18_CLOMR_Report_Body_(minus_appendices)
WEST Consultants, Inc. i CLOMR for Proposed Painted Hills Development
TABLE OF CONTENTS
INTRODUCTION................................................................................................................. 1
BACKGROUND & RESEARCH ........................................................................................ 2 General ............................................................................................................................ 2 Levees ............................................................................................................................. 7
Infiltration Facilities ...................................................................................................... 13
Interior Drainage ........................................................................................................... 17
SITE INVESTIGATION ..................................................................................................... 20
HYDROLOGY ................................................................................................................... 20 HYDRAULICS ................................................................................................................... 23 Duplicate Effective Model (DEM) ............................................................................... 25
Corrected Effective Model (CEM) ............................................................................... 28
Existing (Pre-Project) Conditions Model...................................................................... 30
Proposed or Post-Project Conditions Models ............................................................... 32
FLOODPLAIN MAPPING ................................................................................................. 42 CERTIFICATION FORMS ................................................................................................ 43 SUMMARY ........................................................................................................................ 43
REFERENCES ................................................................................................................... 43
LIST OF TABLES
Table 1. Assumed runoff for Adjacent Subbasins Draining to Project Site. ..................... 18
Table 2. Discharges Used in HEC-RAS Models ............................................................... 20
Table 3. Summary of HEC-RAS Model Plans .................................................................. 23
Table 4. Summary of Model Reaches ................................................................................ 25
Table 5. Summary of 1% Annual Chance Flood Flows for CLOMR Reaches ................. 25
Table 6. Comparison of FIS and DEM model results for Golf Course Overflow Reach ... 27
Table 7. Comparison of FIS and DEM model results for Unnamed Tributary ................... 28
Table 8. DEM and CEM model results for the Golf Course Overflow .............................. 29
Table 9. DEM and CEM model results for the Unnamed Tributary ................................... 30
Table 10. CEM and ECM model results for the Golf Course Overflow ............................ 31
Table 11. CEM and ECM model results for the Unnamed Tributary ................................. 32
Table 12. ECM and PCM model results for the Golf Course Overflow ............................. 36
Table 13. ECM and PCM model results for the Unnamed Tributary ................................. 37
Table 14. Change in Top Width for Golf Course Overflow ............................................... 39
Table 15. Change in Top Width for Unnamed Tributary .................................................... 40
Table 16. Change in Top Width for Unnamed Tributary without DS levee (left overbank
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flowpath) ..................................................................................................................... 42
LIST OF FIGURES
Figure 1. Chester Creek Location Map ................................................................................. 3
Figure 2. Effective FIS stream reaches and storage areas ..................................................... 4
Figure 3. Detail of project area and levees ............................................................................ 6
Figure 4. FEMA Floodplain and levee south of Thorpe Road .............................................. 8
Figure 5 Location of modeled cross sections along levee .................................................. 10
Figure 6. Cross Section Plots of Levee South of Thorpe .................................................... 13
Figure 7. Golf Course Overflow facilities design drawing ................................................. 15
Figure 8. Unnamed Tributary facility design drawing ........................................................ 16
Figure 9. Interior drainage basins ........................................................................................ 19
Figure 10. Layout of HEC-RAS Cross Sections for Chester Creek Golf Course Overflow ..................................................................................................................................... 21
Figure 11. Layout of HEC-RAS Cross Sections for the Unnamed Tributary .................... 22
WEST Consultants, Inc. iii CLOMR for Proposed Painted Hills Development
LIST OF APPENDIXS
Appendix A. FEMA Forms
Appendix B. Survey Maps and Plans of Proposed Project
Appendix C. Duplicate Effective Models
Appendix D. Corrective Effective HEC-RAS Models
Appendix E. Existing Conditions HEC-RAS Models
Appendix F. Post-Project Conditions HEC-RAS Models
Appendix G. Effective Flood Insurance Rate Map (FIRM), Flood Profile, and Floodway
Data Table
Appendix H. Revised Floodplain Boundaries, Flood Profile, and Floodway Data Table
Appendix I. Floodplain Workmap for CLOMR
Appendix J. Geotechnical Evaluation and Levee Certification Reports
Appendix K. Infiltration Facilities Design Report
Appendix L. Biological Opinion
Appendix M. HEC-RAS Models
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INTRODUCTION
A 99 acre mixed used development is proposed for the former Painted Hills Golf Course
property located in Spokane Valley, Washington. The development includes both
residential and commercial property, with 30% of the site preserved as open space. The
property, identified as Storage Area 1 (SA1) within the effective FEMA Flood Insurance Study (FIS) is designated a compensatory storage area. Within a compensatory storage area loss of flood storage capacity due to placement of fill must be mitigated with an
equivalent compensatory volume of storage or through a reduction in flows such that the
net condition causes no adverse impact to the base flood or floodway elevations within
the storage area. In addition, loss of infiltration capacity due to placement of fill or
impervious surfaces must be mitigated such that the decrease in infiltration capacity will cause no adverse impact to the base flood or floodway elevations within the storage area. The overall purpose of the “compensatory” requirement is to ensure that development
activities do not cause an adverse impact on flood elevations within the storage area, or
downstream of the development (e.g. increasing downstream flows due to reduced
infiltration capacity within the storage area.)
Whipple Consulting Engineers, Inc., (WCE) proposes to address the compensatory storage and infiltration requirements by intercepting floodwaters entering the storage area and then storing and infiltrating flood flows and local storm water through the use of a
infiltration and storage facilities. The infiltration facilities will make use of dry wells and
gravel infiltration galleries. Due to the presence of glacially deposited sands and gravels
with high infiltration capacities, dry wells are currently in wide use throughout the
Chester Creek floodplain and are included in the effective FIS hydrologic model. The inclusion of infiltration facilities within the proposed plan will create a net benefit by significantly reducing flood elevations within and nearby the subject property. Two
existing levees are proposed to be enhanced and certified, and one new certified levee is
proposed to be built to help protect the development property. The infiltration facilities
and certified levees will result in approximately 118 acres being removed from the 1%
annual chance floodplain, and the removal of 0.7 river miles of floodway.
Geotechnical analysis and levee investigation was conducted by Inland Pacific Engineering Company (IPEC). Design of the levees and infiltration and storage facilities was conducted by Whipple Consulting Engineers (WCE). WEST Consultants, Inc,
(WEST) conducted the hydraulic analyses to evaluate the effects the proposed
development would have on base flood elevations (BFEs – water surface elevations
associated with the 1% annual chance event), floodway elevations, floodplain boundaries,
and floodway limits of Chester Creek. This report, along with supporting documentation, will be submitted to the Federal Emergency Management Agency (FEMA) through the local communities (City of Spokane Valley, WA, and Spokane County, WA) as a
Conditional Letter of Map Revision (CLOMR).
Pertinent information about the request is provided as follows:
Identifier: Painted Hills Development
Flooding Source: Chester Creek and Unnamed Tributary
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Community: Spokane Valley, WA, Spokane County, WA Community Number: 530342, 430174 FIRM Panels Affected: 0751D
Unless otherwise stated, all elevations within this report are referenced to the North
American Vertical Datum of 1988 (NAVD88).
BACKGROUND & RESEARCH
General
The Chester Creek watershed is located in Spokane County and the City of Spokane
Valley. A location map of the watershed is shown in Figure 1. A map of the effective stream reaches, storage areas, and CLOMR boundary is shown in Figure 2.
The watershed varies in elevation from 1,984 feet at 2nd Avenue (the downstream extent
of the effective study) to a high point of approximately 3,680 feet along the western
watershed boundary. The lower portions of the watershed are underlain by deep glacial
outwash deposits of high infiltration capacity. The upper basin is much steeper and
relatively undeveloped. Due to the high infiltration rates in the lower watershed, the Chester Creek channel is distinct only in the upper reaches of the basin. Chester Creek and its unnamed tributary have no outlet. Historically, both channels transitioned from
channel to pastures where no distinct channel is evident.
An FIS restudy for Chester Creek and its unnamed tributary was conducted in 2005. Due
to the unique infiltration characteristics of the Chester Creek watershed, it was recognized
that the prior effective FIS did not consider the effects of infiltration or available storage
in the watershed. The restudy included an extensive hydrologic modeling effort that considered the effects of infiltration and several storage areas that would serve to attenuate flood flows. The study resulted in significantly reduced flood discharges. Six
primary storage areas were identified, several of which were designated by FEMA as
‘compensatory storage areas’ within which development must compensate equally for
reductions in storage and infiltration capacity.
The main channel of Chester Creek terminates at a large borrow pit (Storage Area 4) which was developed as part of improvements to Dishman-Mica Road (D-M Road) in 1998 and is intended to act as a storm water retention and infiltration facility. The FEMA
regulatory floodplain continues north for approximately 1.5 miles beyond the physical
end of the channel.
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PROJECT LOCATION
NChester Creek Drainage Basin
2 0 2 4 Miles
Figure 1. Chester Creek Location Map
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Figure 2. Effective FIS stream reaches and storage areas
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The Unnamed Tributary channel does not physically connect to the main channel of Chester Creek. The lower portion of the Unnamed Tributary was historically rerouted to higher ground, and currently terminates in a large pit (Storage Area 6 per the effective
FIS) that is east of the proposed project site. Based on the effective FIS, the floodplain of
the Unnamed Tributary continues west from the Storage Area 6 (SA6) until it reaches
SA1, the site of the former Painted Hills Golf Course. For the with-levee condition, the
1% annual chance inflow to SA6 is 16 cfs. Four cfs exits SA6 and continues downstream to SA1 (SA6 outflow reduced due to influence of infiltration and storage within SA6). For the without downstream levee condition in which the levee between Highway 27 and
SA6 is removed, the 1% annual chance flow that could continue to SA1 is 16 cfs.
The development project area is located within SA1 in the right overbank of Chester
Creek. SA1 is physically separated from the main channel of Chester Creek by a levee
along the right bank of the main channel between Thorpe Rd and Dishman-Mica Road (Figure 3). Flood flows can enter the project site from two sources: The Golf Course
Overflow Reach, and the Unnamed Tributary. Due to the natural topography, the Golf
Course Levee, and D-M Road, there is no downstream exit for flows that enter SA1.
Flood flows that enter SA1 pond until they infiltrate. Descriptions of the two primary
flow paths affected by this CLOMR are below.
Golf Course Overflow Reach - Flow escapes the Chester Creek channel approximately 3,000 ft upstream of the golf course due to limited channel capacity, and follows the right
overbank until it crosses Thorpe Road and enters the golf course (SA1). The flow
entering the golf course does not rejoin the main channel due to the topography of the
area and a small levee system along the right bank of the main channel. As the golf
course has no outlet, floodwaters are stored until they infiltrate.
Unnamed Tributary – Based on the effective FIS, flows from the Unnamed Tributary can reach the project site via two paths. First, though SA6 has a noticeable impact on peak
discharge and serves to attenuate flood flows, flow from the 1%-annual-chance-flood
event will fill SA6 and then overflow (4 cfs) and continue to flow west via low ground,
overtopping driveways, and eventually Madison Road, at which point it would enter the
project site. Second, a levee is present along the left bank of the Unnamed Tributary
between SA6 and Highway 27 (Gustin Ditch). As this levee is not certified, a without levee analysis was conducted in the effective FIS. Since the channel is perched at this location, failure of the levee assumed all floodwaters (1% annual chance flow of 16 cfs)
potentially leave the channel. Flood waters would then flow along the low ground of the
left overbank, bypassing SA6 and continue until reaching SA1.
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Figure 3. Detail of project area and levees
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Levees
Four non-certified levees or non-levee embankments are present near the project area based on the effective FIS, that are of concern to the project site. They are described
below from south to north. A map denoting the levees is provided as Figure 3.
A non-certified levee is located along the right bank and overbank of Chester Creek
between Thorpe Road and a private road approximately 1550 feet south of Thorpe Road.
The levee is located along the right bank of the original channel; however, based on the 2005 FIS, the channel was diverted to the left overbank and into a pond. Water exits the pond via a rock spillway and returns to the original channel immediately south of Thorpe
Road. Due to the diversion, a large portion of the floodplain no longer abuts the levee
(Figure 4).
A non-certified levee is located along the east bank (right bank) of Chester Creek between
Thorpe Road and D-M Road. This levee is approximately 1,000 feet in length and protects the project site. A without levee analysis for this levee was not conducted as part of the original FIS since floodwaters are mapped on both sides of the levee (the
floodwaters on the landward side of the levee originating from the Golf Course Overflow
Reach).
Based on current FEMA policy as described in Procedure Memorandum 51 (FEMA,
2009) the portion of D-M Road that borders the northwest corner of the property and
divides the floodwaters of Chester Creek from those of SA1 (Figure 1) is considered a ‘non levee embankment’. For the 1% annual chance flood, water up to three feet deep exists on the west side of D-M Road. Because the effective FIS predated PM51 and
because floodwaters are mapped on both sides of D-M Road, a without levee analysis was
not conducted for the road during the 2005 study.
Another non-certified levee is located along the left bank of the Unnamed Tributary
between SA6 and Highway 27. In this area, the man-made channel is perched and the levee protects the low ground to the south, in the left overbank. A without levee analysis was conducted in the 2005 FIS.
In order to protect the proposed development and remove it from the 1%-annual chance
floodplain, three of the four levees/embankments discussed above are proposed to be
improved and certified. Geotechnical analysis and levee investigation and certification is
being conducted by Inland Pacific Engineering Company. The golf course levee between Thorpe Road and D-M Road, as well as the levee along the Unnamed Tributary are being improved to meet FEMA requirements for certification. Based on the effective FIS, D-M
Road is acting as a levee; however, as certification of roads to provide levee protection is
typically not possible, a new levee is proposed to be built immediately east of and parallel
to D-M Road which would tie into high ground to the north and the existing (to be
certified) levee to the south. Geotechnical evaluation reports for these three levees are provided in Appendix J. Proposed levee design plan sheets were produced by WCE and are provided in Appendix B.
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Figure 4. FEMA Floodplain and levee south of Thorpe Road
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The levee south of Thorpe Road is not proposed to be modified or certified, A review of the effective FIS HEC-RAS model and additional RAS modeling were conducted by WEST, for the area near the levee. To better understand approximate flood velocities and depths along the
portion of the floodplain that abuts the levee, a short HEC-RAS model was developed for the
northern section of the levee. Seven cross sections were cut from the LiDAR data at a spacing of
approximately 100 feet. The 1% annual chance flow was modeled, using the FIS water surface
elevation as the downstream boundary condition at the upstream face of the Thorpe Road Bridge. It should be noted that this model was based on LiDAR only, and did not include channel survey data. The conclusions of the analysis are as follows:
1. Based on the effective FIS. much of the 1% annual chance floodplain no longer abuts
the levee due to historic rerouting of the channel to the nearby pond, in the west
overbank of Chester Creek (Figure 4). Based on the effective FIS, the base flood
floodplain does not touch the levee along the southern 1,100 feet of its 1,500 foot overall length.
2. In the areas where floodwaters do abut the levee, flood velocities are low;
approximately 0.5ft/s – 2.2 ft/s for the 1% annual chance flood. It should be noted
however that velocities are likely higher immediately downstream of the rock
spillway where water is directed towards the levee and that rock protection at that
location would be recommended.
3. Where floodwaters do abut the levee the 1% annual chance flood generally averages 1 foot or less in depth between the base flood elevation and the toe of the levee (see Figure 5 and Figure 6).
Although the levee is not proposed to be certified as the project does not have control of this
property, it is understood that FEMA may still require certification for approval of this CLOMR
and that this matter may be forwarded to the FEMA regional engineer by the CLOMR review
group, for further discussion. It should be noted that although the flood depths along the levee are relatively shallow, and that the flood velocities are generally low, failure of this levee could result in flow from the main channel of Chester Creek joining the Golf Course overflow reach.
Although the infiltration facility maximum design flow of 84 cfs is greater than the 1% flow of
the Golf Course Overflow and local runoff from the area east of S Madison Road (79 cfs total),
the infiltration facility is not designed to carry and infiltrate the combined flows of the Golf
Course Overflow and the main channel of Chester Creek.
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Figure 5 Location of modeled cross sections along levee
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0 200 400 600 800 1000 12002010
2012
2014
2016
2018
Chester Creek Main Stem Plan: Levee Check 2
RS = 89
Station (ft)Elevation (ft)Legend
WS Q100
Ground
Levee
Bank Sta
.05
0 200 400 600 800 1000 12002010
2012
2014
2016
2018
Chester Creek Main Stem Plan: Levee Check 2 RS = 158
Station (ft)Elevation (ft)Legend
WS Q100
Ground
Levee
Bank Sta
.05
0 200 400 600 800 1000 12002010
2012
2014
2016
2018
Chester Creek Main Stem Plan: Levee Check 2
RS = 266
Station (ft)Elevation (ft)Legend
WS Q100
Ground
Levee
Bank Sta
.05 .05
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0 200 400 600 800 1000 12002010
2012
2014
2016
2018
Chester Creek Main Stem Plan: Levee Check 2
RS = 392
Station (ft)Elevation (ft)Legend
WS Q100
Ground
Levee
Bank Sta
.05 .05
0 200 400 600 800 1000 12002010
2012
2014
2016
2018
Chester Creek Main Stem Plan: Levee Check 2 RS = 502
Station (ft)Elevation (ft)Legend
WS Q100
Ground
Levee
Bank Sta
.05
.05 .05
0 200 400 600 800 1000 12002010
2012
2014
2016
2018
Chester Creek Main Stem Plan: Levee Check 2
RS = 622
Station (ft)Elevation (ft)Legend
WS Q100
Ground
Levee
Bank Sta
.05
.05 .05
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0 200 400 600 800 1000 12002010
2012
2014
2016
2018
Chester Creek Main Stem Plan: Levee Check 2
RS = 730
Station (ft)Elevation (ft)Legend
WS Q100
Ground
Levee
Bank Sta
.05
.05 .05
Figure 6. Cross Section Plots of Levee South of Thorpe
Infiltration Facilities
The effective FIS included an extensive hydrologic modeling effort that considered the effects of infiltration and several storage areas that would serve to attenuate flood flows. Nine storage areas were identified and considered in the hydrologic analysis, six of which
have been designated by FEMA as compensatory storage areas within which development
must compensate equally for reductions in storage and infiltration such that there is no
adverse impact on water surface elevations within and downstream of the storage areas.
The proposed development is to occupy a large portion of Storage Area 1.
To mitigate for fill and reduced infiltration WCE proposes to construct two infiltration facilities designed to intercept, and infiltrate flows from the Golf Course Overflow Reach and the Unnamed Tributary before they enter the Project site. This will result in the
entire storage area being removed from the 1% annual chance floodplain. The infiltration
facilities, designed and analyzed by WCE, have several components described below.
Further details regarding the facility design beyond the general description below can be
found in the technical memo, Painted Hills Flood Control Development Narrative
(Storage Area 1, SA1), by WCE (WCE, 2016). The memo and full design plans are provided in Appendix K and Appendix B. A geotechnical investigation was conducted by
IPEC in order to help WCE determine the design infiltration capacity of the proposed
drywells and gravel gallery. More information can be found in the report, Preliminary
Geotechnical Evaluation Phase I, located in Appendix J.
The largest of the two flood sources contributing to SA1 is the Golf Course Overflow Reach. The peak 1%-annual-chance-flood discharge entering SA1 via this reach is 64 cfs based on the effective FIS. Flood flows for the 10-year and greater events overtop the
right bank of Chester Creek approximately 3,000 feet upstream of Thorpe Road, and flow
along low ground in the right overbank before entering the property through two 10”
culverts under Thorpe Road, and via overtopping of the roadway when flows exceed the
capacity of the culverts. The proposed facility that will intercept this flow path includes a
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replacement 30 foot box culvert under Thorpe Road, an open concrete channel, a series of culverts, a bio-infiltration channel, and a series of drywells within gravel infiltration galleries with a design infiltration rate of 118 cfs.
The roadside ditch along the south side of Thorpe will be regraded to drain flows to a 3-
foot-tall by 30-foot-wide box culvert under Thorpe Road which will replace the two
existing 10” culverts. On the north side of Thorpe Road, a 5-foot-wide rectangular
concrete channel will convey flow east along Thorpe Road towards S Madison Road. At S Madison Road, the open channel will turn to the north and enter a 270 foot long 4 foot diameter culvert that will convey flow north along the west side of S Madison Road. The
culvert transitions to a 6-foot diameter and continues to the northeast corner of the project
site where flow exits the pipe into a prismatic open bio-infiltration channel which will be
planted to help filter sediment. At the downstream end of the bio-infiltration channel,
flows will enter two 3-foot diameter culverts which will convey water to the four, 10 foot wide, by 13 foot deep, by 450 foot long drywell and gravel infiltration galleries. The four existing culverts which convey local drainage from the east side of S Madison to the
former golf course will be replaced and connected to the 6 foot diameter north/south
culvert at manholes. Based on the design report from WCE the infiltration galleries have
a design infiltration rate capacity of 118 cfs, and the pipe network leading to the galleries
have a maximum design flow rate of 84 cfs, which is 20 cfs greater than the 1% annual
chance discharge from the Golf Course Overflow Reach and 5 cfs greater than the combined flow of the Golf Course Overflow Reach and the local drainage from east of S Madison Road. A general overview figure of the design is provided in (Figure 7). A more
complete description of the facilities is available in the WCE design report and plan
sheets provided in Appendix K and Appendix B.
The Unnamed Tributary currently terminates in SA6, a large pit. Although no channel
exists downstream of SA6, the FEMA floodplain extends downstream of the pit and connects to SA1. Based on the effective FIS, the 1% annual chance flow entering and leaving the pit is 16 cfs and 4 cfs, respectively. The pit is operated under a drainage
easement purchased by Spokane County in 1983 so that it could be used as a dedicated
infiltration facility. Proposed changes to the existing pit include regrading to increase
overall storage capacity, moving the channel entrance from the south side to the southeast
corner of the pit, construction of a rock spillway, enlargement of the channel immediately upstream of the spillway, and construction of 18 double depth drywells at the bottom of the pit (Figure 8). The channel (Gustin Ditch) between Highway 27 and SA6 will also be
enlarged and regraded to a uniform bottom width of three feet. Based on the effective FIS,
SA6 attenuates the 1% peak flow exiting the pit by 75%, (16 -> 4 cfs). The 18 proposed
drywells have a capacity of 18 cfs, which exceeds the 1% peak discharge of 16 cfs
entering the pit. Further, the storage volume of the pit provides an additional factor of
safety. Based on the proposed design for SA6 and the certification of the levee along the south side of Gustin Ditch, the revised 1% annual chance floodplain along the main channel of the tributary will terminate at SA6. Since the levee will not be certified for the
0.2% annual chance flood, the floodplain south of the levee along the Left Overbank
Flowpath and downstream of SA6 will be mapped as Shaded X/0.2% based on the
hydraulic model output.
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Figure 7. Golf Course Overflow facilities design drawing
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Figure 8. Unnamed Tributary facility design drawing
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Interior Drainage
Figure 9 shows the interior basin areas that could drain to the project site. This basin
area of 0.6 square miles is under the 1 square mile threshold that FEMA requires for
flood inundation analysis; however, this area was considered in order to make sure the
proposed infiltration facility can accommodate nearby storm runoff that naturally drains
to the project site. Approximate runoff values for these watersheds were calculated based
on the unit discharge and drainage area ratios from similar watersheds reported by the
HSPF model in the existing FIS (Table 1).
Runoff from the largely undeveloped 0.34 square mile area east of Madison Road (shown
in pink, labeled ‘Madison East’ in Table 1) will be conveyed to the infiltration facility via
the four existing culverts under S Madison Rd. The four culverts are to be connected to
the proposed north/south trending culvert immediately west of S Madison Rd which
conveys flood flows to the infiltration facility.
The ‘School West’ area (0.03 sq mi) shown in yellow also drains towards the project site;
however, flow from this area would be intercepted by the five drywells located at the
bottom portion of the subbasin, just north of E 40th Ave.
The remaining 0.22 square miles of area west of S Madison Road has no outlet and flows
are not intercepted by the primary infiltration facility. Discharges from this area will be
infiltrated into the ground via additional drywells and gravel galleries as part of the
stormwater system. These drywells are not part of the proposed infiltration facilities
meant to address the Golf Course Overflow and the Unnamed Tributary. As part of the
effective FIS, a drywell analysis was conducted on several highly developed subbasins
within the Chester Creek watershed in order to determine if the existing drywell system
in highly developed portions of the basin could address the 1%, and 0.2% annual chance
flood events (i.e. do these subbasins contribute to flood flows in Chester Creek?) (WEST
2008). Based on HSPF hydrologic model analysis, the highly developed subbasins had a
unit discharge of approximately 150, and 180 cfs/sq mi for the 1% and 0.2% annual
chance events, respectively. The existing drywell network density used in the lower
Chester Creek basin was found to have capacity to address large storm events, and the
basins were assumed to contribute no flow to Chester Creek (further, some of the
subbasins are isolated from Chester Creek due to an existing railroad grade). Using the
1% annual chance flood unit discharge of 150 cfs/sq mi (somewhat conservative since a
third of the proposed development will remain as open space) and a basin area west of
Madison Road of 0.22 sq mi, a peak discharge of 33 cfs was estimated. Currently it is
estimated that 103 drywells will be constructed as part of the stormwater management
plan. The final determination of number, location, and type (single vs double depth) will
be determined during final design and will need to be optimized (i.e. more drywell
capacity in lower lying areas) to ensure there is no risk of interior flooding in order to
meet levee certification guidelines.
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Table 1. Assumed runoff for Adjacent Subbasins Draining to Project Site.
Subbasin
Area
(square
mi)
Assumed 1% annual
chance event
discharge (cfs/sq mi)
Calculated
runoff from
unit discharge
(cfs)
Drywell
Capacity
(cfs)
Final Runoff
(cfs)
Painted Hills (west
of S Madison) 0.22 150 33 minimum of
33 0
School West 0.02 100 3 2.2 <1*
Madison East 0.34 44 15 0.3 14*
* Will be intercepted by primary infiltration facility
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Figure 9. Interior drainage basins
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SITE INVESTIGATION
A site visit was conducted on 12/18/2014 by Ken Puhn of WEST Consultants, Inc.
(WEST) in order to determine site conditions and observe any changes that may have
occurred since the effective FIS was conducted.
Survey data for the site was provided in a xyz format by WCE. Survey data were supplemented by 2003 LiDAR data collected for the effective FIS. Plan views showing the location of the cross sections in the hydraulic models are shown in Figure 10 and
Figure 11.
HYDROLOGY
Hydrology for the effective FIS is based on a detailed hydrologic analysis using the
Hydrologic Simulation Program Fortran (HSPF). Design flows for the proposed
infiltration facility are based on the effective FEMA discharges. The 100-year discharge
was obtained directly from the effective FIS hydraulic model.
Table 2. Discharges Used in HEC-RAS Models
Location 1% Annual Chance Peak Flow (cfs)
Golf Course Overflow Channel 64
Unnamed Tributary 16/4 (upstream/downstream of SA6)
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Figure 10. Layout of HEC-RAS Cross Sections for Chester Creek Golf Course Overflow
WEST Consultants, Inc. 22 CLOMR for Proposed Painted Hills Development
Figure 11. Layout of HEC-RAS Cross Sections for the Unnamed Tributary
HYDRAULICS
Information related to the development of the various hydraulic models required for the
CLOMR application is provided in the following paragraphs. In the effective FIS, Chester Creek (which includes the Golf Course Overflow Reach) and the Unnamed Tributary, were modeled separately. The CLOMR follows this preexisting methodology. The RAS
models for the Golf Course Overflow and the Unnamed Tributary are provided in
separate folders within the digital submittal materials (Appendix M) and are named
CCMain.prj and CCTrib.prj, respectively. A summary list of model plans is provided in
Table 3. A summary of model reaches is provided in Table 4. A summary of modeled
existing and proposed conditions 1% annual-chance flood flows within the CLOMR reaches is provided in Table 5.
Table 3. Summary of HEC-RAS Model Plans
Model Plan Flow File Geometry File Description
CCMain.prj DEM Upper Reach US reach w/levee
(DEM)
Duplicate Effective Model - Upper Reach of
Chester Creek (above SA 5 Borrow Pit), and Golf
Course Overflow
CCMain.prj DEM FW Upper Reach,
floodway
US reach w/levee
(DEM)
Duplicate Effective Model for Enchroachment
Analysis - Upper Reach of Chester Creek (above
SA 5 Borrow Pit), and Golf Course Overflow
CCMain.prj CEM Upper Reach US reach w/levee
(CEM)
Corrected Effective Model - Upper Reach of
Chester Creek (above SA 5 Borrow Pit), and Golf
Course Overflow
CCMain.prj CEM FW Upper Reach,
floodway
US reach w/levee
(CEM)
Corrected Effective Model for Enchroachment Analysis - Upper Reach of Chester Creek (above
SA 5 Borrow Pit), and Golf Course Overflow
CCMain.prj ECM Upper Reach US reach w/levee
(ECM)
Existing Conditions Model - Upper Reach of
Chester Creek (above SA 5 Borrow Pit), and Golf
Course Overflow
CCMain.prj ECM FW Upper Reach,
floodway
US reach w/levee
(ECM)
Existing Conditions Model for Enchroachment
Analysis - Upper Reach of Chester Creek (above
SA 5 Borrow Pit), and Golf Course Overflow
CCMain.prj PCM Upper Reach
Proposed
US reach w/levee
(PCM)
Proposed Conditions Model - Upper Reach of
Chester Creek (above SA 5 Borrow Pit), and Golf
Course Overflow
CCMain.prj PCM FW
Upper Reach
Proposed,
Floodway
US reach w/levee
(PCM)
Proposed Conditions Model for Enchroachment
Analysis - Upper Reach of Chester Creek (above
SA 5 Borrow Pit), and Golf Course Overflow
CCTrib.prj DEM With levee (DEM) With levee flows and
boundary
Duplicate Effective Model - Unnamed Tributary
with levee conditions (with both levees US and
DS of Hwy 27)
CCTrib.prj DEM FW With levee (DEM) With levee flows,
floodway
Duplicate Effective Model for Encroachment
Analysis - Unnamed Tributary with levee
conditions (with both levees US and DS of Hwy
27)
CCTrib.prj DEM w/o DS
lev
Without DS levee
(DEM)
Without ds levee flows
and boundary
Duplicate Effective Model - Unnamed Tributary without downstream levee conditions (with levee
US of Hwy 27 and without levee DS of Hwy 27)
CCTrib.prj DEM w/o DS lev FW Without DS levee (DEM) Without ds levee flows and boundary, fw
Duplicate Effective Model for Encroachment
Analysis - Unnamed Tributary with levee conditions (with levee US of Hwy 27 and without
levee DS of Hwy 27)
CCTrib.prj CEM With levee (CEM) With levee flows and
boundary
Corrected Effective Model - Unnamed Tributary
with levee conditions (with both levees US and
DS of Hwy 27)
CCTrib.prj CEM FW With levee (CEM) With levee flows,
floodway
Corrected Effective Model for Encroachment
Analysis - Unnamed Tributary with levee
conditions (with both levees US and DS of Hwy
27)
CCTrib.prj CEM w/o DS lev Without DS levee (CEM) Without ds levee flows and boundary
Corrected Effective Model - Unnamed Tributary
without downstream levee conditions (with levee
US of Hwy 27 and without levee DS of Hwy 27)
CCTrib.prj CEM w/o DS
lev FW
Without DS levee
(CEM)
Without ds levee flows
and boundary, fw
Corrected Effective Model for Encroachment
Analysis - Unnamed Tributary with levee
conditions (with levee US of Hwy 27 and without
levee DS of Hwy 27)
CCTrib.prj ECM With levee (ECM) With levee flows and
boundary
Existing Conditions Model - Unnamed Tributary
with levee conditions (with both levees US and
DS of Hwy 27)
CCTrib.prj ECM FW With levee (ECM) With levee flows, floodway
Existing Conditions Model for Encroachment
Analysis - Unnamed Tributary with levee conditions (with both levees US and DS of Hwy
27)
CCTrib.prj ECM w/o DS
lev
Without DS levee
(ECM)
Without ds levee flows
and boundary
Existing Conditions Model - Unnamed Tributary
without downstream levee conditions (with levee
US of Hwy 27 and without levee DS of Hwy 27)
CCTrib.prj ECM w/o DS
lev FW
Without DS levee
(ECM)
Without ds levee flows
and boundary, fw
Existing Conditions Model for Encroachment
Analysis - Unnamed Tributary with levee
conditions (with levee US of Hwy 27 and without
levee DS of Hwy 27)
CCTrib.prj PCM With levee (PCM) With levee flows and
boundary (proposed)
Proposed Conditions Model - Unnamed Tributary
with levee conditions (with both levees US and
DS of Hwy 27)
CCTrib.prj PCM FW With levee (PCM) With levee flows,
floodway (proposed)
Proposed Conditions Model for Encroachment
Analysis - Unnamed Tributary with levee
conditions (with both levees US and DS of Hwy
27)
CCTrib.prj PCM w/o DS
lev
Without DS levee
(PCM)
Without ds levee flows
& bndy (proposed)
Proposed Conditions Model - Unnamed Tributary
without downstream levee conditions (with levee
US of Hwy 27 and without levee DS of Hwy 27)
Table 4. Summary of Model Reaches
Model Reach Description
Part of
CLOMR
CCMain.prj Upper
Chester Creek from upstream end of FIS to
FEMA XS AO (culvert under RR tracks
between Bowdish Road and Schafer Rd)
No
CCMain.prj Dredge Channel
Parallel channel of Chester Creek on east
side of RR tracks between Bowdish Road
and Schafer
No
CCMain.prj DS Dredge Channel
Chester Creek from FEMA XS AO to FEMA XS
AD (upstream end of SA 5, Dishman-Mica Rd
Borrow Pit)
No
CCMain.prj Golf Course Golf Course Overflow reach of Chester Creek Yes
CCTrib.prj Trib Main channel of unnamed tributary Yes
CCTrib.prj LOB Overflow
Left overbank flowpath; flowpath is
modeled only for without DS levee
conditions
Yes
Table 5. Summary of 1% Annual Chance Flood Flows for CLOMR Reaches
Watercourse Location RAS Station
(Existing FIS)
RAS Station
(Revised
CLOMR)
1% Annual Chance Flood (cfs)
Effective
Model (DEM,
CEM, ECM)
Revised
Model
(PCM)
Unnamed
Tributary Highway 27 to SA6 2080 - 149 1324 - 0 16 16
Unnamed
Tributary Outflow from SA6 1 - (-1303) n/a 4 0
Unnamed
Tributary
Left Overbank Flowpath
(without levee scenario 1989 - (-1303) 3786 - 535 16 0
Chester
Creek
Golf Course Overflow
Reach 23887 - 20779 2863 - (-2737) 64 64
Duplicate Effective Model (DEM)
The Duplicate Effective Model (DEM) is a copy of the hydraulic model used to create the
effective FIS. Creation of the DEM is required to ensure proper transfer of data from the effective FIS. As the effective FIS model was developed by WEST, the model was obtained from WEST archives. The hydraulic analysis for the effective FIS had been
completed using the Corps of Engineers River Analysis System (HEC-RAS) standard-
step backwater computer program version 3.1.3.
The DEM model was run using HEC-RAS Version 4.1.0. and model output compared to
the effective Floodway Data Tables (FDT). A comparison of water surface elevations
(WSEs) for the 1% annual chance flood (100-year flood) event and the floodway for the
FIS and DEM model output is provided in Table 6 and Table 7. As shown in Table 6 and
Table 7, the water surface elevations computed using the DEM model are nearly identical to elevations published in the effective FISs. For the golf course overflow reach the model reports water surface elevations at RAS cross section 21,013 and 21,128 that differ
from the published FIS elevations. This is due to computational changes between RAS
version 3.1.3 and 4.1. The water surface elevation at cross section 21,013 is fixed in the
RAS model based on the static water elevation reported by the HSPF hydrologic model
for SA1. RAS 4.1 has difficulty converging on a subcritical solution at this cross section and defaults to a critical depth solution, ignoring the fixed elevation. In this case, since the floodplain at this location would reflect the ponded conditions expected within the
storage area, the reported critical depth solution is erroneous and the fixed elevation of
2008.05 (rounded to 2008.1) is the correct elevation. The erroneous solution at cross
section 21,013 results in a slight calculated increase of 0.1 feet at cross section 21,128.
For the Unnamed Tributary RAS cross sections 1,472 and 1,963 are 0.1 foot lower than the FIS. This again is likely due to small computational differences between RAS 3.1.3 and 4.1.
No modifications were made to the DEM because the noted differences were within the
±0.50 ft tolerance required by FEMA Guidelines and Specification for Flood Mapping
Partners (G&S) (FEMA, 2003). HEC-RAS DEM model results are provided in Appendix
C, and an electronic version of the DEM model is included on the CD provided in
Appendix M.
Table 6. Comparison of FIS and DEM model results for Golf Course Overflow Reach
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
Effective
FIS
(FT NAVD)
DEM
(FT NAVD) Difference
(FT)
Effective
FIS
(FT NAVD)
DEM
(FT NAVD) Difference
(FT)
20779 0 A 2008.1 2008.1 0.0 -- 2009.1 --
21013 773 B 2008.1 2007.8 -0.3 -- 2009.1 --
21128 961 C 2008.5 2008.6 0.1 -- 2009.1 --
21229 1145 D 2008.9 2008.9 0.0 2009.6 2009.6 0.0
21385 1425 E 2009.1 2009.1 0.0 2010.0 2010.0 0.0
21445 1600 F 2009.3 2009.3 0.0 2010.3 2010.3 0.0
21548 1800 G 2013.3 2013.3 0.0 2014.1 2014.1 0.0
21924 2123 H 2013.3 2013.3 0.0 2014.2 2014.2 0.0
22423 2704 I 2013.5 2013.5 0.0 2014.5 2014.5 0.0
22972 3144 J 2014.8 2014.8 0.0 2015.3 2015.3 0.0
23050 3287 K 2015.4 2015.4 0.0 2016.2 2016.2 0.0
23090 3387 L 2015.7 2015.7 0.0 2016.6 2016.6 0.0
23446 3721 M 2018.1 2018.1 0.0 2019.0 2019.0 0.0
23887 4318 N 2023.0 2023.0 0.0 2023.8 2023.8 0.0
Table 7. Comparison of FIS and DEM model results for Unnamed Tributary
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
Effective
FIS
(FT NAVD)
DEM
(FT NAVD) Difference
(FT)
Effective
FIS
(FT NAVD)
DEM
(FT NAVD) Difference
(FT)
-1303 0 A 2008.1 2008.1 0.0 2009.1 2009.1 0.0
-1019 283 B 2008.4 2008.4 0.0 2009.1 2009.1 0.0
-880 422 C 2008.4 2008.4 0.0 2009.1 2009.1 0.0
-466 836 D 2008.4 2008.4 0.0 2009.1 2009.1 0.0
-89 910 E 2008.4 2008.4 0.0 2009.1 2009.1 0.0
1 1,378 F 2009.7 2009.7 0.0 2010.7 2010.7 0.0
149 1,525 G 2009.7 2009.7 0.0 2010.7 2010.7 0.0
343 1,720 H 2009.7 2009.7 0.0 2010.7 2010.7 0.0
383 1,760 I 2009.7 2009.7 0.0 2010.7 2010.7 0.0
472 1,849 J 2009.7 2009.7 0.0 2010.7 2010.7 0.0
576 1,952 K 2010.0 2010.0 0.0 2010.7 2010.7 0.0
651 2,028 L 2011.0 2011.0 0.0 2011.1 2011.1 0.0
918 2,295 M 2011.5 2011.5 0.0 2011.6 2011.6 0.0
1472 2,849 N 2012.8 2012.8 0.0 2012.8 2012.7 -0.1
1528 2,905 O 2014.3 2014.3 0.0 2014.3 2014.3 0.0
1557 2,933 P 2014.3 2014.3 0.0 2014.3 2014.3 0.0
1963 3,339 Q 2019.7 2019.7 0.0 2019.7 2019.6 -0.1
2100 3,485 R 2020.8 2020.8 0.0 2020.8 2020.8 0.0
Corrected Effective Model (CEM)
The Corrected Effective Model (CEM) is the model that corrects any errors that occur in
the DEM, adds any additional cross sections needed, and/or incorporates more detailed
topographic information than that used in the DEM. The DEM model review for both the
Unnamed Tributary and the Golf Course Overflow Reach found that the models have
reasonable cross section spacing and contain detailed topographic data based on channel survey and LiDAR capable of supporting 2-foot contours. Further, the floodplain areas within the CLOMR boundary have remained essentially unchanged since the effective
FIS and the effective model topography is representative of current conditions.
For the Unnamed Tributary the topography and ‘n’ values within the effective models are
considered to be reasonable and representative of site conditions based on engineering
judgement and guidance provided in the HEC-RAS Hydraulic Reference Manual; therefore, no changes were made. The CEM model is identical to the DEM with the exception of ineffective flow stations which were added to cross sections 2,028 and 2,849
to better reflect local topography.
For the Golf Course Overflow in order to more accurately assess the impacts of the proposed project, cross sections 22,423, 21,924, and 21,548 were removed and replaced
with five new cross-sections (21,525, 21,609, 21,726, 21,857, and 21,983) the spacing
and alignment of which would better represent the area immediately upstream of Thorpe
Road. Cross section 21,498 was also added to better define the influence of Thorpe
Road. Mannings ‘n’ values for the new cross sections are identical to the ‘n’ values for the cross sections they were replacing, and were considered reasonable based on engineering judgement and the RAS Hydraulic Reference Manual. Mannings ‘n’ values
and topographic data in other portions of the model were considered reasonable;
therefore, no other changes were made. A comparison of DEM and CEM results are
provided in Table 8 and Table 9.
Table 8. DEM and CEM model results for the Golf Course Overflow
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
DEM
(FT NAVD) CEM
(FT NAVD) Difference
(FT) DEM
(FT NAVD) CEM
(FT NAVD) Difference
(FT)
20779 0 A 2008.05 2008.05 0.00 2009.05 2009.05 0.00
21013 773 B 2007.79 2007.79 0.00 2009.05 2009.05 0.00
21128 961 C 2008.60 2008.60 0.00 2009.05 2009.05 0.00
21229 1145 D 2008.87 2008.87 0.00 2009.59 2009.59 0.00
21385 1425 E 2009.12 2009.12 0.00 2010.03 2010.03 0.00
21409 -- -- 2009.17 2009.17 0.00 2010.12 2010.12 0.00
21431 -- -- 2009.23 2009.23 0.00 2010.23 2010.23 0.00
21445 1600 F 2009.31 2009.31 0.00 2010.29 2010.29 0.00
21456 -- -- 2013.13 2013.13 0.00 2013.79 2013.79 0.00
21481 -- -- 2013.25 2013.25 0.00 2014.10 2014.10 0.00
21485 1 -- -- -- 2013.25 -- -- 2014.10 --
21515 -- -- 2013.25 2013.26 0.01 2014.11 2014.11 0.00
21525 1 -- -- -- 2013.26 -- -- 2014.12 --
21548 2 1800 G 2013.26 -- -- 2014.13 --
21609 1 -- -- -- 2013.26 -- -- 2014.14 --
21726 1 -- -- -- 2013.26 -- -- 2014.16 --
21924 3 2123 H 2013.27 2013.27 0.00 2014.23 2014.24 0.01
21983 1 -- n/a -- 2013.31 -- -- 2014.28 --
22423 2704 I 2013.51 2013.64 0.13 2014.48 2014.45 -0.03
22972 3144 J 2014.82 2014.60 -0.22 2015.26 2015.26 0.00
23005 -- -- 2015.25 2015.28 0.03 2016.10 2016.10 0.00
23050 3287 K 2015.38 2015.39 0.01 2016.21 2016.21 0.00
23090 3387 L 2015.65 2015.65 0.00 2016.61 2016.61 0.00
23446 3721 M 2018.10 2018.10 0.00 2019.02 2019.02 0.00
23887 4318 N 2022.99 2022.99 0.00 2023.79 2023.79 0.00
24430 -- -- 2029.56 2029.56 0.00 2029.72 2029.72 0.00
Table 9. DEM and CEM model results for the Unnamed Tributary
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
DEM
(FT NAVD) CEM
(FT NAVD) Difference
(FT) DEM
(FT NAVD) CEM
(FT NAVD) Difference
(FT)
-1303 0 A 2008.05 2008.05 0.00 2009.05 2009.05 0.00
-1019 283 B 2008.36 2008.36 0.00 2009.05 2009.05 0.00
-880 422 C 2008.42 2008.42 0.00 2009.05 2009.05 0.00
-466 836 D 2008.42 2008.42 0.00 2009.05 2009.05 0.00
-89 910 E 2008.42 2008.42 0.00 2009.05 2009.05 0.00
1 1,378 F 2009.70 2009.70 0.00 2010.70 2010.70 0.00
149 1,525 G 2009.70 2009.70 0.00 2010.70 2010.70 0.00
343 1,720 H 2009.70 2009.70 0.00 2010.70 2010.70 0.00
383 1,760 I 2009.70 2009.70 0.00 2010.70 2010.70 0.00
472 1,849 J 2009.70 2009.70 0.00 2010.70 2010.70 0.00
576 1,952 K 2009.95 2009.95 0.00 2010.67 2010.67 0.00
651 2,028 L 2010.95 2011.13 0.18 2011.07 2011.07 0.00
918 2,295 M 2011.45 2011.63 0.18 2011.63 2011.63 0.00
1472 2,849 N 2012.83 2012.72 -0.11 2012.71 2012.71 0.00
1510 -- -- 2013.26 2013.26 0.00 2013.26 2013.26 0.00
1528 2,905 O 2014.30 2014.30 0.00 2014.31 2014.31 0.00
1557 2,933 P 2014.32 2014.32 0.00 2014.32 2014.32 0.00
1963 3,339 Q 2019.66 2019.66 0.00 2019.63 2019.63 0.00
1989 -- -- 2020.06 2020.06 0.00 2020.08 2020.08 0.00
2080 -- -- 2020.61 2020.61 0.00 2020.61 2020.61 0.00
2100 3,485 R 2020.83 2020.83 0.00 2020.83 2020.83 0.00
2110 -- -- 2020.88 2020.88 0.00 2020.88 2020.88 0.00
2120 3,509 S 2020.91 2020.91 0.00 2020.92 2020.92 0.00
2651 4,040 T 2023.40 2023.40 0.00 2023.40 2023.40 0.00
3126 4,515 U 2026.19 2026.19 0.00 2026.18 2026.18 0.00
Existing (Pre-Project) Conditions Model
The Existing or Pre-Project Conditions Model is a modification of the CEM to reflect any
modifications that have occurred within the floodplain since the date of the effective model but prior to the construction of the project for which the revision is being requested. For the Unnamed Tributary, no significant changes have occurred to the
channel or existing floodplain since the time of the effective study; therefore, the Existing Conditions model (ECM) is a duplicate of the CEM.
For the Golf Course Overflow Reach, the only significant modifications known to have
occurred with the floodplain are the addition of two, 10” corrugated metal culverts under
Thorpe Road. The Existing Conditions model was modified to include the culverts.
Comparisons of CEM and ECM model results are summarized in Table 10 and Table 11.
Table 10. CEM and ECM model results for the Golf Course Overflow
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
CEM
(FT NAVD) ECM
(FT NAVD) Difference
(FT) CEM
(FT NAVD) ECM
(FT NAVD) Difference
(FT)
20779 0 A 2008.05 2008.05 0.00 2009.05 2009.05 0.00
21013 773 B 2007.79 2007.79 0.00 2009.05 2009.05 0.00
21128 961 C 2008.60 2008.60 0.00 2009.05 2009.05 0.00
21229 1145 D 2008.87 2008.87 0.00 2009.59 2009.59 0.00
21385 1425 E 2009.12 2009.12 0.00 2010.03 2010.03 0.00
21409 -- -- 2009.17 2009.17 0.00 2010.12 2010.12 0.00
21431 -- -- 2009.23 2009.23 0.00 2010.23 2010.23 0.00
21445 1600 F 2009.31 2009.31 0.00 2010.29 2010.29 0.00
21456 -- -- 2013.13 2013.14 0.01 2013.79 2013.79 0.00
21481 -- -- 2013.25 2013.27 0.02 2014.10 2014.14 0.04
21485 1 -- -- 2013.25 -- -- 2014.10 -- --
21515 -- 2013.26 2013.27 0.01 2014.11 2014.15 0.04
21525 -- -- 2013.26 2013.28 0.02 2014.12 2014.15 0.03
21609 1800 G 2013.26 2013.28 0.02 2014.14 2014.19 0.05
21726 -- -- 2013.26 2013.29 0.03 2014.16 2014.21 0.05
21857 2123 H 2013.27 2013.30 0.03 2014.24 2014.23 -0.01
21983 n/a n/a 2013.31 2013.33 n/a 2014.28 2014.27 n/a
22423 2704 I 2013.64 2013.64 0.00 2014.45 2014.45 0.00
22972 3144 J 2014.60 2014.60 0.00 2015.26 2015.27 0.01
23005 -- -- 2015.28 2015.28 0.00 2016.10 2016.10 0.00
23050 3287 K 2015.39 2015.39 0.00 2016.21 2016.21 0.00
23090 3387 L 2015.65 2015.65 0.00 2016.61 2016.61 0.00
23446 3721 M 2018.10 2018.10 0.00 2019.02 2019.02 0.00
23887 4318 N 2022.99 2022.99 0.00 2023.79 2023.79 0.00
24430 -- -- 2029.56 2029.56 0.00 2029.72 2029.72 0.00
1 XS converted to bridge with culverts in model
Table 11. CEM and ECM model results for the Unnamed Tributary
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
CEM
(FT NAVD) ECM
(FT NAVD) Difference
(FT) CEM
(FT NAVD) ECM
(FT NAVD) Difference
(FT)
-1303 0 A 2008.05 2008.05 0.00 2009.05 2009.05 0.00
-1019 283 B 2008.36 2008.36 0.00 2009.05 2009.05 0.00
-880 422 C 2008.42 2008.42 0.00 2009.05 2009.05 0.00
-466 836 D 2008.42 2008.42 0.00 2009.05 2009.05 0.00
-89 910 E 2008.42 2008.42 0.00 2009.05 2009.05 0.00
1 1,378 F 2009.70 2009.70 0.00 2010.70 2010.70 0.00
149 1,525 G 2009.70 2009.70 0.00 2010.70 2010.70 0.00
343 1,720 H 2009.70 2009.70 0.00 2010.70 2010.70 0.00
383 1,760 I 2009.70 2009.70 0.00 2010.70 2010.70 0.00
472 1,849 J 2009.70 2009.70 0.00 2010.70 2010.70 0.00
576 1,952 K 2009.95 2009.95 0.00 2010.67 2010.67 0.00
651 2,028 L 2011.13 2011.13 0.00 2011.07 2011.07 0.00
918 2,295 M 2011.63 2011.63 0.00 2011.63 2011.63 0.00
1472 2,849 N 2012.72 2012.72 0.00 2012.71 2012.71 0.00
1510 2013.26 2013.26 0.00 2013.26 2013.26 0.00
1528 2,905 O 2014.30 2014.30 0.00 2014.31 2014.31 0.00
1557 2,933 P 2014.32 2014.32 0.00 2014.32 2014.32 0.00
1963 3,339 Q 2019.66 2019.66 0.00 2019.63 2019.63 0.00
1989 2020.06 2020.06 0.00 2020.08 2020.08 0.00
2080 2020.61 2020.61 0.00 2020.61 2020.61 0.00
2100 3,485 R 2020.83 2020.83 0.00 2020.83 2020.83 0.00
2110 2020.88 2020.88 0.00 2020.88 2020.88 0.00
2120 3,509 S 2020.91 2020.91 0.00 2020.92 2020.92 0.00
2651 4,040 T 2023.40 2023.40 0.00 2023.40 2023.40 0.00
3126 4,515 U 2026.19 2026.19 0.00 2026.18 2026.18 0.00
Proposed or Post-Project Conditions Models
The Proposed Conditions models (PCM) reflect the construction of the infiltration
facilities along the Golf Course Overflow Reach and the Unnamed Tributary. The post-
project conditions model was developed by making the following modifications to the Existing Conditions models:
Golf Course Overflow:
• As the proposed infiltration facility will intercept all flow up through and
including the 0.2% annual-chance-flood, the model was shortened and assigned new stationing based on Thorpe Road as the new downstream limit reference
point (Station 0). Table 12 provides a comparison of existing and revised model
and FEMA stationing.
• Existing cross sections north (downstream) of Thorpe Road were removed. This
includes FEMA XS A (RS 0) through F (RS 1600). This corresponds with
effective RAS cross sections 20,779 through 21,481. Cross sections were added
downstream of Thorpe Road in order to represent the proposed concrete open channel and the series of culverts that carry flow from the end of the engineered channel at the southeast corner of the project site, north along the west side of S
Madison Rd, to the proposed bio-infiltration channel and infiltration facility. It
should be noted that the RAS model changes were not used to design the open
channel, culverts, or any other components of the flood control facility (anything
north of Thorpe Road). These components were added to the RAS model based on design plans provided by WCE in order to provide a reasonable starting water surface elevation for the model at Thorpe Road which would properly represent
the proposed water surface elevations within the revised floodplain south of
Thorpe Road.
• The model geometry was modified to include the replacement of existing
culverts under Thorpe Road with a concrete box culvert with a 3 foot rise and 29
foot span.
• Downstream boundary conditions were assigned as a normal depth with a 1%
slope based on the bio-infiltration channel design.
• Floodway stations were maintained at a generally similar width to the effective
FIS; however, since the current/proposed culvert location differs in location from the existing overtopping point of Thorpe Road in the effective FIS, the floodway location was shifted for cross sections 964 through 0 to align with the current
centerline of the overflow path.
Unnamed Tributary:
The Proposed Conditions models (PCM) reflect the construction of the infiltration facility
within SA6 and the proposed certification of the Gustin Ditch levee. The Proposed
Conditions Model was developed by making the following modifications to the Existing Conditions models:
With downstream levee condition
• The model was truncated at SA6. Since the infiltration facility will contain all
flows up to and including the 0.2% annual-chance flood, the main channel of the
tributary no longer continues to Painted Hills. Accordingly, the stationing of the
main tributary channel in the model was revised between SA 6 and Highway 27
such that the entrance to the infiltration facility is now River Station 0. Since the
CLOMR boundary for the tributary ends at Highway 27, the original model stationing was left in place upstream of Highway 27. The revised CLOMR FIS
profile includes a note explaining the revised stationing. Table 13 provides a
comparison of existing and revised model and FEMA stationing.
• Per the levee certification design, the culvert under Highway 27 was extended by
75 feet.
• Per the levee certification design the private culvert at RS 1,520 was removed.
• Three cross sections were added at the downstream end of the model to define the
rock spillway into the infiltration facility (RS 0, -20, -36). The Mannings ‘n’
values of 0.045 for these cross sections were selected to represent the rock lined
channel.
• In order to support design of the Gustin Ditch levee by WCE, the RAS model
cross-sections between Highway 27 and SA6 were revised to provide more even spacing and closer spacing between cross sections. Eight cross sections were removed (RS 651 – 1,989) and replaced with 14 cross sections at a typical spacing
of 100 feet (Revised RS 0 – 1,324). The Mannings ‘n’ values for these cross
sections were selected to represent a vegetated channel assuming reasonable
regular maintenance.
Without downstream levee condition
• The without downstream levee condition is only used for simulating the 0.2%
annual-chance flood since the levee will not be certified for that flood event. Due to the new truncated end point of the main channel of the tributary (at SA 6) the Left Overbank Flowpath used for the without downstream levee scenario was
revised to be a separate reach from the main channel. The centerline reference
stationing for this new reach is based on S Madison Road as Station 0. Table 16
provides a comparison of existing and revised model and FEMA stationing.
• Since the overflow reach in the effective FIS models (and CEM, ECM) is not a
distinct reach separate from the main channel, the cross section locations in the
overbank were somewhat constrained in order to represent both reaches accurately. Free from this constraint, five cross sections (1,103, 1,648, 2,683, 2,975, 3,786) were added to provide better resolution and better alignment than
the ECM model cross sections. Nine cross sections were removed (-89, 149, 343,
472, 918, 1,472, 1,510, 1,528, 1,963), as they were no longer needed based on the
separation of the two reaches and the additional of the new cross sections. The
Mannings ‘n’ values of the new cross sections remain the same as the values of the overbank areas of the existing and removed cross section and are appropriate for the vegetation present in the overflow reach.
Post-Project Conditions model output is provided in Appendix F, and an electronic
version of this model is included on the CD provided in Appendix K.
Existing vs Proposed Conditions model results for the 1% annual chance flood event and the floodway are summarized in Table 12 and Table 13. A comparison of the floodplain
widths is provided in Table 14, Table 15, and Table 16. The proposed infiltration
facilities and levee certifications would have the following impacts:
Golf Course Overflow
• SA1 would be completely removed from the 1% annual chance floodplain. This
includes an area of approximately 94 acres.
• BFEs upstream of Thorpe Road would be reduced by as much as 2-3 feet.
• The portion of the floodway north of Thorpe Rd along the Golf Course Overflow
Reach would be removed from the floodplain mapping.
• The floodway would be 3 feet wider at revised cross section 964.
Unnamed Tributary
• A large portion of the 1% annual chance floodplain (24 acres) will be removed
from the Unnamed Tributary reach. This includes all of the left overbank flowpath
(without DS levee condition) and the floodplain downstream of SA6 between the pre-project SA6 outlet and the Left Overbank Flowpath.
• The floodway downstream of SA6 and along the left overbank would be removed from the mapping.
• 1% annual-chance-flood water surface elevations decrease as much as 2.1 feet in the main channel.
• A slight increase in water surface elevations between existing and proposed conditions (0.01 feet) will occur at two cross sections between RS 2,080 and RS
2,100 immediately upstream of the Highway 27 culvert. The increase is within
the channel banks.
Table 12. ECM and PCM model results for the Golf Course Overflow
Effective Revised 1% Annual Chance Flood Event Floodway
RAS
Station
FEMA
Station
XS
Letter
RAS
Station
FEMA
Station
XS
Letter
ECM
(FT
NAVD)
PCM
(FT
NAVD)
Difference
(FT)
ECM
(FT
NAVD)
PCM
(FT
NAVD)
Difference
(FT)
20779 0 A 1 -- -- -- 2008.05 -- -- 2009.05 -- --
21013 773 B 1 -- -- -- 2007.79 -- -- 2009.05 -- --
21128 961 C 1 -- -- -- 2008.60 -- -- 2009.05 -- --
21229 1145 D 1 -- -- -- 2008.87 -- -- 2009.59 -- --
21385 1425 E 1 -- -- -- 2009.12 -- -- 2010.03 -- --
21409 -- -- -- -- -- 2009.17 -- -- 2010.12 -- --
21431 -- -- -- -- -- 2009.23 -- -- 2010.23 -- --
21445 1600 F 1 -- -- -- 2009.31 -- -- 2010.29 -- --
21456 -- -- -- -- -- 2013.14 -- -- 2013.79 -- --
21481 -- -- -- -- -- 2013.27 -- -- 2014.14 -- --
21485 -- -- -- -- -- 2013.27 -- Culvert --
21515 -- 23 23 A 2013.27 2010.80 -2.47 2014.15 2010.80 -3.35
21525 -- -- 36 36 -- 2013.28 2012.20 -1.08 2014.15 2012.24 -1.91
21609 1800 G 178 178 B 2013.28 2012.63 -0.65 2014.19 2013.09 -1.10
21726 -- -- 276 276 C 2013.29 2012.67 -0.62 2014.21 2013.20 -1.01
21857 2123 H 402 402 D 2013.30 2012.82 -0.48 2014.23 2013.39 -0.84
21983 n/a -- 528 528 E 2013.33 2013.09 -0.24 2014.27 2013.59 -0.68
22423 2704 I 964 964 F 2013.64 2013.62 -0.02 2014.45 2014.11 -0.34
22972 3144 J 1404 1404 G 2014.60 2014.60 0.00 2015.27 2015.27 0.00
23005 -- -- 1489 1489 H 2015.28 2015.28 0.00 2016.10 2016.10 0.00
23050 3287 K 1547 1547 I 2015.39 2015.39 0.00 2016.21 2016.21 0.00
23090 3387 L 1647 1647 J 2015.65 2015.65 0.00 2016.61 2016.61 0.00
23446 3721 M 1981 1981 K 2018.10 2018.10 0.00 2019.02 2019.02 0.00
23887 4318 N 2578 2578 L 2022.99 2022.99 0.00 2023.79 2023.79 0.00
24430 -- -- 2863 2863 -- 2029.56 2029.56 0.00 2029.72 2029.72 0.00
1 Removed XS from PCM
Table 13. ECM and PCM model results for the Unnamed Tributary
Effective Revised (CLOMR) 1% Annual Chance Flood Event Floodway
RAS
Station
FEMA
Station
XS
Letter
RAS
Station
FEMA
Station
XS
Letter
ECM
(FT
NAVD)
PCM
(FT
NAVD)
Difference
(FT)
ECM
(FT
NAVD)
PCM
(FT
NAVD)
Difference
(FT)
-1303 0 A 2 -- -- -- 2008.05 -- -- 2009.05 -- --
-1019 283 B 2 -- -- -- 2008.36 -- -- 2009.05 -- --
-880 422 C 2 -- -- -- 2008.42 -- -- 2009.05 -- --
-466 836 D 2 -- -- -- 2008.42 -- -- 2009.05 -- --
-89 910 E 2 -- -- -- 2008.42 -- -- 2009.05 -- --
1 1,378 F 2 -- -- -- 2009.70 -- -- 2010.70 -- --
149 1,525 G 2 -- -- -- 2009.70 -- -- 2010.70 -- --
343 1,720 H 2 -- -- -- 2009.70 -- -- 2010.70 -- --
383 1,760 I 2 -- -- -- 2009.70 -- -- 2010.70 -- --
472 1,849 J 2 -- -- -- 2009.70 -- -- 2010.70 -- --
576 1,952 K -20 -20 -- 2009.95 1999.27 -10.68 2010.67 1999.26 -11.41
-- -- -- 0 0 A 1 -- 2009.26 -- -- 2009.24 --
651 2,028 L 75 75 B 2011.13 2009.91 -1.22 2011.07 2010.08 -0.99
-- -- -- 174 174 -- 1 -- 2009.99 -- -- 2010.13 --
-- -- -- 275 275 -- 1 -- 2010.00 -- -- 2010.14 --
918 2,295 M 2 -- -- -- 2011.63 2010.01
3 -1.62 2011.63 2010.14
3 -1.49
-- -- -- 374 374 C 1 -- 2010.01 -- -- 2010.14 --
-- -- -- 474 474 -- 1 -- 2010.06 -- -- 2010.18 --
-- -- -- 574 574 -- 1 -- 2010.12 -- -- 2010.23 --
-- -- -- 674 674 D 1 -- 2010.17 -- -- 2010.27 --
-- -- -- 774 774 -- 1 -- 2010.65 -- -- 2010.65 --
1472 2,849 N 874 874 -- 2012.72 2011.93 -0.79 2012.71 2011.93 -0.78
1510 -- -- 2 -- -- -- 2013.26 2012.11
3 -1.15 2013.26 2012.11
3 -1.15
1528 2,905 O -- -- -- 2014.30 2012.18
3 -2.12 2014.31 2012.18
3 -2.13
1557 2,933 P 974 974 E 2014.32 2012.34 -1.98 2014.32 2012.34 -1.98
-- -- -- 1073 1073 -- 1 -- 2013.35 -- -- 2013.35 --
-- -- -- 1174 1174 -- 1 -- 2014.94 -- -- 2014.94 --
-- -- -- 1274 1274 -- 1 -- 2016.45 -- -- 2016.45 --
-- -- -- 1324 1324 F1 -- 2017.66 -- -- 2017.66 --
1963 3,339 Q 2 -- -- -- 1 2019.66 -- -- 2019.63 -- --
1989 -- -- 2 -- -- -- 1 2020.06 -- -- 2020.08 -- --
2080 -- -- 2080 2080 Q 2020.61 2020.62 0.01 2020.61 2020.62 0.01
2100 3,485 R 2100 2100 R 2020.83 2020.83 0.00 2020.83 2020.84 0.01
2110 -- -- 2110 2110 -- 2020.88 2020.88 0.00 2020.88 2020.88 0.00
2120 3,509 S 2120 2120 S 2020.91 2020.92 0.01 2020.92 2020.92 0.00
2651 4,040 T 2651 2651 T 2023.40 2023.39 -0.01 2023.40 2023.40 0.00
3126 4,515 U 3126 3126 U 2026.19 2026.19 0.00 2026.19 2026.19 0.00
Table 14. Change in Top Width for Golf Course Overflow
Effective Revised Top Width Base Flood (ft) Top Width Floodway (ft)
RAS
Station
FEMA
Station
XS
Letter
RAS
Station
FEMA
Station
XS
Letter
ECM
(FT)
PCM
(FT)
Difference
(FT)
ECM
(FT)
PCM
(FT)
Difference
(FT)
20779 0 A 1 -- -- -- 2279 0 -2279 -- -- --
21013 773 B 1 -- -- -- 566 0 -566 -- -- --
21128 961 C 1 -- -- -- 624 0 -624 -- -- --
21229 1145 D 1 -- -- -- 586 0 -586 35 0 -35
21385 1425 E 1 -- -- -- 336 0 -336 30 0 -30
21409 -- --1 -- -- -- 385 0 -385 25 0 -25
21431 -- --1 -- -- -- 326 0 -223 25 0 -25
21445 1600 F 1 -- -- -- 252 0 -143 20 0 -20
21456 -- --1 -- -- -- 430 0 -115 30 0 -30
21481 -- --1 -- -- -- 657 0 -657 45 0 -45
21515 -- 23 23 A 521 29 -492 29 29 -29
21525 2 1800 G 36 36 -- 521 133 -388 29 29 0
-- -- -- 178 178 B 484 441 -43 44 44 0
21726 -- -- 276 276 C 364 245 -119 44 44 0
21857 2123 H 402 402 D 250 204 -46 43 43 0
21983 -- -- 528 528 E 180 141 -39 43 43 0
22423 2704 I 964 964 F 203 202 -1 40 43 3
22972 3144 J 1404 1404 G 54 54 0 14 14 0
23005 -- -- 1489 1489 H 136 136 0 27 27 0
23050 3287 K 1547 1547 I 152 152 0 22 22 0
23090 3387 L 1647 1647 J 155 155 0 20 20 0
23446 3721 M 1981 1981 K 240 240 0 20 20 0
23887 4318 N 2578 2578 L 160 160 0 20 20 0
24430 -- -- 2863 2863 -- 37 37 0 20 20 0
1 Removed XS from PCM
2 XS 21609 in DEM model
Table 15. Change in Top Width for Unnamed Tributary
Effective Revised Top Width Base Flood (ft) Top Width Floodway (ft)
RAS
Station
FEMA
Station
XS
Letter
RAS
Station
FEMA
Station
XS
Letter
ECM
(FT)
PCM
(FT)
Difference
(FT)
ECM
(FT)
PCM
(FT)
Difference
(FT)
-1303 0 A 2 -- -- -- 104 0 -104 31 0 -31
-1019 283 B 2 -- -- -- 48 0 -48 21 0 -21
-880 422 C 2 -- -- -- 362 0 -362 40 0 -40
-466 836 D 2 -- -- -- 389 0 -389 46 0 -46
-89 910 E 2 -- -- -- 346 0 -346 40 0 -40
1 1,378 F 2 -- -- -- 102 --4 -- -- 1 --4 --
149 1,525 G 2 -- -- -- 176 --4 -- -- 1 --4 --
343 1,720 H 2 -- -- -- 267 --4 -- -- 1 --4 --
383 1,760 I 2 -- -- -- 280 --4 -- -- 1 --4 --
472 1,849 J 2 -- -- -- 193 --4 -- -- 1 --4 --
576 1,952 K -20 -20 -- 6 --4 -- 5 --4 --
-- -- -- 0 0 A 1 -- 4 -- -- 4 --
651 2,028 L 75 75 B 64 11 -53 16 11 -5
-- -- -- 174 174 -- 1 -- 23 -- -- 23 --
-- -- -- 275 275 -- 1 -- 22 -- -- 22 --
918 2,295 M 2 -- -- -- 43 19 4 -24 8 16 8
-- -- -- 374 374 C 1 -- 14 -- -- 14 --
-- -- -- 474 474 -- 1 -- 16 -- -- 16 --
-- -- -- 574 574 -- 1 -- 13 -- -- 13 --
-- -- -- 674 674 D 1 -- 8 -- -- 8 --
-- -- -- 774 774 -- 1 -- 10 -- -- 10 --
1472 2,849 N 874 874 -- 5 11 6 5 11 6
1510 -- -- 2 -- -- -- 6 11 4 5 6 10 4 4
1528 2,905 O -- -- -- 10 11 4 6 10 8 4 -2
1557 2,933 P 974 974 E 8 7 -1 8 7 -1
-- -- -- 1073 1073 -- 1 -- 9 -- -- 9 --
-- -- -- 1174 1174 -- 1 -- 15 -- 7 --
-- -- -- 1274 1274 -- 1 -- 15 -- -- 9 --
-- -- -- 1324 1324 F1 -- 15 -- -- 15 --
1963 3,339 Q 2 -- -- -- 1 15 -- -- 9 -- --
1989 -- -- 2 -- -- -- 1 48 -- -- 9 -- --
2080 -- -- 2080 2080 Q 15 15 0 15 15 0
2100 3,485 R 2100 2100 R 15 15 0 15 15 0
2110 -- -- 2110 2110 -- 16 16 0 15 15 0
2120 3,509 S 2120 2120 S 16 16 0 15 15 0
2651 4,040 T 2651 2651 T 8 8 0 8 8 0
3126 4,515 U 3126 3126 U 10 10 0 10 10 0
Table 16. Change in Top Width for Unnamed Tributary without DS levee (left overbank flowpath)
Effective Revised Top Width Base Flood (ft) Top Width Floodway (ft)
RAS Station FEMA Station XS Letter RAS Station FEMA Station XS Letter ECM (FT) PCM (FT) Difference (FT) ECM (FT) PCM (FT) Difference (FT)
-1303 0 A 535 535 A 113 0 -113 31 0 -31
-1019 283 B 817 817 B 98 0 -98 21 0 -21
-880 422 C 957 957 C 369 0 -369 40 0 -40
-- -- -- 1103 1103 D -- 0 -- -- 0 --
-466 836 D 1388 1388 E 404 0 -404 46 0 -46
-- -- -- 1648 1648 F -- 0 -- -- 0 --
-89 910 E -- -- -- 389 0 -389 40 0 -40
1 1,378 F 1868 1868 G 211 0 -211 15 0 -15
149 1,525 G -- -- -- 271 0 -271 15 0 -15
343 1,720 H -- -- -- 298 0 -298 15 0 -15
383 1,760 I 2078 2078 H 216 0 -216 15 0 -15
472 1,849 J -- -- -- 236 0 -236 15 0 -15
576 1,952 K 2285 2285 I 165 0 -165 15 0 -15
651 2,028 L 2417 2417 J 99 0 -99 15 0 -15
918 2,295 M 2683 2683 K 233 0 -233 15 0 -15
-- -- -- 2975 2975 L -- 0 -- -- 0 --
1472 2,849 N -- -- -- 119 0 -119 15 0 -15
1510 -- -- -- -- -- 137 0 -137 15 0 -15
1528 2,905 O -- -- -- 139 0 -139 3 0 -3
1557 2,933 P 3407 3407 M 162 0 -162 15 0 -15
1963 3339 Q -- -- -- 522 0 -522 137 0 -137
-- -- -- 3786 3786 N -- 0 -- -- 0 --
FLOODPLAIN MAPPING
An annotated FIRM showing existing and proposed floodplain mapping is provided in Appendix H along with revised flood profiles and floodway data tables. Workmaps are
provided in Appendix I. The proposed floodplain mapping reflects the proposed
infiltration facilities and levee certifications. In the effective FIS, the 0.2% annual chance
floodplain within SA1 was based on model output for the Golf Course Overflow Reach.
Since the infiltration facility will be intercepting all but 4 cfs of the 0.2% annual change
flood, and since the D-M Road levee, and the golf course levee will not be certified for the 0.2% annual chance flood event the mapping for that flood in SA1 is based on projection of the water surface elevations from the riverward side of the levees. The
downstream end of the Unnamed Tributary (mapping and profile) reflect this condition.
CERTIFICATION FORMS
Completed FEMA forms are included in Appendix A. Appendix A contains MT2 Forms
1, 2 and 3. The Biological Opinion required for ESA compliance is provided in Appendix L.
SUMMARY
A hydraulic analysis was conducted for a proposed development within the Chester Creek floodplain. The hydraulic analysis was completed to support a Conditional Letter of Map Revision (CLOMR) application for the proposed development per requirements of the
City of Spokane Valley and FEMA. The revised hydraulic models and mapping products
reflect the proposed certification of 2 levees, construction and certification of one
additional levee, and construction of two infiltration and storage facilities. The results of
the analysis indicate that with limited exceptions, the proposed development will
significantly reduce the floodplain extent and water surface elevations for both the base flood and floodway conditions within the extents of the CLOMR.
REFERENCES
Federal Emergency Management Agency, Procedure Memorandum No. 51, Guidance for
Mapping of Non-Levee Embankments, February 27, 2009
Federal Emergency Management Agency, Flood Insurance Study, Spokane County,
Washington and Incorporated Areas, July 6, 2010.
Federal Emergency Management Agency, Guidelines and Specifications for Flood
Hazard Partners, July 6, 2003.
U.S. Army Corps of Engineers, HEC-RAS River Analysis System User’s Manual, Version
4.1, January 2010.
Whipple Consulting Engineers, Inc., Painted Hills Flood Control Development
Narrative, October 13, 2016.
WEST Consultants. Inc., Technical Memorandum Chester Creek Flood Insurance Study
Hydrology Re-evaluation, January 14, 2008.
WEST Consultants. Inc., Flood Insurance Study Hydrologic Analysis for Chester Creek, December 8, 2004.
Inland Pacific Engineering Company, Preliminary Geotechnical Evaluation Phase I
Painted Hills Golf Course Property, December 31, 2013.
Inland Pacific Engineering Company, Geotechnical Evaluation Levee Evaluation and
Certification 4403 South Dishman-Mica Road Spokane County, Washington,
February 12, 2015
Biology Soil & Water, Inc, Critical Areas Assessment, Buffer Averaging, and Habitat
Management Plan for the Painted Hills PRD, May 14, 2015