Lane 16 feet
Minor Arterial, 2 Lane 24 feet
Other road types Per local jurisdiction
The non-flooded width shall be evaluated at low points and at proposed inlet locations.
The non-flooded width shall also be evaluated at intersections. Bypass flow shall be
limited to 0.1 cfs at intersections and at the project boundary.
Non-flooded width and flow depth at the curb are often used as criteria for spacing
pavement drainage inlets (curb or grate inlets). Drainage inlets shall be spaced so that the
non-flooded width requirements are met and stormwater does not flow over the back of
the curb. Spacing shall not exceed 300 feet regardless of flooded width and flow depth
compliance.
Generally, inlets shall be placed in the uphill side of the curb return. Additionally, the
first inlet shall not be located more 500 feet from the point where the gutter flow path
originates.
8.6.1 GUTTER ANALYSIS
When applicable, the drainage report shall include complete gutter calculations that
state the design peak flow rates, design flow depth, road cross slope, road grade, and
non-flooded width.
The equation for calculating gutter flow is a modified version of Manning's equation.
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SPOKANE REGIONAL STORMWATER MANUAL
�.�� �x.67��.5�2.67
Q = (8-7)
n
Where: Q = flow rate (cfs);
n = Manning's coefficient(from Table 8-8);
SL = longitudinal slope of the gutter(feet/foot);
SX = cross slope (feet/foot); and,
T = spread (feet)
TABLE 8-8
MANNING'S ROUGHNESS COEFFICIENTS (/1�
FOR STREET & PAVEMENT GUTTERS
Type of G utter or pavement n
Concrete gutter, troweled finish 0.012
Asphalt Pavement
Smooth Texture 0.013
Rough Texture 0.016
Concrete pavement
Float finish 0.014
Broom finish 0.016
Source: Federal Highway Administration(FHWA),Aydraulic Engineering
Circular No. 22, Second Edition
8.6.2 GUTTER DESIGN
Unifo�m Gutter Section
Uniform gutter sections have a cross slope that is equal to the cross slope of the
shoulder or travel lane adjacent to the gutter (see Figure 8-3). The spread (T) in a
uniform gutter section can be calculated using Equation 8-7 and solving for T
(spread) as follows:
0375
Q n � �
T 0.56 Sz.b'SL.s 8-8
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T
0
5�
Figure 8-3 —Uniform Gutter Section
An example calculation for determining the non-flooded width and the depth of flow
for a uniform gutter section is provided in Appendix 8A.
Composite Gutter Section
Gutters with composite sections have has a cross slope that is steeper than that of the
adjacent pavement (see Figure 8-4). The design of composite gutters requires
consideration of flow in the depressed segment of the gutter.
T
W Ts
0
o.s
sX
a� ,-
,,,
a � s�
Figure 8-4 —Composite Gutter Section
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The spread (T)in composite gutter sections cannot be determined by a direct solution;
an iterative approach following the procedure outlined below must be used. An
example calculation for determining the spread for a composite gutter section is
included in Appendix 8B.
1. Assume a flow rate above the depressed gutter section, Qs.
2. Compute QW using the following:
Qw = Q—QS (8-9)
Where: QW = flow rate in the depressed section of the gutter(cfs);
Q = design flow rate (cfs);
QS = flow rate in the gutter section beyond the depressed
section (cfs);
3. Compute the gutter cross slope (if it is not given), SW, using following
equation:
5,�, = Sx +�� (8-10)
Where: SW = cross slope of the depressed gutter(feet/foot);
SX = road cross slope (feet/foot);
W = gutter width (feet); and,
a = gutter depression (feet).
4. Compute Eo using the following equation:
�. = Q—QS = Qw (8-11)
� Q Q
Where: Eo = ratio of flow in a chosen width (the width of a
depressed gutter or grate) to the total gutter flow.
5. Solve for T using following equation:
Sx,
T - YY 1+ �x 3 �O-IG�
�
S"' E° +l —1
S� 1—Eo
6. Compute TS using following equation:
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TS = T—W (8-13)
Where: TS = the width of the spread from the junction of the
gutter with the edge of pavement to the edge of the
spread (feet).
7. Use Equation 8-7 to determine QS for TS and compare to estimated QS from
Step l. Steps 1 through 6 shall be repeated until the estimated and computed
Qs are approximately the same.
8.7 DRAINAGE INLETS
Drainage inlets are used to collect runoff and discharge it to a storm drainage system.
They are typically located in gutter sections, paved medians, and roadside and median
ditches. Inlets most commonly used in the Spokane Region are as follows:
• Grate Inlets consist of an opening in the gutter or ditch covered by a grate. They
perform satisfactorily over a wide range of longitudinal slopes. Grate inlets
generally lose capaciry as the grade of the road, gutter or ditch increases.
• Curb Inlets are vertical openings in the curb. They are most effective on flat
grades, in sumps, and where flows are found to carry significant amounts of
floating debris. Curb inlets lose interception capacity as the gutter grade increases;
therefore, the use of curb inlets is recommended in sumps and on grades less than
3%.
• Combination Inlets consist of both a curb-opening and a grate inlet. They offer the
advantages of both grate and curb inlets, resulting in a high capacity inlet.
There are many variables involved in designing the number and placement of inlets, and
in determining the hydraulic capacity of an inlet. The hydraulic capacity of a storm drain
inlet depends upon its geometry as well as the characteristics of the gutter flow. Inlet
capaciry governs both the rate of water removal from the gutter and the amount of water
that can enter the storm drainage system. Inadequate inlet capacity or poor inlet location
may cause flooding on the roadway resulting in a hazard to the traveling public.
8.7.1 MINIMUM REQUIREMENTS
Peak Flow Rate
The capacity of drainage inlets shall be determined using the design peak flow rates.
These rates can be calculated using the methods described in Chapter 5 and the design
storm criteria specified in Chapter 2.
Bypass flow shall be limited to 01 cfs at intersections and at the project boundary.
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Structures
Catch basins, inlets and storm manholes shall conform to the standard plans of the
local jurisdiction, or the standard plans jointly published by WSDOT and APWA
(M21-O l).
Catch basins shall be used in all public and private roads unless utiliry conflicts
prohibit their use.
WSDOT/County Type 1 Catch Basins shall not be used where invert elevation depths
are more than 5 feet below lid elevations. Manholes shall be used in these situations.
Catch basins, inlets, and storm manholes shall be placed at all breaks in grade and
horizontal alignments. Pipe runs shall not exceed 300 feet for all pipe sizes.
Horizontal and vertical angle points shall not be allowed in a storm system unless a
manhole is provided for cleaning.
Grates
Herringbone grates are no longer accepted in roadway applications.
All grate inlets constructed at low points shall be combination inlets. The most
commonly used combination inlet is a vaned grate with a hooded curb cut area.
Grate inlets on grade shall have a minimum spacing of 20 feet to enable the bypass
water to reestablish its flow against the face of curb. Drainage inlets shall not be
located on the curved portion of a curb return.
Grates shall be depressed to ensure satisfactory operation; the maximum depression is
2 inches.
Inlets with larger openings may be used for additional capacity, such as WSDOT
Grate Inlet Type 2 (WSDOT Standard Plan B-4035-00) with frame and vaned grate
(WSDOT Standard Plan B-40.40-00). WSDOT Grate Inlet Type 1 and Grates A and
B shall not be used in areas of pedestrian or vehicular traffic. Refer to WSDOT
Manual and Standard Plans if any of the WSDOT inlets are proposed.
Curb Inlets
Concrete curb inlets (i.e. aprons) shall be used at the entrances to all stormwater
facilities to aid stormwater conveyance into the facility and to suppress grass growth
at the inlet.
The curb inlet shall have a 2-inch depression at the curb line and a maximum length
of 6 feet.
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At a minimum (where space constraints allow), curb inlets shall be placed at the most
upstream and downstream point along the road adjacent to the treatment or disposal
facility, regardless of the flow directed to the curb inlet. In many cases, when a long
drainage facility is proposed, and the engineering calculations support it, additional
intermediate curb inlets may be required.
Overflow structures, such as drywells or catch basins, shall be located away from the
point or points where runoff flows into the facility. When the overflow structure is
located within the facility, slopes around the structure shall be no greater than 4:1
(horizontal to vertical).
8.7.2 DRAINAGE INLET DESIGN
Grate Inlets, Continuous Grade
The capacity of an inlet on a continuous grade can be found by determining the
portion of the gutter discharge directly over the width of the inlet. On continuous
grades (assuming that the grate has the capacity to intercept the entire flow rate
directed toward it), the amount of stormwater intercepted by a grate is equal to the
amount of stormwater runoff flowing directly over the grate plus the amount that
flows in over the side of the grate through the slats/bars. The analysis shall include a
35% clogging factor. The use of formulas for side flow interception for grate inlets
found in FHWA Hydraulic Engineering Circular No. 22 (HEC-22)will be accepted.
The following procedure is most accurate when velocities are in the range of 3 to 5
feet/second at a 2% or 3% longitudinal slope. For instances where the velocity is
found to exceed 5 feet/second, additional intermediate inlets can be added,
contributing basins redefined, and the associated velocities recalculated. While
adding inlets is one solution to reducing the velocity, more information may be found
regarding the affect of side flow by consulting the HEC-22 Circular, Section 4.4
Drainage Inlet Design. Note that commercially available software may be used to
determine grate inlet capacity.
The capacity of a grate inlet on a continuous grade may be calculated using the
procedure outlined below. Figure 8-5 identifies key parameters. Example calculations
for grate inlets on a continuous grade for a uniform gutter section and a composite
gutter section are provided in Appendices 8C and 8D.
l. Determine the runoff from the contributing basin at the high point to the first
inlet. This is the amount of runoff that could be intercepted by the first inlet.
2. Select an inlet and note the grate width (GW) in the calculations (refer to
Table 8-9).
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T
d ' QBP
V� � S
x
1
GW
Figure 8-5–Typical Grate Inlet Cross-Section
TABLE 8-9
ALLOWABLE WIDTH AND PERIMETER
FOR GRATE CAPACITY ANALYSIS
Allowable W idth Allowable Perimeter
on a Continuous in a Sump Condition
Str ucture T ype G rade (feet) (feet)
Vaned Grate for Catch Basin and Inlet 1.67 —
Metal Frame and Grate for Catch Basin
and Inlet(Herringbone Pattern)i 1.67 —
Metal Frame with Hood and 1.67 3.134,s
Bi-Directional Vaned Grate
Frame and Vaned Grates for Grate Inlet 1.75�
Type 2 (WSDOT B-40.40-00) 3.503 2.964,s
1 Not recommended for new construction.Values are presented for evaluation of existing conditions.
�Normal Installation—see Figure 5-5.5 of WSDOT Hydraulics Manual
3 Rotated Installation—see Figure 5-5.5 of WSDOT Hydraulics Manual
4 This perimeter value has already been reduced by 50%for clogging.
5 This perimeter value has also been reduced far bar area.
Note: Readers should review the most current versions of the local jurisdiction's standard plans far any
revisions that may have been made to values provided in this table.
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3. Analyze the most upstream inlet. The width of flow (T) is calculated using the
procedure described in Section 8.6.2. Verify that T is within the allowable
limit (see Table 8-7), then determine the amount of flow intercepted by the
grate (basin flow—bypass flow).
4. The inlet bypass flow on a continuous grade is computed as follows:
s
(T—GW) 3
QBP = Q �T� �8-14)
Where: QBP = portion of flow outside the grate width (cfs);
Q = total flow of gutter approaching the inlet (cfs);
T = spread, calculated from the gutter section upstream
of the inlet (feet); and
GW = grate inlet width perpendicular to the direction of
flow (feet), see Table 8-9.
5. The velocity shall not exceed 5 feet/second. The velocity of flow directly over
the inlet is calculated as follows:
V� Q—QBP (8-15)
(GW)[d—0.5(GW)(Sx)]
Where: Vi = velocity over the inlet(feet/second);
SX = cross slope (feet/foot); and,
d = depth of flow at the face of the curb (feet), given by:
d = (T)(Sx) (8-16)
If the non-flooded road width does not meet the minimum criteria, an
additional inlet should be placed at an intermediate location and the procedure
repeated. If the velocity exceeds 5 feet/second then side flow shall be
considered using the method outlined in HEC-22.
6. The analysis is then repeated with the next inlet. The bypass flow (QBp) from
the previous inlet shall be added to the flow from the contributing basin to
determine the total flow (to the inlet at the station being analyzed.
7. The last inlet may require an adjustment of spacing (usually smaller spacing)
in order to prevent a bypass flow to the project boundaries.
Curb Inlets, Continuous G�ade
The capacity of a curb inlet on a continuous grade depends upon the length of
opening and the depth of flow at the opening. This depth in turn depends upon the
amount of depression of the flow line at the inlet, the cross slope, the longitudinal
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slope, and the roughness of the gutter. The analysis shall include a 35% clogging
factor.
The capacity of a curb inlet on a continuous grade may be calculated using the
procedure outlined below. Example calculations for curb inlets on a continuous grade
for a uniform gutter section and a composite gutter section are provided in
Appendices 8E and 8F.
1. Determine the runoff from the contributing basin at the high point to the first
curb inlet. This is the amount of runoff that could be intercepted by the first
curb inlet.
2. Analyze the most upstream inlet. The width of flow (T) is calculated using the
procedure described in Section 8.6.2. Verify that T is within the allowable
limit(Table 8-7).
3. The length of the curb-opening inlet required for total interception of gutter
flow is calculated as follows:
0.6
LT - �.6Q0.42SL3 � �g-17�
nse
Where: LT = curb opening length required to intercept 100% of
the flow(feet);
Se = equivalent cross slope (feet/foot);
for uniform gutter sections: Se = SX; and,
for composite gutter sections:
Se = Sx +Eo�Sw —Sx�= Sx + i2W (8-18)
where: a = gutter depression (inches);
Eo = ratio of flow in the depressed section to total gutter
flow, calculated in the gutter configuration
upstream of the inlet; and,
W = gutter width (feet).
4. When the actual curb inlet is shorter than the length required for total
interception, calculate the efficiency of the curb inlet using Equation 8-19.
i.s
E = 1— 1—� (8-19)
T
Where: E = efficiency; and,
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L = actual curb opening length (feet).
5. Compute the interception capacity of the curb inlet using the following
relationship:
Q� _ �E)�Q) �8-20)
Where: Q� = curb inlet capacity (cfs),
6. The analysis is then repeated with the next inlet. The bypass flow (QBP) from
the previous inlet shall be added to the flow from the contributing basin to
determine the total flow (Q) to the inlet at the station being analyzed.
QBP = Q—Q� (8-21)
7. The last inlet may require an adjustment of spacing (usually smaller spacing)
in order to prevent a bypass flow to the project boundaries.
Combination Inlets,Sump Conclition
Inlets in sump locations perform differently than inlets on a continuous grade. Inlets
in sump locations operate in one of two ways: 1) as a weir, at low ponding depths; or
2) as an orifice, at high ponding depths (1.4 times the grate opening length). It is very
rare that ponding on a roadway will become deep enough to force the inlet to operate
as an orifice; therefore, this section will focus on the inlet operating as a weir.
The interception capaciry of a combination inlet in a sump is equal to that of a grate
inlet alone in weir flow. Design procedures presented here are a conservative
approach to estimating the capacity of inlets in sump locations. All inlets in a sump
condition shall be evaluated using a 50% clogging factor.
The analysis shall include an evaluation of the inlet and the surrounding street, gutter,
curb and adjacent properties for storm events exceeding the required level of service.
An emergency overIlow path shall be provided.
The capacity of a combination inlet operating in a sump as a weir may be estimated
using the following procedure. There are also commercially available software
programs that will analyze combination inlets in a sump location. An example
calculation for a combination inlet in a sump location is provided in Appendix 8G.
1. Determine the runoff contributing to the combination inlet. This is the sum of
the bypassed flows from all upstream inlets and the runoff generated from the
basin contributing directly to the combination inlet.
2. Determine the allowable spread (Tall) based on the non-flooded width
requirements in Table 8-7.
3. Calculate the depth of flow at the curb (d)using Equation 8-16.
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4. Determine the average depth of flow over the grate using one of the following
relationships:
For uniform gutter sections:
daVe = d—Sx���+Y �8-22)
For composite gutter sections:
daVe = d+��Sx, — 2Sx�+ Y (8-23)
Where: y = local depression (feet), Spokane County Standard
Plans B-7 and B-18 show a 1-inch local depression
at the grate.
5. Calculate the allowable flow (Qali)using the following relationship:
3
Qall = CPd z (8-24)
Where: Qall = allowable flow based upon the maximum allowable
spread (cfs);
P = perimeter of the grate inlet (refer to Table 8-9 for
projects in Spokane Counry and the City of Spokane
Valley);
d = average depth of water across the grate (feet); and,
C = may be taken as 3.0.
6. Compare the allowable flow to the actual flow. If the actual flow is less than
the allowable flow then the combination inlet capaciry is adequate. Otherwise,
changes shall be made to the design and steps 1 through 5 repeated.
Curb Inlets,Sump Condition
The procedure below assumes that the curb inlet is operating as a weir and the depth
of flow is less than the height of the curb opening.
The capaciry of a concrete curb inlet (no grate)in a sump condition may be calculated
by the method described below. An example calculation for a curb inlet in a sump
location is provided in Appendix 8H.
1. Determine the runoff contributing to the curb inlet. This is the sum of the
bypassed flows from all upstream inlets and the runoff generated from the
basin contributing directly to the combination inlet.
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2. Determine the allowable spread (Tall) based upon the non-flooded width
requirements found in Table 8-7.
3. Calculate the depth of flow at the curb (d).
4. Calculate the allowable flow(Qall)using one of the following relationships:
For a depressed curb opening inlet:
3
QQ�, = 23�L+1.8W�d � (8-25)
Where: Qall = allowable flow based upon the maximum allowable
spread (cfs);
W = lateral width of depression (feet);
L = length of curb opening (feet); and,
d = depth of flow at the curb (feet).
For a curb opening inlet without a depression:
T�,7 3 p r�L
�a11 �.�L(�G 2 �O-GV�
5. Compare the allowable flow to the actual flow. If the actual flow is less than
the allowable flow then the curb inlet capacity is adequate. Otherwise,
changes shall be made to the design and steps 1 through 4 repeated.
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APPENDIX 8A - EXAMPLE CALCULATION :
NON -FLOODED WIDTH (UNIFORM GUTTER
SE CT I ON)
Gl VEN
• A crowned private road with a uniform gutter section (as illustrated), assuming an
equal flow rate on each side of the road.
o Flow rate(Q)=42 cfs T �
li �
o Gutter width(W)= 1.5 feet
o Road/Gutter cross slope (S� = 0.02
feet/foot
5'h
o Longitudinal slope(SL)=0.01 feet/ft
o Manning's friction coefficient,n=0.016
o Road width(RV�=30 feet
CA L CUL A TIONS
1. Calculate the spread(T) for half of the roadway using Equation 8-8.
o.��s o.��s
Q n �42��0.016�
� 0.56 Sx�67 Si�s 0.56 �0.02�i��'�0.01�0�5 —12.4 feet
2. Calculate the non-flooded width using the following relationship for crowned
roadways, and then verify that the non-flooded width is within the allowable limit
(refer to Table 8-7):
Non-flooded width = 2[(�/z)(RW) +W—T)]
= z���iz�(3o� + l.s — 12.4)]
= 8.2 feet< 12 feet FAIL*
* Table 8-7 indicates that the minimum non-flooded width is 12 feet for private roads.
Therefore, the design fails to meet the required non-flooded road width criteria. The
design will need to be altered (i.e. try an additional inlet placed at an intermediate
location, contributing basins redefined, new flow rates calculated, and the above steps
repeated).
Apii12008 Appendix 8A—Example Calculation:
Non-Flooded Width(LTnifarm Gutter Section)
8-33
SPOKANE REGIONAL STORMWATER MANUAL
THIS PAGE IS INTENTIONALLY LEFT BLANK
Apii12008 Appendix 8A—Example Calculation:
Non-Flooded Width(LTnifarm Gutter Section)
8-34
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8B - EXAMPLE CALCULATION :
NON -F L OODE D W I DT H (C OM POSI T E G UTT E R
SE CT I ON)
Gl VEN
• A super-elevated local access road with a composite gutter section (as illustrated).
o Flow rate(Q)=42 cfs T
o Gutter width(VV)= 1.5 feet W Ts
o Road cross slope (SX)=0.02 feet/foot
o Gutter cross slope(S�,)_ .081 feet/foot as
sx
o Longitudinal slope(SL)=0.01 feet/foot a�"''
'�� s.�
o Manning's friction coefficient,n=0.016 �
o Road width(RV�=30 feet
CA L CUL A TIONS
1. Assume a flow rate (QS) for that portion of the flow above the depressed gutter
section.
Assume QS = 1.4 cfs
2. Calculate QW using Equation 8-9.
Q,�, = Q—QS = 42-1.4 = 2.8cfs
3. Calculate Eo using Equation 8-11.
�. = Q—QS = QW = 2_g - 0.67
° Q Q 42
4. Calculate the spread (T)using Equation 8-12.
Sw 0.081
T = W 1+ Sx = 1.5 l+ ���� = 6.17ft
Sw E� +l �8 -1 �.�gl �.67 +1 38 -1
S. 1—E � 0.02 ��1-0.67�
x o
Apii12008 Appendix 8B—Example Calculation:
Non-Flooded Width(Composite Gutter Section)
8-35
SPOKANE REGIONAL STORMWATER MANUAL
5. Calculate TS using Equation 8-13.
TS = T—W = 6.17-1.5 = 4.67ft
6. Use Equation 8-7 to compute Q� for the calculated Ts, then compare to the estimated
QS from Step l.
0.56 S'6'�osl.z.6� 0.56 0.020�'67 O.Ol�os�4 67)z.6�
Qs(computed)= x L s = =0.31 cfs < 1.4 cfs
n 0.016
Since Q� (estimated) and Q� (computed) are not approximately equal, repeat Steps 1
through 6 until the estimated and computed Q� are numerically closer in value.
7. Assume a new QS and repeat steps 2 through 6. The following parameters are
calculated using QS =2.6 cfs.
QW = 1.6 cfs
Eo = 0.38
T = 11.68 feet
TS = 10.18 feet
QS = 2.5 cfs (computed)
QS (estimated) � Qs (computed)
Note that a spreadsheet can be set up to perform the above calculations, and
commercially available software can calculate spread in composite gutters.
8. Now that T has been found for the relationship: Qs (estimat�i�s (calculated),
calculate the non-flooded width using the following relationship for super-elevated
roadways, and then verify that the non-flooded width is within the allowable limit
(refer to Table 8-7):
Non-flooded width = RW+2W - T
= 30 +2(1.5) - 11.68
= 21.3 feet> 12 feet OK*
* Table 8-7 indicates that the minimum non-flooded width is 12 feet for local access
roads. Therefore, the design has met the required non-flooded road width criteria.
Apii12008 Appendix 8B—Example Calculation:
Non-Flooded Width(Composite Gutter Section)
8-36
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8C - EXAMPLE CALCULATION :
GRATE INLET CAPACITY (UNIFORM GUTTER
SE CT I ON)
Gl VEN
• A crowned private road with a uniform gutter section (as illustrated), assuming an
equal flow rate on each side of the road.
o Flow rate(Q)=2.5 cfs T �
li �
o Gutter width(W)= 1.5 ft
o Spokane County Type 1 Grate(Standard
Plan B-12) Grate width(GV�= 1.67 feet
5+
o Road/Gutter cross slope (SX)=0.02
feet/foot
o Longitudinal slope(SL)=0.03 feet/foot
o Manning's friction coefficient,n=0.016
o Road width(RV�=30 feet
CA L CUL A TIONS
l. Determine the runoff from the contributing basin at the high point to the first inlet;
For this example, the design flow rate (Q)is given as 2.5 cfs
2. Select an inlet and note the grate width.
For this example, the grate width (GW)is given as 1.67 ft
3. Calculate the spread (T) for half of the roadway using Equation 8-8.
0375 0375
Q n (2.5�(0.016�
� 0.56 Sx�67SL�s 0.56 �0.02�i��'�0.03�0�5 — g�3lfeet
4. Calculate the non-flooded width using the following relationship, and then verify that
the non-flooded width is within the allowable limit(refer to Table 8-7):
Non-flooded width = 2[(�/z)(RW) +W—T )]
= z���iz��3o� + l.s —s.31)]
= 16.38 feet> 12 feet OK*
'� Table 8-7 indicates that the minimum non-flooded width is 12 feet for private roads.
Therefore, design has met the required non-flooded road width criteria.
Apii12008 Appendix 8C—Example Calculation:
Grate Inlet Capacity(LTnifarm Gutter Section)
8-37
SPOKANE REGIONAL STORMWATER MANUAL
5. Calculate the inlet bypass flow using Equation 8-14:
With 35% clogging factor, grate width (GW) = 1.67(1 - 0.35)= 1.09'
s 8
QBP =Q ��� �T��� 3 z.s s.3 s.31.o913 1.�z�fs
_ � J =
Therefore the capacity of the inlet= 2.5 — 1.72= 0.78 cfs
6. Verify that the velocity does not exceed 5 feet/second. The velocity of flow directly
over the inlet is calculated using Equation 8-15 (where d= T SX):
V� (>—(>BP 2.5—1.72 —4.61ft/s < 5 feet/second OK��
(UW)[d—o.s�UW)(�'x)] 1.o9[�g.31��0.02�—o.s(1.o9X.o2)]
**Refer to Section 8.7.2 for guidance when the velocity exceeds 5 feet/second.
7. The analysis is then repeated with the next inlet. The bypass flow (QBP) from the
previous inlet shall be added to the flow from the contributing basin to determine the
total flow(Q) to the inlet at the station being analyzed.
Note that the City of Spokane requires the analysis to include a 50% clogging factor.
Apii12008 Appendix 8C—Example Calculation:
Grate Inlet Capacity(LTnifarm Gutter Section)
8-38
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8D - EXAMPLE CALCULATION :
GRATE INLET CAPACITY , CONTINUOUS
GRADE (COMPOSITE GUTTER SECTION)
Gl VEN
• A super-elevated local access road with a composite gutter section (as illustrated)
o Flow rate (Q) = 42 cfs
o Gutter width(VV)= 1.5 feet T
o Spokane County Type 1 Grate (Standard W Ts
Plan B-12) Grate Width(GV�= 1.67 feet
o Road cross slope (SX)=0.02 feet/foot as
sx
o Gutter cross slope(S�,)_ .081 feet/foot o� ,-
o Longitudinal slope(SL)=0.01 feet/foot a ' s�
o Manning's friction coefficient,n=0.016 �
o Road width(RV�=30 feet
CA L CUL A TIONS
l. Determine the runoff from the contributing basin at the high point to the first inlet;
For this example, the design flow rate is given as 4.2 cfs
2. Select an inlet and note the grate width.
For this example, the grate width (GW)is given as 1.67 feet
3. Calculate the spread (T) for half of the roadway using the method outlined in
Appendix 8B and verify that the non-flooded width is within the allowable limit
(Table 8-7).
T = 11.68 feet
(Solution from Appendix 8B)
Non-flooded width = 21.3 feet> 12 feet OK�
(Solution from Appendix 8B)
'� Table 8-7 indicates that the minimum non-flooded width is 12 feet for private roads.
Therefore, design has met the required non-flooded road width criteria.
4. Calculate the inlet bypass flow using Equation 8-14:
With 35% clogging factor, grate width (GW) = 1.67(1 - 0.35)= 1.09'
Apii12008 Appendix 8D—Example Calculation:Grate Inlet Capacity,
Continuous Grade(Composite Gutter Section)
8-39
SPOKANE REGIONAL STORMWATER MANUAL
8 g
QBP =Q �T� ���W� � —42 11.6s.6s.o913 3.23�fs
— � J =
Therefore the capacity of the inlet= 42—3.23 = 0.97 cfs
5. Verify that the velocity does not exceed 5 feet/second. The velocity of flow directly
over the inlet is calculated using Equation 8-15:
V _ Q—QBP _ 42—323 —4.00ft/s < 5 feet/second OK
�GW�[a—o.s�Gw)(sx)] 1.09[�l 1.6s��o.o2�—o.s(1.o9)(0.02)]
6. The analysis is then repeated with the next inlet. The bypass flow (QBP) from the
previous inlet shall be added to the flow from the contributing basin to determine the
total flow(Q) to the inlet at the station being analyzed.
Apii12008 Appendix 8D—Example Calculation:Grate Inlet Capacity,
Continuous Grade(Composite Gutter Section)
8-40
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8E - EXAMPLE CALCULATION :
CUR B I NL ET CAPACITY , CONTI NUOUS GRADE
(UNIFORM GUTTER SECTION)
Gl VEN
• A crowned private road with a uniform gutter section (as illustrated), assuming an
equal flow rate on each side of the road.
o Flow rate(Q)= 1.5 cfs T �
li �
o Gutter width(W)= 1.5 feet
o Curb Inlet Length(L)=3 feet
o Road/Gutter cross slope (S� = 0.02 5.�
feet/foot
o Longitudinal slope(SL)=0.03 feet/foot
o Manning's friction coefficient,n=0.016
o Road width(RV�=30 feet
CA L CUL A TIONS
l. Determine the runoff from the contributing basin at the high point to the first inlet;
For this example, the design flow rate is given as 1.5 cfs
2. Calculate the spread (T) for half of the roadway using Equation 8-8 and verify that
the non-flooded width is within the allowable limit (Table 8-7).
0.375 0375
Q n (1.5�(0.016�
T 0.56 Sx b'Si s 0.56 �0.02�167�0.03�°5 —6.86feet
Non-flooded width = 2[(�/z)(RW) +W—T )]
= z���iz��3o� + l.s —6.s6)]
= 19.3 feet> 12 feet OK*
'� Table 8-7 indicates that the minimum non-flooded width is 12 feet for private roads.
Therefore, design has met the required non-flooded road width criteria.
3. Calculate the length of curb inlet required for total interception of gutter flow using
Equation 8-17:
Apii12008 Appendix 8E—Example Calculation:Grate Inlet Capacity,
Continuous Grade(LTniform Gutter Section)
8-41
SPOKANE REGIONAL STORMWATER MANUAL
0.6 0.6
LT = 0.6Qo.4zSi�3 1 =(0.6)(1.5o.4z)(0.0303) 1 =31.lfeet
nSe �.�16'��.�2
4. Calculate the efficiency of the curb inlet using Equation 8-19.
i.s i.s
E =1— 1—L =1— 1— 3.0 = 0.167
LT 31.1
5. Compute the interception capacity and the bypass flow of the curb inlet using
Equations 8-20 and 8-21.
Q� _(E)(Q)_(0.16���1.s�=o.2s�fs
QBP =Q—Q� =1.5 —0.25 =125cfs
6. The analysis is then repeated with the next curb inlet. The bypass flow (QBP) from the
previous inlet shall be added to the flow from the contributing basin to determine the
total flow(Q) to the next inlet.
Note that the City of Spokane requires the analysis to include a 50% clogging factor.
Apii12008 Appendix 8E—Example Calculation:Grate Inlet Capacity,
Continuous Grade(LTniform Gutter Section)
8-42
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8F - EXAMPLE CALCULATION :
CUR B I NL ET CAPACITY , CONTI NUOUS GRADE
(COMPOSITE GUTTER SECTION)
Gl VEN
• A super-elevated local access road with a composite gutter section (as illustrated)
o Flow rate (Q) = 42 cfs
o Gutter width(VV)= 1.5 feet T
o Curb Inlet Width(GV�=3 feet W Ts
o Road cross slope (SX)=0.02 feet/foot
o Gutter cross slope(S�,)_ .081 feet/foot as
sx
o Longitudinal slope(SL)=0.01 feet/foot a�"'"�
o Manning's friction coefficient,n=0.016 a s�
o Road width(RV�=30 feet �
CA L CUL A TIONS
l. Determine the runoff from the contributing basin at the high point to the first inlet;
For this example, the design flow rate is given as 4.2 cfs
2. Calculate the spread (T) for half of the roadway using the method outlined in
Appendix 8B and verify that the non-flooded width is within the allowable limit
(Table 8-7).
T = 11.68 feet
(Solution from Appendix 8B)
Non-flooded width = 21.3 feet> 12 feet OK�
(Solution from Appendix 8B)
* Table 8-7 indicates that the minimum non-flooded width is 12 feet for private roads.
Therefore, design has met the required non-flooded road width criteria.
3. Calculate the equivalent cross slope (Se) using Equation 8-18 and the length of curb
inlet required for total interception of gutter flow (LT)using Equation 8-17.
Se = Sx +Eo�Sw —Sx�= 0.02+0.38(0.081—0.02) = 0.043
Where, Eo = 0.38 (Solution from Appendix 8B)
Apii12008 Appendix 8F—Example Calculation:Curb Inlet Capacity,
Continuous Grade(Composite Gutter Section)
8-43
SPOKANE REGIONAL STORMWATER MANUAL
0.6 0.6
LT = 0.6Qo.4zSi�3 1 = (0.6)(4.2o.4z)(O.Olo 3) 1 = 21.8feet
nSe (0.016)(0.043)
4. Calculate the efficiency of the curb inlet using Equation 8-19.
i.s i.s
E = 1— 1—L =1— 1— 3.0 = 0.23
LT 21.8
5. Compute the interception capacity and the bypass flow of the curb inlet using
Equations 8-20 and 8-21.
Q� _ (E)(Q) _ (0.23��4.2� = o.9��fs
QBP = Q—Q� = 4.2—0.97 = 323cfs
6. The analysis is then repeated with the next curb inlet. The bypass flow (QBP) from the
previous inlet shall be added to the flow from the contributing basin to determine the
total flow(Q) to the next inlet.
Note that the City of Spokane requires the analysis to include a 50% clogging factor.
Apii12008 Appendix 8F—Example Calculation:Curb Inlet Capacity,
Continuous Grade(Composite Gutter Section)
8-44
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8G - EXAMPLE CALCULATION :
COMBINATION INLET CAPACITY , SUMP
Gl VEN
• A crowned private road with a uniform gutter section (as illustrated).
o Inlet: Metal Frame with Hood,Type 2 and T �
Bi-Directional Vaned Grate,Type 3 — �
Spokane County Standard Plans B-11 and o
B-14
o Gutter Width(W)= 1.5 feet 5.�
o Local depression= 1 inch
o Cross slope(S�=0.02 feet/foot
o Road width(RV�=30 feet
o Qgp=0.68 cfs=Upstream inlets total
bypass flow rate
� QBASIN—0.82 cfs=Contributing
drainage basin direct flow rate
CA L CUL A TIONS
l. Determine the total runoff contributing and bypassed to the combination inlet.
QrorAi — QaP +QBasrv = 0.68cfs+0.82cfs = 1.Scfs
2. From Table 8-7, the non-flooded width for a private road is 12 feet minimum.
Determine the allowable spread (T) for the roadway using the following relationship
for a crowned roadway:
T _ RW+2W—Non-flooded Width _ 30+(2)(1.5)—12 =10.5 feet
aii — 2 2
3. Calculate the depth of flow at the curb (d)using Equation 8-16.
a = (T)(sx) _ (lo.s��o.o2� = 0.21 feet
4. Determine the average depth of flow over the grate using Equation 8-22.
daVe = d—S����+y = 021—0.02�1��+�� = 0.28 feet
5. Calculate the allowable flow(Qall)using Equation 8-24.
Apri12008 Appendix 8G—Example Calculation:Combination Inlet Capacity,Sump
8-45
SPOKANE REGIONAL STORMWATER MANUAL
Q��� = CPd 3� _ (3.0)(3.13)�0.28�3� =138 cfs
6. Compare the allowable flow to the actual flow.
138 cfs(QQ����1.5 cfs�Q� FAIL*
* The actual flow rate directed at the given metal frame and grate inlet combination
exceeds the calculated allowable flow capacity of the structure. The design will need
to be altered (i.e. try an additional inlet placed at an intermediate location,
contributing basins redefined, new flow rates calculated, and the above steps
repeated).
Note that grate perimeter used in this example includes a 50% clogging factor (refer to
Table 8-9).
Apri12008 Appendix 8G—Example Calculation:Combination Inlet Capacity,Sump
8-46
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8H - EXAMPLE CALCULATION :
CURB INLET CAPACITY , SUMP
Gl VEN
• A crowned private road with a composite gutter section (as illustrated).
o Curb opening length(L)=3.0 feet(reduce T
by half—clogging safety factor) w Ts
o Local depression= 1 inch
o Cross slope(S�=0.02 feet/foot as
sx
o Gutter cross slope(S�,)=0.081 feet/foot o�,_-�
o Gutter Width= 1.5 feet a �� s�
o Road width(RV�=30 feet �
o Qgp=0.68 cfs=Upstream inlets total
bypass
� QBASIN—0.82 cfs=Contributing
drainage basin direct flow rate
CA L CUL A TIONS
1. Determine the total runoff contributing and bypassed to the curb inlet.
QrorAi — QsP +QBASrN = 0.68cfs+0.82cfs = 1.Scfs
2. From Table 8-7, the non-flooded width for a private road is 12 feet minimum.
Determine the allowable spread (T) for the roadway using the following relationship
for crowned roadways:
T _ RW+2W—Non-flooded Width _ 30+(2)(1.5)—12 =10.5 feet
°'i � 2 2
3. Calculate the depth of flow at the curb (d).
a=�l.s��o.osl�+�lo.s —l.s��o.o2�= 0.3o feet
4. Calculate the allowable flow(Qall)using Equation 8-25.
Qa�� =2.3(L +l.sw)a� =2.3�(l.s +�l.s��l.$)](o.30)� =1.s9�fs
5. Compare the allowable flow to the actual flow.
Apri12008 Appendix 8H—Example Calculation:Curb Inlet Capacity,Sump
8-47
SPOKANE REGIONAL STORMWATER MANUAL
1.59cfs�QQ���>1.5 cfs�Q� OK*
* The actual flow rate directed at the curb inlet is less than the calculated allowable
flow capacity of the structure. The design is adequate.
Apri12008 Appendix 8H—Example Calculation:Curb Inlet Capacity,Sump
8-48
SPOKANE REGIONAL STORMWATER MANUAL
CHAPTER 9 - EROSION AND SEDIMENT
CONTROL DESIGN
v �
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.�` y
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Chapter Organization
9.1 Introduction......................................................................................................................9-1
9.2 Applicability ....................................................................................................................9-1
9.3 Exemptions ......................................................................................................................9-2
9.4 Erosion and Sediment Control (ESC) Plan......................................................................9-2
9.4.1 Introduction..............................................................................................................9-2
9.42 Minimum Requirements for ESC Plans...................................................................9-3
9.43 Best Management Practices for ESC Plans .............................................................9-3
9.4.4 Modification to ESC Plans.....................................................................................9-11
9.5 Additional Information Regarding ESC Plans...............................................................9-11
9.5.1 Performance Standards ..........................................................................................9-ll
9.52 Maintenance Responsibility...................................................................................9-12
9.53 Enforcement and Appeals Process.........................................................................9-12
Appendix 9A —ESC Standard Plan Notes.................................................................................9-13
Apri12008 Chapter 9—Erosion and Sediment Control Design
9-i
SPOKANE REGIONAL STORMWATER MANUAL
THIS PAGE IS INTENTIONALLY LEFT BLANK
Apri12008 Chapter 9—Erosion and Sediment Control Design
9-ii
SPOKANE REGIONAL STORMWATER MANUAL
9.1 INTRODUCTION
This chapter lists the steps for developing a Erosion and Sediment Control (ESC) plan,
which is a required component of a Drainage Submittal or permit. Controlling erosion
and preventing sediment and other pollutants from leaving the project site during
construction can be achieved by implementing the best management practices (BMPs)
identified in this chapter. The ESC plan shall outline specific construction BMPs for a
project site to a�oid adverse stormwater impacts from construction activities on water
resources, roads, drainage facilities, surrounding properties and other improvements.
Potential impacts due to erosion and sedimentation include:
• Sediment accumulation in culverts, storm drains and open channels, resulting in
decreased capacities and the potential for increased flooding and increased
maintenance frequency;
• Sedimentation of storage ponds and swales, resulting in decreased infiltrative and
storage capacity, and the potential for increased flooding and failure;
• Clogging and failure of Underground Injection Control (UIC) facilities;
• Destruction of vegetation, topsoil and seeds, making re-establishment of
vegetation difficult;
• Increased turbidity, reducing water quality in water bodies; and,
• Air pollution due to fugitive dust.
Implementation of an effective ESC plan may help to reduce these potential impacts as
well as other unforeseen environmental impacts and associated costs.
Although the construction phase of a project is usually considered a temporary condition,
construction work may take place over several seasons. All BMPs used in the course of
construction should be of sufficient size, strength, and durability to readily outlast the
expected construction schedule and operate properly during the design storm rainfall
conditions (see Basic Requirement No. 6 in Chapter 2 for design storm criteria).
9.2 APPLICABILITY
Land-disturbing activities are activities that result in a change in existing soil cover
(vegetative or non-vegetative) or site topography. Land-disturbing activities include, but
are not limited to, demolition, construction, clearing and grubbing, grading and logging.
The following land-disturbing activities require an ESC plan:
• Major land-disturbing activities involving 1 acre or more of disturbed area; or,
• Minor land-disturbing activities, such as grading, involving less than 1 acre of
disturbed area but requiring a permit by the local jurisdiction.
Apri12008 Chapter 9—Erosion and Sediment Cont��ol Design
9-1
SPOKANE REGIONAL STORMWATER MANUAL
An ESC plan, when required, shall be submitted with either the road and drainage plans
or the permit application, prior to any land-disturbing activity. Clearing and grading
activities for developments will be permitted only if conducted pursuant to an accepted
site development plan that establishes permitted areas of clearing, grading, cutting, and
filling. When establishing these permitted clearing and grading areas, consideration shall
be given to minimizing removal of existing trees and minimizing disturbance and
compaction of native soils except as needed for building purposes. These permitted
clearing and grading areas and any other areas with a preservation requirement, such as
critical or sensitive areas, buffers, native growth protection easement areas or tree
retention areas, shall be delineated on the site plans and development site plan. ESC plans
are only required to address the area of land that is subject to the land-disturbing activity
for which a permit is being requested and the area of land that will serve as the stockpile
or staging area for materials.
9.3 EXEMPTIONS
An ESC plan is typically not required for the following activities:
• Commercial agriculture as regulated under RCW Chapter 84.34.020;
• Forest practices regulated under WAC Title 222, except for Class IV General
Forest Practices that are conversions from timberland to other uses;
• Actions by a public utiliry or any other governmental agency to remove or
alleviate an emergency condition, restore utility service, or reopen a public
thoroughfare to traffic;
• Land divisions, interior improvements to an existing structure, and other activities
requiring permits or approvals for which there is no physical disturbance to the
surface of the land; and,
• Minor land-disturbing activities that do not require a permit.
Although an ESC plan may not be required for the situations noted above, that does not
relieve the proponent from the responsibility of controlling erosion and sediment during
construction nor the liability of damage claims associated with adverse impacts on off-
site properties.
9.4 EROSION AND SEDIMENT CONTROL (ESC ) PLAN
9.4.1 INTRODUCTION
The ESC plan must be prepared by a professional engineer currently licensed in the
State of Washington with a good working knowledge of hydrology and ESC
practices, or a Certified Erosion and Sedimentation Control Technician. A copy of the
Apri12008 Chapter 9—Erosion and Sediment Control Design
9-2
SPOKANE REGIONAL STORMWATER MANUAL
ESC plan must be located on the construction site or within reasonable access to the
site. As site construction progresses, the ESC plan may require modification to reflect
changes in site conditions.
The ESC plan must accompany the road and drainage plans, grading plan, or permit
request and should be integrated into the grading plan whenever possible. It must
contain sufficient information to demonstrate to the local jurisdiction that potential
problems associated with erosion, sediment, and pollution have been adequately
addressed for the proposed project. The drawings and notes should be clear and
concise and describe when and where each BMP is to be implemented.
9.4.2 MINIMUM REQUIREMENTS FOR ESC PLANS
At a minimum, all ESC plans must be legible, reproducible and on good quality 24" x
36" bond paper, and must contain the following information:
• Title block, north arrow, scale and plan preparation date;
• Name of property owner, permit applicant, anticipated contact person on-
site, and the stamp and signature of the engineer who prepared the plan
(note that for municipal projects, this information will be not be available
until the pre-construction conference);
• Vicinity map, section, township and range, project address, project
boundaries and dimensions;
• Description of project, list of on-site soils and existing vegetation, location
of any existing water bodies and/or critical areas;
• Summary description of ESC BMPs utilized (see Section 9.4.3);
• ESC Standard Plan Notes (see Appendix 9A);
• Construction Sequence (see Section 9.4.3)
9.4.3 BEST MANAGEMENT PRACTICES FOR ESC PLANS
BMPs must be used to comply with the requirements of this chapter. It is not the
intent of this chapter to limit innovative or creative efforts to effectively control
erosion and sedimentation. Experimental ESC management practices to improve
erosion control technology and meet the purpose and intent of this chapter are
encouraged as a means of solving erosion and sedimentation problems. Minor
modifications to standard BMPs are considered experimental ESC management
practices and, as with any proposed BMP, must be reviewed and accepted by the local
jurisdiction. It is important to note that not only do new facilities and off-site
properties need to be protected from erosion and sedimentation, but existing facilities
on-site or downstream also need to be evaluated and protected if there is potential for
damage due to lack of erosion control.
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As the season and subsequent site conditions dictate, alterations to existing ESC
BMPs may be warranted or additional ESC measures may be required. Note that
items below that are shown in italics are considered General Erosion and
Sedimentation Control Notes (see Appendix 9A for complete list). These notes shall
be shown on the ESC plan, when applicable to the given project site.
BMPs are referenced in this chapter by their identification code in the September
2004 Stormwater Management Manual for Eastern Washington (e.g., BMP C101,
BMP C 102). Detailed examples and descriptions of these BMPs are included in
Chapter 7 of the Eastern Washington manual. At a minimum, the following items
shall be addressed in the ESC plan:
1. Construction Sequence
• The following construction sequence shall be followed in order to best
minimize the potential for erosion and sedimentation control problems:
a) Clear and grub sufficiently for installation of temporary ESC BMPs;
b) Install temporary ESC BMPs; constructing sediment trapping BMPs
as one of the first steps prior to grading;
c) Clear, grub and rough grade for roads, temporary access points and
utility locations;
d) Stabilize roadway approaches and temporary access points with the
appropriate construction entry BMP;
e) Clear, grub and grade individual lots or groups of lots;
� Temporarily stabilize, through re-vegetation or other appropriate
BMPS, lots or groups of lots in situations where substantial cut or fill
slopes are a result of the site grading;
g) Construct roads, buildings,permanent stormwater facilities (i.e. inlets,
ponds, UIC facilities, etc.);
h) Protect all permanent stormwater facilities utilizing the appropriate
BMPs;
i) Install permanent ESC controls, when applicable; and,
j) Remove temporary ESC controls when:
♦ Permanent ESC contr�ols, when applicable, have been
completely installed;
♦ All land-disturbing activities that have the potential to cause
erosion or sedimentation problems have ceased; and,
♦ i�egetation had been established in the areas noted as
requiring vegetation on the accepted ESC plan on file with the
local jurisdiction.
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2. Clearing Limits
• Distinctly mark all clearing limits, both on the plans and in the field—
taking precaution to visibly mark separately any sensitive or critical areas,
and their buffers, and trees that are to be preserved prior to beginning
any land-disturbing activities, including clearing and grubbing; and,
• If clearing and grubbing has occurred, there is a window of 15 days in
which construction activiry must begin, otherwise the cleared area must be
stabilized.
• Suggested BMPs:
o BMP C 101: Preserving Natural Vegetation
o BMP C 102: Buffer Zones
o BMP C103: High Visibility Plastic or Metal Fence
o BMP C 104: Stake and Wire Fence
3. ConstNUCtion Access Route
• Limit access for construction vehicles to one route whenever possible;
• Stabilize the construction access route with quarry spalls or crushed rock
to minimize the tracking of sediment onto roadways;
• Inspect all roadways, at the end of each day, adjacent to the construction
access route. If it is evident that sediment has been tr°acked offsite and/or
beyond the roadway approach, removal and cleaning is required.
• If sediment removal is necessary prior to street washing, it shall be
removed by shoveling or pickup sweeping and transported to a controlled
sediment disposal area.
• If street washing is required to clean sediment tracked offsite, once
sediment has been removed, street wash wastewater shall be controlled by
pumping back on-site or othenvise prevented from discharging into
systems tributary to waters of the state;
• Locate wheel washes or tire baths, if applicable to ESC plan, on site.
Dispose of wastewater into a separate temporary on-site treatment facility
in a location other than where a permanent stormwater facility is
proposed; and,
• Restore construction access route equal to or better than the pre-
construction condition.
• Suggested BMPs:
o BMP C 105: Stabilized Construction Entrance
o BMP C106: Wheel Wash
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o BMP C107: Construction Road/Parking Area Stabilization
4. Install Sediment Cont�ols
• Retain the duff layer, native topsoil, and natural vegetation in an
undisturbed state to the maximum extent practical;
• Pass stormwater runoff from disturbed areas through a sediment pond
prior to leaving a construction site or discharging to an infiltration facility,
• Keep sediment on the project site, to the maximum extent practical, in
order to protect adjacent properties, water bodies, and roadways;
• Stabilize earthen structures such as dams, dikes, and diversions with either
quarry spalls, seed or mulch, or a combination thereof;
• Locate sediment facilities such that they will not interfere with natural
drainage channels or streams; and,
• Inspect sediment control BMPs weekly at a minimum, daily during a storm
event, and after any discharge from the site (stormwater or non-
stormwater). The inspection frequency may be reduced to once a month if
the site is stabilized and inactive.
• Suggested BMPs:
o BMP C230: Straw Bale Barrier
o BMP C231: Brush Barrier
o BMP C232: Gravel Filter Berm
o BMP C233: Silt Fence
o BMP C234: Vegetated Strip
o BMP C235: Straw Wattles
o BMP C240: Sediment Trap
o BMP C241: Temporary Sediment Pond
5. Soil Stabilization
• Select appropriate BMPs to protect the soil from the erosive forces of
raindrop impact, flowing water and wind, taking into account the expected
construction season, site conditions and estimated duration of use;
• Control fugitive dust from construction activity in accordance with state
and local air quality control authorities with jurisdiction over the project
area;
• Stabilize exposed unworked soils (including stockpiles), whether at final
grade or not, within 10 days during the regional dry season (July 1
through September 30) and within 5 days during the regional wet season
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(October 1 through June 30). Soils must be stabilized at the end of a shift
before a holiday weekend if needed based on the weather forecast. This
time limit may only be adjusted by a local jurisdiction with a "Qualified
Local Program, " if it can be demonstrated that the recent precipitation
justifies a different standard and meets the requirements set fourth in the
Construction S'tormwater General Permit; and,
• Stabilization practices include, but are not limited to, temporary and
permanent seeding, sodding, mulching, plastic covering, erosion control
fabric and mats, soil application of polyacrylamide (PAM) and the early
application of gravel base on areas to be paved, and dust control.
• Suggested BMPs:
o BMP C 120: Temporary and Permanent Seeding
o BMP C 121: Mulching
o BMP C 122: Nets and Blankets
o BMP C 123: Plastic Covering
o BMP C 124: Sodding
o BMP C125: Topsoiling
o BMP C126: Polyacrylamide (PAM) for Soil Erosion Protection
o BMP C 130: Surface Roughening
o BMP C 131: Gradient Terraces
o BMP C140: Dust Control
6. Protection of Inlets
• Protect inlets, drywells, catch basins and other stormwater management
facilities from sediment, whether or not facilities are operable, so that
stormwater runoff does not enter the conveyance system (both on and off
site)without being treated or filtered to remove sediment;
• Keep roads adjacent to inlets clean; sediment and street wash water shall
not be allowed to enter the conveyance system (both on and offsite)
without prior treatment;
• Inspect inlets weekly at a minimum and daily during storm events. Inlet
protection devices shall be cleaned or removed and replaced before
6 inches of sediment can accumulate.
• Suggested BMP:
o BMP C220: Storm Drain Inlet Protection
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7. Runoff from Construction Sites
• Protect down-gradient properties, waterways, and stormwater facilities
from possible impacts due to increased flow rates, volumes, and velocities
of stormwater runoff from the project site that may temporarily occur
during construction;
• Construct stormwater control facilities (detention/retention storage pond
or swales) before grading begins. These facilities shall be operational
before the construction of impervious site improvements; and,
• Protect permanent infiltration ponds that are used for flow control during
construction.
• Suggested BMPs:
o BMP C240: Sediment Trap
o BMP C241: Temporary Sediment Pond
8. Washout Site for Concrete Trucks and Equipment
• Designate the location of a slurry pit where concrete trucks and equipment
can be washed out. Slurry pits are not to be located in or upstream of a
swale, drainage area, stormwater facility or water body, or in an area
where a stormwater facility is existing or proposed.
• Suggested BMP:
o BMP C151: Concrete Handling
9. Mate�ial Sto�age/Stockpile
• Identify locations for storage/stockpile areas, within the proposed ESC
plan boundaries, for any soil, earthen and landscape material that is used
or will be used on-site;
• Stockpile materials (such as topsoil) on-site, keeping off roadway and
sidewalks; and,
• Maintain on-site, as feasible, items such as gravel and a roll of plastic, for
emergency soil stabilization during a heavy rain event, or for emergency
berm construction.
• Suggested BMP:
o BMP C 150: Materials On Hand
10. Cut and Fill Slopes
• Consider soil type and its erosive properties;
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• Divert any off-site stormwater run-on or groundwater away from slopes
and disturbed areas with interceptor dikes, pipes or temporary swales. Off-
site stormwater shall be managed separately from stormwater generated
on-site;
• Reduce slope runoff velocities by reducing the continuous length of slope
with terracing and diversion, and roughening the slope surface;
• Place check dams at regular intervals within ditches and trenches that are
cut into a slope; and,
• Stabilize soils on slopes,where appropriate.
• Suggested BMPs:
o BMP C120: Temporary and Permanent Seeding
o BMP C 130: Surface Roughening
o BMP C 131: Gradient Terraces
o BMP C200: Interceptor Dike and Swale
o BMP C201: Grass-Lined Channels
o BMP C204: Pipe Slope Drains
o BMP C205: Subsurface Drains
o BMP C206: Level Spreader
o BMP C207: Check Dams
o BMP C208: Triangular Silt Dike (Geotextile-Encased Check Dam)
I1. Stabilization of Temporary Conveyance Channels and Outlets
• Design, construct and stabilize all temporary on-site conveyance channels
to prevent erosion from the expected flow velociry of a 2-year, NRCS
Type II, 24-hour frequency storm or 2-year Rational Method event, in the
post-developed condition; and,
• Stabilize outlets of all conveyance systems adequately to prevent erosion
of outlets, adjacent streambanks, slopes and downstream reaches.
• Suggested BMPs:
o BMP C202: Channel Lining
o BMP C209: Outlet Protection
12. Dewatering Const�uction Site
• Discharge any effluent of dewatering operations that has similar
characteristics to stormwater runoff at the site, such as foundation, vault,
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and trench dewatering, into a controlled system prior to discharge into a
sediment trap or sediment pond; and,
• Handle highly turbid or otherwise contaminated dewatering effluent, such
as from a concrete pour, construction equipment operation, or work inside
a coffer dam, separately from stormwater disposed of on-site.
• Consider other disposal options such as:
o infiltration;
o transportation off site for legal disposal in a way that does not pollute;
o treatment and disposal on-site with chemicals or other technologies;
and,
13. Cont�ol of Pollutants Othe� Than Sediment on Construction Sites
• Control on-site pollutants, such as waste materials and demolition debris,
in a way that does not cause contamination of stormwater or groundwater.
Woody debris may be chopped or mulched and spread on-site;
• Cover, contain and protect all chemicals, liquid products, petroleum
products, and non-inert wastes present on-site from vandalism (see
Chapter 173-304 WAC for the definition of inert waste), use secondary
containment for on-site fueling tanks;
• Conduct maintenance and repair of heavy equipment and vehicles
involving oil changes, hydraulic system repairs, solvent and de-greasing
operations, fuel tank drain down and removal, and other activities that
may result in discharge or spillage of pollutants to the ground or into
stormwater runoff using spill prevention measures, such as drip pans.
Clean all contaminated surfaces immediately following any discharge or
spill incident. If raining,perform on-site emergency repairs on vehicles or
equipment using temporary plastic over and beneath the vehicle;
• Conduct application of agricultural chemicals, including fertilizers and
pesticides, in such a manner, and at application rates, that inhibits the
loss of chemicals into stormwater runoff facilities. Amend manufacturer's
recommended application rates and procedures to meet this requirement,
if necessary; and,
• Locate pH-modifying sources, such as bulk cement, cement kiln dust, fly
ash, new concrete washing and curing waters, waste streams generated
from concrete grinding and cutting, exposed aggregate processes, and
concrete pumping and mixer washout waters, downstream and away from
any stormwater facilities or location of proposed stormwater facilities.
Adjust pH if necessary to prevent violations of water quality standards.
Obtain approval from Ecology for using chemicals other than liquid COz
or dry ice to adjust pH.
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• Suggested BMPs:
o See also Chapter 10 —Source Control
o BMP C151: Concrete Handling
o BMP C 152: Sawcutting and Surfacing Pollution Prevention
14. Permanent BMPs
• Include permanent BMPs, if necessary, in the ESC plan to ensure the
successful transition from temporary BMPs to permanent BMPs; and,
• Restore and rehabilitate temporary BMPs that are proposed to remain in
place after construction as permanent BMPs.
I5. Maintenance of BMPs
• Inspect on a regular basis (at a minimum weekly, and daily during/after a
runoff producing storm event) and maintain all ESC BMPs to ensure
successful performance of the BMPs. Conduct maintenance and repair in
accordance with individual ESC BMPs outlined in this section; and,
• Remove temporary ESC BMPs within 30 days after they are no longer
needed. Permanently stabilize areas that are disturbed during the removal
process.
9.4.4 MODIFICATION TO ESC PLANS
ESC plans may be modified after submittal to the reviewing agency. An amended
plan shall be submitted to illustrate any modifications to the methods used to prevent
and control erosion and sedimentation.
9.5 ADDITIONAL INFORMATION REGARDING ESC PLANS
9.5.1 PERFORMANCE STANDARDS
The following performance standards represent a minimum threshold for controlling
soil erosion and sedimentation caused by land-disturbing activities and will be used to
determine if the requirements of this chapter have been met:
L Minimize Tracking onto Roadways
This performance standard has not been met if soil, dirt, mud or debris is visibly
tracked onto the road area and a reasonable attempt to control it through the use of
ESC BMPs is not evident.
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2. PNOtection of Ro�dways, Properties and Sto�mw�ter Facilities
This performance standard has not been met if there is visible downstream deposition
of soil, dirt, mud or debris, originating from the project site, on adjacent or down-
gradient roads,properties or stormwater systems.
3. Proper Washout of Concrete Trucks and Ec�uipment
This performance standard has not been met if there is observation or evidence of
concrete washout outside the area designated for concrete washout on the accepted
ESC plan.
4. Protection of Water Bodies, Streams and Wetlands
This performance standard has not been met if there is obvious turbidity or deposition
of soil, dirt, mud, or debris from the project site into adjacent water bodies or into
sensitive or critical areas and their buffers. In addition, the performance standard
requires that no construction activity, material or equipment encroach into sensitive or
critical areas.
9.5.2 MAINTENANCE RESPONSIBILITY
The proponent is responsible to ensure that BMPs are used, maintained, and repaired
so that the performance standards continue to be met. After all land-disturbing
activiry is complete and the site has been permanently stabilized, maintenance and the
prevention of erosion and sedimentation is the responsibility of the property owner.
Special criteria regarding the degradation of water resources are found in the
Washington Administrative Code of various state agencies such as the Departments
of Ecology, Natural Resources, and Fish and Wildlife.
9.5.3 ENFORCEMENT AND APPEALS PROCESS
Review the local jurisdiction's code to determine the enforcement and appeal
processes for violation of the above performance standards.
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APPENDIX 9A - ESC STANDARD PLAN NOTES
The following ESC Standard Plan Notes originate from Section 9.4.3. These notes are an
overall set; use only what applies to the given project.
l. The following construction sequence shall be followed in order to best minimize the
potential for erosion and sedimentation control problems:
(a) Clear and grub sufficiently for installation of temporary ESC BMPs;
(b) Install temporary ESC BMPs, constructing sediment trapping BMPs as one of the
first steps prior to grading;
(c) Clear, grub and rough grade for roads, temporary access points and utiliry
locations;
(d) Stabilize roadway approaches and temporary access points with the appropriate
construction entry BMP;
(e) Clear, grub and grade individual lots or groups of lots;
(f) Temporarily stabilize, through re-vegetation or other appropriate BMPS, lots or
groups of lots in situations where substantial cut or fill slopes are a result of the
site grading;
(g) Construct roads, buildings, permanent stormwater facilities (i.e. inlets, ponds,
UIC facilities, etc.);
(h) Protect all permanent stormwater facilities utilizing the appropriate BMPs;
(i) Install permanent ESC controls, when applicable; and,
(j) Remove temporary ESC controls when:
2. Permanent ESC controls, when applicable, have been completely installed;
3. All land-disturbing activities that have the potential to cause erosion or sedimentation
problems have ceased; and,
4. Vegetation had been established in the areas noted as requiring vegetation on the
accepted ESC plan on file with the local jurisdiction.
5. Inspect all roadways, at the end of each day, adjacent to the construction access route. If
it is evident that sediment has been tracked off site and/or beyond the roadway approach,
cleaning is required.
6. If sediment removal is necessary prior to street washing, it shall be removed by shoveling
or pickup sweeping and transported to a controlled sediment disposal area.
7. If street washing is required to clean sediment tracked off site, once sediment has been
removed, street wash wastewater shall be controlled by pumping back on-site or
otherwise prevented from discharging into systems tributary to waters of the state.
8. Restore construction access route equal to or better than the pre-construction condition.
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SPOKANE REGIONAL STORMWATER MANUAL
9. Retain the duff layer, native topsoil, and natural vegetation in an undisturbed state to the
maximum extent practical.
10. Inspect sediment control BMPs weekly at a minimum, daily during a storm event, and
after any discharge from the site (stormwater or non-stormwater). The inspection
frequency may be reduced to once a month if the site is stabilized and inactive.
1l. Control fugitive dust from construction activiry in accordance with the state and/or local
air quality control authorities with jurisdiction over the project area.
12. Stabilize exposed unworked soils (including stockpiles), whether at final grade or not,
within 10 days during the regional dry season (July 1 through September 30) and within 5
days during the regional wet season (October 1 through June 30). Soils must be stabilized
at the end of a shift before a holiday weekend if needed based on the weather forecast.
This time limit may only be adjusted by a local jurisdiction with a "Qualified Local
Program," if it can be demonstrated that the recent precipitation justifies a different
standard and meets the requirements set fourth in the Construction Stormwater General
Permit.
13. Protect inlets, drywells, catch basins and other stormwater management facilities from
sediment, whether or not facilities are operable.
14. Keep roads adjacent to inlets clean.
15. Inspect inlets weekly at a minimum and daily during storm events.
16. Construct stormwater control facilities (detention/retention storage pond or swales)
before grading begins. These facilities shall be operational before the construction of
impervious site improvements.
17. Stockpile materials (such as topsoil) on site, keeping off of roadway and sidewalks.
18. Cover, contain and protect all chemicals, liquid products, petroleum product, and non-
inert wastes present on site from vandalism (see Chapter 173-304 WAC for the definition
of inert waste), use secondary containment for on-site fueling tanks.
19. Conduct maintenance and repair of heavy equipment and vehicles involving oil changes,
hydraulic system repairs, solvent and de-greasing operations, fuel tank drain down and
removal, and other activities that may result in discharge or spillage of pollutants to the
ground or into stormwater runoff using spill prevention measures, such as drip pans.
Clean all contaminated surfaces immediately following any discharge or spill incident. If
raining over equipment or vehicle, perform emergency repairs on site using temporary
plastic beneath the vehicle.
20. Conduct application of agricultural chemicals, including fertilizers and pesticides, in such
a manner, and at application rates, that inhibits the loss of chemicals into stormwater
runoff facilities. Amend manufacturer's recommended application rates and procedures
to meet this requirement, if necessary.
21. Inspect on a regular basis (at a minimum weekly, and daily during/after a runoff
producing storm event) and maintain all erosion and sediment control BMPs to ensure
successful performance of the BMPs. Note that inlet protection devices shall be cleaned
or removed and replace before six inches of sediment can accumulate.
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SPOKANE REGIONAL STORMWATER MANUAL
22. Remove temporary ESC BMPs within 30 days after the temporary BMPs are no longer
needed. Permanently stabilize areas that are disturbed during the removal process.
Apri12008 Appendix 9A—ESC Standard Plan Notes
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Apri12008 Appendix 9A—ESC Standard Plan Notes
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SPOKANE REGIONAL STORMWATER MANUAL
CHAPTER 10 - SOURCE CONTROL
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Chapter Organization
10.1 Introduction.................................................................................................................... 10-1
10.2 Applicability .................................................................................................................. 10-1
Apri12008 Chapter 10—Source Control
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Apri12008 Chapter 10—Source Control
10-ii
SPOKANE REGIONAL STORMWATER MANUAL
10.1 INTRODUCTION
Source control consists of ineasures taken to prevent pollutants from entering stormwater
and thus affecting the water quality of surface water and groundwater. Source control
measures are typically in the form of best management practices (BMPs) to keep the
common pollutants generated in an urban environment from contacting stormwater,
either through physical separation of areas or through careful management of activities
that generate pollutants. Water pollutants are generally defined as hazardous or toxic
solids that are water soluble or transportable, or substances that are liquids at ambient
temperatures and pressures. Insoluble gases and vapors are not considered water
pollutants.
The main purpose of source control BMPs is to prevent pollutants from coming into
contact with stormwater through physical separation and/or management of activities that
produce pollutants. Guidance for selecting BMPs to satisfy this basic requirement is
presented in Chapter 8 of the Stormwater Management Manual for Eastern Washington.
For more information regarding source control and the recommended BMPs, visit the
Washington State Department of Ecology website at the following address:
• http://www.ecy.wa.gov/programs/wq/stormwater/index.html.
10.2 APPLICABILITY
All projects, unless exempted in Section 2.1.4, shall comply with this Basic Requirement.
Project proponents are required to implement applicable source controls through the use
of BMPs as specified in Chapter 8 of the Stormwater Management Manual for Eastern
Washington.
A project proponent is not relieved from the responsibility of preventing pollutant release
from coming in contact with stormwater, whether or not the project triggers the
regulatory threshold.
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Apri12008 Chapter 10—Source Control
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CHAPTER 11 - MAINTENANCE, TRACTS
AND EASEMENTS
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Chapter Organization
11.1 Maintenance................................................................................................................... 11-1
11.1.1 Introduction............................................................................................................ 11-1
11.12 Applicability .......................................................................................................... 11-1
11.13 Homeowners' and Property Owners' Associations ............................................... ll-2
11.1.4 Operation and Maintenance Manual...................................................................... 11-2
11.1.5 Financial Plan......................................................................................................... 11-3
11.1.6 Maintenance Access Requirements ....................................................................... ll-3
112 Tracts and Easements..................................................................................................... 11-4
11.2.1 Tracts...................................................................................................................... 11-4
11.22 Easements .............................................................................................................. 11-5
Appendix 1lA —Example Calculation: Sinking Fund .............................................................. 11-7
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11 .1 MAINTENANCE
11.1.1 INTRODUCTION
Insufficient maintenance of stormwater control facilities can lead to poor
performance, shortened life, increased maintenance and replacement costs, and
property damage.
The local jurisdiction maintains the stormwater system structures located within the
public road right of way and structures located within border easements that serve
public road runoff, unless a separate agreement exists whereby the homeowner,
property owner or other independent entity is responsible for the maintenance.
Drainage tracts created by public projects will be maintained by the local jurisdiction.
The project proponent is to provide for the perpetual maintenance of all elements of
the stormwater system located outside the public right of way. The high-frequency
maintenance of vegetated cover, turf grass and other landscaping within the public
right of way and within border easements that accommodate public road runoff is the
responsibility of the adjacent property owner. When applicable, the following
maintenance-related items shall be submitted with the Drainage Submittal (refer to
Chapter 3)for all projects:
• A copy of the conditions, covenants and restrictions (CC&Rs) for the
homeowners' association (HOA) in charge of operating and maintaining
all elements of the stormwater system;
• A Financial Plan outlining the funding mechanism for the operation,
maintenance, repair, and replacement of the private stormwater system,
including contingencies; and,
• An Operations and Maintenance (O&M) Manual.
Appendix SA and 6A of the Stormwater Management Manual for Eastern
Washington outline facility maintenance recommendations and frequencies.
11.1.2 APPLICABILITY
All projects that meet the regulatory threshold and that propose drainage facilities or
structures shall comply with the Basic Requirement for operation and maintenance.
All projects that propose UIC facilities also must comply with the operation and
maintenance requirements, regardless of whether they meet the regulatory threshold.
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11.1.3 HOMEOWNERS' AND PROPERTY OWNERS' ASSOCIATIONS
For privately maintained stormwater systems in residential neighborhoods, a
homeowner's association, or alternate entity acceptable to the local jurisdiction, shall
be formed to maintain the facilities located outside of the public right of way.
A draft copy of the CC&Rs for the HOA in charge of operating and maintaining the
facilities associated with the stormwater system shall be submitted as part of the
Drainage Submittal review package. The CC&Rs shall summarize the maintenance
and fiscal responsibilities of the HOA, reference the O&M Manual (Section 11.1.4),
and include a copy of the sinking fund calculations and Financial Plan
(Section 11.1.5). Annual HOA dues shall provide funding for the annual operation
and maintenance of all facilities associated with the stormwater system and for the
eventual replacement of these facilities.
For commercial/industrial and multi-family residential developments with joint
stormwater systems and multiple owners, a property owners' association (POA) or
similar entity such as a business shall be formed, or a reciprocal-use agreement
executed.
Homeowners' associations and property owners' associations are to be non-profit
organizations accepted by the Washington Secretary of State. A standard business
license is not acceptable for this purpose.
11.1.4 OPE RATION AND MAI NTE NANCE MANUAL
For stormwater systems operated and maintained by a HOA or POA, an O&M
Manual is required. The O&M Manual summarizes the tasks required to ensure the
proper operation of all facilities associated with the stormwater system and must
include, as a minimum:
• Description of the entiry responsible for the perpetual maintenance of all
facilities associated with the stormwater system, including legal means of
successorship;
• Description of maintenance tasks to be performed and their frequency;
• A list of the expected design life and replacement schedule of each
component of the stormwater system;
• A general site plan (drawn to scale) showing the overall layout of the site
and all the facilities associated with the stormwater system; and,
• A description of the source control BMPs.
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11.1.5 FINANCIAL PLAN
A Financial Plan is required in order to provide the entity responsible for maintenance
with guidance with regard to financial planning for maintenance and replacement
costs. The Financial Plan shall include the following items:
• A list of all stormwater-related facilities and their expected date of
replacement and associated costs;
• Sinking fund calculations that take into consideration probable inflation
over the life of the infrastructure and estimates the funds that need to be
set aside annually (an example is provided in Appendix 11A); and,
• A mechanism for initiating and sustaining the sinking fund account
demonstrating that perpetual maintenance of all facilities associated with
the stormwater system will be sustained.
11.1.6 MAINTENANCE ACCESS REQUIREMENTS
An access road is required when the stormwater system facilities/structures are
located 8 feet or more from an all weather drivable surface and are maintained by the
local jurisdiction. Privately maintained facilities located 15 feet or more from an all
weather drivable surface are also required to have an access road. When required,
maintenance access roads shall meet the following minimum requirements:
• The horizontal alignment of all access roads shall be designed and
constructed to accommodate the turning movements of a Single-Unit
Truck (as defined by AASHTO Geometric Design of Highways and
Streets, Exhibit 2-4, 2004 Edition). The minimum outside turning radius
shall be 50 feet. The minimum width shall be 12 feet on straight sections
and 15 feet on curves;
• Access roads shall consist of an all weather, drivable surface;
• Access roads shall be located within a 20-foot-minimum-width (or as
required by the horizontal alignment requirements) tract or easement,
extending from a public or private road;
• Access roads shall have a maximum grade of 10 percent;
• A paved apron must be provided where access roads connect to paved
public roads; and,
• Gravel access roads shall have a minimum of 6 inches of crushed
surfacing top course, in accordance with WSDOT Standard Specifications
and shall be designed to support the heaviest anticipated maintenance
vehicle year round.
The following access road requirements apply only when the local jurisdiction has
assumed the responsibility of the maintenance and operation of the facilities, though
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it is recommended that access roads for privately maintained facilities also be
designed to meet these criteria:
• If the maintenance access road is longer than 150 feet, a turn-around is
required at or near the terminus of the access road. Turn-arounds are
required for long, winding, or steep conditions, regardless of the length of
the drive, where backing up would otherwise be difficult; and,
• Turn-arounds shall conform to the jurisdiction's standard plan.
11 .2 TRACTS AND EASEMENTS
Flow control and treatment facilities must be located within the right of way, within a
border easement parallel to the road or within an individual tract. For lots larger than 1
acre, the drainage facility may be located within a drainage easement if the facility does
not occupy more than 10% of the lot and does not straddle private property lines.
Stormwater facilities serving commercial projects do not generally require separate tracts
or easements unless they serve more than one parcel.
A stormwater faciliry, as defined for this section, is a swale or pond. It is acceptable for
other types of facilities, such as a pipe, to be in a drainage easement.
11.2.1 TRACTS
A drainage tract for access, maintenance, operation, inspection and repair shall be
dedicated to the entity in charge of the maintenance and operation of the stormwater
system. Unless otherwise approved by the local jurisdiction, a tract will be dedicated
when any of the following situations are present:
• Facilities associated with a stormwater system serving a residential
development are located outside of the public right of way;
• Drainage ditches are located in residential neighborhoods. The limits of
the tract may have to be delineated with a permanent fence when the ditch
is located near property lines; or,
• A drainageway is present on a lot of 1 acre or smaller (refer to
Section 83.4).
Tracts shall be of sufficient width to provide access to, and maintain, repair or replace
elements of, the stormwater system without risking damage to adjacent structures,
utilities and normal property improvements, and without incurring additional costs for
shoring or specialized equipment.
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11.2.2 EASEMENTS
A drainage easement for access, maintenance, operation, inspection and repair shall
be granted to the entity in charge of the maintenance and operation of the stormwater
system. The easement shall grant to the local jurisdiction the right to ingress/egress
over the easement for purposes of inspection or emergency repair. If not in a tract, the
following infrastructure shall be placed within drainage easements:
• Elements of a stormwater system, such as a pipe, located outside the
public right of way. Easements for stormwater conveyance pipes shall be
of sufficient width to allow construction of all improvements, including
any associated site disturbances, and access to maintain, repair or replace
the pipe and appurtenances without risking damage to adjacent structures
or incurring additional costs for shoring or special equipment. No storm
pipe in a drainage easement shall have its centerline closer than 5 feet to a
private rear or side property line. The storm drain shall be centered in the
easement. The minimum drainage easement shall be 20 feet;
• For drainage ditches and natural drainageways, the easement width shall
be wide enough to contain the runoff from a 50-year storm event for the
contributing stormwater basin, plus a 30% freeboard or 1 foot, whichever
is greater. Constructed drainage ditches will not typically be allowed to
straddle lot lines. Natural drainageways (refer to Section 83.4) located on
lots larger than 1 acre may be placed in an easement; and,
• Easements for access roads and turnarounds shall be at least 20 feet wide.
Easement documents shall be drafted by the project proponent for review by the local
jurisdiction and recorded by the project proponent.
Off-Site Easements
When a land action proposes infrastructure outside the property boundaries, an off-
site easement shall be recorded separately from plat documents, with the auditor's
recording number placed on the face of the plat. The easement document shall include
language prescribed by the local jurisdiction. The easement language shall grant the
local jurisdiction the right to ingress and egress for purposes of routine or emergency
inspection and maintenance. The following will be submitted to the local jurisdiction
for review:
• A legal description of the site stamped and signed by a surveyor;
• An exhibit showing the entire easement limits and easement bearings,
stamped and signed by a surveyor;
• Proof of ownership for the affected parcel and a list of signatories; and,
• Copy of the draft easement.
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The legal exhibit and description shall have 1-inch margins for all four sides of the
page. All text shall be legible and at least 8 point.
For plats and binding site plans, the off-site drainage facility must be clearly
identified on the plans and operation and maintenance responsibilities must be clearly
defined prior to acceptance of the project.
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APPENDIX 11A - EXAMPLE CALCULATION :
SI NK I NG FUND
LIST OF QUANTITIES
Description Units Quantity Unit Price Total
24" Pipe LF 175 $40.00 $ 7,000.00
21"Pipe LF 50 $40.00 $ 2,000.00
18"Pipe LF 700 $26.00 $ 18,200.00
15"Pipe LF 650 $24.00 $ 15,600.00
12"Pipe LF 1600 $22.00 $ 35,200.00
10" Pipe LF 50 $20.00 $ 1,000.00
Pipe Total $ 79,000.00
Inlets EA 22 $500.00 $ 11,000.00
Type B Drywells EA 4 $2,500.00 $ 10,000.00
Structure Total $ 21,000.00
ANNUAL MAINTENANCE AND OPERATION COSTS
Description Units Quantity Unit Price Total
Inspect Structures DAY 4 $50.00 $ 200.00
Flush/Clean Inlets EA 26 $100.00 $ 2,600.00
Flush Pipes $ -
Inspect Ponds and Clean Outlets LS 1 $500.00 $ 500.00
Mowing&Irrigation of Ponds DAY 34 $250.00 $ 8,500.00
Annual Maintenance Cost � 11,800.00
REPLACEMENT COST & ANNUAL COST PER LOT
Description Total
Assume 50%of Pipe is Replaced in 20 years (=Pipe Total*0.5) $ 39,500.00
Assume 25%of Structures are Replaced in 20 years(=Structure Total*025) $ 5,250.00
Total Present Value(P� of Replaced Pipe and Structures � 44,750.00
Future Value of Pipe and Structures(F�,assume inflation=4°/o,n=20
FV=PV(F/P,4%,n=20) $ 98,052.76
Annual Set-Aside for Future Replacement(A),assume interest=6°/o,n=20
A=FV(A/F,6%,n=20) $ 2,665.52
Annual Maintenance and Operation Costs(from subtotal above) $ 11,800.00
Total Annual Costs � 14,465.52
Total Charge per Lot, assume 100 Lots
Charge per Lot=Total Annual Costs/#of Lots
Charge per Lot $ 144.66
NOTE: F/P,A/F factors are from interest tables
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CHAPTER 7 - FLOW CONTROL
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SPOKANE REGIONAL STORMWATER MANUAL
7.7 Evaporation Facilities ....................................................................................................7-15
7.7.1 Introduction............................................................................................................7-15
7.72 Minimum Requirements ........................................................................................7-15
7.8 Additional Requirements for All Facilities....................................................................7-17
7.8.1 General...................................................................................................................7-17
7.82 Setbacks .................................................................................................................7-18
7.8.3 Drawdown Time ....................................................................................................7-18
7.8.4 Side Slopes.............................................................................................................7-18
7.8.5 Emergency Overflow Spillway..............................................................................7-19
7.8.6 Embankments.........................................................................................................7-19
7.8.7 Fencing...................................................................................................................7-20
7.8.8 Planting Requirements...........................................................................................7-21
7.8.9 Landscaping...........................................................................................................7-21
7.8.10 Maintenance...........................................................................................................7-22
7.8.11 Dam Safety.............................................................................................................7-22
7.9 Special Requirements.....................................................................................................7-23
7.9.1 Special Drainage Areas..........................................................................................7-23
7.92 Floodplains.............................................................................................................7-23
7.9.3 Wetlands and Classified Streams...........................................................................7-26
7.9.4 Closed Depressions................................................................................................7-27
7.10 Regional Stormwater Facilities......................................................................................7-27
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7.1 INTRODUCTION
This chapter outlines the minimum requirements for sizing flow control facilities.
Standard flow control facilities are detention/retention ponds, drywells, and evaporation
ponds. Any other faciliry is considered a non-standard system, and shall be evaluated
individually by the local jurisdiction. Flow control facilities are necessary to mitigate
potential adverse impacts on down-gradient properties due to the increase in stormwater
runoff caused by land development.
Unless specifically approved by the local jurisdiction, the peak rate and volume of
stormwater runoff from any proposed land development to any natural or constructed
point of discharge downstream shall not exceed the pre-development peak rate or volume
of runoff. A down-gradient analysis demonstrating that there will be no expected adverse
impacts on downgradient properties will be required (refer to Section 3.4.5 for down-
gradient analysis criteria). Exceptions with regard to rate and volume control can be made
for regional facilities planned by the local jurisdiction.
All engineering work for flow control facilities shall be performed by, or under the
direction of, a professional engineer currently licensed in the State of Washington.
Refer to Chapter 11 for maintenance requirements.
7.2 APPLICABILITY AND EXEMPTIONS
7.2.1 APPLICABILITY
All projects that meet the regulatory threshold shall comply with this Basic
Requirement.
7.2.2 EXEMPTIONS
Projects are exempt from flow control if they discharge to any of the following:
• The Spokane River or other exempt water bodies, which are defined in the
Stormwater Management Manual for Eastern Washington as fifth-order or
greater stream channels, as determined from a 1:150,000 scale map;
• A river or stream that is fifth-order or greater as determined from a
1:24,000 scale map;
• A river or stream that is fourth-order or greater as determined from a
1:100,000 scale map;
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• A stream that flows only during runoff-producing events. These streams
are defined as those that do not discharge via surface flow to a non-exempt
surface water following receipt of the 2-year, NRCS Type lA, 24 hour
rainfall event. In addition, for the stream to be exempt, it shall be able to
carry the runoff from an average snowmelt event, but shall not have a
period of base flow during a year of normal precipitation;
• A lake or reservoir with a contributing watershed areas greater than 100
square miles;
• A reservoir with outlet controls that are operated for varying discharges to
the down-gradient reaches as for hydropower, flood control, irrigation or
drinking water supplies (discharges to uncontrolled flow-through
impoundments are not exempt).
In order to be exempted the discharge shall meet all of the following requirements:
• The project area must be drained by a conveyance system that consists
entirely of manmade conveyance elements (i.e. pipes, ditches, outfall
protection); and,
• The conveyance system must extend to the ordinary high water mark line
of the receiving water, or (in order to avoid construction activities in
sensitive areas) flows are properly dispersed before reaching the buffer
zone of the sensitive or critical area; and,
• Any erodible elements of the conveyance system for the project area must
be adequately stabilized to prevent erosion; and,
• Surface water from the project area must not be diverted from or increased
to an existing wetland, stream, or near-shore habitat sufficient to cause a
significant adverse impact. Adverse impacts are expected from
uncontrolled flows causing a significant increase or decrease in the 1.5- to
2-year peak flow rate.
Maps shall be standard U.S. Geological Survey (USGS) maps or geographic
information system (GIS) data sets derived from USGS base maps.
7.3 DETENTION FACILITIES
7.3.1 INTRODUCTION
A detention system is a storage facility that has a surface discharge. Detention ponds,
vaults and underground storage tanks are all considered to be detention facilities.
Refer to the Stormwater Management Manual for Eastern Washington for design
criteria for vaults and underground storage tanks.
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A detention facility is intended to control peak stormwater runoff rates, and if
designed per the criteria in this chapter, does not control volume. If the subgrade soils
meet the drawdown criteria specified in Section 7.8.3, the engineer may choose to
propose a system that uses infiltration in conjunction with a detention pond as a
means to control volume. Otherwise, the engineer can use evaporation to control
volume, in conjunction with a detention pond (refer to Section 7.7.2).
7.3.2 MINIMUM REQUIREMENTS
The following minimum requirements shall be met. Additional requirements are
specified in Section 7.8.
Design Criteria
Detention facilities shall be designed such that the release rate does not exceed the
pre-developed conditions for multiple storm events. The analysis of multiple design
storms is needed to control and attenuate both low and high flow storm events.
The total post-developed discharge rate leaving the site (including bypass flow) shall
be limited to the pre-development rates outlined in Table 7-l. Bypass flow is the
runoff that leaves the site without being conveyed through the drainage system.
TABLE 7-1
ALLOWABLE DISCHARGE RATES
Design F requency (24 hr storm) Post-Developed Discharge R ate'
2-year <2-year pre-developed
25-year <25-year pre-developed
100-year2 (Emergency Overflow) Overflow route only
�Post-developed flow is equal to the release from detention facility plus the bypass flow.
�The emergency overflow shall direct the 100-year post-developed flow safely towards the
downstream conveyance system
Detention systems that store any stormwater below the first overflow shall adhere to
the subgrade infiltrative criteria specified in Table 6-1. Unless waived by the local
jurisdiction, the subgrade infiltration rate shall be verified through testing.
If the detention facility is also proposed to function as a water quality treatment
facility, the following criteria must be met:
• The first orifice or outlet from the facility must be placed 6 inches above
the pond bottom; and,
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• The treatment zone shall meet the requirements specified in Table 6-1 and
be verified through testing, unless waived by the local jurisdiction.
The NRCS Type lA 24 hour storm event is the design storm to be used for all flow
control facilities that use a surface discharge; for flow control facilities that involve
infiltration into the subsurface, the NRCS Type II 24 hour storm event can be used for
design.
A wetland may also be considered for use as a flow control facility, if approved by
Ecology. Refer to Section 7.9.3 for additional information.
Setbacks
When a detention faciliry is proposed upslope of developed property or at the top of a
slope inclined 10% or greater, down-gradient impacts shall be evaluated and the
minimum setback from the slope must be greater than or equal to the height of the
slope. The distance between the outlet structure and the inlet into the detention
facility shall be maximized.
Release Point
Stormwater runoff from a developed site shall leave the site in the same manner and
location as it did in the pre-developed condition. Therefore, a detention system may
be used only when a well-defined natural drainage course is present prior to
development.
7.4 OUTFLOW CONTROL STRUCTURES
7.4.1 INTRODUCTION
Control structures are weirs, orifices, culverts, or manholes with a restrictor device
that is used for controlling outflow from a facility to meet a desired standard. This
section presents a general overview of flow control structures. For additional
information, the engineer should consult a hydraulics reference.
7.4.2 OUTFLOW CONTROL STRUCTURE TYPES
WeiNS and Orifices
Weirs and orifices are partial obstructions in an open channel or in a detention facility
at the point of discharge, typically used to limit and measure flow rates. Weirs have
openings with no top, referred to as a notch, through which the water flows when its
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surface elevation is above the bottom of the opening (the weir invert). An orifice is
typically a circular opening cut into the structure obstructing the stream. The
following are common features of weirs:
• Weir Length: The weir length is the length of the notch in the direction
perpendicular to the flow:
o Contracted weirs (see Figure 7-1) have weir lengths less than the
channel width or pond wall, and the falling liquid sheet (called the
nappe) decreases in width as it falls.
o Su�ressed weirs (see Figure 7-2) extend the full channel width.
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Figure 7-2 —Suppressed Weir
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SPOKANE REGIONAL STORMWATER MANUAL
• Weir Crest: The weir crest is the surface of the weir invert in the direction
of the flow:
o Broad-crested weirs (see Figure 7-3) have a crest that extends
horizontally in the direction of flow far enough that the flow leaves the
weir in essentially a horizontal direction. A weir is broad-crested if the
length of the crest in the direction of flow is greater than half of the
head(H).
o Shar�-crested weirs (see Figure 7-4) have a narrow crest with a sharp
upstream edge so that water flows clear of the crest. The weir invert or
top of the crest should be set above the pond bottom a height of at least
twice the maximum head,preferably more.
HEAD
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• Weir Notch Shape: The following are the common shapes of weir
openings, as viewed looking in the direction of the flow:
o V-notch: The opening has two sloped sides coming together at a point
at the bottom.
o Rectangular: The opening has two vertical sides and a horizontal
invert.
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o Trapezoidal (Cipoletti): The opening has a horizontal invert and two
sloped sides.
Table 7-2 provides equations and coefficients for calculating flow through the most
common types of weirs and orifices used for flow control.
TABLE 7-2
FLOW EQUATIONS FOR VARIOUS WEIR AND ORIFICE TYPES
Weir/OrificeType Equation C
Sharp Crested V-notch s
i Q = C�Tan��FI� 0.60
weir
Broad Crested Suppressed Q = CLH� 0.33
Rectangular weir
Rectangular Sharp Crested
W eirs i: �
Q = C�L—02H�H � 327+0.40 y
Contracted
Suppressed Q = CLH 3�
Sharp Crested Cipoletti
(Trapezoidal)i Q = CLH� 3.367
Side slopes are 1:4
wo ad Crested Trapezoidal Q = C(2g)����3 LH� +g�Tan,��H�� 0.60
Orifice Q = CA 2gH 0.62
1 The weir inverts should be set above the pond bottom a height of at least twice the maximum head.
Q=flow(cfs); C=coefficient of discharge;A=area of orifice(square feet);H=hydraulic head(feet);
g=gravity (32.2 feet/second�); 8 =angle of side slopes(degrees); Y=starage depth(feet);L=weir length
or opening(feet)
Risers
A riser typically consists of a circular pipe or box inlet with its opening oriented
parallel to the water surface. A riser operates under three hydraulic flow regimes in
this order as the water surface elevation rises: weir, orifice, and full barrel. Full barrel
flow occurs when the downstream conduit is undersized with respect to the riser
capaciry and when the water surface elevation rises high enough.
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SPOKANE REGIONAL STORMWATER MANUAL
Figure 7-5 can be used to determine the head (in feet) above a riser of given diameter
and for a given flow (usually the 100-year peak flow for developed conditions). For
additional information, consult a hydraulics reference.
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7.4.3 MINIMUM REQUIREMENTS
Outflow control structures shall meet the following requirements:
• A weir used as a flow control structure shall be made of non-erosive
material that is resistant to alteration or vandalism, such as reinforced
concrete or metal with a non-corrosive surface. An emergency overflow
weir can be made of soil with revetment;
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• The inverts for sharp-crested weirs should be set above the pond bottom a
height of at least twice the maximum head;
• The crest length for broad-crested weirs should be at least 3 times the
maximum head and preferably 4 times the maximum head, or more;
• Runoff shall enter the detention facility through a conveyance system
separate from the control and outflow conveyance system. The distance
between the inlet and outlet shall be maximized to reduce sediment from
accumulating in the outflow structure;
• Flow control structures discharging from a high use site to a drainage
facility shall include an oil control BMP that meets the requirements
outlined in Chapter 6;
• Control structures shall be selected taking into consideration the expected
hydraulic heads. Table 7-3 presents typical control structures and their
applicability.
TABLE 7-3
OPTIMAL APPLICATION OF CONTROL STRUCTURES
Control Structure Pond Head
outlet Pipe very Low
V-Notch Weir Low
Slotted Weir Moderate
Multi-Stage Orifice High
• Circular orifices shall be 3 inches in diameter or greater. Slotted weirs can
be used in lieu of smaller orifices to reduce the occurrence of plugging;
• The top of manhole/catch basin grates used for control structures shall be
placed 2 inches above the finish grade when located in earth or gravel
locations.
Figure 7-6 shows a typical flow control structure.
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MANHOLE LID(OR GRATE IF APPLICABLE)
SHALL BE PLACED DIRECTLY OVER LADDER
"„–�-I 1-1 I 1-1 I 1 1 I 1=1 I 1=1 I 1-1 I 1-1 I- -
_ _ _ _ _ _ _ – –
=— ____
=
OVERFLOW PIPE AND OUTLET PIPE
LL MUST HAVE CAPACITY EQUAL TO OR
�z GREATER THAN COMBINED INLET
o�
STANDARD MANHOLE LADDER STEPS OVERFLOW ELEVATION
CHAIN-200#CAPACITY SLACK WHEN–
GATE IS DOWN.N/ELD TO MANHOLE.
STRAP TO MANHOLE WALL
OUTFLOW ORIFICE LOCATION, ---_ —_
FOR EXAMPLE --== –=–
___- __–
OVERFLOW PIPE OUTLET PIPE
_ _ _ _ _
_ _ _ =IIII �
-�_ _= F
– - �
N
WATERTIGHT CLEAN OUT PLATE
WITH MINIMUM 8 GAUGE SLIDE z
�
�
N
48 IN DIAMETER MANHOLE
NOTES: RESTRICTOR PLATE ORIFICE CUT
ALL METAL PARTS AND SURFACES MUST BE MADE OF CORROSION RESISTANT MATERIAL IN PLATE 3 IN MIN.DIAMETER
OR GALVANIZED. COMPLETE CORROSION PROTECTION MUST BE ASSURED.
TOP OF MANHOLE OR GRATE SHALL BE PLACED 2 IN ABOVE FINISH GRADE UNiEN LOCATED
IN EARTH OR GRAVEL.
Figure 7-6—Flow Control Structure Example
7.5 INFILTRATION FACILITIES
7.5.1 INTRODUCTION
An infiltration facility is used for disposing of stormwater runoff into the subsurface
and can be used for flow control provided that:
• The discharge is uncontaminated or properly treated so that it does not
violate water quality criteria per Chapter 6. For additional information
regarding discharges to drywells, refer to Ecology's Guidance for UIC
Wells that Manage Stormwater;
• The Geotechnical Site Characterization demonstrates the suitability of the
soil for subsurface disposal;
• The down-gradient analysis indicates that adverse impacts are not
anticipated; and,
• The discharge does not violate UIC regulations.
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Drywells are considered standard infiltration facilities. The engineer shall consider
the impact of infiltration on groundwater elevations both on site and on down-
gradient properties.
For discharges to UIC facilities, the best management practices chosen for the site
must remove or reduce the target pollutants to levels that will comply with state
groundwater quality standards when the discharge reaches the water table or first
comes into contact with an aquifer (see WAC 173-200). Pre-treatment is required
prior to discharging to a UIC facility (refer to Chapter 6 for additional information).
Discharges to surface waters shall comply with WAC 173-201A, Water Quality
Standards for Surface Waters of the State of Washington. Refer to Chapter 6 for BMP
selection.
7.5.2 MINIMUM REQUIREMENTS
In addition to the requirements specified in Section 7.8, infiltration facilities shall
meet the minimum requirements described below.
Swale Sizing
The methodology for sizing swales is in Section 5.6.
Location
Drywells shall be spaced at least 30 feet center-to-center or twice the depth of the
drywell, whichever is greater.
If the site has the potential for contaminated or unstable soil, then these conditions
shall be investigated and appropriate mitigating measures taken before designing
infiltration facilities in theses areas.
Infiltration facilities shall not be placed on or above a landslide hazard area or slopes
greater than 15 percent without evaluation by a geotechnical engineer and
jurisdictional approval.
Outflow Rates
Outflow rates shall be determined using the field methods presented in Section 4.3.1.
The active barrel of the drywell shall be installed within the target soil layer. Target
soils with more than 12% fines (percent passing the No. 200 sieve) are not suitable
for drywells.
The proponent may assume a maximum outflow rate of 03 cfs and 1.0 cfs for Type A
and Type B drywells, respectively, if all of the following conditions are met:
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• The drywells are located within the NRCS Garrison or Springdale soil
groups. A full-scale drywell test may still be required;
• The soils are verified by a qualified professionaL Field verification should
include classifying the target layer soils, obtaining soil gradation data and
confirming that the site soils are consistent with the design outflow rates.
• There is no history of drainage problems in the vicinity of the drywell
location;
• The anticipated rise in the elevation of the local groundwater table
resulting from the disposal faciliry will not significantly impact adjacent
properties or structures; and,
• The local jurisdiction does not have concerns regarding the soil's abiliry to
drain.
Limiting Layer
The optimal separation between the bottom of the drywell and the limiting layer
(bedrock, groundwater, clay lens, etc.) shall equal the maximum drywell head, which
is 6 feet for single-depth drywells and 10 feet for double-depth drywells. The limiting
layer separation can be reduced to 4 feet when the factors of safety specified in
Appendix 4A are applied.
For a pond or swale with no infiltrative structure, the separation shall be a minimum
of 4.5 feet below the pond bottom, to account for the 6-inch treatment zone and 48
inches of subgrade infiltrative soil.
The local jurisdiction reserves the authority to increase the required depth to the
limiting layer should there be evidence that the functionality of the faciliry will be
negatively impacted.
Setback
Drywells shall be placed with the following setback distances:
• At least 100 feet from water wells;
• At least 200 feet from springs used for drinking water supplies,
• At least 100 feet from septic tanks or drainfields;
• At least 100 feet up-slope and 20 feet down-slope from building
foundations, unless a reduction is geotechnically justified;
• At least 20 feet from a Native Growth Protection Easement; and,
• Per the geotechnical engineer's recommendations when located up-slope
from a structure or behind the top of a slope inclined in excess of 15
percent.
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7.6 NATURAL DISPERSION
7.6.1 INTRODUCTION
Natural dispersion attempts to minimize the hydrologic changes created by new
impervious surfaces by restoring the natural drainage patterns of sheet flow and
infiltration. There are three types of natural dispersion:
• Concentrated Flow Dis�ersion: Used for steep driveways or other small
pavement areas, concentrated flow dispersion uses berms or drains to
direct runoff from the paved area to a vegetated pervious area (the
"dispersal area") that slows entry of the runoff into a conveyance system,
allows for some infiltration and provides some water quality enhancement.
• Sheet Flow Dis�ersion: Used for flat or moderately sloped paved or
cleared areas, sheet flow dispersion consists of a vegetated buffer zone
through which sheet flow from the pavement passes, providing flow
attenuation and treatment.
• Full Dispersion: Use for larger areas of new residential or commercial
development, full dispersion is the preservation of native vegetation on
some portion of the site to allow dispersion of runoff from roofs,
driveways and roads within the site.
Natural dispersion can be used for impervious or pervious surfaces that are graded to
avoid concentrating flows. This flow control method shall only be considered for use
on rural projects, including linear roadway projects, large lot subdivision, short plat
roads, driveways, sport courts, parking lots and roofs that are not guttered. This flow
control method is not intended for use prior to discharge to a lake, stream or water
body.
7.6.2 MINIMUM REQUIREMENTS
This section describes a sheet-flow dispersion technique; concentrated flow
dispersion is not allowed in the Spokane Region at this time.
In addition to the requirements specified in Section 7.8, as applicable, the following
minimum requirements shall be met:
• The dispersal area shall be well-vegetated;
• A vegetated dispersal width of 10 feet must be provided for every 20 feet
of width of impervious surface that drains to the dispersal area, with
10 feet the minimum width in all cases. An additional 0.25 feet of
vegetated dispersal width shall be provided for each additional foot of
impervious surface;
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• A vegetated dispersal width of 1 foot must be provided for every 6 feet of
disturbed pervious area(i.e. bare soil and non-native landscaping);
• Natural dispersion areas (perpendicular to the impervious area) shall have
a slope no steeper than 14% (7H:1 V). If this criterion cannot be met due to
site constraints, the dispersal width must be increased 1.5 feet for each
percent increase in slope above 14%, and in no case shall the slope exceed
20%;
• The average longitudinal slope (roughly parallel to the road or diagonally
away from the road) of the dispersal area shall be no more than 15%;
• The longitudinal slope of the contributing impervious or pervious drainage
area(parallel to the edge of the dispersal area) shall be 5% or less;
• The lateral slope of the contributing impervious or pervious drainage area
(perpendicular to the dispersal area, typically the road cross-slope) shall be
4.5% or less;
• The sheet flow path leading to the natural dispersal area shall not be
longer than 75 feet for impervious areas or 150 feet for pervious areas;
• The longitudinal length of the dispersal area shall be equivalent to or
greater than the longitudinal length of impervious area that is contributing
the sheet flow;
• A 2-foot-wide transition zone (to discourage channeling) shall be provided
between the edge of the impervious surface and the vegetated dispersal
area, or under the eaves of a roof that has not been guttered. This may be
an extension of the sub-grade material (crushed rock), modular pavement,
or drain rock;
• The dispersal area shall have a minimum infiltration rate of 4 inches per
hour;
• Clearing and grubbing of native dispersal area shall be minimized in order
to help maintain the existing root systems that are vital to the success of
natural dispersion;
• The area around the dispersal zones shall not be compacted;
• For sites with septic systems, the dispersal area must be downgradient of
the drain field primary and reserve areas. This requirement may be waived
by the local jurisdiction if the site topography clearly indicates that flow is
prohibited from intersecting the drain field;
• The dispersal area shall be located down-gradient from building sites;
• The dispersal area shall be clearly identified on all construction plans,
including grading plans, so that the area is not cleared, grubbed or
compacted, and shall be clearly delineated on the site;
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• Native vegetation and existing trees should not be removed from the
natural growth retention areas except where required to meet sight
distance, clear-zone or other traffic-related requirements, or if the
vegetation is diseased;
• Dispersal is not allowed across, over or toward a landslide or
geohazardous area; and,
• The dispersal area shall be preserved within the road right of way, a
separate dedicated tract or an easement in order to ensure that treatment
and flow control are not interrupted.
7.7 EVAPORATION FACILITIES
7.7.1 INTRODUCTION
Evaporation systems are used to collect and dispose of stormwater runoff when soils
are not conducive to infiltration, shallow groundwater is present, or there is the
potential for negative impacts due to post-developed stormwater runoff being injected
into the subsurface.
The locally developed spreadsheet described in Section 5.7.3 is the most common
tool used to perform evaporative pond capacity calculations.
7.7.2 MINIMUM REQUIREMENTS
Liner
Geosynthetic or natural liners may be required to limit infiltration in areas where
there is the potential for down-gradient impacts or where the water table may
adversely impact the pond via seepage or mounding. The liner shall be a product
suitable for stormwater storage and installed per the geotechnical engineer's or
manufacturer's recommendation.
When an evaporative pond is proposed, a geotechnical engineer shall provide
evaluation of the following:
• Liner materials and installation;
• The potential for groundwater seepage into the pond from the surrounding
area;
• The potential for any down-gradient adverse impacts due to the injection
of developed stormwater volume into the subsurface; and,
• The potential for groundwater mounding or uplift for a lined pond.
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Based upon the information in these evaluations, the geotechnical engineer shall
make recommendations regarding the following, if applicable:
• Liner materials and installation; and,
• Any proposed mitigation measures.
Pond Geometry
Evaporative systems designed with the Preferred Method (refer to Section 5.7.2) shall
have an evaporation volume separate from the detention volume that provides
attenuation of peak flows. Depending on the site conditions and limitations, the
proponent can provide separated cells or stacked cells to satisfy this requirement.
Separated svstem: This type of facility has one evaporation cell (upstream cell)
followed by a detention or infiltration cell (downstream cell). The storage volume and
design depth of the evaporation cell is determined by a water budget analysis as
described in Section 5.7. A factor of safety of at least 1.2 is applied to the required
evaporative volume or design depth. The invert of the overflow to the detention or
infiltration faciliry is placed at or above the maximum surface water elevation of the
evaporative system (including the factor of safety).
The downstream cell is designed per the criteria for detention facilities (refer to
Section 7.3) or infiltration facilities (refer to Section 7.5). In order to allow a point
discharge from a detention facility, it must be established that there is an existing,
well-defined natural drainage course. A 1-foot freeboard above the maximum surface
elevation of the detention or infiltration cell is required. Figure 7-7 shows a typical
cross-section of a separated system.
1 FT FREEBOARD
----
EVAPOR4TIVE CELL
EXISTING NATUR4L
DETENTION OR DR4INAGE COURSE
INFILTR4TION
CELL
OUTFLOW
STRUCTURE
Figure 7-7 —Separated Evaporative/Detention Facility Cross-Section
A stacked svstem: This type of facility has the evaporative cell (lower cell) below the
detention cell (upper cell). The storage volume and design depth of the evaporation
cell are determined by a water budget analysis as described in Section 5.7. A factor of
safety of at least 12 is applied to the required evaporative volume or design depth.
Once the dimensions of the lower cell are determined, the upper cell is placed on top
of the lower cell. Thus, the detention cell "bottom" and outflow structure must be
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placed at or above the maximum surface water elevation of the evaporative system
(including the factor of safety).
The detention cell is designed per the criteria specified in Section 73. In order to
allow a point discharge from a detention facility, it must be established that there is an
existing, well-defined natural drainage course. A 1-foot freeboard above the
maximum surface elevation of the detention or infiltration cell is required. Figure 7-8
shows a typical cross-section of a stacked system.
1 FT
1F FREEBOARD
DETENTION CELL EXISTING NATUR4L
DR4INAGE COURSE
EVAPOR4TIVE CELL
OUTFALL
STRUCTURE
Figure 7-8 —Stacked Evaporative/Detention System Cross-Section
T�eatment
Evaporative systems designed with the Alternative Method are not subject to water
quality treatment requirements. Evaporative systems designed with the Preferred
Method are required to provide water quality treatment per the goals, applicability
and criteria specified in Chapter 6.
7.8 ADDITIONAL REQUIREMENTS FOR ALL FACILITIES
The following minimum requirements shall be met for all flow control facilities:
7.8.1 G E N E R A L
The design of flow control facilities shall adhere to the following:
• Pond bottoms shall be located a minimum of 0.5 feet below the outlet to
provide sediment storage; and,
• In general, all pond bottoms shall be flat. Roadside swales are considered
flat if the swale bottom slope is 1% or less. When calculating treatment
volume, the designer can assume a flat bottom for swale/pond bottom
slopes up to 1%. Note that if treatment volume versus area is the
methodology used, the volume may be calculated assuming a flat bottom
even if the roadside swale bottom has a slope (maximum of 1%).
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However, for the calculation of stormwater disposal volume, the grade of
the roadside swale bottom shall factor into the geometry used to size the
facility. The drainage facility bottom shall slope away from the pond inlet
and toward the control structure at 1% for a maximum distance of 20 feet.
• Drainage facilities shall be located within the right of way, within a border
easement parallel to the road or within a drainage tract. In unincorporated
Spokane County, drainage facilities may also be located in a drainage
easement on private property (refer to Chapter 11 for specific
information).
7.8.2 SETBACKS
Setbacks for any pond, swale or ditch (measured from the maximum design operating
depth) shall be at least 30 feet when located up-gradient or 10 feet when located
down-gradient from septic tanks or drainfields.
Pond overflow structures shall be located a minimum of 10 feet from any structure or
property line. The toe of the berm or top of bank shall be a minimum of 5 feet from
any structure or property line.
7.8.3 DRAWDOWN TIME
Detention and infiltration facilities shall have a minimum subgrade infiltration rate of
0.15 inches/hour and drain completely within 72 hours after a storm event.
7.8.4 SIDE SLOPES
Pond side slopes shall meet one of the following requirements:
• Interior side slopes shall not be steeper than 3:1 (horizontal to vertical);
• Interior side slopes may be increased to a maximum of 2:1 (horizontal to
vertical) if the surrounding grade creates a cut or fill with no greater depth
than 1.0 foot;
• Exterior side slopes shall not be steeper than 2:1 (horizontal to vertical)
unless analyzed for stability by a geotechnical engineer.
• Pond walls may be vertical retaining walls,provided that:
o A fence is provided along the top of the wall for walls 2.5 feet or
taller;
o A 4-foot-wide access ramp to the pond bottom is provided, with slopes
less than 4:1 (horizontal to vertical); and,
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o The design is stamped by an engineer with structural expertise if the
wall is surcharged or if it is 4 feet or more in height. A separate
building permit may be required by the local jurisdiction if the wall
height exceeds 4 feet.
7.8.5 EMERGENCY OVERFLOW SPILLWAY
An emergency overflow spillway shall be provided, whenever reasonable, to bypass
the 100-year developed peak flow toward the downstream conveyance system in the
event of plugged orifices or high flows that exceed the design storm.
Emergency overflow spillways shall be provided for detention ponds with constructed
berms 2 feet or more in height and for ponds located on grades in excess of 5%.
Emergency overflow spillways shall be analyzed as broad crested trapezoidal weirs
and comply with the following requirements:
• The spillway shall have the capacity to pass the 100 year-developed peak
flow with a 30% freeboard;
• The full width of the spillway shall be armored with riprap and extend
downstream to where emergency overflows enter the conveyance system;
• If the detention facility is located on an embankment, the overflow
spillway shall be armored to a minimum of 10 feet beyond the toe of the
embankment; and;
• The overflow path shall be identified on the construction plans and
easements shall be provided as necessary.
Engineers may choose to design the detention pond multi-stage outflow structure with
an emergency bypass that can route the 100-year storm through the structure and out
of the pond directly into the conveyance channel. However, due to the high potential
for sedimentation and plugged orifices within these structures, an emergency
overflow spillway shall still be provided in order to reduce the potential for a pond
berm breach for detention ponds that require an emergency overflow spillway.
7.8.6 EMBANKMENTS
The height of an embankment is measured from the top of the berm to the catch point
of the native soil at the lowest elevation. Embankments shall meet the following
minimum requirements:
• Embankments 4 feet or more in height shall be constructed as
recommended by a geotechnical engineer. Depending upon the site,
geotechnical recommendations may be necessary for lesser embankment
heights;
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• Embankments shall be constructed on native consolidated soil, free of
loose surface soil materials, fill, roots, and other organic debris or as
recommended by the geotechnical engineer;
• Energy dissipation and erosion control shall be provided to stabilize the
berm and its overflow;
• The embankment compaction shall produce a dense, low permeability
engineered fill that can tolerate post-construction settlements with minimal
cracking. The embankment fill shall be placed on a stable subgrade and
compacted to a minimum of 95% of the Modified Proctor Densiry (ASTM
Procedure D1557);
• Anti-seepage filter-drain diaphragms shall be considered on all outflow
pipes and are required on outflow pipes when design water depths are 8
feet or greater;
• Embankments must be constructed by excavating a key. The key width
shall equal 50 percent of the berm base width, and the key depth shall
equal 50 percent of the berm height; and,
• The berm top width shall be a minimum of 4 feet.
7.8.7 FENCING
Fencing or other barriers may be required to protect the health, welfare and safety of
the public. In general, fencing is required for the following:
• Drainage facilities with the first overflow at 2 or more feet above the pond
bottom;
• Drainage facilities with retaining walls 2.5 feet high or taller;
• Drainage facilities located at, or adjacent to, schools, nursing homes, day-
cares, or similar facilities; and,
• Evaporation Ponds.
Fencing is not required for a typical bio-infiltration swale. However, the local
jurisdiction reserves the authority to require a fence along any swale or pond should
there be a concern for safety.
At the discretion of the local jurisdiction, if a pond is proposed as an amenity (i.e.
enhancements to the disposal facility are proposed, such as rocks, boulders,
waterfalls, fountains, creative landscaping or plant materials), the design will be
reviewed on a case-by-case basis, such that the fencing requirements may be reduced
or waived.
At the discretion of the local jurisdiction, marking fences, terraces, shallower side-
slopes, egress bars, etc. may be allowed instead of fencing.
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The minimum fencing requirements are as follows:
• The fencing shall be at least 4 feet tall unless otherwise specified by the
local jurisdiction, and provide visual access; and,
• Gates are to be provided where drainage facilities are fenced. The gates
shall be a minimum of 12 feet wide and have locks.
The City of Spokane Valley reserves the authoriry to waive any and all fencing in
commercial areas, as reviewed and accepted on a case-by-case basis by City staff.
7.8.8 PLANTING REQUIREMENTS
Exposed earth on the pond bottom and interior side slopes shall be sodded, seeded or
vegetated in a timely manner, taking into account the current season. Unless a dryland
grass or other drought tolerant plant material is proposed, irrigation shall be provided.
All remaining areas of the tract or easement shall be sodded or planted with dryland
grass or landscaped.
7.8.9 LANDSCAPING
Where space and circumstances allow, the landscaping scheme and common use
areas should be integrated with the open drainage features and into the overall
stormwater plan. Plants other than turf grass have characteristics that can provide
additional stormwater management benefits such as enhanced evapotranspiration and
improved soil-holding capabilities.
However, in all cases the landscaping and other uses must be subservient to the
primary stormwater needs and functions. Landscaping that does not conflict with the
collection, conveyance, treatment, storage, and disposal of stormwater is encouraged.
The following general principles should guide the landscaping and selection of plants
in conjunction with stormwater facilities:
• Supplemental landscaping areas should be grouped into irregular islands
and borders outside of the immediate stormwater facilities and not
uniformly dispersed throughout them. The constructed stormwater features
should be irregular and curved in shape to look more natural. Avoid
straight lines and regular shapes where and when possible;
• Trees and shrubs shall not be planted on pond liners due to potential
leakage from root penetration;
• Trees and shrubs shall not be planted near drainage appurtenances such as
outlet control structures, manholes, catch basins, inlets, storm drain lines,
and underground disposal structures such as drywells or drain-fields. The
minimum spacing between the tree or shrub and the drainage structure
shall be equal to the crown diameter of the mature plant;
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• Trees and shrubs shall not be planted within the treatment, storage, and
conveyance zones of swales, ponds, and open channels, unless treatment
and storage calculations take into account the mature tree size and allow
runoff to reach the drainage facilities;
• Self-limiting plants shall be used, not spreading or self-seeding types.
• Full-size forest trees and trees with aggressive root systems should not be
used except where space and circumstances allow. Deciduous trees with
heavy shade and leaf-fall should also be avoided to allow the survival of
the surrounding grass areas and not plug drainage facilities. Evergreens
and smaller ornamental trees are normally better suited to urban
conditions;
• Shrubs should be upright in form and groundcovers should have neat
growth patterns to assist in their maintenance and that of the surrounding
grass areas; and,
• The plant selection needs to consider the native soil conditions and altered
moisture conditions created by the stormwater facilities. The plants need
to be adaptable to the changes in site conditions. Plants that are self-
sufficient and self-limiting, do not require year-round irrigation and
require minimal care are encouraged.
7.8.10 MAINTENANCE
Maintenance is of primary importance for drainage facilities to operate as designed.
The requirements of Chapter 11 shall be met as applicable.
7.8.11 DAM SAFETY
Detention facilities that can impound 10 acre-feet (435,600 cubic feet) or more with
the water level at the embankment crest are subject to the state's dam safety
requirements, even if water storage is intermittent and infrequent (WAC 173-175-
020(1)). The principal safety concern is for the downstream population at risk if the
dam should breach and allow an uncontrolled stormwater release. Peak flows from
dam failures are typically much larger than the 100-year flows which these ponds are
typically designed to accommodate.
Dam safety considerations generally apply only to the volume of water stored above
natural ground level. Per the definition of dam height in WAC 173-175-030, natural
ground elevation is measured from the downstream toe of the dam. If a trench is cut
through natural ground to install an outlet pipe for a spillway or low-level drain, the
natural ground elevation is measured from the base of the trench where the natural
ground remains undisturbed.
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Ecology's Dam Safety Office is available to provide written guidance documents and
technical assistance for owners and engineers to address dam safety requirements. If
the pond exceeds the volume criteria for dam safety, Ecology shall be contacted early
in the facilities planning process.
7.9 SPECIAL REQUIREMENTS
7.9.1 SPECIAL DRAINAGE AREAS
Special Drainage Areas (SDAs) are designated areas with shallow soils, bedrock near
the surface of the land, and soils or geological features that may make long-term
infiltration of stormwater difficult or areas where infiltration may pose potential
problems for on-site or adjacent properties. These areas may also contain steep slopes
where infiltration of water and dispersion of water into the soils may be difficult or
delayed, creating drainage problems such as erosion. Known areas of flooding or
areas that historically have had drainage or high groundwater problems (mapped or
unmapped) are also SDAs.
SDAs in the Ciry of Spokane are described in SMC 17D.060 "Stormwater Facilities."
Additional requirements for development in these areas are included in this
ordinance.
Spokane County has mapped several SDAs. Among the mapped SDAs are portions of
the Glenrose/Central Park Watershed, the North Spokane Stormwater Planning Area
and the West Plains Stormwater Planning Areas. The Spokane County Stormwater
Utility Section maintains and updates these maps. At the discretion of the local
jurisdiction, an area can be designated as an SDA if it is determined that development
may have adverse impacts on existing or future down-gradient or adjacent properties.
Unless specifically approved by the local jurisdiction, the peak rate and volume of
stormwater runoff from any proposed land development to any natural or constructed
point of discharge downstream shall not exceed the pre-development peak rate or
volume of runoff. A down-gradient analysis demonstrating that there will be no
expected adverse impacts on downgradient properties will be required. Exceptions
with regard to rate and volume control can be made for regional facilities planned by
the local jurisdiction.
7.9.2 FLOODPLAINS
In the City of Spokane and the City of Spokane Valley, floodplain requirements are
administered by the planning department. Check with the local jurisdiction for more
information and specific requirements.
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When any property is developed in and around identified Areas of Special Flood
Hazard (100-year floodplains) all work must conform to the requirements of the
National Flood Insurance Program and the flood ordinance of the local jurisdiction.
This section summarizes the general requirements for projects located within a
floodplain. Specific requirements and additional information can be obtained from the
local jurisdiction.
Land-actions located within a floodplain (A and B Flood Zones only) shall conform
with the following requirements:
• A Floodplain Development permit shall be obtained from the local
jurisdiction before any development (including structures, manufactured
homes, bridges, culverts, grading, excavation or fill) is undertaken,
constructed, located, extended, connected or altered on any properry that is
partly or entirely located in a floodplain;
• The 100-year Base Flood Elevation (BFE) shall not increase at any point
by more than 1.0 foot within Unnumbered A Zones and B Zones; increase
in other designated flood hazard areas (numbered A zones and floodways)
may be further restricted.
• Projects proposing any increases in BFEs or in the way floodwaters enter
and exit the property may require approval from the impacted property
owners.
• Disposal of increases in stormwater runoff may not be allowed in an
identified 100-year floodplain.
• The lowest floor (including basement floor) shall be elevated to a
minimum of 1.0 foot above the BFE. Flood Insurance Rate Maps (FIIZMs)
provide the BFEs for some flood zones. Development in areas without
established BFEs may be inspected by the local jurisdiction. When it is not
evident that the proposed building will be outside the flood zone or if a
subdivision is proposed, a flood study may be required to establish the
100-year BFE and delineate the 100-year floodplain;
• Commercial, industrial, or other nonresidential buildings may be
floodproofed to 1 foot above the BFE in lieu of elevating the lowest floor
elevation to a minimum of 1.0 foot above the BFE. Floodproofing
techniques shall be certified by an engineer or architect licensed in the
State of Washington;
• Residential emergency access and egress shall be provided for the 100-
year event;
• The plat dedication of all subdivision proposals associated with
floodplains shall contain language prescribed by the local jurisdiction.
A floodplain study is required when development impacts floodplains or may impact
floodplains in an unnumbered A Flood Zone or when BFEs have not been
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SPOKANE REGIONAL STORMWATER MANUAL
established. Disturbance to the floodplain may include filling, excavating, etc. The
floodplain study shall meet the following requirements:
• The 100-year peak flows and volumes shall be determined for each basin.
The engineer shall review FEMA studies, previously accepted floodplain
studies, USGS studies and gage data, or watershed plans for already
established 100-year flows. If 100-year flows are not available from other
sources acceptable to the local jurisdiction, the engineer shall calculate the
required flow by comparison with similar watersheds where flows have
been determined or the use of regression equations (see USGS Water
Resources Investigations Report 97-4277, Magnitude and Frequency of
Floods in Washington or the most current version), or by running a
hydraulic model per the requirements of this Manual. Contact the local
jurisdiction for guidance on the appropriate storm type and duration to use.
• The study shall include all relevant calculations for determining the 100-
year flow. The study shall be presented in a rational format so as to allow
a reviewer to reproduce the same results; a basin map showing the site
boundary and the limits of the watershed contributing to the floodplain
shall be included. Topographic contours shall extend beyond the
floodplain's watershed boundary, as needed, to confirm the basin limits.
The basin map shall meet the requirements of Section 3.43;
• In determining the BFE, the study shall use field-surveyed cross-sections
of the floodplain in the project area. The cross-sections shall extend
offsite, as necessary, to delineate the floodplain in the area of the proposal.
FEMA-generated cross-sections may be available for use, but these shall
be supplemented with field-surveyed cross-sections for the specific site;
• The BFE shall be determined and the floodplain shall be delineated for the
pre-developed and post-developed conditions. The BFE shall be tabulated
by station in order to estimate any change to the BFE and delineate
modifications to the floodplain. The analysis shall calculate the pre-
developed and post-developed BFEs as follows:
o To the nearest 1/10 of a foot in unnumbered A and B zones;
o To the nearest 1/100 of a foot in numbered A zones; and,
o To the nearest 1/1,000 of a foot (as required by FEMA) in floodway
areas.
• Floodplain analysis maps shall be prepared for the pre-developed and
post-developed conditions and shall meet the following requirements:
o The maps shall show the BFEs on-site to the nearest 1/lOth of a foot
and clearly delineate the 100-year floodplain;
o Topographic contours shall be clearly marked, a bench mark shall be
identified for the topographic work and the details of the bench mark
shall be discussed;
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SPOKANE REGIONAL STORMWATER MANUAL
o Maps shall clearly show no violations to the requirements of the local
jurisdiction's Flood Ordinance;
o All lots and development, a north arrow, and a scale bar shall be
shown on the map; and,
o The map must be stamped and signed by an engineer.
In unincorporated Spokane County, plats, short plats and commercial project
floodplain requirements shall be coordinated during the pre-design meeting and
submitted with the Drainage Submittal. For single-family residential projects, the
engineer shall work directly with the Environmental Programs section of the
Engineering Department as soon as possible in the planning process.
7.9.3 WETLANDS AND CLASSIFIED STREAMS
Wetlands and classified streams are regulated by the Department of Ecology, the
Department of Fish and Wildlife and the local jurisdiction's critical areas ordinance.
Classified streams are those identified and classified under the Washington
Department of Natural Resources' water typing system. This section provides criteria
for using a wetland for stormwater treatment or disposal. The engineer shall
coordinate with the local building and planning department for further requirements.
The term wetland encompasses a variety of aquatic habitats including swamps,
marshes, bogs or floodplains. Wetlands have a natural supply of water, from flooding
rivers, streams, natural drainage channels, connections to groundwater, or a perched
shallow groundwater table, and are typically inundated with water for a portion of the
year. Wetlands are often vegetated with aspen, cattails, cottonwoods, willows, reed
grasses and other aquatic plants.
Sites with a wetland or a classified stream often feature other Natural Location of
Drainage Systems as well. In addition to the requirements in Section 8.3, the
following are required for sites with a wetland or classified stream:
• A qualified wetland biologist shall categorize the wetland, according to
the local jurisdiction's critical areas ordinance and Ecology's Wetland
Rating System for Eastern Washington, and delineate the wetland
boundaries and buffer areas. More information can be found at:
http://www.ecy.wa.gov/programs/sea/wetlan.html;
• The proponent shall submit to the local jurisdiction a Mitigation Plan,
accepted by the Department of Ecology, if the wetland is to be disturbed
due to construction activity or if any natural source of recharge to the
wetland will be eliminated or altered;
• A Hydraulic Permit shall be obtained when work is proposed within the
normal high-water level of classified streams. Site alterations within the
buffers of regulated streams are generally limited to essential access and
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SPOKANE REGIONAL STORMWATER MANUAL
utiliry needs or restoration plans as reviewed and accepted by the State
Department of Fish and Wildlife and under the local jurisdiction's critical
areas ordinance; and,
• The local planning department and state and federal agencies shall be
contacted for permitting and buffer requirements, etc.
Requirements for hydrologic modification of a wetland for stormwater treatment or
disposal are presented in Section 6.7.5.
7.9.4 CLOSED DEPRESSIONS
Closed depressions are natural low areas that hold a fixed volume of surface water.
Depending upon soil characteristics, a closed depression may or may not accumulate
surface water during wet periods of the year. Some closed depressions may be
classified as wetlands. If so, the engineer shall comply with the wetland criteria
specified in this chapter and in Chapter 6. Analysis of closed depressions shall
include the following at a minimum:
• Identification of the location of the closed depression on the pre-developed
basin map;
• A routing analysis of the drainage basins contributing to the closed
depression to estimate the peak flow rates and volumes leaving the site in
the pre-developed condition;
• An estimation of the storage capacity of the closed depression for the 100-
year storm event;
If the closed depression will be filled in, a facility shall be provided that has the
capacity to store the 100-year volume that was historically intercepted by the closed
depression. This is in addition to the drainage facilities required for flow control and
treatment due to the increase in stormwater runoff. The construction plans shall
include a grading plan of any closed depression areas to be filled in. The grading plan
shall show both existing and finish grade contours. The plans shall also specify
compaction and fill material requirements.
7.10 REGIONAL STORMWATER FACILITIES
Regional stormwater facilities are grass-lined ditches, natural drainageways, ponds, pipes
and various other means of conveying, treating and disposing of stormwater runoff that
serve as the "backbone" of a system to which smaller drainage elements can be
connected. Most regional facilities serve more than a single development within a given
contributing drainage basin. Regional facilities have the potential to lessen flooding in
existing drainage problem areas and to provide new development with an alternative to
on-site stormwater disposal.
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SPOKANE REGIONAL STORMWATER MANUAL
If regional facilities consist of pipes or other non-infiltrative conveyance facilities, they
have the potential to significantly increase stormwater runoff and contaminants going
into selected discharge areas. The location of such discharges, and pretreatment levels,
must be carefully considered to avoid adverse impacts on water resources.
Regional facilities may reduce a communiry's long term costs for stormwater
management because they can free up buildable land for development and can be less
expensive to build, operate, and maintain than multiple individual facilities. The local
jurisdiction may assume responsibiliry, or form a partnership, for the design,
construction, operation and maintenance of regional facilities.
Studies are currently being performed and completed for several planned regional
facilities in the Spokane region. In addition, local jurisdictions have begun mapping
natural stormwater features that will need to be incorporated into future regional
stormwater systems. Due to this recent progress, developments in the near future may be
allowed to discharge stormwater into regional systems. As regional facilities come "on-
line," the requirements for on-site treatment and detention may vary from the basic
requirements in this manual. Close coordination with the local jurisdiction will be
required in order to determine the location and timing of any planned regional system,
and to learn the specific design criteria for on-site stormwater facilities that may
discharge into the system.
All projects shall be reviewed for the presence of natural drainageways, and a
determination will be made as to their significance with regard to preservation of natural
conveyance and potential use as part of a regional system.
When a local jurisdiction assumes the responsibility for any or all portions of the design,
construction, operation, and maintenance of the drainage facilities, project proponents
shall be required to contribute a pro-rated share of the cost (via system development
charges or other related fees) based on the estimated cost of improvements the project
proponent would otherwise have been required to install. The proponent shall supply the
information to justify the estimated costs of the foregone individual improvements.
While opportunities may be available for private developments to use public regional
stormwater facilities to accommodate runoff, local jurisdictions reserve the authority to
limit or restrict discharge to public facilities.
Spokane County has completed Stormwater Management Plans for Chester Creek and the
Glenrose, Central Park, North Spokane and West Plains Stormwater Planning Areas. The
Ciry of Spokane has completed a City Stormwater Management Plan and the City of
Spokane Valley may also identify needed regional stormwater facilities in the near future.
Project proponents shall coordinate with the appropriate local jurisdiction early in the
project proposal process if the project is in an area for which natural drainage features
with potential regional significance have been identified where regional facilities have
been proposed, or where capital improvement plans have been adopted.
Apri12008 Chapter 7—Flow Control
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SPOKANE REGIONAL STORMWATER MANUAL
CHAPTER 8 - CONVEYANCE
.��
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Chapter Organization
8.1 Introduction......................................................................................................................8-1
8.2 Applicability ....................................................................................................................8-1
8.3 Natural and Constructed Channels...................................................................................8-1
8.3.1 Channel Analysis .....................................................................................................8-1
8.32 Minimum Requirements ..........................................................................................8-3
8.3.3 Channel Design........................................................................................................8-5
8.3.4 Preservation of Natural Location of Drainage Systems (NLDS).............................8-8
8.4 Culverts..........................................................................................................................8-11
8.4.1 Culvert Analysis.....................................................................................................8-11
8.42 Minimum Requirements for Culverts....................................................................8-11
8.43 Culvert Design.......................................................................................................8-15
8.5 Storm Drain Systems .....................................................................................................8-15
8.5.1 Pipe Analysis .........................................................................................................8-15
8.52 Minimum Requirements ........................................................................................8-16
8.5.3 Pipe Design............................................................................................................8-19
8.6 Gutters............................................................................................................................8-19
8.6.1 Gutter Analysis ......................................................................................................8-20
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SPOKANE REGIONAL STORMWATER MANUAL
8.62 Gutter Design.........................................................................................................8-21
8.7 Drainage Inlets...............................................................................................................8-24
8.7.1 Minimum Requirements ........................................................................................8-24
8.72 Drainage Inlet Design............................................................................................8-26
Appendix 8A —Example Calculation: Non-Flooded Width (Uniform Gutter Section)............8-33
Appendix 8B —Example Calculation: Non-Flooded Width (Composite Gutter Section).........8-35
Appendix 8C —Example Calculation: Grate Inlet Capacity (Uniform Gutter Section)............8-37
Appendix 8D —Example Calculation: Grate Inlet Capaciry, Continuous Grade (Composite Gutter
Section)......................................................................................................................................8-39
Appendix 8E — Example Calculation: Grate Inlet Capacity, Continuous Grade (Uniform Gutter
Section)......................................................................................................................................8-41
Appendix 8F —Example Calculation: Curb Inlet Capacity, Continuous Grade (Composite Gutter
Section)......................................................................................................................................8-43
Appendix 8G—Example Calculation: Combination Inlet Capacity, Sump ..............................8-45
Appendix 8H—Example Calculation: Curb Inlet Capacity, Sump ...........................................8-47
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SPOKANE REGIONAL STORMWATER MANUAL
8.1 INTRODUCTION
A conveyance system includes all natural or constructed components of a storm drain
system that collect stormwater runoff and convey it away from structures, minimizing the
potential for flooding and erosion.
Conveyance facilities consist of curbs and gutters, inlets, storm drains, catch basins,
channels, ditches, pipes and culverts. The placement and hydraulic capacities of storm
drain structures and conveyance systems shall consider the potential for damage to
adjacent properties and minimize flooding within traveled roadways. The conveyance
system shall also provide discharge capacity sufficient to convey the design flow at
velocities that are self-cleaning without being destructive to the conveyance facilities.
These objectives are achieved by designing all conveyance facilities using the design
storm event specified for the given facility and by adhering to requirements such as
minimum velocity, freeboard, cover, etc.
A properly designed conveyance system maximizes hydraulic efficiency by using the
proper material, slope and size. Constructed conveyance systems should emulate natural,
pre-developed conditions to the maximum extent feasible. Field-verified defined natural
drainageways must be preserved and protected; filling them in and building on top of
them is not an acceptable practice. In addition, some drainageways may be required for
regional use(refer to Section 83.4 for criteria).
Inflow and discharge from the system shall occur at the natural drainage points in the
same manner as the pre-developed condition as determined by topography and existing
drainage patterns.
8.2 APPLICABILITY
All projects shall comply with this Basic Requirement regardless of whether the project
they meet the regulatory threshold.
8.3 NATURAL AND CONSTRUCTED CHANNELS
8.3.1 CHANNEL ANALYSIS
A channel analysis shall be performed for all constructed channels proposed for a
project and for all field-verified existing natural drainageways/channels present on-
site (refer to Section 83.4 for details). The following requirements apply to the
Drainage Report and the road and drainage plans, when applicable:
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SPOKANE REGIONAL STORMWATER MANUAL
• Complete channel calculations shall be provided, indicating the design
peak flow rates and assumptions, such as channel shape, slope and
Manning's coefficient(see Table 5-4);
• Calculations, including the velocity, capacity, and Froude number
shall be provided for each distinct channel segment whenever the
geometry of the channel changes (i.e. if the slope, shape or roughness
changes significantly);
• The centerline and direction of flow for all constructed drainage
ditches or natural channels within the project limits are to be clearly
shown on the construction plans and basin map. For all proposed
channels, locating information shall be provided at all angle points;
• Calculations shall support the riprap area, thickness, riprap size and
gradation, and filter blanket reinforcement for all channel protection,
which shall be provided when permissible velocities are exceeded (see
Table 8-1). This information shall be included in the plans;
TABLE 8-1
PERMISSIBLE VELOCITIES FOR CHANNELS WITH ERODIBLE LININGS,
BASED ON UNIFORM FLOW IN CONTINUOUSLY WET,AGED CHANNELS
Maximum Permissible
Velocities (feet/second)
Soil TypeOf Lining Clear Water Carrying Water Carrying
(Earth; NoVegetation) Water FineSilts Sand & Gravel
Fine sand(non-colloidal) 1.5 2.5 1.5
Sandy loam (non-colloidal) 1.7 2.5 2.0
Silt loam (non-colloidal) 2.0 3.0 2.0
Ordinary firm loam 2.5 3.5 2.2
Volcanic ash 2.5 3.5 2.0
Fine gravel 2.5 5.0 3.7
Stiff clay (very colloidal) 3.7 5.0 3.0
Graded, loam to cobbles (non-colloidal) 3.7 5.0 5.0
Graded, silt to cobbles (colloidal) 4.0 5.5 5.0
Alluvial silts (non-colloidal) 2.0 3.5 2.0
Alluvial silts (colloidal) 3.7 5.0 3.0
Coarse gravel (non-colloidal) 4.0 6.0 6.5
Cobbles and shingles 5.0 5.5 6.5
Shales and hard pans 6.0 6.0 5.0
Source: Special Committee on Irrigation Research,American Society of Civil Engineers, 1926.
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SPOKANE REGIONAL STORMWATER MANUAL
• The Froude number shall be checked near the beginning and near the
end of a channel that has significant grade changes to determine if a
hydraulic jump occurs (as indicated by the Froude number changing
from <1 to >1, or vice versa). Since it is difficult to correlate the
location of a hydraulic jump to the actual location in the field, the
engineer shall propose evenly spaced riprap berms, check dams, or
other protective measures to ensure that the jump does not erode the
conveyance facility;
• When geosynthetics are used for channel protection, the plans shall
clearly specify fabric type, placement, and anchoring requirements.
Installation shall be per the manufacturer's recommendation; and,
• Plans for grass-lined channels shall specify seed mixture and irrigation
requirements, as applicable.
8.3.2 MINIMUM REQUIREMENTS
Slope
Minimum grades for constructed channels shall be as follows:
• 1.0%for asphalt concrete; and,
• 0.5%for cement concrete, graded earth or close-cropped grass.
Side Slopes
Ditch cross-sections may be V-shaped or trapezoidal. However, V-ditches are not
recommended in easily erodible soils or where problems establishing vegetation are
anticipated.
The side slope of roadside ditches shall conform to the requirements for clear zone of
the local jurisdiction and WSDOT design standards.
No ditches or channels shall have side slopes that exceed the natural angle of repose
for a given material or per Table 8-2.
Location
Constructed channels shall not be placed within or between residential lots. Ditches
and channels shall be located within a drainage tract or within a border easement.
Ditches or channels may be allowed to traverse through lots in large-lot subdivisions
(lots of 1 acre or more) and consideration may be given to placement within an
easement versus a tract. The local jurisdiction will review these proposals on a case-
by-case basis.
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SPOKANE REGIONAL STORMWATER MANUAL
TABLE 8-2
MAXIMUM DITCH OR CHANNEL SIDE SLOPES
Typeof Channel SideSlope (Horizontal: Vertical)
Firm rock �/4:1 to Vertical
Concrete-lined stiff clay �/z:l
Fissured rock 1/2:1
Firm earth with stone lining 1�/z:l
Firm earth, large channels 1�/z:l
Firm earth, small channels 2:1
Loose, sandy earth 2:1
Sandy,porous loam 3:1
Source: Civil Engineering Reference Manual, 8th Edition
Depth
The minimum depth of open channels shall be 1.3 times the flow depth or 1 foot;
whichever is greater.
Velocity
Table 8-1 lists the maximum permissible mean channel velocities for various types of
soil and ground cover. If inean channel velocities exceed these values, channel
protection is required (refer to Section 83.3). In addition, the following criteria shall
apply:
• Where only sparse vegetative cover can be established or maintained,
velocities should not exceed 3 feet/second;
• Where the vegetation is established by seeding, velocities in the range
of 3 to 4 feet/second are permitted;
• Where dense sod can be developed quickly or where the normal flow
in the channel can be diverted until a vegetative cover is established,
velocities of 4 to 5 feet/second are permitted; and,
• On well established sod of good quality, velocities in the range of 5 to
6 feet/second are permitted.
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SPOKANE REGIONAL STORMWATER MANUAL
8.3.3 CHANNEL DESIGN
Channel Capacity
Open channels shall be sized using the following variation of Manning's formula.
Q = VA — 1.486 A R�3 S�� (8-1)
n
Where: Q = rate of flow (cfs);
V = mean velocity in channel (feet/second);
A = cross-sectional area of flow in the channel (square
feet);
R = hydraulic radius (feet); where R=A/P, and
P =wetted perimeter(feet)
S = channel slope (feet/foot);
n = Manning's roughness coefficient (Table 5-4); and,
Note: Manning's equation will give a reliable estimate of velocity only if the
discharge, channel cross-section, roughness, and slope are constant over a sufficient
distance to establish uniform flow conditions. Uniform flow conditions seldom, if
ever, occur in nature because channel sections change from point to point. For
practical purposes, however, Manning's equation can be applied to most open
channel flow problems by making judicious assumptions.
Energy Dissipc�tion Design
An energy dissipater is useful in reducing excess velociry, as a means of preventing
erosion below an outfall or spillway. Common rypes of energy dissipaters for small
hydraulic works are: hydraulic jumps, stilling wells, riprap outfall pads, and gabion
weirs.
Channel Protection
Channel velocities shall be analyzed at the following locations, and if they are found
to be erosive, channel protection shall be provided:
• At the top of a watershed, at the point where the stormwater runoff
becomes concentrated into a natural or constructed channel;
• At all changes in channel configuration (grade, side slopes, depth,
shape, etc.), if an erosive velocity is determined at a change in channel
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SPOKANE REGIONAL STORMWATER MANUAL
configuration, the velocity shall be evaluated up the channel until the
point at which the velociry is determined not to be erosive; and,
• At periodic locations along the entire channelized route.
A material shall be selected that has revetment and armoring capabilities, and the
channel shall be analyzed using the Manning's "n" value for that material to
determine if the material will reduce the velocity in the channel. In some cases,
vegetative cover (natural grasses, etc.) may provide excellent protection without
changing the flow characteristics and should be evaluated. If the calculations reveal
that common materials such as riprap are not adequate, stronger protection such as
gabions and/or stilling pools may be necessary.
RipNap PNOtection at Outlets
If the velocity at a channel or culvert outlet exceeds the maximum permissible
velocity for the soil or channel lining, channel protection is required. The protection
usually consists of a reach between the outlet and the stable downstream channel
lined with an erosion-resistant material such as riprap.
The ability of riprap revetment to resist erosion is related to the size, shape and
weight of the stones. Most riprap-lined channels require either a gravel filter blanket
or filter fabric under the riprap.
Riprap material shall be blocky in shape rather than elongated. The riprap stone shall
have sharp, angular, clean edges. Riprap stone shall be reasonably well-graded.
Apron Dimensions: The length of an apron (La) is determined using the following
empirical relationships that were developed for the U.S. Environmental Protection
Agency (ASCE, 1992):
L� _ �gQ +�7Do� for TW < �° (8-2)
0
Or
LQ = ��z +�7Do� for TW >_ �° (8-3)
0
Where: Do = maximum inside culvert width (feet);
Q = pipe discharge (cfs); and,
TW = tailwater depth (feet).
When there is no well-defined channel downstream of the apron, the width, W, of the
apron outlet as shown in Figure 8-1, shall be calculated using Equation 8-4 or 8-5:
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SPOKANE REGIONAL STORMWATER MANUAL
�� � I
°1 3Do
�
I
La
I
W
Figure 8-1 —Riprap Revetment at Outfall Schematic
W = 3Do +0.4LQ for TW >_�° (8-4)
W = 3Do +LQ for TW <�° (8-5)
When there is a well-defined channel downstream of the apron, the bottom width of
the apron should be at least equal to the bottom width of the channel and the lining
should extend at least 1 foot above the tailwater elevation.
The width of the apron at a culvert outlet should be at least 3 times the culvert width.
A�ron Materials: The median stone diameter, Dso is determined from the following
equation:
� _ �.�ZQ4/3 g-6
50 �,7j7T(DO\ � �
�� l 1
Where: DSO = the diameter of rock, for which 50% of the particles
are finer.
The riprap should be reasonably well graded, within the following gradation
parameters:
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SPOKANE REGIONAL STORMWATER MANUAL
I.ZJ � �max � I.J� al1U �15 = �.J� al1U �min = �.ZJ
Dso Dso Dso
Where: DmaX = the maximum particle size;
Dm�n = the minimum particle size; and,
Dis = the diameter of rock, for which 15% of the particles
are finer.
Minimum Thickness: The minimum thickness of the riprap layer shall be 12 inches,
DmaX or 1.SDso, whichever is greater.
Filter Blanket: A filter fabric blanket under the riprap is normally needed. If a gravel
or sand filter blanket is used, then it shall conform to the gradation parameters listed
in Table 8-3.
TABLE 8-3
CRITERIA FOR GRAVEL OR
SAND FILTER BLANKET GRADATION
Primary Criterion D15 < Sdgs
SCI15 <D15 <4�CI15
Recommended Secondary Criteria
DSp�CI50< $�
Guidelines far Stormwater Management, Spokane County,February 1998
The size of the filter blanket material is designated dXX, the size of the riprap is
designated DXX, and the size of the subgrade is designated d'XX. The thickness of each
filter blanket should be one-half that of the riprap layer. If it is found that D15/d'gs< 2
then no filter blanket is needed. Where very large riprap is used, it is sometimes
necessary to use two filter blanket layers between the sub-grade and the riprap.
8.3.4 PRESERVATION OF NATURAL LOCATION OF DRAINAGE
SY ST E M S (N L DS)
New development shall be designed to protect certain natural drainage features that
convey or store water or allow it to infiltrate into the ground in its natural location,
including drainageways, floodplains (Section 7.9.2), wetlands and streams (including
classified streams) (Section 7.9.3), and natural closed depressions (Section 7.9.4).
These features are collectively referred to as the Natural Location of Drainage
Systems (NLDS). Preserving the NLDS will help ensure that stormwater runoff can
continue to be conveyed and disposed of at its natural location. Preservation will also
increase the ability to use the predominant systems as regional stormwater facilities.
A regional stormwater faciliry is typically defined as a system designed and built by a
local jurisdiction to receive an agreed-upon rate and volume of stormwater from a
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SPOKANE REGIONAL STORMWATER MANUAL
defined contributing drainage area, but it can also refer to a private system that serves
multiple developments.
Projects located within the City of Spokane shall refer to the City of Spokane's
Stormwater Ordinance for specific requirements with regard to the Natural Location
of Drainage Systems that may differ from the information found in this section.
Definitions
Some of the drainageways that need to be evaluated for preservation purposes or for
potential use as part of a regional facility have been mapped. These drainageways are
generally defined as Type A and Type B:
• Tvpe A drainageways are predominant systems that are considered a
significant part of a larger existing natural conveyance system.
• Tvpe B drainageways are systems that are generally less prominent,
but are deemed to perform important functions in the existing
management of stormwater runoff and may be necessary for managing
stormwater as part of a larger regional or natural system.
Because every site is unique, the local jurisdiction shall make interpretations, as
necessary, based on site visits and technical information as to the exact location and
type of drainageways or any NLDS on a project site. The local jurisdiction may also
require the project proponent to provide engineering information to assist in this
determination.
The maps denoting these drainageways are not definitive; a computer program was
used to generate the contours and identify the drainageways. The Type A/B
designations are not concrete labels nor are they all inclusive. The maps are only one
tool that may be used to identify existing natural drainageways; field verification will
typically be required to fully identify the existence of a drainageway and its
significance with regard to a natural conveyance system. The Spokane County
Stormwater Utility Section maintains maps of drainageways identified within the
Spokane County Stormwater Service Areas. The criteria for analysis and preservation
of all other NLDS (floodplains, wetlands, closed depressions and wetlands/streams)
are covered in Chapter 7.
Protection
No cuts or fills shall be allowed in predominant drainageways except for
perpendicular driveway or road crossings with engineering plans showing
appropriately sized culverts or bridges. Predominant drainageways shall be preserved
for stormwater conveyance in their existing location and state, and shall also be
considered for use as regional facilities.
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Less prominent drainageways in a non-residential development and in a residential
development containing lots 1 acre or smaller may be realigned within the
development provided that the drainageway will enter and exit the site at the pre-
developed location and that discharge will occur in the same manner as prior to
development.
Realignment of a less prominent drainageway shall be defined as still following the
"basic" flow path of the original drainageway. An acceptable example would be if the
drainageway is proposed to be realigned such that it will follow a new road within the
proposed development, and will be left in its existing state or utilized as part of the
project's on-site stormwater system.
Stormwater leaving the site in the same manner shall be defined as replicating the
way the stormwater left the site in its existing condition. If the drainageway is
preserved in its existing location and is left undisturbed, this goal should be met. If
the local jurisdiction accepts the proposal to allow a less predominant drainageway to
be routed through the site via a pipe, the following additional criteria shall be met:
• Where the less prominent drainageway enters the site, the design shall
ensure that the entire drainageway is "captured" as it enters the site;
i.e. the surrounding property shall not be regraded to "neck-down" the
drainageway so that it fits into a drainage easement or tract or structure
intended to capture and reroute the off-site stormwater runoff.
• Where the less prominent drainageway exits the site, the design shall
ensure that the stormwater leaves the pipe, pond or structure a
significant distance from the edge of the adjacent property so that by
the time the stormwater reaches the property boundary, its dispersal
shall mimic that of the pre-developed condition.
Since some of the less prominent drainageways may also be useful for managing
regional stormwater, if identified as a significant drainageway (i.e. necessary
conveyance for flood control, or being considered as a connection to a planned
regional facility or conveyance route), then the drainageway may be subject to the
same limitations and criteria as a predominant drainageway.
The size of the tract or easement containing the drainageway shall be determined
based on an analysis of the existing and proposed stormwater flows directed to these
drainage systems and any access and maintenance requirements found in this Manual.
This analysis shall be performed as per the criteria found in Basic Requirement No. 5,
Section 2.2.5.
All new development containing lots that are 1 acre or smaller shall be required to set
aside the drainageway as open space in a separate tract. For new development
containing lots that are greater than 1 acre, the drainageway may be set aside in either
a tract or an easement.
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All projects shall be reviewed for the presence of any NLDS and a determination will
be made as to their significance with respect to preservation for continued natural
conveyance and for potential use as part of a regional system.
8.4 CULVERTS
A culvert is a short pipe used to convey flow under a roadway or embankment. A culvert
shall convey flow without causing damaging backwater flow constriction, or excessive
outlet velocities. Factors to be taken into consideration in culvert design include design
flows, the culvert's hydraulic performance, the economy of alternative pipe materials and
sizes, horizontal and vertical alignment, and environmental concerns.
8.4.1 CULVERT ANALY515
When applicable, the following items shall be included in the Drainage Report, or on
road and drainage plans:
• Complete culvert calculations that state the design peak flow rates,
velocities at the inlet and outlet, flow control type, and design
information for the culvert such as size, slope, length, material type,
and Manning's coefficient(refer to Table 8-4);
• Headwater depths and water surface elevations for the design flow
rate;
• Roadway cross-section and roadway profile;
• Location information for each of the culvert inverts and invert
elevations;
• Type of end treatment (wingwall, flared end sections, etc); and,
• Wall thickness.
8.4.2 MINIMUM REQUIREMENTS FOR CULVERTS
Peak Flow Rate
Culverts shall be sized to handle the design peak flow rates calculated using the
methods described in Chapter 5 and the design criteria specified in Chapter 2.
To avoid saturation of the road base, culverts shall be designed such that the water
surface elevation for the design storm event does not exceed the elevation of the base
course of the roadway.
Culverts shall be designed to withstand the 100-year storm event without damage.
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TABLE 8-4
MANNING'S ROUGHNESS COEFFICIENT (n)
FOR CULVERTS
Material Type n
Concrete pipe 0.013
Ductile iron 0.013
HDPE(only allowed in private roads) 0.013
CMP 0.024
HDPE=high-density polyethylene; CMP=corrugated metal pipe;PVC=polyvinyl
chloride
Allowable Headwater Elevation
Headwater is the depth of water at the culvert entrance at a given design flow.
Headwater depth is measured from the invert of the culvert to the water surface.
Culverts shall be designed to carry the design runoff with a headwater depth less than
2 times the culvert diameter for culverts 18 inches or less in diameter, and less than
1.5 times the culvert diameter for culverts more than 18 inches in diameter.
Velocity and Slope
To avoid silting, the minimum velocity of flow through culverts shall be 4 feet/second
and the minimum slope shall be 0.5%.
Diameter
Table 8-S lists required minimum culvert diameters.
TABLE 8-5
MINIMUM CULVERT SIZES
Culvert Location Minimum Size (inches)
Under public roads 18
Under private roads 12
Under driveways/approaches 12
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Material and Anchoring
Corrugated metal pipe, ductile iron, or concrete boxes can be used for all culverts.
High-density polyethylene (HDPE) is only allowed in private roads. For grades
greater than or equal to 20%, anchors are required unless calculations or the
manufacturer's recommendations show that they are not necessary.
Placement/Alignment
Generally, culverts shall be placed on the same alignment and grade as the
drainageway. Consideration should also be given to changes of conditions over time
by using design measures such as:
• Cambering or crowning under high tapered fill zones;
• Raising intakes slightly above the flow line to allow for sedimentation;
• Using cantilevered outfalls away from road banks to allow for toe
erosion; and,
• Using drop inlets or manholes to reduce exit velocities on steep terrain.
Angle Points
The slope of a culvert shall remain constant throughout the entire length of the
culvert. However, in situations where existing roadways are to be widened, it may be
necessary to extend an existing culvert at a different slope; the location where the
slope changes is referred to as the angle point. The change in slope tends to create a
location in the culvert that catches debris and sediment. If an extension of a culvert is
to be placed at a different grade than the existing culvert, a manhole shall be provided
at the angle point to facilitate culvert maintenance.
Outfalls
Outfalls shall conform to the requirements of all federal, state, and local regulations.
Erosion control shall be provided at the culvert outfall. Refer to Section 8.3.3 for
additional information regarding outfall protection.
Culvert Debris and Safety
The engineer shall evaluate the site to determine whether debris protection shall be
provided for culverts. Debris protection shall be provided in areas where heavy debris
flow is a concern, for example, in densely wooded areas. Methods for protecting
culverts from debris problems include: upsizing the culvert and installing debris
deflectors, trash racks or debris basins. Section 3.4.8 of the WSDOT Hydraulic
Manual has additional information on debris protection.
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Safety bars to prevent unauthorized individuals from entering the culvert shall be
provided for culverts with a diameter greater than 36 inch (see WSDOT standard
drawings).
When a trash rack is proposed, the effects of plugging shall be evaluated.
Consideration should be given to the potential degree of damage to the roadway and
adjacent property, potential hazard and inconvenience to the public, and the number
of users of the roadway.
Structural Design
The WSDOT Hydraulics Manual, Tables 8-11.1 through 8-11.18, shows the
maximum cover for different pipe materials and sizes.
For culverts under roadways, the amount of cover over the culvert is defined as the
distance from the top of the pipe to the bottom of the pavement. It does not include
asphalt or concrete paving above the base. The minimum amount of cover is 2 feet for
culverts, unless proposing ductile iron pipe. The minimum cover for ductile iron pipe
is 1 foot.
The minimum cover for culverts under private driveways is 1 foot from the top of the
pipe to the finish grade of the drivable surface. Driveway culverts shall be a
minimum of 12" CMP or ductile iron pipe.
If the depth of cover is shallow (less than 1 foot) and truck wheel loads are present, it
will be necessary to propose a design to prevent structural damage to the pipe or to
implement the manufacturer's recommendations. Also, extreme fill heights (20 feet or
greater) may cause structural damage to pipes and will require a special design or
adherence to the manufacturer's recommendations.
End T�eatments
The type of end treatment used on a culvert depends on many interrelated and often
conflicting considerations:
• Projectin_g Ends is a treatment in which the culvert is simply allowed
to protrude out of the embankment. This is the simplest and most
economical. There are several disadvantages such as susceptibility to
flotation and erosion, safery when projecting into a roadway clear zone
(an area beyond the traveled roadway provided for recovery of errant
vehicles), and aesthetic concerns;
• Beveled End Sections consist of cutting the end of the culvert at an
angle to match the embankment slope surrounding the culvert. Beveled
ends should be considered for culverts 6 feet in diameter or less.
Structural problems may be encountered for larger culverts not
reinforced with a headwall or slope collar;
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• Flared End Sections are manufactured culvert ends that provide a
simple transition from culvert to a drainage way. Flared end sections
are typically only used on circular pipe or pipe arches. This end
treatment is typically the most feasible option in pipes up to 48 inches
in diameter. Safety concerns generally prohibit their use in the clear
zone for all but the smallest diameters;
• Headwalls are concrete frames poured around a beveled or projecting
culvert. They provide structural support and eliminate the tendency for
buoyancy. They are considered feasible for metal culverts that range
from 6 to 10 feet in diameter. For larger diameters, a slope collar is
recommended. A slope collar is a reinforced concrete ring that
surrounds the exposed culvert end; or,
• Win�walls and A�rons are intended for use on reinforced concrete box
culverts. Their purpose is to retain and protect the embankment, and
provide a smooth transition between the culvert and the channel.
8.4.3 CULVERT DESIGN
Culvert analysis is typically performed using commercially available computer
software. If hand calculations are proposed, example calculations can be found in
several technical publications and open channel hydraulics manuals.
8.5 STORM DRAIN SYSTEMS
A storm drain system is a network of pipes that convey surface drainage from catch
basins or other surface inlets, through manholes, to an outfall.
The design of storm drain systems shall take into consideration runoff rates, pipe flow
capaciry, hydraulic grade line, soil characteristics, pipe strength, potential construction
problems, and potential impacts on down-gradient properties.
8.5.1 PIPE ANALYSIS
The following items shall be included in the Drainage Report, or on road and
drainage plans:
• A basin map showing on-site and off-site basins contributing runoff to
each inlet, which includes a plan view of the location of the
conveyance system;
• Complete pipe calculations that state the design peak flow rates and
design information for each pipe run, such as size, slope, length,
material type, and Manning's coefficient (see Table 8-6);
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• Velocities at design flow for each pipe run;
• The hydraulic grade line at each inlet, angle point, and outlet; and,
TABLE 8-6
MANNING'S ROUGHNESS COEFFICIENTS (n)
FOR CLOSED SYSTEMS
Material Type n
Concrete pipe 0.013
Ductile iron 0.013
HDPEi 0.013
PVC (only allowed in closed system) 0.013
1 Contact the local jurisdiction far additional requirements when
using HDPE pipe.
For lateral pipe connections to storm drain lines in existing rights-of-way (i.e. from a
catch basin to a drywell, a main line stormwater system, a pond or a swale), fixed
invert elevations are preferred but not required. The minimum depth from finish
grade to pipe invert and the minimum pipe slope necessary to satisfy the freeboard
and self-cleaning velociry requirements shall be provided. If necessary, invert
elevations may be adjusted during construction to avoid potential conflicts with
existing utilities in the right of way.
8.5.2 MINIMUM REQUIREMENTS
Peak Flow Rate
Closed pipe systems shall be sized to handle the design peak flow rates. These peak
rates can be calculated using the methods described in Chapter 5 and the design
criteria specified in Chapter 2.
Hyd�aulic Grn�le Line
The hydraulic grade line (HGL) represents the free water surface elevation of the
flow traveling through a storm drain system. Pipes in closed systems will be sized by
calculating the HGL in each catch basin or manhole. A minimum of 0.5 feet of
freeboard shall be provided between the HGL in a catch basin or manhole and the top
of grate or cover.
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Pipe Velocities and Slope
In Spokane County and the Ciry of Spokane Valley pipe systems shall be designed to
have a self-cleaning velociry of 2.5 feet/second at design flow. In the City of
Spokane, pipe systems shall be designed to have a self-cleaning velocity of 3
feet/second or greater calculated under full flow conditions even if the pipe is only
flowing partially full during the design storm.
Pipe velocities should not be excessively high since high flow velocities (approaching
and above 10 feet/second) cause abrasion of the pipes. When the design velocities are
10 feet/second or greater, manufacturer's recommendations demonstrating that the
pipe material can sustain the proposed velocities shall be provided.
When the grade of a storm pipe is greater than or equal to 20%, then pipe anchors are
required at the joints, at a minimum, unless calculations and manufacturer's
recommendations demonstrate that pipe anchors are not needed. Pipe anchor
locations are to be defined on the plans, and a pipe anchor detail shall be referenced
or provided.
Pipe material shall meet the WSDOT standards for storm sewer pipe. All pipe
segments shall be pressure tested, according to WSDOT testing procedures and
standards
Pipe Diameter and Length
The minimum pipe diameter shall be 12 inches, except that single pipe segments less
than 50 feet long may be 8 inches in diameter. The maximum length of pipe between
junctions shall be no greater than 300 feet. No pipe segment shall have a diameter
smaller than the upstream segments.
Placement and Alignment
No storm drain pipe in a drainage easement shall have its centerline closer than 5 feet
to a private rear or side property line. A storm drain located under a road shall be
placed in accordance with the local jurisdiction's requirements or standard plans.
If it is anticipated that a storm drain system may be expanded in the future, provisions
for the expansion shall be incorporated into the current design.
Outfalls
Pipe outfalls shall be placed on the same alignment and grade as the drainage way.
Outfalls shall conform to the requirements of all federal, state, and local regulations.
Erosion control is required at the storm system outfalls. Refer to Section 8.3.3 for
additional information regarding outfall protection.
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Storm DNain Debris �nd Safety
The engineer shall evaluate the site to determine whether debris protection shall be
provided for storm drain systems. Debris protection shall be provided in areas where
heavy debris flow is a concern, for example, in densely wooded areas. Methods for
protecting storm drain systems from debris problems include debris deflectors, trash
racks and debris basins. The WSDOT Hydraulic Manual has additional information
on debris protection.
For enclosed storm drain systems in urban locations, safety bars shall be provided for
outfalls with a diameter 18 inches or greater, in order to prevent unauthorized
individuals from entering the storm drain system. Outfalls within a fenced area are
not required to have safery bars. The clear space between bars shall be 4 inches
maximum.
Structural Design
The WSDOT Hydraulics Manual, Tables 8-11.1 through 8-11.18, shows the
maximum cover for different pipe materials and sizes.
In unincorporated Spokane Counry and the City of Spokane Valley, the amount of
cover over the pipe is defined as the distance from the top of the pipe to the bottom of
the pavement. It does not include asphalt or concrete paving above the base. The
minimum amount of cover is 2 feet, unless proposing ductile iron. The minimum
cover for ductile iron pipe is 1 foot.
In the City of Spokane, cover is measured from the top of pipe to the top of the
pavement. The minimum amount of cover is 3 feet, unless proposing ductile iron. The
minimum cover for ductile iron pipe is 1 foot.
If the depth of cover is shallow (less than 1 foot) and truck wheel loads are present, it
will be necessary to propose a design to prevent structural damage to the pipe or to
implement manufacturer's recommendations. Extreme fill heights (20 feet or greater)
may also cause structural damage to pipes and will thus require a special design or
adherence to the manufacturer's recommendations.
Inve�ts at Junctions
Whenever two pipes of the same size meet at a junction, the downstream pipe shall be
placed with its invert 0.1 feet below the upstream pipe invert. When two different
sizes of pipes are joined, pipe crowns shall be placed at the same elevation. The
exception to this rule is at drop manholes. Exceptions may be allowed by the local
jurisdiction when topographic conditions will significantly impact the depth of the
disposal location.
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Combined Systems
Combined sanitary and stormwater sewer systems are prohibited.
8.5.3 PIPE DESIGN
To analyze the conveyance capacity of a closed pipe system, the following general
steps may be followed when steady flow conditions exist, or conditions can be
accurately approximated assuming steady flow conditions:
1. Estimate the size of the pipes assuming a uniform flow condition, using
Equation 8-l. Refer to Table 8-6 for Manning's coefficient values.
2. For the pipe sizes chosen, determine uniform and critical flow depth;
3. Determine if upstream (accelerated) flow conditions or downstream (retarded)
flow conditions exist. Subcritical flow occurs when downstream conditions
control, supercritical flow occurs when upstream conditions control.
Determine what flow regime will occur by comparing uniform flow depth,
critical flow depth, and initial flow depth. Identify hydraulic jump locations,
and where any other discontinuity of flow depth will occur.
4. Conduct a more detailed analysis by computing the hydraulic grade line. The
direct step method or standard step method is often used to calculate the
hydraulic grade line. For supercritical flow, begin at the upstream end and
compute flow sections in consecutive order heading downstream. For sub-
critical flow, begin at the downstream end and compute flow sections in
consecutive order heading upstream.
The analysis of closed pipe systems is typically done using commercially available
computer software packages. If hand calculations are proposed, example calculations
can be found in several technical publications on open channel hydraulics, such as:
"Handbook of Hydraulics", by Brater and King; and "Open-Channel Hydraulics" by
French.
8.6 GUTTERS
A gutter is a section of pavement adjacent to a roadway that conveys water during a
storm runoff event. Gutter flow calculations are necessary to establish the spread of water
onto the shoulder, parking lane, or travel lane. Roadways shall have an adequate non-
flooded width to allow for the passing of vehicular traffic during the design storm event.
The non-flooded width (L) is shown in Figure 8-2 and the minimum non-flooded widths
for various road classifications are outlined in Table 8-7.
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L
STORMWATER RUNOFF
GUTTER FLOW� �CURB
L SIDEWALK
Figure 8-2 —Non-Flooded Road Width (L)
TABLE 8-7
NON-FLOODED ROAD WIDTH REQUIREMENTS
R oad C lassification Non-F looded W idth (L)
Private Road 12 feet
Local Access 12 feet
Collector Arterial, 2 Lane 16 feet
Minor Arterial, 2 Lane 24 feet
Other road types Per local jurisdiction
The non-flooded width shall be evaluated at low points and at proposed inlet locations.
The non-flooded width shall also be evaluated at intersections. Bypass flow shall be
limited to 0.1 cfs at intersections and at the project boundary.
Non-flooded width and flow depth at the curb are often used as criteria for spacing
pavement drainage inlets (curb or grate inlets). Drainage inlets shall be spaced so that the
non-flooded width requirements are met and stormwater does not flow over the back of
the curb. Spacing shall not exceed 300 feet regardless of flooded width and flow depth
compliance.
Generally, inlets shall be placed in the uphill side of the curb return. Additionally, the
first inlet shall not be located more 500 feet from the point where the gutter flow path
originates.
8.6.1 GUTTER ANALYSIS
When applicable, the drainage report shall include complete gutter calculations that
state the design peak flow rates, design flow depth, road cross slope, road grade, and
non-flooded width.
The equation for calculating gutter flow is a modified version of Manning's equation.
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�.�� �x.67��.5�2.67
Q = (8-7)
n
Where: Q = flow rate (cfs);
n = Manning's coefficient(from Table 8-8);
SL = longitudinal slope of the gutter(feet/foot);
SX = cross slope (feet/foot); and,
T = spread (feet)
TABLE 8-8
MANNING'S ROUGHNESS COEFFICIENTS (/1�
FOR STREET & PAVEMENT GUTTERS
Type of G utter or pavement n
Concrete gutter, troweled finish 0.012
Asphalt Pavement
Smooth Texture 0.013
Rough Texture 0.016
Concrete pavement
Float finish 0.014
Broom finish 0.016
Source: Federal Highway Administration(FHWA),Aydraulic Engineering
Circular No. 22, Second Edition
8.6.2 GUTTER DESIGN
Unifo�m Gutter Section
Uniform gutter sections have a cross slope that is equal to the cross slope of the
shoulder or travel lane adjacent to the gutter (see Figure 8-3). The spread (T) in a
uniform gutter section can be calculated using Equation 8-7 and solving for T
(spread) as follows:
0375
Q n � �
T 0.56 Sz.b'SL.s 8-8
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T
0
5�
Figure 8-3 —Uniform Gutter Section
An example calculation for determining the non-flooded width and the depth of flow
for a uniform gutter section is provided in Appendix 8A.
Composite Gutter Section
Gutters with composite sections have has a cross slope that is steeper than that of the
adjacent pavement (see Figure 8-4). The design of composite gutters requires
consideration of flow in the depressed segment of the gutter.
T
W Ts
0
o.s
sX
a� ,-
,,,
a � s�
Figure 8-4 —Composite Gutter Section
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The spread (T)in composite gutter sections cannot be determined by a direct solution;
an iterative approach following the procedure outlined below must be used. An
example calculation for determining the spread for a composite gutter section is
included in Appendix 8B.
1. Assume a flow rate above the depressed gutter section, Qs.
2. Compute QW using the following:
Qw = Q—QS (8-9)
Where: QW = flow rate in the depressed section of the gutter(cfs);
Q = design flow rate (cfs);
QS = flow rate in the gutter section beyond the depressed
section (cfs);
3. Compute the gutter cross slope (if it is not given), SW, using following
equation:
5,�, = Sx +�� (8-10)
Where: SW = cross slope of the depressed gutter(feet/foot);
SX = road cross slope (feet/foot);
W = gutter width (feet); and,
a = gutter depression (feet).
4. Compute Eo using the following equation:
�. = Q—QS = Qw (8-11)
� Q Q
Where: Eo = ratio of flow in a chosen width (the width of a
depressed gutter or grate) to the total gutter flow.
5. Solve for T using following equation:
Sx,
T - YY 1+ �x 3 �O-IG�
�
S"' E° +l —1
S� 1—Eo
6. Compute TS using following equation:
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TS = T—W (8-13)
Where: TS = the width of the spread from the junction of the
gutter with the edge of pavement to the edge of the
spread (feet).
7. Use Equation 8-7 to determine QS for TS and compare to estimated QS from
Step l. Steps 1 through 6 shall be repeated until the estimated and computed
Qs are approximately the same.
8.7 DRAINAGE INLETS
Drainage inlets are used to collect runoff and discharge it to a storm drainage system.
They are typically located in gutter sections, paved medians, and roadside and median
ditches. Inlets most commonly used in the Spokane Region are as follows:
• Grate Inlets consist of an opening in the gutter or ditch covered by a grate. They
perform satisfactorily over a wide range of longitudinal slopes. Grate inlets
generally lose capaciry as the grade of the road, gutter or ditch increases.
• Curb Inlets are vertical openings in the curb. They are most effective on flat
grades, in sumps, and where flows are found to carry significant amounts of
floating debris. Curb inlets lose interception capacity as the gutter grade increases;
therefore, the use of curb inlets is recommended in sumps and on grades less than
3%.
• Combination Inlets consist of both a curb-opening and a grate inlet. They offer the
advantages of both grate and curb inlets, resulting in a high capacity inlet.
There are many variables involved in designing the number and placement of inlets, and
in determining the hydraulic capacity of an inlet. The hydraulic capacity of a storm drain
inlet depends upon its geometry as well as the characteristics of the gutter flow. Inlet
capaciry governs both the rate of water removal from the gutter and the amount of water
that can enter the storm drainage system. Inadequate inlet capacity or poor inlet location
may cause flooding on the roadway resulting in a hazard to the traveling public.
8.7.1 MINIMUM REQUIREMENTS
Peak Flow Rate
The capacity of drainage inlets shall be determined using the design peak flow rates.
These rates can be calculated using the methods described in Chapter 5 and the design
storm criteria specified in Chapter 2.
Bypass flow shall be limited to 01 cfs at intersections and at the project boundary.
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Structures
Catch basins, inlets and storm manholes shall conform to the standard plans of the
local jurisdiction, or the standard plans jointly published by WSDOT and APWA
(M21-O l).
Catch basins shall be used in all public and private roads unless utiliry conflicts
prohibit their use.
WSDOT/County Type 1 Catch Basins shall not be used where invert elevation depths
are more than 5 feet below lid elevations. Manholes shall be used in these situations.
Catch basins, inlets, and storm manholes shall be placed at all breaks in grade and
horizontal alignments. Pipe runs shall not exceed 300 feet for all pipe sizes.
Horizontal and vertical angle points shall not be allowed in a storm system unless a
manhole is provided for cleaning.
Grates
Herringbone grates are no longer accepted in roadway applications.
All grate inlets constructed at low points shall be combination inlets. The most
commonly used combination inlet is a vaned grate with a hooded curb cut area.
Grate inlets on grade shall have a minimum spacing of 20 feet to enable the bypass
water to reestablish its flow against the face of curb. Drainage inlets shall not be
located on the curved portion of a curb return.
Grates shall be depressed to ensure satisfactory operation; the maximum depression is
2 inches.
Inlets with larger openings may be used for additional capacity, such as WSDOT
Grate Inlet Type 2 (WSDOT Standard Plan B-4035-00) with frame and vaned grate
(WSDOT Standard Plan B-40.40-00). WSDOT Grate Inlet Type 1 and Grates A and
B shall not be used in areas of pedestrian or vehicular traffic. Refer to WSDOT
Manual and Standard Plans if any of the WSDOT inlets are proposed.
Curb Inlets
Concrete curb inlets (i.e. aprons) shall be used at the entrances to all stormwater
facilities to aid stormwater conveyance into the facility and to suppress grass growth
at the inlet.
The curb inlet shall have a 2-inch depression at the curb line and a maximum length
of 6 feet.
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At a minimum (where space constraints allow), curb inlets shall be placed at the most
upstream and downstream point along the road adjacent to the treatment or disposal
facility, regardless of the flow directed to the curb inlet. In many cases, when a long
drainage facility is proposed, and the engineering calculations support it, additional
intermediate curb inlets may be required.
Overflow structures, such as drywells or catch basins, shall be located away from the
point or points where runoff flows into the facility. When the overflow structure is
located within the facility, slopes around the structure shall be no greater than 4:1
(horizontal to vertical).
8.7.2 DRAINAGE INLET DESIGN
Grate Inlets, Continuous Grade
The capacity of an inlet on a continuous grade can be found by determining the
portion of the gutter discharge directly over the width of the inlet. On continuous
grades (assuming that the grate has the capacity to intercept the entire flow rate
directed toward it), the amount of stormwater intercepted by a grate is equal to the
amount of stormwater runoff flowing directly over the grate plus the amount that
flows in over the side of the grate through the slats/bars. The analysis shall include a
35% clogging factor. The use of formulas for side flow interception for grate inlets
found in FHWA Hydraulic Engineering Circular No. 22 (HEC-22)will be accepted.
The following procedure is most accurate when velocities are in the range of 3 to 5
feet/second at a 2% or 3% longitudinal slope. For instances where the velocity is
found to exceed 5 feet/second, additional intermediate inlets can be added,
contributing basins redefined, and the associated velocities recalculated. While
adding inlets is one solution to reducing the velocity, more information may be found
regarding the affect of side flow by consulting the HEC-22 Circular, Section 4.4
Drainage Inlet Design. Note that commercially available software may be used to
determine grate inlet capacity.
The capacity of a grate inlet on a continuous grade may be calculated using the
procedure outlined below. Figure 8-5 identifies key parameters. Example calculations
for grate inlets on a continuous grade for a uniform gutter section and a composite
gutter section are provided in Appendices 8C and 8D.
l. Determine the runoff from the contributing basin at the high point to the first
inlet. This is the amount of runoff that could be intercepted by the first inlet.
2. Select an inlet and note the grate width (GW) in the calculations (refer to
Table 8-9).
Apri12008 Chapter 8—Conveyance
8-26
SPOKANE REGIONAL STORMWATER MANUAL
T
d ' QBP
V� � S
x
1
GW
Figure 8-5–Typical Grate Inlet Cross-Section
TABLE 8-9
ALLOWABLE WIDTH AND PERIMETER
FOR GRATE CAPACITY ANALYSIS
Allowable W idth Allowable Perimeter
on a Continuous in a Sump Condition
Str ucture T ype G rade (feet) (feet)
Vaned Grate for Catch Basin and Inlet 1.67 —
Metal Frame and Grate for Catch Basin
and Inlet(Herringbone Pattern)i 1.67 —
Metal Frame with Hood and 1.67 3.134,s
Bi-Directional Vaned Grate
Frame and Vaned Grates for Grate Inlet 1.75�
Type 2 (WSDOT B-40.40-00) 3.503 2.964,s
1 Not recommended for new construction.Values are presented for evaluation of existing conditions.
�Normal Installation—see Figure 5-5.5 of WSDOT Hydraulics Manual
3 Rotated Installation—see Figure 5-5.5 of WSDOT Hydraulics Manual
4 This perimeter value has already been reduced by 50%for clogging.
5 This perimeter value has also been reduced far bar area.
Note: Readers should review the most current versions of the local jurisdiction's standard plans far any
revisions that may have been made to values provided in this table.
Apri12008 Chapter 8—Conveyance
8-27
SPOKANE REGIONAL STORMWATER MANUAL
3. Analyze the most upstream inlet. The width of flow (T) is calculated using the
procedure described in Section 8.6.2. Verify that T is within the allowable
limit (see Table 8-7), then determine the amount of flow intercepted by the
grate (basin flow—bypass flow).
4. The inlet bypass flow on a continuous grade is computed as follows:
s
(T—GW) 3
QBP = Q �T� �8-14)
Where: QBP = portion of flow outside the grate width (cfs);
Q = total flow of gutter approaching the inlet (cfs);
T = spread, calculated from the gutter section upstream
of the inlet (feet); and
GW = grate inlet width perpendicular to the direction of
flow (feet), see Table 8-9.
5. The velocity shall not exceed 5 feet/second. The velocity of flow directly over
the inlet is calculated as follows:
V� Q—QBP (8-15)
(GW)[d—0.5(GW)(Sx)]
Where: Vi = velocity over the inlet(feet/second);
SX = cross slope (feet/foot); and,
d = depth of flow at the face of the curb (feet), given by:
d = (T)(Sx) (8-16)
If the non-flooded road width does not meet the minimum criteria, an
additional inlet should be placed at an intermediate location and the procedure
repeated. If the velocity exceeds 5 feet/second then side flow shall be
considered using the method outlined in HEC-22.
6. The analysis is then repeated with the next inlet. The bypass flow (QBp) from
the previous inlet shall be added to the flow from the contributing basin to
determine the total flow (to the inlet at the station being analyzed.
7. The last inlet may require an adjustment of spacing (usually smaller spacing)
in order to prevent a bypass flow to the project boundaries.
Curb Inlets, Continuous G�ade
The capacity of a curb inlet on a continuous grade depends upon the length of
opening and the depth of flow at the opening. This depth in turn depends upon the
amount of depression of the flow line at the inlet, the cross slope, the longitudinal
Apri12008 Chapter 8—Conveyance
8-28
SPOKANE REGIONAL STORMWATER MANUAL
slope, and the roughness of the gutter. The analysis shall include a 35% clogging
factor.
The capacity of a curb inlet on a continuous grade may be calculated using the
procedure outlined below. Example calculations for curb inlets on a continuous grade
for a uniform gutter section and a composite gutter section are provided in
Appendices 8E and 8F.
1. Determine the runoff from the contributing basin at the high point to the first
curb inlet. This is the amount of runoff that could be intercepted by the first
curb inlet.
2. Analyze the most upstream inlet. The width of flow (T) is calculated using the
procedure described in Section 8.6.2. Verify that T is within the allowable
limit(Table 8-7).
3. The length of the curb-opening inlet required for total interception of gutter
flow is calculated as follows:
0.6
LT - �.6Q0.42SL3 � �g-17�
nse
Where: LT = curb opening length required to intercept 100% of
the flow(feet);
Se = equivalent cross slope (feet/foot);
for uniform gutter sections: Se = SX; and,
for composite gutter sections:
Se = Sx +Eo�Sw —Sx�= Sx + i2W (8-18)
where: a = gutter depression (inches);
Eo = ratio of flow in the depressed section to total gutter
flow, calculated in the gutter configuration
upstream of the inlet; and,
W = gutter width (feet).
4. When the actual curb inlet is shorter than the length required for total
interception, calculate the efficiency of the curb inlet using Equation 8-19.
i.s
E = 1— 1—� (8-19)
T
Where: E = efficiency; and,
Apri12008 Chapter 8—Conveyance
8-29
SPOKANE REGIONAL STORMWATER MANUAL
L = actual curb opening length (feet).
5. Compute the interception capacity of the curb inlet using the following
relationship:
Q� _ �E)�Q) �8-20)
Where: Q� = curb inlet capacity (cfs),
6. The analysis is then repeated with the next inlet. The bypass flow (QBP) from
the previous inlet shall be added to the flow from the contributing basin to
determine the total flow (Q) to the inlet at the station being analyzed.
QBP = Q—Q� (8-21)
7. The last inlet may require an adjustment of spacing (usually smaller spacing)
in order to prevent a bypass flow to the project boundaries.
Combination Inlets,Sump Conclition
Inlets in sump locations perform differently than inlets on a continuous grade. Inlets
in sump locations operate in one of two ways: 1) as a weir, at low ponding depths; or
2) as an orifice, at high ponding depths (1.4 times the grate opening length). It is very
rare that ponding on a roadway will become deep enough to force the inlet to operate
as an orifice; therefore, this section will focus on the inlet operating as a weir.
The interception capaciry of a combination inlet in a sump is equal to that of a grate
inlet alone in weir flow. Design procedures presented here are a conservative
approach to estimating the capacity of inlets in sump locations. All inlets in a sump
condition shall be evaluated using a 50% clogging factor.
The analysis shall include an evaluation of the inlet and the surrounding street, gutter,
curb and adjacent properties for storm events exceeding the required level of service.
An emergency overIlow path shall be provided.
The capacity of a combination inlet operating in a sump as a weir may be estimated
using the following procedure. There are also commercially available software
programs that will analyze combination inlets in a sump location. An example
calculation for a combination inlet in a sump location is provided in Appendix 8G.
1. Determine the runoff contributing to the combination inlet. This is the sum of
the bypassed flows from all upstream inlets and the runoff generated from the
basin contributing directly to the combination inlet.
2. Determine the allowable spread (Tall) based on the non-flooded width
requirements in Table 8-7.
3. Calculate the depth of flow at the curb (d)using Equation 8-16.
Apri12008 Chapter 8—Conveyance
8-30
SPOKANE REGIONAL STORMWATER MANUAL
4. Determine the average depth of flow over the grate using one of the following
relationships:
For uniform gutter sections:
daVe = d—Sx���+Y �8-22)
For composite gutter sections:
daVe = d+��Sx, — 2Sx�+ Y (8-23)
Where: y = local depression (feet), Spokane County Standard
Plans B-7 and B-18 show a 1-inch local depression
at the grate.
5. Calculate the allowable flow (Qali)using the following relationship:
3
Qall = CPd z (8-24)
Where: Qall = allowable flow based upon the maximum allowable
spread (cfs);
P = perimeter of the grate inlet (refer to Table 8-9 for
projects in Spokane Counry and the City of Spokane
Valley);
d = average depth of water across the grate (feet); and,
C = may be taken as 3.0.
6. Compare the allowable flow to the actual flow. If the actual flow is less than
the allowable flow then the combination inlet capaciry is adequate. Otherwise,
changes shall be made to the design and steps 1 through 5 repeated.
Curb Inlets,Sump Condition
The procedure below assumes that the curb inlet is operating as a weir and the depth
of flow is less than the height of the curb opening.
The capaciry of a concrete curb inlet (no grate)in a sump condition may be calculated
by the method described below. An example calculation for a curb inlet in a sump
location is provided in Appendix 8H.
1. Determine the runoff contributing to the curb inlet. This is the sum of the
bypassed flows from all upstream inlets and the runoff generated from the
basin contributing directly to the combination inlet.
Apri12008 Chapter 8—Conveyance
8-31
SPOKANE REGIONAL STORMWATER MANUAL
2. Determine the allowable spread (Tall) based upon the non-flooded width
requirements found in Table 8-7.
3. Calculate the depth of flow at the curb (d).
4. Calculate the allowable flow(Qall)using one of the following relationships:
For a depressed curb opening inlet:
3
QQ�, = 23�L+1.8W�d � (8-25)
Where: Qall = allowable flow based upon the maximum allowable
spread (cfs);
W = lateral width of depression (feet);
L = length of curb opening (feet); and,
d = depth of flow at the curb (feet).
For a curb opening inlet without a depression:
T�,7 3 p r�L
�a11 �.�L(�G 2 �O-GV�
5. Compare the allowable flow to the actual flow. If the actual flow is less than
the allowable flow then the curb inlet capacity is adequate. Otherwise,
changes shall be made to the design and steps 1 through 4 repeated.
Apri12008 Chapter 8—Conveyance
8-32
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8A - EXAMPLE CALCULATION :
NON -FLOODED WIDTH (UNIFORM GUTTER
SE CT I ON)
Gl VEN
• A crowned private road with a uniform gutter section (as illustrated), assuming an
equal flow rate on each side of the road.
o Flow rate(Q)=42 cfs T �
li �
o Gutter width(W)= 1.5 feet
o Road/Gutter cross slope (S� = 0.02
feet/foot
5'h
o Longitudinal slope(SL)=0.01 feet/ft
o Manning's friction coefficient,n=0.016
o Road width(RV�=30 feet
CA L CUL A TIONS
1. Calculate the spread(T) for half of the roadway using Equation 8-8.
o.��s o.��s
Q n �42��0.016�
� 0.56 Sx�67 Si�s 0.56 �0.02�i��'�0.01�0�5 —12.4 feet
2. Calculate the non-flooded width using the following relationship for crowned
roadways, and then verify that the non-flooded width is within the allowable limit
(refer to Table 8-7):
Non-flooded width = 2[(�/z)(RW) +W—T)]
= z���iz�(3o� + l.s — 12.4)]
= 8.2 feet< 12 feet FAIL*
* Table 8-7 indicates that the minimum non-flooded width is 12 feet for private roads.
Therefore, the design fails to meet the required non-flooded road width criteria. The
design will need to be altered (i.e. try an additional inlet placed at an intermediate
location, contributing basins redefined, new flow rates calculated, and the above steps
repeated).
Apii12008 Appendix 8A—Example Calculation:
Non-Flooded Width(LTnifarm Gutter Section)
8-33
SPOKANE REGIONAL STORMWATER MANUAL
THIS PAGE IS INTENTIONALLY LEFT BLANK
Apii12008 Appendix 8A—Example Calculation:
Non-Flooded Width(LTnifarm Gutter Section)
8-34
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8B - EXAMPLE CALCULATION :
NON -F L OODE D W I DT H (C OM POSI T E G UTT E R
SE CT I ON)
Gl VEN
• A super-elevated local access road with a composite gutter section (as illustrated).
o Flow rate(Q)=42 cfs T
o Gutter width(VV)= 1.5 feet W Ts
o Road cross slope (SX)=0.02 feet/foot
o Gutter cross slope(S�,)_ .081 feet/foot as
sx
o Longitudinal slope(SL)=0.01 feet/foot a�"''
'�� s.�
o Manning's friction coefficient,n=0.016 �
o Road width(RV�=30 feet
CA L CUL A TIONS
1. Assume a flow rate (QS) for that portion of the flow above the depressed gutter
section.
Assume QS = 1.4 cfs
2. Calculate QW using Equation 8-9.
Q,�, = Q—QS = 42-1.4 = 2.8cfs
3. Calculate Eo using Equation 8-11.
�. = Q—QS = QW = 2_g - 0.67
° Q Q 42
4. Calculate the spread (T)using Equation 8-12.
Sw 0.081
T = W 1+ Sx = 1.5 l+ ���� = 6.17ft
Sw E� +l �8 -1 �.�gl �.67 +1 38 -1
S. 1—E � 0.02 ��1-0.67�
x o
Apii12008 Appendix 8B—Example Calculation:
Non-Flooded Width(Composite Gutter Section)
8-35
SPOKANE REGIONAL STORMWATER MANUAL
5. Calculate TS using Equation 8-13.
TS = T—W = 6.17-1.5 = 4.67ft
6. Use Equation 8-7 to compute Q� for the calculated Ts, then compare to the estimated
QS from Step l.
0.56 S'6'�osl.z.6� 0.56 0.020�'67 O.Ol�os�4 67)z.6�
Qs(computed)= x L s = =0.31 cfs < 1.4 cfs
n 0.016
Since Q� (estimated) and Q� (computed) are not approximately equal, repeat Steps 1
through 6 until the estimated and computed Q� are numerically closer in value.
7. Assume a new QS and repeat steps 2 through 6. The following parameters are
calculated using QS =2.6 cfs.
QW = 1.6 cfs
Eo = 0.38
T = 11.68 feet
TS = 10.18 feet
QS = 2.5 cfs (computed)
QS (estimated) � Qs (computed)
Note that a spreadsheet can be set up to perform the above calculations, and
commercially available software can calculate spread in composite gutters.
8. Now that T has been found for the relationship: Qs (estimat�i�s (calculated),
calculate the non-flooded width using the following relationship for super-elevated
roadways, and then verify that the non-flooded width is within the allowable limit
(refer to Table 8-7):
Non-flooded width = RW+2W - T
= 30 +2(1.5) - 11.68
= 21.3 feet> 12 feet OK*
* Table 8-7 indicates that the minimum non-flooded width is 12 feet for local access
roads. Therefore, the design has met the required non-flooded road width criteria.
Apii12008 Appendix 8B—Example Calculation:
Non-Flooded Width(Composite Gutter Section)
8-36
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8C - EXAMPLE CALCULATION :
GRATE INLET CAPACITY (UNIFORM GUTTER
SE CT I ON)
Gl VEN
• A crowned private road with a uniform gutter section (as illustrated), assuming an
equal flow rate on each side of the road.
o Flow rate(Q)=2.5 cfs T �
li �
o Gutter width(W)= 1.5 ft
o Spokane County Type 1 Grate(Standard
Plan B-12) Grate width(GV�= 1.67 feet
5+
o Road/Gutter cross slope (SX)=0.02
feet/foot
o Longitudinal slope(SL)=0.03 feet/foot
o Manning's friction coefficient,n=0.016
o Road width(RV�=30 feet
CA L CUL A TIONS
l. Determine the runoff from the contributing basin at the high point to the first inlet;
For this example, the design flow rate (Q)is given as 2.5 cfs
2. Select an inlet and note the grate width.
For this example, the grate width (GW)is given as 1.67 ft
3. Calculate the spread (T) for half of the roadway using Equation 8-8.
0375 0375
Q n (2.5�(0.016�
� 0.56 Sx�67SL�s 0.56 �0.02�i��'�0.03�0�5 — g�3lfeet
4. Calculate the non-flooded width using the following relationship, and then verify that
the non-flooded width is within the allowable limit(refer to Table 8-7):
Non-flooded width = 2[(�/z)(RW) +W—T )]
= z���iz��3o� + l.s —s.31)]
= 16.38 feet> 12 feet OK*
'� Table 8-7 indicates that the minimum non-flooded width is 12 feet for private roads.
Therefore, design has met the required non-flooded road width criteria.
Apii12008 Appendix 8C—Example Calculation:
Grate Inlet Capacity(LTnifarm Gutter Section)
8-37
SPOKANE REGIONAL STORMWATER MANUAL
5. Calculate the inlet bypass flow using Equation 8-14:
With 35% clogging factor, grate width (GW) = 1.67(1 - 0.35)= 1.09'
s 8
QBP =Q ��� �T��� 3 z.s s.3 s.31.o913 1.�z�fs
_ � J =
Therefore the capacity of the inlet= 2.5 — 1.72= 0.78 cfs
6. Verify that the velocity does not exceed 5 feet/second. The velocity of flow directly
over the inlet is calculated using Equation 8-15 (where d= T SX):
V� (>—(>BP 2.5—1.72 —4.61ft/s < 5 feet/second OK��
(UW)[d—o.s�UW)(�'x)] 1.o9[�g.31��0.02�—o.s(1.o9X.o2)]
**Refer to Section 8.7.2 for guidance when the velocity exceeds 5 feet/second.
7. The analysis is then repeated with the next inlet. The bypass flow (QBP) from the
previous inlet shall be added to the flow from the contributing basin to determine the
total flow(Q) to the inlet at the station being analyzed.
Note that the City of Spokane requires the analysis to include a 50% clogging factor.
Apii12008 Appendix 8C—Example Calculation:
Grate Inlet Capacity(LTnifarm Gutter Section)
8-38
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8D - EXAMPLE CALCULATION :
GRATE INLET CAPACITY , CONTINUOUS
GRADE (COMPOSITE GUTTER SECTION)
Gl VEN
• A super-elevated local access road with a composite gutter section (as illustrated)
o Flow rate (Q) = 42 cfs
o Gutter width(VV)= 1.5 feet T
o Spokane County Type 1 Grate (Standard W Ts
Plan B-12) Grate Width(GV�= 1.67 feet
o Road cross slope (SX)=0.02 feet/foot as
sx
o Gutter cross slope(S�,)_ .081 feet/foot o� ,-
o Longitudinal slope(SL)=0.01 feet/foot a ' s�
o Manning's friction coefficient,n=0.016 �
o Road width(RV�=30 feet
CA L CUL A TIONS
l. Determine the runoff from the contributing basin at the high point to the first inlet;
For this example, the design flow rate is given as 4.2 cfs
2. Select an inlet and note the grate width.
For this example, the grate width (GW)is given as 1.67 feet
3. Calculate the spread (T) for half of the roadway using the method outlined in
Appendix 8B and verify that the non-flooded width is within the allowable limit
(Table 8-7).
T = 11.68 feet
(Solution from Appendix 8B)
Non-flooded width = 21.3 feet> 12 feet OK�
(Solution from Appendix 8B)
'� Table 8-7 indicates that the minimum non-flooded width is 12 feet for private roads.
Therefore, design has met the required non-flooded road width criteria.
4. Calculate the inlet bypass flow using Equation 8-14:
With 35% clogging factor, grate width (GW) = 1.67(1 - 0.35)= 1.09'
Apii12008 Appendix 8D—Example Calculation:Grate Inlet Capacity,
Continuous Grade(Composite Gutter Section)
8-39
SPOKANE REGIONAL STORMWATER MANUAL
8 g
QBP =Q �T� ���W� � —42 11.6s.6s.o913 3.23�fs
— � J =
Therefore the capacity of the inlet= 42—3.23 = 0.97 cfs
5. Verify that the velocity does not exceed 5 feet/second. The velocity of flow directly
over the inlet is calculated using Equation 8-15:
V _ Q—QBP _ 42—323 —4.00ft/s < 5 feet/second OK
�GW�[a—o.s�Gw)(sx)] 1.09[�l 1.6s��o.o2�—o.s(1.o9)(0.02)]
6. The analysis is then repeated with the next inlet. The bypass flow (QBP) from the
previous inlet shall be added to the flow from the contributing basin to determine the
total flow(Q) to the inlet at the station being analyzed.
Apii12008 Appendix 8D—Example Calculation:Grate Inlet Capacity,
Continuous Grade(Composite Gutter Section)
8-40
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8E - EXAMPLE CALCULATION :
CUR B I NL ET CAPACITY , CONTI NUOUS GRADE
(UNIFORM GUTTER SECTION)
Gl VEN
• A crowned private road with a uniform gutter section (as illustrated), assuming an
equal flow rate on each side of the road.
o Flow rate(Q)= 1.5 cfs T �
li �
o Gutter width(W)= 1.5 feet
o Curb Inlet Length(L)=3 feet
o Road/Gutter cross slope (S� = 0.02 5.�
feet/foot
o Longitudinal slope(SL)=0.03 feet/foot
o Manning's friction coefficient,n=0.016
o Road width(RV�=30 feet
CA L CUL A TIONS
l. Determine the runoff from the contributing basin at the high point to the first inlet;
For this example, the design flow rate is given as 1.5 cfs
2. Calculate the spread (T) for half of the roadway using Equation 8-8 and verify that
the non-flooded width is within the allowable limit (Table 8-7).
0.375 0375
Q n (1.5�(0.016�
T 0.56 Sx b'Si s 0.56 �0.02�167�0.03�°5 —6.86feet
Non-flooded width = 2[(�/z)(RW) +W—T )]
= z���iz��3o� + l.s —6.s6)]
= 19.3 feet> 12 feet OK*
'� Table 8-7 indicates that the minimum non-flooded width is 12 feet for private roads.
Therefore, design has met the required non-flooded road width criteria.
3. Calculate the length of curb inlet required for total interception of gutter flow using
Equation 8-17:
Apii12008 Appendix 8E—Example Calculation:Grate Inlet Capacity,
Continuous Grade(LTniform Gutter Section)
8-41
SPOKANE REGIONAL STORMWATER MANUAL
0.6 0.6
LT = 0.6Qo.4zSi�3 1 =(0.6)(1.5o.4z)(0.0303) 1 =31.lfeet
nSe �.�16'��.�2
4. Calculate the efficiency of the curb inlet using Equation 8-19.
i.s i.s
E =1— 1—L =1— 1— 3.0 = 0.167
LT 31.1
5. Compute the interception capacity and the bypass flow of the curb inlet using
Equations 8-20 and 8-21.
Q� _(E)(Q)_(0.16���1.s�=o.2s�fs
QBP =Q—Q� =1.5 —0.25 =125cfs
6. The analysis is then repeated with the next curb inlet. The bypass flow (QBP) from the
previous inlet shall be added to the flow from the contributing basin to determine the
total flow(Q) to the next inlet.
Note that the City of Spokane requires the analysis to include a 50% clogging factor.
Apii12008 Appendix 8E—Example Calculation:Grate Inlet Capacity,
Continuous Grade(LTniform Gutter Section)
8-42
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8F - EXAMPLE CALCULATION :
CUR B I NL ET CAPACITY , CONTI NUOUS GRADE
(COMPOSITE GUTTER SECTION)
Gl VEN
• A super-elevated local access road with a composite gutter section (as illustrated)
o Flow rate (Q) = 42 cfs
o Gutter width(VV)= 1.5 feet T
o Curb Inlet Width(GV�=3 feet W Ts
o Road cross slope (SX)=0.02 feet/foot
o Gutter cross slope(S�,)_ .081 feet/foot as
sx
o Longitudinal slope(SL)=0.01 feet/foot a�"'"�
o Manning's friction coefficient,n=0.016 a s�
o Road width(RV�=30 feet �
CA L CUL A TIONS
l. Determine the runoff from the contributing basin at the high point to the first inlet;
For this example, the design flow rate is given as 4.2 cfs
2. Calculate the spread (T) for half of the roadway using the method outlined in
Appendix 8B and verify that the non-flooded width is within the allowable limit
(Table 8-7).
T = 11.68 feet
(Solution from Appendix 8B)
Non-flooded width = 21.3 feet> 12 feet OK�
(Solution from Appendix 8B)
* Table 8-7 indicates that the minimum non-flooded width is 12 feet for private roads.
Therefore, design has met the required non-flooded road width criteria.
3. Calculate the equivalent cross slope (Se) using Equation 8-18 and the length of curb
inlet required for total interception of gutter flow (LT)using Equation 8-17.
Se = Sx +Eo�Sw —Sx�= 0.02+0.38(0.081—0.02) = 0.043
Where, Eo = 0.38 (Solution from Appendix 8B)
Apii12008 Appendix 8F—Example Calculation:Curb Inlet Capacity,
Continuous Grade(Composite Gutter Section)
8-43
SPOKANE REGIONAL STORMWATER MANUAL
0.6 0.6
LT = 0.6Qo.4zSi�3 1 = (0.6)(4.2o.4z)(O.Olo 3) 1 = 21.8feet
nSe (0.016)(0.043)
4. Calculate the efficiency of the curb inlet using Equation 8-19.
i.s i.s
E = 1— 1—L =1— 1— 3.0 = 0.23
LT 21.8
5. Compute the interception capacity and the bypass flow of the curb inlet using
Equations 8-20 and 8-21.
Q� _ (E)(Q) _ (0.23��4.2� = o.9��fs
QBP = Q—Q� = 4.2—0.97 = 323cfs
6. The analysis is then repeated with the next curb inlet. The bypass flow (QBP) from the
previous inlet shall be added to the flow from the contributing basin to determine the
total flow(Q) to the next inlet.
Note that the City of Spokane requires the analysis to include a 50% clogging factor.
Apii12008 Appendix 8F—Example Calculation:Curb Inlet Capacity,
Continuous Grade(Composite Gutter Section)
8-44
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8G - EXAMPLE CALCULATION :
COMBINATION INLET CAPACITY , SUMP
Gl VEN
• A crowned private road with a uniform gutter section (as illustrated).
o Inlet: Metal Frame with Hood,Type 2 and T �
Bi-Directional Vaned Grate,Type 3 — �
Spokane County Standard Plans B-11 and o
B-14
o Gutter Width(W)= 1.5 feet 5.�
o Local depression= 1 inch
o Cross slope(S�=0.02 feet/foot
o Road width(RV�=30 feet
o Qgp=0.68 cfs=Upstream inlets total
bypass flow rate
� QBASIN—0.82 cfs=Contributing
drainage basin direct flow rate
CA L CUL A TIONS
l. Determine the total runoff contributing and bypassed to the combination inlet.
QrorAi — QaP +QBasrv = 0.68cfs+0.82cfs = 1.Scfs
2. From Table 8-7, the non-flooded width for a private road is 12 feet minimum.
Determine the allowable spread (T) for the roadway using the following relationship
for a crowned roadway:
T _ RW+2W—Non-flooded Width _ 30+(2)(1.5)—12 =10.5 feet
aii — 2 2
3. Calculate the depth of flow at the curb (d)using Equation 8-16.
a = (T)(sx) _ (lo.s��o.o2� = 0.21 feet
4. Determine the average depth of flow over the grate using Equation 8-22.
daVe = d—S����+y = 021—0.02�1��+�� = 0.28 feet
5. Calculate the allowable flow(Qall)using Equation 8-24.
Apri12008 Appendix 8G—Example Calculation:Combination Inlet Capacity,Sump
8-45
SPOKANE REGIONAL STORMWATER MANUAL
Q��� = CPd 3� _ (3.0)(3.13)�0.28�3� =138 cfs
6. Compare the allowable flow to the actual flow.
138 cfs(QQ����1.5 cfs�Q� FAIL*
* The actual flow rate directed at the given metal frame and grate inlet combination
exceeds the calculated allowable flow capacity of the structure. The design will need
to be altered (i.e. try an additional inlet placed at an intermediate location,
contributing basins redefined, new flow rates calculated, and the above steps
repeated).
Note that grate perimeter used in this example includes a 50% clogging factor (refer to
Table 8-9).
Apri12008 Appendix 8G—Example Calculation:Combination Inlet Capacity,Sump
8-46
SPOKANE REGIONAL STORMWATER MANUAL
APPENDIX 8H - EXAMPLE CALCULATION :
CURB INLET CAPACITY , SUMP
Gl VEN
• A crowned private road with a composite gutter section (as illustrated).
o Curb opening length(L)=3.0 feet(reduce T
by half—clogging safety factor) w Ts
o Local depression= 1 inch
o Cross slope(S�=0.02 feet/foot as
sx
o Gutter cross slope(S�,)=0.081 feet/foot o�,_-�
o Gutter Width= 1.5 feet a �� s�
o Road width(RV�=30 feet �
o Qgp=0.68 cfs=Upstream inlets total
bypass
� QBASIN—0.82 cfs=Contributing
drainage basin direct flow rate
CA L CUL A TIONS
1. Determine the total runoff contributing and bypassed to the curb inlet.
QrorAi — QsP +QBASrN = 0.68cfs+0.82cfs = 1.Scfs
2. From Table 8-7, the non-flooded width for a private road is 12 feet minimum.
Determine the allowable spread (T) for the roadway using the following relationship
for crowned roadways:
T _ RW+2W—Non-flooded Width _ 30+(2)(1.5)—12 =10.5 feet
°'i � 2 2
3. Calculate the depth of flow at the curb (d).
a=�l.s��o.osl�+�lo.s —l.s��o.o2�= 0.3o feet
4. Calculate the allowable flow(Qall)using Equation 8-25.
Qa�� =2.3(L +l.sw)a� =2.3�(l.s +�l.s��l.$)](o.30)� =1.s9�fs
5. Compare the allowable flow to the actual flow.
Apri12008 Appendix 8H—Example Calculation:Curb Inlet Capacity,Sump
8-47
SPOKANE REGIONAL STORMWATER MANUAL
1.59cfs�QQ���>1.5 cfs�Q� OK*
* The actual flow rate directed at the curb inlet is less than the calculated allowable
flow capacity of the structure. The design is adequate.
Apri12008 Appendix 8H—Example Calculation:Curb Inlet Capacity,Sump
8-48
SPOKANE REGIONAL STORMWATER MANUAL
CHAPTER 9 - EROSION AND SEDIMENT
CONTROL DESIGN
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Chapter Organization
9.1 Introduction......................................................................................................................9-1
9.2 Applicability ....................................................................................................................9-1
9.3 Exemptions ......................................................................................................................9-2
9.4 Erosion and Sediment Control (ESC) Plan......................................................................9-2
9.4.1 Introduction..............................................................................................................9-2
9.42 Minimum Requirements for ESC Plans...................................................................9-3
9.43 Best Management Practices for ESC Plans .............................................................9-3
9.4.4 Modification to ESC Plans.....................................................................................9-11
9.5 Additional Information Regarding ESC Plans...............................................................9-11
9.5.1 Performance Standards ..........................................................................................9-ll
9.52 Maintenance Responsibility...................................................................................9-12
9.53 Enforcement and Appeals Process.........................................................................9-12
Appendix 9A —ESC Standard Plan Notes.................................................................................9-13
Apri12008 Chapter 9—Erosion and Sediment Control Design
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SPOKANE REGIONAL STORMWATER MANUAL
9.1 INTRODUCTION
This chapter lists the steps for developing a Erosion and Sediment Control (ESC) plan,
which is a required component of a Drainage Submittal or permit. Controlling erosion
and preventing sediment and other pollutants from leaving the project site during
construction can be achieved by implementing the best management practices (BMPs)
identified in this chapter. The ESC plan shall outline specific construction BMPs for a
project site to a�oid adverse stormwater impacts from construction activities on water
resources, roads, drainage facilities, surrounding properties and other improvements.
Potential impacts due to erosion and sedimentation include:
• Sediment accumulation in culverts, storm drains and open channels, resulting in
decreased capacities and the potential for increased flooding and increased
maintenance frequency;
• Sedimentation of storage ponds and swales, resulting in decreased infiltrative and
storage capacity, and the potential for increased flooding and failure;
• Clogging and failure of Underground Injection Control (UIC) facilities;
• Destruction of vegetation, topsoil and seeds, making re-establishment of
vegetation difficult;
• Increased turbidity, reducing water quality in water bodies; and,
• Air pollution due to fugitive dust.
Implementation of an effective ESC plan may help to reduce these potential impacts as
well as other unforeseen environmental impacts and associated costs.
Although the construction phase of a project is usually considered a temporary condition,
construction work may take place over several seasons. All BMPs used in the course of
construction should be of sufficient size, strength, and durability to readily outlast the
expected construction schedule and operate properly during the design storm rainfall
conditions (see Basic Requirement No. 6 in Chapter 2 for design storm criteria).
9.2 APPLICABILITY
Land-disturbing activities are activities that result in a change in existing soil cover
(vegetative or non-vegetative) or site topography. Land-disturbing activities include, but
are not limited to, demolition, construction, clearing and grubbing, grading and logging.
The following land-disturbing activities require an ESC plan:
• Major land-disturbing activities involving 1 acre or more of disturbed area; or,
• Minor land-disturbing activities, such as grading, involving less than 1 acre of
disturbed area but requiring a permit by the local jurisdiction.
Apri12008 Chapter 9—Erosion and Sediment Cont��ol Design
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SPOKANE REGIONAL STORMWATER MANUAL
An ESC plan, when required, shall be submitted with either the road and drainage plans
or the permit application, prior to any land-disturbing activity. Clearing and grading
activities for developments will be permitted only if conducted pursuant to an accepted
site development plan that establishes permitted areas of clearing, grading, cutting, and
filling. When establishing these permitted clearing and grading areas, consideration shall
be given to minimizing removal of existing trees and minimizing disturbance and
compaction of native soils except as needed for building purposes. These permitted
clearing and grading areas and any other areas with a preservation requirement, such as
critical or sensitive areas, buffers, native growth protection easement areas or tree
retention areas, shall be delineated on the site plans and development site plan. ESC plans
are only required to address the area of land that is subject to the land-disturbing activity
for which a permit is being requested and the area of land that will serve as the stockpile
or staging area for materials.
9.3 EXEMPTIONS
An ESC plan is typically not required for the following activities:
• Commercial agriculture as regulated under RCW Chapter 84.34.020;
• Forest practices regulated under WAC Title 222, except for Class IV General
Forest Practices that are conversions from timberland to other uses;
• Actions by a public utiliry or any other governmental agency to remove or
alleviate an emergency condition, restore utility service, or reopen a public
thoroughfare to traffic;
• Land divisions, interior improvements to an existing structure, and other activities
requiring permits or approvals for which there is no physical disturbance to the
surface of the land; and,
• Minor land-disturbing activities that do not require a permit.
Although an ESC plan may not be required for the situations noted above, that does not
relieve the proponent from the responsibility of controlling erosion and sediment during
construction nor the liability of damage claims associated with adverse impacts on off-
site properties.
9.4 EROSION AND SEDIMENT CONTROL (ESC ) PLAN
9.4.1 INTRODUCTION
The ESC plan must be prepared by a professional engineer currently licensed in the
State of Washington with a good working knowledge of hydrology and ESC
practices, or a Certified Erosion and Sedimentation Control Technician. A copy of the
Apri12008 Chapter 9—Erosion and Sediment Control Design
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SPOKANE REGIONAL STORMWATER MANUAL
ESC plan must be located on the construction site or within reasonable access to the
site. As site construction progresses, the ESC plan may require modification to reflect
changes in site conditions.
The ESC plan must accompany the road and drainage plans, grading plan, or permit
request and should be integrated into the grading plan whenever possible. It must
contain sufficient information to demonstrate to the local jurisdiction that potential
problems associated with erosion, sediment, and pollution have been adequately
addressed for the proposed project. The drawings and notes should be clear and
concise and describe when and where each BMP is to be implemented.
9.4.2 MINIMUM REQUIREMENTS FOR ESC PLANS
At a minimum, all ESC plans must be legible, reproducible and on good quality 24" x
36" bond paper, and must contain the following information:
• Title block, north arrow, scale and plan preparation date;
• Name of property owner, permit applicant, anticipated contact person on-
site, and the stamp and signature of the engineer who prepared the plan
(note that for municipal projects, this information will be not be available
until the pre-construction conference);
• Vicinity map, section, township and range, project address, project
boundaries and dimensions;
• Description of project, list of on-site soils and existing vegetation, location
of any existing water bodies and/or critical areas;
• Summary description of ESC BMPs utilized (see Section 9.4.3);
• ESC Standard Plan Notes (see Appendix 9A);
• Construction Sequence (see Section 9.4.3)
9.4.3 BEST MANAGEMENT PRACTICES FOR ESC PLANS
BMPs must be used to comply with the requirements of this chapter. It is not the
intent of this chapter to limit innovative or creative efforts to effectively control
erosion and sedimentation. Experimental ESC management practices to improve
erosion control technology and meet the purpose and intent of this chapter are
encouraged as a means of solving erosion and sedimentation problems. Minor
modifications to standard BMPs are considered experimental ESC management
practices and, as with any proposed BMP, must be reviewed and accepted by the local
jurisdiction. It is important to note that not only do new facilities and off-site
properties need to be protected from erosion and sedimentation, but existing facilities
on-site or downstream also need to be evaluated and protected if there is potential for
damage due to lack of erosion control.
Apri12008 Chapter 9—Erosion and Sediment Control Design
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SPOKANE REGIONAL STORMWATER MANUAL
As the season and subsequent site conditions dictate, alterations to existing ESC
BMPs may be warranted or additional ESC measures may be required. Note that
items below that are shown in italics are considered General Erosion and
Sedimentation Control Notes (see Appendix 9A for complete list). These notes shall
be shown on the ESC plan, when applicable to the given project site.
BMPs are referenced in this chapter by their identification code in the September
2004 Stormwater Management Manual for Eastern Washington (e.g., BMP C101,
BMP C 102). Detailed examples and descriptions of these BMPs are included in
Chapter 7 of the Eastern Washington manual. At a minimum, the following items
shall be addressed in the ESC plan:
1. Construction Sequence
• The following construction sequence shall be followed in order to best
minimize the potential for erosion and sedimentation control problems:
a) Clear and grub sufficiently for installation of temporary ESC BMPs;
b) Install temporary ESC BMPs; constructing sediment trapping BMPs
as one of the first steps prior to grading;
c) Clear, grub and rough grade for roads, temporary access points and
utility locations;
d) Stabilize roadway approaches and temporary access points with the
appropriate construction entry BMP;
e) Clear, grub and grade individual lots or groups of lots;
� Temporarily stabilize, through re-vegetation or other appropriate
BMPS, lots or groups of lots in situations where substantial cut or fill
slopes are a result of the site grading;
g) Construct roads, buildings,permanent stormwater facilities (i.e. inlets,
ponds, UIC facilities, etc.);
h) Protect all permanent stormwater facilities utilizing the appropriate
BMPs;
i) Install permanent ESC controls, when applicable; and,
j) Remove temporary ESC controls when:
♦ Permanent ESC contr�ols, when applicable, have been
completely installed;
♦ All land-disturbing activities that have the potential to cause
erosion or sedimentation problems have ceased; and,
♦ i�egetation had been established in the areas noted as
requiring vegetation on the accepted ESC plan on file with the
local jurisdiction.
Apri12008 Chapter 9—Erosion and Sediment Control Design
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2. Clearing Limits
• Distinctly mark all clearing limits, both on the plans and in the field—
taking precaution to visibly mark separately any sensitive or critical areas,
and their buffers, and trees that are to be preserved prior to beginning
any land-disturbing activities, including clearing and grubbing; and,
• If clearing and grubbing has occurred, there is a window of 15 days in
which construction activiry must begin, otherwise the cleared area must be
stabilized.
• Suggested BMPs:
o BMP C 101: Preserving Natural Vegetation
o BMP C 102: Buffer Zones
o BMP C103: High Visibility Plastic or Metal Fence
o BMP C 104: Stake and Wire Fence
3. ConstNUCtion Access Route
• Limit access for construction vehicles to one route whenever possible;
• Stabilize the construction access route with quarry spalls or crushed rock
to minimize the tracking of sediment onto roadways;
• Inspect all roadways, at the end of each day, adjacent to the construction
access route. If it is evident that sediment has been tr°acked offsite and/or
beyond the roadway approach, removal and cleaning is required.
• If sediment removal is necessary prior to street washing, it shall be
removed by shoveling or pickup sweeping and transported to a controlled
sediment disposal area.
• If street washing is required to clean sediment tracked offsite, once
sediment has been removed, street wash wastewater shall be controlled by
pumping back on-site or othenvise prevented from discharging into
systems tributary to waters of the state;
• Locate wheel washes or tire baths, if applicable to ESC plan, on site.
Dispose of wastewater into a separate temporary on-site treatment facility
in a location other than where a permanent stormwater facility is
proposed; and,
• Restore construction access route equal to or better than the pre-
construction condition.
• Suggested BMPs:
o BMP C 105: Stabilized Construction Entrance
o BMP C106: Wheel Wash
Apri12008 Chapter 9—Erosion and Sediment Control Design
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SPOKANE REGIONAL STORMWATER MANUAL
o BMP C107: Construction Road/Parking Area Stabilization
4. Install Sediment Cont�ols
• Retain the duff layer, native topsoil, and natural vegetation in an
undisturbed state to the maximum extent practical;
• Pass stormwater runoff from disturbed areas through a sediment pond
prior to leaving a construction site or discharging to an infiltration facility,
• Keep sediment on the project site, to the maximum extent practical, in
order to protect adjacent properties, water bodies, and roadways;
• Stabilize earthen structures such as dams, dikes, and diversions with either
quarry spalls, seed or mulch, or a combination thereof;
• Locate sediment facilities such that they will not interfere with natural
drainage channels or streams; and,
• Inspect sediment control BMPs weekly at a minimum, daily during a storm
event, and after any discharge from the site (stormwater or non-
stormwater). The inspection frequency may be reduced to once a month if
the site is stabilized and inactive.
• Suggested BMPs:
o BMP C230: Straw Bale Barrier
o BMP C231: Brush Barrier
o BMP C232: Gravel Filter Berm
o BMP C233: Silt Fence
o BMP C234: Vegetated Strip
o BMP C235: Straw Wattles
o BMP C240: Sediment Trap
o BMP C241: Temporary Sediment Pond
5. Soil Stabilization
• Select appropriate BMPs to protect the soil from the erosive forces of
raindrop impact, flowing water and wind, taking into account the expected
construction season, site conditions and estimated duration of use;
• Control fugitive dust from construction activity in accordance with state
and local air quality control authorities with jurisdiction over the project
area;
• Stabilize exposed unworked soils (including stockpiles), whether at final
grade or not, within 10 days during the regional dry season (July 1
through September 30) and within 5 days during the regional wet season
Apri12008 Chapter 9—Erosion and Sediment Control Design
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SPOKANE REGIONAL STORMWATER MANUAL
(October 1 through June 30). Soils must be stabilized at the end of a shift
before a holiday weekend if needed based on the weather forecast. This
time limit may only be adjusted by a local jurisdiction with a "Qualified
Local Program, " if it can be demonstrated that the recent precipitation
justifies a different standard and meets the requirements set fourth in the
Construction S'tormwater General Permit; and,
• Stabilization practices include, but are not limited to, temporary and
permanent seeding, sodding, mulching, plastic covering, erosion control
fabric and mats, soil application of polyacrylamide (PAM) and the early
application of gravel base on areas to be paved, and dust control.
• Suggested BMPs:
o BMP C 120: Temporary and Permanent Seeding
o BMP C 121: Mulching
o BMP C 122: Nets and Blankets
o BMP C 123: Plastic Covering
o BMP C 124: Sodding
o BMP C125: Topsoiling
o BMP C126: Polyacrylamide (PAM) for Soil Erosion Protection
o BMP C 130: Surface Roughening
o BMP C 131: Gradient Terraces
o BMP C140: Dust Control
6. Protection of Inlets
• Protect inlets, drywells, catch basins and other stormwater management
facilities from sediment, whether or not facilities are operable, so that
stormwater runoff does not enter the conveyance system (both on and off
site)without being treated or filtered to remove sediment;
• Keep roads adjacent to inlets clean; sediment and street wash water shall
not be allowed to enter the conveyance system (both on and offsite)
without prior treatment;
• Inspect inlets weekly at a minimum and daily during storm events. Inlet
protection devices shall be cleaned or removed and replaced before
6 inches of sediment can accumulate.
• Suggested BMP:
o BMP C220: Storm Drain Inlet Protection
Apri12008 Chapter 9—Erosion and Sediment Control Design
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7. Runoff from Construction Sites
• Protect down-gradient properties, waterways, and stormwater facilities
from possible impacts due to increased flow rates, volumes, and velocities
of stormwater runoff from the project site that may temporarily occur
during construction;
• Construct stormwater control facilities (detention/retention storage pond
or swales) before grading begins. These facilities shall be operational
before the construction of impervious site improvements; and,
• Protect permanent infiltration ponds that are used for flow control during
construction.
• Suggested BMPs:
o BMP C240: Sediment Trap
o BMP C241: Temporary Sediment Pond
8. Washout Site for Concrete Trucks and Equipment
• Designate the location of a slurry pit where concrete trucks and equipment
can be washed out. Slurry pits are not to be located in or upstream of a
swale, drainage area, stormwater facility or water body, or in an area
where a stormwater facility is existing or proposed.
• Suggested BMP:
o BMP C151: Concrete Handling
9. Mate�ial Sto�age/Stockpile
• Identify locations for storage/stockpile areas, within the proposed ESC
plan boundaries, for any soil, earthen and landscape material that is used
or will be used on-site;
• Stockpile materials (such as topsoil) on-site, keeping off roadway and
sidewalks; and,
• Maintain on-site, as feasible, items such as gravel and a roll of plastic, for
emergency soil stabilization during a heavy rain event, or for emergency
berm construction.
• Suggested BMP:
o BMP C 150: Materials On Hand
10. Cut and Fill Slopes
• Consider soil type and its erosive properties;
Apri12008 Chapter 9—Erosion and Sediment Control Design
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SPOKANE REGIONAL STORMWATER MANUAL
• Divert any off-site stormwater run-on or groundwater away from slopes
and disturbed areas with interceptor dikes, pipes or temporary swales. Off-
site stormwater shall be managed separately from stormwater generated
on-site;
• Reduce slope runoff velocities by reducing the continuous length of slope
with terracing and diversion, and roughening the slope surface;
• Place check dams at regular intervals within ditches and trenches that are
cut into a slope; and,
• Stabilize soils on slopes,where appropriate.
• Suggested BMPs:
o BMP C120: Temporary and Permanent Seeding
o BMP C 130: Surface Roughening
o BMP C 131: Gradient Terraces
o BMP C200: Interceptor Dike and Swale
o BMP C201: Grass-Lined Channels
o BMP C204: Pipe Slope Drains
o BMP C205: Subsurface Drains
o BMP C206: Level Spreader
o BMP C207: Check Dams
o BMP C208: Triangular Silt Dike (Geotextile-Encased Check Dam)
I1. Stabilization of Temporary Conveyance Channels and Outlets
• Design, construct and stabilize all temporary on-site conveyance channels
to prevent erosion from the expected flow velociry of a 2-year, NRCS
Type II, 24-hour frequency storm or 2-year Rational Method event, in the
post-developed condition; and,
• Stabilize outlets of all conveyance systems adequately to prevent erosion
of outlets, adjacent streambanks, slopes and downstream reaches.
• Suggested BMPs:
o BMP C202: Channel Lining
o BMP C209: Outlet Protection
12. Dewatering Const�uction Site
• Discharge any effluent of dewatering operations that has similar
characteristics to stormwater runoff at the site, such as foundation, vault,
Apri12008 Chapter 9—Erosion and Sediment Control Design
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SPOKANE REGIONAL STORMWATER MANUAL
and trench dewatering, into a controlled system prior to discharge into a
sediment trap or sediment pond; and,
• Handle highly turbid or otherwise contaminated dewatering effluent, such
as from a concrete pour, construction equipment operation, or work inside
a coffer dam, separately from stormwater disposed of on-site.
• Consider other disposal options such as:
o infiltration;
o transportation off site for legal disposal in a way that does not pollute;
o treatment and disposal on-site with chemicals or other technologies;
and,
13. Cont�ol of Pollutants Othe� Than Sediment on Construction Sites
• Control on-site pollutants, such as waste materials and demolition debris,
in a way that does not cause contamination of stormwater or groundwater.
Woody debris may be chopped or mulched and spread on-site;
• Cover, contain and protect all chemicals, liquid products, petroleum
products, and non-inert wastes present on-site from vandalism (see
Chapter 173-304 WAC for the definition of inert waste), use secondary
containment for on-site fueling tanks;
• Conduct maintenance and repair of heavy equipment and vehicles
involving oil changes, hydraulic system repairs, solvent and de-greasing
operations, fuel tank drain down and removal, and other activities that
may result in discharge or spillage of pollutants to the ground or into
stormwater runoff using spill prevention measures, such as drip pans.
Clean all contaminated surfaces immediately following any discharge or
spill incident. If raining,perform on-site emergency repairs on vehicles or
equipment using temporary plastic over and beneath the vehicle;
• Conduct application of agricultural chemicals, including fertilizers and
pesticides, in such a manner, and at application rates, that inhibits the
loss of chemicals into stormwater runoff facilities. Amend manufacturer's
recommended application rates and procedures to meet this requirement,
if necessary; and,
• Locate pH-modifying sources, such as bulk cement, cement kiln dust, fly
ash, new concrete washing and curing waters, waste streams generated
from concrete grinding and cutting, exposed aggregate processes, and
concrete pumping and mixer washout waters, downstream and away from
any stormwater facilities or location of proposed stormwater facilities.
Adjust pH if necessary to prevent violations of water quality standards.
Obtain approval from Ecology for using chemicals other than liquid COz
or dry ice to adjust pH.
Apri12008 Chapter 9—Erosion and Sediment Control Design
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• Suggested BMPs:
o See also Chapter 10 —Source Control
o BMP C151: Concrete Handling
o BMP C 152: Sawcutting and Surfacing Pollution Prevention
14. Permanent BMPs
• Include permanent BMPs, if necessary, in the ESC plan to ensure the
successful transition from temporary BMPs to permanent BMPs; and,
• Restore and rehabilitate temporary BMPs that are proposed to remain in
place after construction as permanent BMPs.
I5. Maintenance of BMPs
• Inspect on a regular basis (at a minimum weekly, and daily during/after a
runoff producing storm event) and maintain all ESC BMPs to ensure
successful performance of the BMPs. Conduct maintenance and repair in
accordance with individual ESC BMPs outlined in this section; and,
• Remove temporary ESC BMPs within 30 days after they are no longer
needed. Permanently stabilize areas that are disturbed during the removal
process.
9.4.4 MODIFICATION TO ESC PLANS
ESC plans may be modified after submittal to the reviewing agency. An amended
plan shall be submitted to illustrate any modifications to the methods used to prevent
and control erosion and sedimentation.
9.5 ADDITIONAL INFORMATION REGARDING ESC PLANS
9.5.1 PERFORMANCE STANDARDS
The following performance standards represent a minimum threshold for controlling
soil erosion and sedimentation caused by land-disturbing activities and will be used to
determine if the requirements of this chapter have been met:
L Minimize Tracking onto Roadways
This performance standard has not been met if soil, dirt, mud or debris is visibly
tracked onto the road area and a reasonable attempt to control it through the use of
ESC BMPs is not evident.
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SPOKANE REGIONAL STORMWATER MANUAL
2. PNOtection of Ro�dways, Properties and Sto�mw�ter Facilities
This performance standard has not been met if there is visible downstream deposition
of soil, dirt, mud or debris, originating from the project site, on adjacent or down-
gradient roads,properties or stormwater systems.
3. Proper Washout of Concrete Trucks and Ec�uipment
This performance standard has not been met if there is observation or evidence of
concrete washout outside the area designated for concrete washout on the accepted
ESC plan.
4. Protection of Water Bodies, Streams and Wetlands
This performance standard has not been met if there is obvious turbidity or deposition
of soil, dirt, mud, or debris from the project site into adjacent water bodies or into
sensitive or critical areas and their buffers. In addition, the performance standard
requires that no construction activity, material or equipment encroach into sensitive or
critical areas.
9.5.2 MAINTENANCE RESPONSIBILITY
The proponent is responsible to ensure that BMPs are used, maintained, and repaired
so that the performance standards continue to be met. After all land-disturbing
activiry is complete and the site has been permanently stabilized, maintenance and the
prevention of erosion and sedimentation is the responsibility of the property owner.
Special criteria regarding the degradation of water resources are found in the
Washington Administrative Code of various state agencies such as the Departments
of Ecology, Natural Resources, and Fish and Wildlife.
9.5.3 ENFORCEMENT AND APPEALS PROCESS
Review the local jurisdiction's code to determine the enforcement and appeal
processes for violation of the above performance standards.
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APPENDIX 9A - ESC STANDARD PLAN NOTES
The following ESC Standard Plan Notes originate from Section 9.4.3. These notes are an
overall set; use only what applies to the given project.
l. The following construction sequence shall be followed in order to best minimize the
potential for erosion and sedimentation control problems:
(a) Clear and grub sufficiently for installation of temporary ESC BMPs;
(b) Install temporary ESC BMPs, constructing sediment trapping BMPs as one of the
first steps prior to grading;
(c) Clear, grub and rough grade for roads, temporary access points and utiliry
locations;
(d) Stabilize roadway approaches and temporary access points with the appropriate
construction entry BMP;
(e) Clear, grub and grade individual lots or groups of lots;
(f) Temporarily stabilize, through re-vegetation or other appropriate BMPS, lots or
groups of lots in situations where substantial cut or fill slopes are a result of the
site grading;
(g) Construct roads, buildings, permanent stormwater facilities (i.e. inlets, ponds,
UIC facilities, etc.);
(h) Protect all permanent stormwater facilities utilizing the appropriate BMPs;
(i) Install permanent ESC controls, when applicable; and,
(j) Remove temporary ESC controls when:
2. Permanent ESC controls, when applicable, have been completely installed;
3. All land-disturbing activities that have the potential to cause erosion or sedimentation
problems have ceased; and,
4. Vegetation had been established in the areas noted as requiring vegetation on the
accepted ESC plan on file with the local jurisdiction.
5. Inspect all roadways, at the end of each day, adjacent to the construction access route. If
it is evident that sediment has been tracked off site and/or beyond the roadway approach,
cleaning is required.
6. If sediment removal is necessary prior to street washing, it shall be removed by shoveling
or pickup sweeping and transported to a controlled sediment disposal area.
7. If street washing is required to clean sediment tracked off site, once sediment has been
removed, street wash wastewater shall be controlled by pumping back on-site or
otherwise prevented from discharging into systems tributary to waters of the state.
8. Restore construction access route equal to or better than the pre-construction condition.
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9. Retain the duff layer, native topsoil, and natural vegetation in an undisturbed state to the
maximum extent practical.
10. Inspect sediment control BMPs weekly at a minimum, daily during a storm event, and
after any discharge from the site (stormwater or non-stormwater). The inspection
frequency may be reduced to once a month if the site is stabilized and inactive.
1l. Control fugitive dust from construction activiry in accordance with the state and/or local
air quality control authorities with jurisdiction over the project area.
12. Stabilize exposed unworked soils (including stockpiles), whether at final grade or not,
within 10 days during the regional dry season (July 1 through September 30) and within 5
days during the regional wet season (October 1 through June 30). Soils must be stabilized
at the end of a shift before a holiday weekend if needed based on the weather forecast.
This time limit may only be adjusted by a local jurisdiction with a "Qualified Local
Program," if it can be demonstrated that the recent precipitation justifies a different
standard and meets the requirements set fourth in the Construction Stormwater General
Permit.
13. Protect inlets, drywells, catch basins and other stormwater management facilities from
sediment, whether or not facilities are operable.
14. Keep roads adjacent to inlets clean.
15. Inspect inlets weekly at a minimum and daily during storm events.
16. Construct stormwater control facilities (detention/retention storage pond or swales)
before grading begins. These facilities shall be operational before the construction of
impervious site improvements.
17. Stockpile materials (such as topsoil) on site, keeping off of roadway and sidewalks.
18. Cover, contain and protect all chemicals, liquid products, petroleum product, and non-
inert wastes present on site from vandalism (see Chapter 173-304 WAC for the definition
of inert waste), use secondary containment for on-site fueling tanks.
19. Conduct maintenance and repair of heavy equipment and vehicles involving oil changes,
hydraulic system repairs, solvent and de-greasing operations, fuel tank drain down and
removal, and other activities that may result in discharge or spillage of pollutants to the
ground or into stormwater runoff using spill prevention measures, such as drip pans.
Clean all contaminated surfaces immediately following any discharge or spill incident. If
raining over equipment or vehicle, perform emergency repairs on site using temporary
plastic beneath the vehicle.
20. Conduct application of agricultural chemicals, including fertilizers and pesticides, in such
a manner, and at application rates, that inhibits the loss of chemicals into stormwater
runoff facilities. Amend manufacturer's recommended application rates and procedures
to meet this requirement, if necessary.
21. Inspect on a regular basis (at a minimum weekly, and daily during/after a runoff
producing storm event) and maintain all erosion and sediment control BMPs to ensure
successful performance of the BMPs. Note that inlet protection devices shall be cleaned
or removed and replace before six inches of sediment can accumulate.
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22. Remove temporary ESC BMPs within 30 days after the temporary BMPs are no longer
needed. Permanently stabilize areas that are disturbed during the removal process.
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CHAPTER 10 - SOURCE CONTROL
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Chapter Organization
10.1 Introduction.................................................................................................................... 10-1
10.2 Applicability .................................................................................................................. 10-1
Apri12008 Chapter 10—Source Control
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10.1 INTRODUCTION
Source control consists of ineasures taken to prevent pollutants from entering stormwater
and thus affecting the water quality of surface water and groundwater. Source control
measures are typically in the form of best management practices (BMPs) to keep the
common pollutants generated in an urban environment from contacting stormwater,
either through physical separation of areas or through careful management of activities
that generate pollutants. Water pollutants are generally defined as hazardous or toxic
solids that are water soluble or transportable, or substances that are liquids at ambient
temperatures and pressures. Insoluble gases and vapors are not considered water
pollutants.
The main purpose of source control BMPs is to prevent pollutants from coming into
contact with stormwater through physical separation and/or management of activities that
produce pollutants. Guidance for selecting BMPs to satisfy this basic requirement is
presented in Chapter 8 of the Stormwater Management Manual for Eastern Washington.
For more information regarding source control and the recommended BMPs, visit the
Washington State Department of Ecology website at the following address:
• http://www.ecy.wa.gov/programs/wq/stormwater/index.html.
10.2 APPLICABILITY
All projects, unless exempted in Section 2.1.4, shall comply with this Basic Requirement.
Project proponents are required to implement applicable source controls through the use
of BMPs as specified in Chapter 8 of the Stormwater Management Manual for Eastern
Washington.
A project proponent is not relieved from the responsibility of preventing pollutant release
from coming in contact with stormwater, whether or not the project triggers the
regulatory threshold.
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Apri12008 Chapter 10—Source Control
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CHAPTER 11 - MAINTENANCE, TRACTS
AND EASEMENTS
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Chapter Organization
11.1 Maintenance................................................................................................................... 11-1
11.1.1 Introduction............................................................................................................ 11-1
11.12 Applicability .......................................................................................................... 11-1
11.13 Homeowners' and Property Owners' Associations ............................................... ll-2
11.1.4 Operation and Maintenance Manual...................................................................... 11-2
11.1.5 Financial Plan......................................................................................................... 11-3
11.1.6 Maintenance Access Requirements ....................................................................... ll-3
112 Tracts and Easements..................................................................................................... 11-4
11.2.1 Tracts...................................................................................................................... 11-4
11.22 Easements .............................................................................................................. 11-5
Appendix 1lA —Example Calculation: Sinking Fund .............................................................. 11-7
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11 .1 MAINTENANCE
11.1.1 INTRODUCTION
Insufficient maintenance of stormwater control facilities can lead to poor
performance, shortened life, increased maintenance and replacement costs, and
property damage.
The local jurisdiction maintains the stormwater system structures located within the
public road right of way and structures located within border easements that serve
public road runoff, unless a separate agreement exists whereby the homeowner,
property owner or other independent entity is responsible for the maintenance.
Drainage tracts created by public projects will be maintained by the local jurisdiction.
The project proponent is to provide for the perpetual maintenance of all elements of
the stormwater system located outside the public right of way. The high-frequency
maintenance of vegetated cover, turf grass and other landscaping within the public
right of way and within border easements that accommodate public road runoff is the
responsibility of the adjacent property owner. When applicable, the following
maintenance-related items shall be submitted with the Drainage Submittal (refer to
Chapter 3)for all projects:
• A copy of the conditions, covenants and restrictions (CC&Rs) for the
homeowners' association (HOA) in charge of operating and maintaining
all elements of the stormwater system;
• A Financial Plan outlining the funding mechanism for the operation,
maintenance, repair, and replacement of the private stormwater system,
including contingencies; and,
• An Operations and Maintenance (O&M) Manual.
Appendix SA and 6A of the Stormwater Management Manual for Eastern
Washington outline facility maintenance recommendations and frequencies.
11.1.2 APPLICABILITY
All projects that meet the regulatory threshold and that propose drainage facilities or
structures shall comply with the Basic Requirement for operation and maintenance.
All projects that propose UIC facilities also must comply with the operation and
maintenance requirements, regardless of whether they meet the regulatory threshold.
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11.1.3 HOMEOWNERS' AND PROPERTY OWNERS' ASSOCIATIONS
For privately maintained stormwater systems in residential neighborhoods, a
homeowner's association, or alternate entity acceptable to the local jurisdiction, shall
be formed to maintain the facilities located outside of the public right of way.
A draft copy of the CC&Rs for the HOA in charge of operating and maintaining the
facilities associated with the stormwater system shall be submitted as part of the
Drainage Submittal review package. The CC&Rs shall summarize the maintenance
and fiscal responsibilities of the HOA, reference the O&M Manual (Section 11.1.4),
and include a copy of the sinking fund calculations and Financial Plan
(Section 11.1.5). Annual HOA dues shall provide funding for the annual operation
and maintenance of all facilities associated with the stormwater system and for the
eventual replacement of these facilities.
For commercial/industrial and multi-family residential developments with joint
stormwater systems and multiple owners, a property owners' association (POA) or
similar entity such as a business shall be formed, or a reciprocal-use agreement
executed.
Homeowners' associations and property owners' associations are to be non-profit
organizations accepted by the Washington Secretary of State. A standard business
license is not acceptable for this purpose.
11.1.4 OPE RATION AND MAI NTE NANCE MANUAL
For stormwater systems operated and maintained by a HOA or POA, an O&M
Manual is required. The O&M Manual summarizes the tasks required to ensure the
proper operation of all facilities associated with the stormwater system and must
include, as a minimum:
• Description of the entiry responsible for the perpetual maintenance of all
facilities associated with the stormwater system, including legal means of
successorship;
• Description of maintenance tasks to be performed and their frequency;
• A list of the expected design life and replacement schedule of each
component of the stormwater system;
• A general site plan (drawn to scale) showing the overall layout of the site
and all the facilities associated with the stormwater system; and,
• A description of the source control BMPs.
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11.1.5 FINANCIAL PLAN
A Financial Plan is required in order to provide the entity responsible for maintenance
with guidance with regard to financial planning for maintenance and replacement
costs. The Financial Plan shall include the following items:
• A list of all stormwater-related facilities and their expected date of
replacement and associated costs;
• Sinking fund calculations that take into consideration probable inflation
over the life of the infrastructure and estimates the funds that need to be
set aside annually (an example is provided in Appendix 11A); and,
• A mechanism for initiating and sustaining the sinking fund account
demonstrating that perpetual maintenance of all facilities associated with
the stormwater system will be sustained.
11.1.6 MAINTENANCE ACCESS REQUIREMENTS
An access road is required when the stormwater system facilities/structures are
located 8 feet or more from an all weather drivable surface and are maintained by the
local jurisdiction. Privately maintained facilities located 15 feet or more from an all
weather drivable surface are also required to have an access road. When required,
maintenance access roads shall meet the following minimum requirements:
• The horizontal alignment of all access roads shall be designed and
constructed to accommodate the turning movements of a Single-Unit
Truck (as defined by AASHTO Geometric Design of Highways and
Streets, Exhibit 2-4, 2004 Edition). The minimum outside turning radius
shall be 50 feet. The minimum width shall be 12 feet on straight sections
and 15 feet on curves;
• Access roads shall consist of an all weather, drivable surface;
• Access roads shall be located within a 20-foot-minimum-width (or as
required by the horizontal alignment requirements) tract or easement,
extending from a public or private road;
• Access roads shall have a maximum grade of 10 percent;
• A paved apron must be provided where access roads connect to paved
public roads; and,
• Gravel access roads shall have a minimum of 6 inches of crushed
surfacing top course, in accordance with WSDOT Standard Specifications
and shall be designed to support the heaviest anticipated maintenance
vehicle year round.
The following access road requirements apply only when the local jurisdiction has
assumed the responsibility of the maintenance and operation of the facilities, though
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it is recommended that access roads for privately maintained facilities also be
designed to meet these criteria:
• If the maintenance access road is longer than 150 feet, a turn-around is
required at or near the terminus of the access road. Turn-arounds are
required for long, winding, or steep conditions, regardless of the length of
the drive, where backing up would otherwise be difficult; and,
• Turn-arounds shall conform to the jurisdiction's standard plan.
11 .2 TRACTS AND EASEMENTS
Flow control and treatment facilities must be located within the right of way, within a
border easement parallel to the road or within an individual tract. For lots larger than 1
acre, the drainage facility may be located within a drainage easement if the facility does
not occupy more than 10% of the lot and does not straddle private property lines.
Stormwater facilities serving commercial projects do not generally require separate tracts
or easements unless they serve more than one parcel.
A stormwater faciliry, as defined for this section, is a swale or pond. It is acceptable for
other types of facilities, such as a pipe, to be in a drainage easement.
11.2.1 TRACTS
A drainage tract for access, maintenance, operation, inspection and repair shall be
dedicated to the entity in charge of the maintenance and operation of the stormwater
system. Unless otherwise approved by the local jurisdiction, a tract will be dedicated
when any of the following situations are present:
• Facilities associated with a stormwater system serving a residential
development are located outside of the public right of way;
• Drainage ditches are located in residential neighborhoods. The limits of
the tract may have to be delineated with a permanent fence when the ditch
is located near property lines; or,
• A drainageway is present on a lot of 1 acre or smaller (refer to
Section 83.4).
Tracts shall be of sufficient width to provide access to, and maintain, repair or replace
elements of, the stormwater system without risking damage to adjacent structures,
utilities and normal property improvements, and without incurring additional costs for
shoring or specialized equipment.
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11.2.2 EASEMENTS
A drainage easement for access, maintenance, operation, inspection and repair shall
be granted to the entity in charge of the maintenance and operation of the stormwater
system. The easement shall grant to the local jurisdiction the right to ingress/egress
over the easement for purposes of inspection or emergency repair. If not in a tract, the
following infrastructure shall be placed within drainage easements:
• Elements of a stormwater system, such as a pipe, located outside the
public right of way. Easements for stormwater conveyance pipes shall be
of sufficient width to allow construction of all improvements, including
any associated site disturbances, and access to maintain, repair or replace
the pipe and appurtenances without risking damage to adjacent structures
or incurring additional costs for shoring or special equipment. No storm
pipe in a drainage easement shall have its centerline closer than 5 feet to a
private rear or side property line. The storm drain shall be centered in the
easement. The minimum drainage easement shall be 20 feet;
• For drainage ditches and natural drainageways, the easement width shall
be wide enough to contain the runoff from a 50-year storm event for the
contributing stormwater basin, plus a 30% freeboard or 1 foot, whichever
is greater. Constructed drainage ditches will not typically be allowed to
straddle lot lines. Natural drainageways (refer to Section 83.4) located on
lots larger than 1 acre may be placed in an easement; and,
• Easements for access roads and turnarounds shall be at least 20 feet wide.
Easement documents shall be drafted by the project proponent for review by the local
jurisdiction and recorded by the project proponent.
Off-Site Easements
When a land action proposes infrastructure outside the property boundaries, an off-
site easement shall be recorded separately from plat documents, with the auditor's
recording number placed on the face of the plat. The easement document shall include
language prescribed by the local jurisdiction. The easement language shall grant the
local jurisdiction the right to ingress and egress for purposes of routine or emergency
inspection and maintenance. The following will be submitted to the local jurisdiction
for review:
• A legal description of the site stamped and signed by a surveyor;
• An exhibit showing the entire easement limits and easement bearings,
stamped and signed by a surveyor;
• Proof of ownership for the affected parcel and a list of signatories; and,
• Copy of the draft easement.
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The legal exhibit and description shall have 1-inch margins for all four sides of the
page. All text shall be legible and at least 8 point.
For plats and binding site plans, the off-site drainage facility must be clearly
identified on the plans and operation and maintenance responsibilities must be clearly
defined prior to acceptance of the project.
Apri12008 Chapter 11—Maintenance,Tracts and Easements
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APPENDIX 11A - EXAMPLE CALCULATION :
SI NK I NG FUND
LIST OF QUANTITIES
Description Units Quantity Unit Price Total
24" Pipe LF 175 $40.00 $ 7,000.00
21"Pipe LF 50 $40.00 $ 2,000.00
18"Pipe LF 700 $26.00 $ 18,200.00
15"Pipe LF 650 $24.00 $ 15,600.00
12"Pipe LF 1600 $22.00 $ 35,200.00
10" Pipe LF 50 $20.00 $ 1,000.00
Pipe Total $ 79,000.00
Inlets EA 22 $500.00 $ 11,000.00
Type B Drywells EA 4 $2,500.00 $ 10,000.00
Structure Total $ 21,000.00
ANNUAL MAINTENANCE AND OPERATION COSTS
Description Units Quantity Unit Price Total
Inspect Structures DAY 4 $50.00 $ 200.00
Flush/Clean Inlets EA 26 $100.00 $ 2,600.00
Flush Pipes $ -
Inspect Ponds and Clean Outlets LS 1 $500.00 $ 500.00
Mowing&Irrigation of Ponds DAY 34 $250.00 $ 8,500.00
Annual Maintenance Cost � 11,800.00
REPLACEMENT COST & ANNUAL COST PER LOT
Description Total
Assume 50%of Pipe is Replaced in 20 years (=Pipe Total*0.5) $ 39,500.00
Assume 25%of Structures are Replaced in 20 years(=Structure Total*025) $ 5,250.00
Total Present Value(P� of Replaced Pipe and Structures � 44,750.00
Future Value of Pipe and Structures(F�,assume inflation=4°/o,n=20
FV=PV(F/P,4%,n=20) $ 98,052.76
Annual Set-Aside for Future Replacement(A),assume interest=6°/o,n=20
A=FV(A/F,6%,n=20) $ 2,665.52
Annual Maintenance and Operation Costs(from subtotal above) $ 11,800.00
Total Annual Costs � 14,465.52
Total Charge per Lot, assume 100 Lots
Charge per Lot=Total Annual Costs/#of Lots
Charge per Lot $ 144.66
NOTE: F/P,A/F factors are from interest tables
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