2008, 06 Technical Support Data Notebook Chester Creek Flood Insurance Restudy Draft ReportL'Ut S` eo /-t - ou_k_boo k. pa,(+ 1 of) 2 . d o c_
Federal Emergency Management Agency
Region X
Technical Support Data Notebook
For
Chester Creek
City of Spokane Valley
and
Unincorporated Spokane County, Washington
Flood Insurance Restudy
DRAFT REPORT
DRAFT
JUNE 2008
WEST
CO N S U L T A NTS, INC.
Prepared by WEST Consultants, Inc. under contract EMS -2001-00-0068
Table of Contents
SECTION 1 - GENERAL DOCUMENTATION 2
Special Problems Reports 2
General Correspondence 10
SECTION 2 - ENGINEERING ANALYSES 11
Hydrologic Analysis 11
General 11
Hydraulic Analyses 16
General 16
Boundary Conditions, Sensitivity Analyses, and Calibration Data 23
Profile Plots 23
Levees 23
Floodway Analyses 24
RAS Models 24
Water Surface Elevation Profiles 25
References 32
Key to Cross -Section Labeling Error! Bookmark not defined.
Hydraulic Model Files Index Error! Bookmark not defined.
SECTION 3 - DRAFT FIS REPORT TEXT ERROR! BOOKMARK NOT
DEFINED.
Water Surface Profiles Index Error! Bookmark not defined.
Floodway Tables Error! Bookmark not defined.
MAPPING INFORMATION ERROR! BOOKMARK NOT DEFINED.
Work maps Error! Bookmark not defined.
Elevation Reference Marks Error! Bookmark not defined.
SECTION 4 - MISCELLANEOUS REFERENCE MATERIALS ERROR!
BOOKMARK NOT DEFINED.
WEST
CO NS U I T A NT S. INC.
Prepared by WEST Consultants, Inc. under contract EMS -2001 -CO -0068
SECTION 5- CERTIFICATION ERROR! BOOKMARK NOT DEFINED.
List of Figures
Figure 1. Study area location 17
Figure 2. Location of model reaches 21
Figure 3. Location of storage areas. 22
List of Tables
Table 1. Flood Frequency Flows for the Chester Creek and Unnamed Tributary 14
Table 2. Storage area water surface elevation magnitude frequency estimates. 15
Table 3. Comparison between effective and proposed hydrology. 15
Table 4. Water surface elevations for the 10, 50, 100, and 500 -year events 25
Table 5. Floodway table. Error! Bookmark not defined.
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
Introduction
This Technical Support Data Notebook (TSDN) was prepared by the Study Contractor (SC),
WEST Consultants, Inc. (WEST), in concordance with the submittal requirements for a Federal
Emergency Management Agency (FEMA) Flood Insurance Study (FIS). The TSDN
specifications are detailed in the Guidelines and Specifications for Flood Hazard Mapping
Partners, Federal Emergency Management Agency, February 2002.
Study Contractor: WEST Consultants, Inc. Page 1
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
General Documentation
Special Problems Reports
Study Contractor: WEST Consultants, Inc.
Special Problems Report Index
Chester Creek — Spokane County, Washington
FLOOD INSURANCE STUDY
Contact
SPR# Report Date Name, Firm/Agency Subject
1
Jul. 03, 2003
WEST Consultants
Additional Hydrologic Analysis
2
Sep. 24, 2003
WEST Consultants
Schedule Revision
3
Aug. 18, 2004
WEST Consultants
Additional Hydrologic Analysis
4
Mar. 25, 2005
WEST Consultants
Schedule Revision
5
Mar. 22, 2007
WEST Consultants
Modify Hydrologic and Hydraulic
Modeling
Study Contractor:• WEST Consultants, Inc. Page 2
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
SPR #1
SPECIAL PROBLEM REPORT
FLOOD INSURANCE STUDIES
Date: 30 -Jul -03
Contract Number: EMS -2001 -CO -0068
Name of Community: Spokane County, WA
Study Contractor: WEST Consultants, Inc.
PROBLEM AREA
TECHNICAL COORDINATION
UNRESOLVED
DISCREPANCIES
UNREALISTIC SCHEDULE
OTHER
X
DETAILED EXPLANATION
Additional Hydrologic Analysis for Determining Flood Frequency Analysis Values: Under the
original scope, an existing HSPF model of the Chester Creek basin was assumed to adequately
represent current watershed conditions except for an existing borrow pit. Review of the existing model
revealed numerous problems and errors in the input file. These included erroneous IMPLND, PERLND,
and FTABLE values. Consequently, the existing model is not accurately calculating runoff to the stream
channel due to the IMPLND and PERLND errors, or correctly routing flow through the stream channel
network due to FTABLE stage -storage -discharge relationship errors. As a result of these errors, the
results of the existing model should not be used.
PROPOSED SOLUTION
To correct the noted deficiencies, the following work is required: infiltration will be added to the lower
basin stream reaches underlain by glacial outwash, routing tables will be revised based on recent
topographic survey data, climatic time series developed for recent watershed studies in the Spokane
area will be incorporated, runoff parameters developed as part of other recent watershed hydrology
studies in north Spokane will be incorporated and revisions to land use designations will be made to
reflect current watershed conditions. The revised hydrologic model will be calibrated using available
streamflow data and anecdotal information. Following calibration, a long simulation will be performed
usig climatic timeseries information from the Spokane airport. This will provide a 49 -year flow timeseries
that will be used to compute floo magnitude frequency estimates for the study area. The sensitivity of
flood magnitude -frequency estimates to assumed infiltration rates in the lower reach will be examined.
Coordination between WEST and Spokane County will conducted to review hydrologic modeling results.
Estimated cost: $13, 582.12 (see detailed cost sheet)
Study Contractor: WEST Consultants, Inc.
Page 3
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
SPR #2
SPECIAL PROBLEM REPORT
FLOOD INSURANCE STUDIES
Date: 24 -Sep -03
Contract Number: EMS -2001 -CO -0068
Name of Community: Spokane County, WA
Study Contractor: WEST Consultants, Inc.
PROBLEM AREA
TECHNICAL COORDINATION
UNRESOLVED
DISCREPANCIES
UNREALISTIC SCHEDULE
OTHER
X
DETAILED EXPLANATION
Unrealistic Schedule: Under the original project schedule, all work was to be completed by September
30, 2003. Additional hydrologic analysis was required to complete the project that was not anticipated in
the original scope of work.
PROPOSED SOLUTION
To allow time for completion of hydrologic and hydraulic analyses, preparation of draft work maps, and
review by involved parties it is recommended that the completion date for the project be extended by six
months until 1 April 2004.
Estimated cost: No cost modification to contract.
Study Contractor: WEST Consultants, Inc.
Page 4
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
SPR #3
SPECIAL PROBLEM REPORT
FLOOD INSURANCE STUDIES
Date: 18-Auq-04
Contract Number: EMS -2001-00-0068
Name of Community: Spokane County, WA
Study Contractor: WEST Consultants, Inc.
PROBLEM AREA
TECHNICAL COORDINATION
UNRESOLVED
DISCREPANCIES
UNREALISTIC SCHEDULE
OTHER
X
DETAILED EXPLANATION
As a result of review comments received from Michael Baker Jr., Inc. (MB) on the draft Hydrology
Summary Report, various modifications to the developed HSPF model were recommended. The
modifications included reduced estimates of infiltration for consistency with regional gage records.
Modification of infiltration rates will require revision of stage -storage relationships for storage areas
located downstream of the existing detention facility along Chester Creek. Additional HEC -RAS modeling
and HSPF modeling will be required to finalize the hydrology estimates.
PROPOSED SOLUTION
Additional HEC -RAS modeling will be conducted to define stage/discharge relations for locations
downstream of the existing detention facility along Chester Creek. Addtional HSPF modeling will be
conducted to incorporate the results of the new stage/discharge relations. Infiltration values used in the
HSPF model will be adjusted to be consistent with gage records for similar regional basins. The HSPF
model adjusted by Michael Baker Jr. as part of the review process will be reviewed for quality assurance.
Results of the hydrologic analysis will be summarized in a revised Hydrology Summary report and
resubmitted to Michael Baker Jr. for approval.
Estimated cost: $15,405.92 (see detailed cost sheet)
Study Contractor: WEST Consultants, Inc.
Page 5
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
SPR #4
Study Contractor: WEST Consultants, Inc.
Page 6
SPECIAL PROBLEM
FLOOD INSURANCE
Date:
REPORT
STUDIES
March 25, 2005
Contract Number: EMS -2001 -CO -0068
Name of Community: Chester Creek, Spokane County, WA
Study Contractor: WEST Consultants, Inc.
TECHNICAL
UNRESOLVED
DISCREPANCIES
UNREALISTIC
OTHER
PROBLEM AREA
COORDINATION r -i
SCHEDULE n
X
DETAILED EXPLANATION
Funding to cover an SPR to complete the hydrology is expected to the study contractor by the end of
March 2005.
PROPOSED SOLUTION
The date for delivery for Activity 4 - Hydrology Analyses should be postponed to May 31, 2005 to allow
sufficient time for finalizing the Hydrology Analysis. Subsequent deliverable deadlines should be equally
extended.
REVISED SCHEDULE REMAINING
ACTIVITY
RESPONSIBLE
MAPPING PARTNER(S)
DUE DATE
Activity 4 — Hydrologic Analyses
FEMA (SC)
5/31/2005
Activity 5— Independent QA/QC Review of
Hydrologic Analyses
FEMA (NSP)
6/30/2005
Activity 6 —Hydraulic Analyses
FEMA (SC)
5/31/2005
Activity 7 — Independent QA/QC Review of
Hydraulic Analyses
FEMA (NSP)
6/30/2005
Activity 8 — Floodplain Mapping (Detailed Riverine
Analysis)
FEMA (SC)
8/31/2005
Activity 9 — Independent QA/QC Review of
Floodplain Mapping (Revised Areas)
FEMA (NSP)
9/30/2005
Activity 11 — DFIRM Production (Non -Revised
Areas)
FEMA (NSP)
10/31/2005
Activity 12 — DFIRM Production (Merging Revised
and Non -Revised Information)
FEMA (NSP)
10/31/2005
Activity 12A — DFIRM Production (Application of
DFIRM Graphics and Database Specifications
FEMA (NSP)
10/31/2005
Activity 13 — Preliminary DFIRM and FIS Report
Distribution
FEMA (NSP)
11/30/2005
Activity 14 — Post -Preliminary Processing
FEMA (SC), and FEMA
(NSP)
11/30/2006
Study Contractor: WEST Consultants, Inc.
Page 6
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
SPR #5
SPECIAL PROBLEM REPORT
FLOOD INSURANCE STUDIES
Date: 22 -Mar -07
Contract Number: EMS -2001 -CO -0068
Name of Community: Spokane County, WA
Study Contractor: WEST Consultants, Inc.
PROBLEM AREA
TECHNICAL COORDINATION
UNRESOLVED
DISCREPANCIES
UNREALISTIC SCHEDULE
OTHER
X
DETAILED EXPLANATION
Modify Hydrologic and Hydraulic Modeling: The City of Spokane Valley has submitted data during
the 90 -day appeal period regarding several dry wells located in the downstream portion of the study area.
They would like to investigate the impact of the dry wells on the proposed floodplain.
PROPOSED SOLUTION
To investigate the impact of the dry wells, the following work is required: The HSPF model will be
revised to include the dry wells and Sub basin 2 (located to the east of Dishman Mica Rd.). Additionally,
the downstream portion of the study area will be modeled in HSPF as a series reaches and storage
areas formed from east -west road crossings. Appropriate revisions to the hydrologic report will be made
and the results submitted to FEMA for review. The hydraulic model will be modified to incorporate these
changes and the newly revised discharges from HSPF. SFHAs will be redelineated in the downstream
portion of Chester Creek based on the revised modeling. Appropriate changes will be made to the
Floodway Tables, Flood Profiles and the TSDN. Workmaps will be created and submitted to FEMA for
review.
Study Contractor: WEST Consultants, Inc.
Page 7
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
Contact Reports (Telephone Conversations and E-mails)
Study Contractor: WEST Consultants, Inc.
Contact (Telephone Conversation) Reports Index
Chester Creek — Spokane County, Washington
FLOOD INSURANCE STUDY
Contact
CR# Report Date Name, Firm/Agency Subject
1
12/02/03
WEST Consultants, Inc.
Levee certification
2
11/17/03
Steve Worley, City of
Spokane Valley
Flooding photo dates
3
02/11/04
Wilbert Thomas, Michael
Baker Corporation
Hydrology Draft Review
4
02/19/04
Wilbert Thomas, Michael
Baker Corporation
Flooding Video
5
03/05/04
WEST Consultants, Inc.
Hydrology Draft
6
03/08/04
Wilbert Thomas, Michael
Baker Corporation
Flooding Video
7
05/18/04
Larry Johnson, NRCS
Hydrology Draft Review
8
06/07/04
Wilbert Thomas, Michael
Baker Corporation
Hydrology Draft Review
9
06/29/04
Wilbert Thomas, Michael
Baker Corporation
Hydrology Draft Review
10
06/29/04
Joseph Weber, Department
of Homeland Security,
FEMA Region 10
Hydrology Draft Review
11
07/14/04
Wilbert Thomas, Michael
Baker Corporation
Hydrology Draft Review
12
11/23/04
Wilbert Thomas, Michael
Baker Corporation
Hydrology Draft Review
13
01/07/05
Wilbert Thomas, Michael
Baker Corporation
Hydrology Draft Review Approval
14
7/15/05
Joseph Weber, Department
of Homeland Security,
FEMA Region 10
Floodplain Mapping Zone Designation
15
11/21/05
Gaston Cabanilla, Michael
Baker Corporation
Hydraulics Draft Review
Study Contractor: WEST Consultants, Inc.
Page 8
Chester Creek Flood Insurance Study
Spokane County, Washington
DRAFT
16
11/23/05
Gaston Cabanilla, Michael
Hydraulics Draft Review Approval
Baker Corporation
Copies of the Contact Reports are provided at the end of this document.
Study Contractor: WEST Consultants, Inc. Page 9
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
General Correspondence
The purpose of this section is to provide FEMA with a comprehensive chronology of written
correspondence generated or received by the Contractor during the preparation of the FIS.
Study Contractor: WEST Consultants, Inc.
General Correspondence Index
Chester Creek — Spokane County, Washington
INSURANCE STUDY
GC# Meetings/Report Corresponding Topic
Date Firm/Agency
1
12/05/03
WEST Consultants, Inc.
Hydrology/Hydraulics
Discussion
2
03/10/04
Thomas Jackson, Michael
Baker Corporation
Hydrology Draft Review
3
03/24/04
Mark Mastin, USGS
Hydrology Draft Review
4
05/25/04
Wilbert Thomas, Michael
Baker Corporation
Hydrology Draft Review
5
06/29/04
Wilbert Thomas, Michael
Baker Corporation
Hydrology Draft Review
6
11/05/04
WEST Consultants, Inc.
Hydrology/Hydraulics
Discussion
7
10/13/05
Gaston Cabanilla, Michael
Baker Corporation
Hydraulics Draft Review
8
11/04/05
WEST Consultants, Inc.
Hydraulics Draft Review
Copies of the General Correspondence are provided at the end of this document.
Study Contractor: WEST Consultants, Inc. Page 10
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
Engineering Analyses
Hydrologic Analysis
General
A detailed hydrologic analysis of Chester Creek in Spokane County and the City of Spokane
Valley, Washington was conducted. The objective of the analysis was to establish flood
magnitude -frequency estimates for use in a Flood Insurance Restudy of the watercourse. The
analysis was conducted using the Hydrological Simulation Program Fortran (HSPF; EPA, 1997).
The hydrology of Chester Creek was previously analyzed using HSPF by HYDMET (1997) for
watershed planning purposes (CH2M Hill, 1997). The model developed for that analysis was
refined for use in the current study. The refinements included incorporating more detailed
channel survey information, updating the land use to reflect current conditions, including the
effects of infiltration from the stream channel and six storage areas identified in the lower reaches
of the basin, extending the simulation through year 2002 for a total simulation period of 54 -years,
and recalibrating the model to recorded stream flow and regional information from adjacent
basins. Frequency analyses were performed using the 54 years of simulated stream flow
information and are presented in this report for each subbasin.
An HSPF model of Chester Creek calibrated to historical anecdotal evidence was submitted as a
draft report for review to Michael Baker Jr., Inc. in February 2004 (WEST, 2004). That review
concluded that the infiltration rates used were consistent with optimal infiltration conditions but
did not consider reduced infiltration resulting from frozen soil conditions or sealing of pond and
ditch bottoms by fine sediment over time. Based on an analysis of regional gage data, Michael
Baker Jr., Inc. recommended that the HSPF model be revised to reflect infiltration rates that are
33 percent of the "optimum" values previously considered (MBJr., 2004).
During the 90 day appeal period the City of Spokane Valley filed an appeal based on several dry
wells located in the downstream portion of the study area. Additional funding was provided (SPR
#5) to modify the HSPF model to determine the impacts of the dry wells as well as changing the
downstream reach to model the areas immediately upstream of 2nd Ave., 8th Ave. 16th Ave. and
24th Ave. as storage areas. The modifications to the HSPF model were approved the discharge
estimates listed in the report reflect the dry well and storage modifications. Additional details on
Study Contractor: WEST Consultants, Inc. Page 11
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
the dry well analyses can be found in the technical memo "Chester Creek Flood Insurance Study
Hydrology Re-evaluation" dated March 12, 2008 (WEST, 2008).
The key findings of the hydrologic analysis include:
• Simulated and recorded discharges compare well with the limited data available for the
Dishman-Mica Road gage location.
• The discharge along the mainstem of Chester Creek reaches a maximum in the central
portion of the watershed at the outlet of Subbasin 9 near Thorpe Road. A 100 -year return
period flood discharge of 211 cfs was predicted at that location. Floods equal to or
greater than a 10 -year return period event overflow into the Painted Hills Golf Course
across Thorpe Road. Floodwaters entering the golf course were determined to infiltrate
and not return to Chester Creek. Downstream of Thorpe Road, the remaining flow in
Chester Creek infiltrates into glacial outwash and the discharge decreases with distance
downstream.
• Chester Creek enters a borrow pit (Storage Area 2) constructed in 1998 as part of road
improvements to Dishman-Mica Road. Stream flow entering the borrow pit infiltrates
and discharge to downstream reaches has not occurred since the borrow pit was
constructed. Hydrologic simulations showed some discharge occurring from this storage
area for all simulated flood events starting at the 2 -year recurrence interval. During flood
events water will overflow from the borrow pit and result in discharge to the downstream
channel.
• The 100 -year return period discharge rate at 24th Avenue (Cross Section K) downstream
of the borrow pit was estimated at approximately 55 cfs with a discharge of 4 cfs at 2nd
Avenue indicating only shallow sheet flow at the downstream extent of the study area.
• Uncertainty is associated with the developed hydrologic estimates due to the lack of long-
term records of precipitation and flow measurements specific to the Chester Creek
watershed. The HSPF results were calibrated to a limited record of stream flow data and
regional basin data and downstream flooding conditions based on assumed soil
infiltration rates. It should be recognized that the long-term infiltration characteristics of
the soil can change due to sedimentation along the channel, vegetation, and land use
changes.
• There are significant differences between the magnitude of hydrologic estimates defined
by prior investigations and those developed by the current study. These differences have
significant implications for water surface elevations and the extent of the floodplain. The
major differences between the prior and current studies are related to assumptions
regarding land use conditions and soil infiltration characteristics. It is evident that these
assumptions have a significant effect on the resulting hydrologic estimates.
Recommendations of the study to address the potential uncertainty in the hydrologic estimates
include the following:
Study Contractor: WEST Consultants, Inc. Page 12
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
• Watershed changes that would affect soil infiltration characteristics in the lower basin should
be monitored. This should include monitoring of land use conditions and sedimentation
conditions along channels and within the borrow pit.
• Additional precipitation and stream flow measurements should be collected that would allow
improvement of model calibration and verification.
• Future reanalysis of the hydrologic estimates should be conducted if soil infiltration
conditions in the basin change or if an improved record of basin specific precipitation and
flow records is developed. Additionally, if land use conditions in the basin change
significantly the hydrologic estimates should be updated.
• In recognition of the uncertainties involved with the hydrologic analysis of the basin, until a
long term record of basin specific precipitation and stream flow data is developed,
consideration should be given to defining hydrologic estimates based on future land use
conditions that consider the influence of the recently constructed borrow pit. It is recognized
that both existing and future land use condition floodplains could be delineated on FEMA
FIRM maps. A future land use condition floodplain would be expected to define a
conservatively larger estimate of the 100 -year floodplain extent.
The final hydrologic discharges for Chester Creek are listed in Table 1. Calculated water surface
elevation magnitude frequency estimates for the six storage areas are listed in Table 2. A
comparison of the effective flows and the final proposed flows are presented in Table 3.
Complete details on the hydrologic analyses (prior to the dry well analysis funded by SPR #5) can
be found in the final hydrologic report "Flood Insurance Study Hydrologic Analysis for Chester
Creek, Spokane County, Washington" (WEST, 2004).
Study Contractor: WEST Consultants, Inc. Page 13
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
Table 1. Flood Frequency Flows for the Chester Creek and Unnamed Tributary.
Location
10 -yr
50 -yr
100 -yr
500 -yr
38025 — Chester Creek
58
89
101
131
26315 — Chester Creek
87
135
155
201
23887 — Chester Creek
75
104
116
144
21515 — Chester Creek
118
139
147
167
24430 — Golf Course
30
54
64
88
20492 — Chester Creek
115
138
148
170
18180 — Chester Creek
113
137
147
169
Storage Area 2 Outlet
60
100
117
155
14412 — West of Borrow Pit
14
26
32
44
SA5 Outflow — Chester
Creek
30
40
44
53
SA 7 Outflow — Chester
Creek (24th Ave)
25
42
49
67
SA 8 Outflow — Chester
Creek (16`h Ave)
9
25
32
48
SA 9 Outflow — Chester
Creek (8th Ave)
0
0
0
0
7220 - Tributary
7
14
18
28
6553 - Tributary
11
22
28
45
4704 - Tributary
12
24
30
46
472 - Tributary
10
14
16
20
Storage Area 6 Outflow
0
2
4
7
Storage Area 1 Outflow
0
0
0
0
Study Contractor: WEST Consultants, Inc. Page 14
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
Table 2. Storage area water surface elevation magnitude frequency estimates.
Location
10 -yr
50 -yr
100 -yr
500 -yr
Overflow
Elevation (ft)
Storage Area 1
2007.19
2007.80
2008.05
2008.64
2009.0
Storage Area 2
2000.81
2001.06
2001.16
2001.78
2000.81
Storage Area 3
1999.99
2000.74
2001.05
2001.78
1999.99
Storage Area 4
2018.30
2019.23
2019.47
2020.03
2025.0
Storage Area 5
1999.99
2000.74
2001.05
2001.78
2001.9
Storage Area 6
2005.00
2009.60
2009.70
2010.00
2009.5
Storage Area 7
1997.14
1997.19
1997.21
1997.26
1996.9
Storage Area 8
1993.54
1993.65
1993.70
1993.78
1993.4
Storage Area 9
1988.54
1990.75
1990.79
1990.86
1992.5
Table 3. Comparison between effective and proposed hydrology.
Flooding Sources and Location
Drainage Area
(Square Miles)
Peak Discharges
(cfs)
100 -Year
Chester Creek
Thorpe Rd.*
Effective
405
Proposed
12.4
211
Difference
(%)
-48
Schafer Rd.
Effective
644
Proposed
18.4
178
Difference
(%)
-72
24th Ave.
Effective
722
Proposed
22.0
49
Difference
(%)
-93
* This value includes flow continuing downstream from Thorpe and flow diverted to golf course
Study Contractor: WEST Consultants, Inc. Page 15
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
Hydraulic Analyses
Study Contractor: WEST Consultants, Inc.
Hydraulic Analyses Index
Chester Creek — Spokane County, Washington
FLOOD INSURANCE STUDY
Flooding Source/Stream Hydraulic Method/Model Method/Model Analysis
Name Used Date
Chester Creek and Unnamed
Tributary
HEC -RAS Version 3.1.3
June 2008
General
WEST Consultants, Inc. developed a steady flow model of Chester Creek and it's
Unnamed Tributary using HEC -RAS Version 3.1.3 (HEC, 2005), as part of a Flood Insurance
Study for Federal Emergency Management Agency, Region X (FEMA) and modeled the 10, 50,
100 and 500 -year events. Figure 1 shows the study area location. The HEC -RAS models consist
of 5 reaches, 6 storage areas and almost 265 cross sections contrasted with the effective study
which characterized the system using two reaches with no storage areas. The reaches and storage
areas, in approximately downstream to upstream order, are described below and are presented in
Figure 2 and Figure 3.
Study Contractor: WEST Consultants, Inc. Page 16
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
Figure 1. Study area location.
Study Contractor: WEST Consultants, Inc.
Page 17
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
Chester Creek, Main Channel - This reach contains the main channel of Chester Creek from S.
Sands Road to 8th Ave. in Dishman, a distance of approximately 7 miles. Notably, though the
floodplain extends downstream to 8th Ave., no defined channel exists for Chester Creek
downstream of the Borrow Pit just south of 28th Ave (Storage Area 2). Downstream of 28th Ave.
flood discharges spread across relatively flat fields and pastures and continues downstream to
residential and commercial property.
Chester Creek, West of Borrow Pit — This reach begins just downstream of Shaffer Rd. Flood
discharge in this reach do not originate from the upper reaches of Chester Creek, but rather from
sub -basin C4 (See WEST, 2004). Discharge from this basin is prevented from reaching the main
channel of Chester Creek by the Railroad Bed running parallel to the Channel and Borrow Pit.
Discharge from the basin flows in the left overbank downstream across 28th Ave. until it joins the
discharge from the Chester Creek (outflow from Storage Area 5 culvert under RR tracks).
Chester Creek, Dredge Channel — This short reach parallels Chester Creek on the east side of the
railroad tracks between Bowdish Rd. and Shaffer Rd. It provides additional area for discharge to
flow under the railroad through the series of additional culverts installed as part of a flood control
plan.
Chester Creek, Golf Course Reach — Approximate 2500 feet upstream of Thorpe Road, a flow
split occurs due to limited channel capacity. For flood discharges equal to and greater than the
10 -yr event, flow overtops the right channel bank, and flows downstream in the overbank area
until it crosses Thorpe Road and enters the Painted Hills Golf Course (Storage Area 1).
Unnamed Tributary, Main Channel — This reach extends from the Painted Hills Golf Course
upstream for approximately 1.7 miles to a private crossing, roughly 0.5 miles upstream of E. 46th
Ave. The channel terminates in a gravel pit ( Storage Area 6) just north of E. 40th Ave and
approximately 1500 downstream of the Pines Road crossing. The gravel pit contains flows up to
the 25 year event. Small discharges of 2, 4 and 7 cfs overtop the pit for the 50, 100 and 500 yr
event, respectively and continue downstream flowing across fields until joining the floodwaters
from Chester Creek in the Painted Hills Golf Course (Storage Area 1) where it is expected to
pond until it infiltrates.
Study Contractor: WEST Consultants, Inc. Page 18
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
Storage Area 1 (SA1) is the Painted Hills Golf Course. It is located near the midpoint of the
study area. Available historic photographs dated 1949 and 1950 and anecdotal reports indicate
the golf course to be a site of frequent flood storage. 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. 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, the flow is
stored until it infiltrates.
Storage Area 2 (SA2) is the borrow pit that was constructed in 1998 between Schaffer Road and
28th Avenue as part of improvements to the Dishman-Mica Road. The borrow pit was
constructed to retain and infiltrate floodwaters from Chester Creek that pass downstream of
Schaffer Road. Since the borrow pit was constructed, no overflows from the borrow pit have
been observed.
Storage Area 3 (SA3) is located in the Kokomo residential area, just downstream of the borrow
pit and to the east of Dishman-Mica Road. Dishman-Mica road was recently raised to help
mitigate potential flooding of the Kokomo area (Spokane County, 1997). As part of this
construction, an 18" culvert was installed to allow the roadside ditch on the east side of Dishman-
Mica road to drain to the borrow pit (Chad Coles, County Roads Division, personal
communication, November 19, 2003). When the water surface elevation in the borrow pit
exceeds elevation 1999.0 ft, the culvert would allow flow, estimated to be a maximum of
approximately 5 cfs for the 100 -year recurrence interval flood event, under Dishman-Mica Road
and into the Kokomo residential area where it would pond and infiltrate.
Storage Area 4 (SA4) is located in the right overbank area to the east of State Highway 27 along
an unnamed tributary to Chester Creek. Water escapes the main channel of this tributary just
upstream of Pines Road due to limited channel capacity. Flow in the right overbank is expected
to pond in the Storage Area 4 until it infiltrates.
Storage Area 5 (SA5) is located just to the west of Dishman Mica Rd. and just north of 28th
Avenue, immediately downstream of Storage Area 2. This storage area is bounded by 24th Ave to
the north, the railroad track embankment to the west, and Dishman Mica Road to the east. Water
Study Contractor: WEST Consultants, Inc. Page 19
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
can pond in this area during larger flood events due to the limited capacity of a culvert through
the railroad embankment, the only downstream outlet.
Storage Area 6 (SA6) is a large gravel pit which serves as the terminus of the channel of the
Unnamed Tributary on Chester Creek.
Storage Area 7 (SA7) is the area immediately upstream of 24th Ave. 24th Ave crosses the
floodplain obstructing downstream flow until flood waters overtop the road and continue
downstream.
Storage Area 8 (SA8) is the area immediately upstream of 16th Ave. 16th Ave crosses the
floodplain obstructing downstream flow until flood waters overtop the road and continue
downstream.
Storage Area 9 (SA9) is the area immediately upstream of 8th Ave. 8th Ave crosses the floodplain
obstructing downstream flow until flood waters overtop the road and continue downstream.
Study Contractor: WEST Consultants, Inc. Page 20
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
Unnamed Tributary
Main Channel
Without DS levee
iv Without US levee
Chester Creek
A/W. of Borrow Pit
A` Golf Course
Dredge Channel
A/ Main Channel
Figure 2. Location of model reaches.
Study Contractor: WEST Consultants, Inc.
Page 21
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
Storage
Area 5
Storage
Area 4
N
No Defined Channel
f\J Unnamed Tributary
Ovl Chester Creek
Storage Areas
12 Drainage Basin ID
0
2 Miles
Figure 3. Location of storage areas.
Study Contractor: WEST Consultants, Inc.
Page 22
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
We developed cross-section geometry and bridge and culvert structure data for the HEC -
RAS models from 2' contours, provided by TerraPoint, and ground surveys conducted by Stratton
Surveying and Mapping. LiDAR flights were conducted in March 2003 and the surveys occurred
in December 2002 and March 2003. The vertical datum is NAVD 88. We converted the
topographic data to RAS format using HEC-GeoRAS. We specified flows in the model based on
the previously discussed hydrologic analyses (Table 1).
Boundary Conditions, Sensitivity Analyses, and Calibration Data
We specified normal depth at the downstream boundary for the main stem of Chester
Creek based on the ground slope near the downstream end of the reach. The boundary conditions
for the unnamed tributary, golf course overflow, and main stem reach flowing into the borrow pit
(dredge channel reach) were assigned fixed water surface elevations determined from the HSPF
hydrologic model. Manning's 'n' values were based on field inspections during the cross-section
survey and from 1993 aerial orthophotos received from the city of Spokane. No calibration data
were identified for this study. We evaluated the HEC -RAS models using CHECK -RAS.
Profile Plots
We have generated profile plots, using RASPLOT, for all study reaches. The water
surface elevations for the area between river stations 10162 (AC) and 13616 (AD) on Chester
Creek were determined from HSPF model output (Storage Areas 2, 3 and 5). The downstream
extent of the `without levee US of Pines Rd.' plot for the Unnamed Tributary is based on Storage
Area 4 (east of Pines Road) output from HSPF. The profile plot ends near the upstream end of
the storage area and does not represent the full length of the storage area. Likewise, the water
surface elevations at the downstream extent of the main channel of the Unnamed Tributary and
the Golf Course reach of Chester Creek are based on Storage Area 1 output (Painted Hills Golf
Course) from HSPF. The profile plots do not represent the full length of the storage area.
Levees
Five levees were identified within the project area and four of these were modeled using
both "with" and "without levee" analyses. None of the levees were determined to be FEMA
certified. On the Unnamed Tributary a levee exists on the right bank between E 46th and Pines
Rd. This levee is highly variable in nature, almost disappearing completely in places. This levee
was modeled as a lateral weir in the "with levee" analysis. All flow overtopping the levee flows
in the right overbank and ponds in Storage Area 4 until it infiltrates. The channel between E 46th
and Pines road is "perched" above the right overbank. For the "without levee" analysis, all flow
was assumed to leave the channel and flow in the right overbank to Storage Area 6. A second
Study Contractor: WEST Consultants, Inc. Page 23
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
"without levee" analysis was conducted on the tributary just downstream of the Pines Road
crossing. Flow leaves the channel and flows along left overbank and continues downstream until
merging with outflow from Storage Area 4.
Three levees were identified along the main Channel of Chester Creek. The golf course
levee is located on the right bank between Thorpe Road and Dishman-Mica Road. It was
determined that a "without levee" analysis was not necessary because the water surface elevations
in the channel are at or below the base of the levee. The water surface elevations on the golf
course side of the levee (Storage Area 1) are also below the base of the levee. A second levee is
located just downstream on the right bank between the Dishman-Mica Rd. crossing and Bowdish
Rd and a third levee located just downstream of the Bowdish Road crossing on the left bank.
Both of these areas were modeled using "with" and "without levee" analysis.
Floodway Analyses
The Regulatory Floodway is defined as the portion of the river and adjacent land area that
must remain unobstructed to ensure that the water surface elevation does not cumulatively
increase more than one foot above the base flood elevation (BFE). No existing floodway had
been previously defined for Chester Creek. Typically, an encroachment analysis is computed from
both the left and right channel banks towards the river channel using an "equal conveyance
reduction" method. This approach was utilized where appropriate, however floodway analyses
were conducted in multiple areas where no channel exists. Notably, the downstream reach of
Chester Creek extends approximately 2 miles beyond the end of the channel in the borrow pit at 28th
Ave. Downstream of 28th Ave. flood discharges flows through fields as well as residential and
commercial areas. In these areas the floodway was routed along existing surface streets where
possible to avoid existing structures. Throughout much of the study area, and particularly in the
upstream reaches, the 100 -year water surface is confined within the main channel and therefore
typically an energy increase of zero is listed. The Floodway Tables from RAS are shown later in
this document.
RAS Models
A separate RAS model was created for the main stem of Chester Creek and it's unnamed
tributary. The RAS project files provided to FEMA are listed in the "Hydraulic Model Files
Index". Each of these RAS projects includes a RAS plan for the 10-, 50-, 100-, and 500 -year
flows, as well as the encroachment analysis.
Study Contractor: WEST Consultants, Inc. Page 24
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
Water Surface Elevation Profiles
Table 4 presents the simulated water surface elevations for the 10, 50, 100 and 500 -year
events. These results are from both "with" and "without levee" analysis models. The final work
maps plot the highest water surface elevations. RASPLOT was used to generate water surface
profiles and automatically removes any negative surcharges (E.g., a drop in water surface as the
water is "squeezed" passing though a bridge opening). RASPLOT profiles are provided at the
end of this document.
Table 4. Water surface elevations for the 10, 50, 100, and 500 -year events.
RAS
Station
Cross
Section
10 -Year
50 -Year
100 -Year
500 -Year
WSELEV (ft)
(NAVD)
WSELEV (ft)
(NAVD)
WSELEV (ft)
(NAVD)
WSELEV (ft)
(NAVD)
Chester Creek - Main Channel
4201.7
A
1988.78
1990.75
1990.79
1990.86
4787.8
B
1990.13
1990.79
1990.84
1990.96
5319.4
C
1990.47
1990.92
1991.02
1991.21
5836.2
D
1990.87
1991.35
1991.5
1991.76
6348.0
E
1991.27
1991.72
1991.87
1992.15
6738.0
1991.82
1992.17
1992.28
1992.52
6788.0
F
1993.54
1993.65
1993.7
1993.78
7197.6
G
1993.54
1993.65
1993.7
1993.78
7685.8
H
1993.54
1993.65
1993.7
1993.79
8194.9
1
1993.56
1993.69
1993.75
1993.85
8714.4
J
1994.17
1994.32
1994.37
1994.48
9215.5
K
1995.12
1995.22
1995.26
1995.34
9340.0
1995.55
1995.7
1995.75
1995.86
9395.0
L
1997.14
1997.19
1997.21
1997.26
9676.0
M
1997.14
1997.2
1997.22
1997.28
9916.0
N
1997.16
1997.23
1997.26
1997.34
10162.0
0
1997.36
1997.46
1997.54
1997.66
13616.0
P
2000.81
2001.06
2001.16
2001.78
14037.1
Q
2000.83
2001.09
2001.19
2001.8
14344.1
R
2002.26
2002.35
2002.38
2002.38
14412.7
S
2002.46
2002.55
2002.58
2002.63
14460.8
2002.51
2002.6
2002.63
2002.69
14534.3
T
2003.42
2003.68
2003.78
2004.00
Study Contractor: WEST Consultants, Inc. Page 25
Chester Creek Flood Insurance Study
Spokane County, Washington
DRAFT
14583.4
U
2003.50
2003.77
2003.87
2004.10
15031.5
V
2003.66
2003.89
2003.99
2004.19
15266.6
W
2003.86
2004.05
2004.13
2004.31
15707.7
X
2004.04
2004.22
2004.30
2004.46
16067.4
Y
2004.29
2004.44
2004.51
2004.64
16140.0
2004.64
2004.66
2004.69
2004.76
16170.0
2005.24
2005.55
2005.68
2005.99
16212.1
Z
2005.25
2005.56
2005.69
2006.00
16684.6
AA
2005.49
2005.75
2005.86
2006.13
17202.3
AB
2005.87
2006.09
2006.19
2006.31
17443.6
AC
2006.15
2006.37
2006.45
2006.44
17655.0
AD
2006.37
2006.60
2006.68
2006.75
18180.1
AE
2006.88
2007.12
2007.21
2007.35
18716.0
AF
2007.41
2007.66
2007.76
2007.94
18755.8
2007.44
2007.69
2007.79
2007.97
18804.3
AG
2007.50
2007.76
2007.86
2008.06
18849.8
AH
2007.57
2007.84
2007.95
2008.16
19395.1
Al
2008.16
2008.46
2008.58
2008.81
19427.6
2008.20
2008.49
2008.61
2008.83
19444.0
AJ
2008.35
2008.65
2008.77
2009.01
19460.2
AK
2008.41
2008.71
2008.84
2009.08
19493.4
AL
2008.44
2008.75
2008.88
2009.13
19887.6
AM
2009.10
2009.37
2009.48
2009.71
20263.2
AN
2009.68
2009.95
2010.05
2010.27
20427.8
2009.92
2010.21
2010.33
2010.57
20492.8
AO
2009.95
2010.23
2010.35
2010.59
20554.7
AP
2009.95
2010.23
2010.35
2010.59
20779.1
AQ
2010.47
2010.73
2010.83
2011.05
20828.3
2010.57
2010.82
2010.91
2011.12
20838.5
2010.57
2010.82
2010.91
2011.12
20868.1
2010.62
2010.87
2010.97
2011.18
20895.9
2010.65
2010.91
2011.00
2011.21
20928.9
2010.68
2010.94
2011.03
2011.24
20967.2
2010.72
2010.97
2011.06
2011.27
20976.0
2010.74
2010.99
2011.08
2011.29
21013.8
AR
2010.82
2011.07
2011.16
2011.37
Study Contractor: WEST Consultants, Inc. Page 26
Chester Creek Flood Insurance Study
Spokane County, Washington
DRAFT
21231.7
AS
2011.44
2011.68
2011.77
2011.97
21456.4
AT
2011.78
2012.02
2012.11
2012.32
21481.4
2011.78
2012.02
2012.11
2012.32
21515.0
2012.52
2012.77
2012.87
2013.09
21548.4
2012.78
2013.07
2013.18
2013.44
21584.0
2012.78
2013.07
2013.18
2013.44
21607.0
AU
2014.51
2014.62
2014.65
2014.74
21924.1
AV
2014.65
2014.78
2014.82
2014.91
22423.7
AW
2014.69
2014.82
2014.86
2014.96
22972.9
AX
2018.40
2018.47
2018.49
2018.53
23005.8
2018.98
2019.41
2019.45
2019.58
23050.2
AY
2020.14
2020.44
2020.51
2020.62
23090.9
AZ
2020.31
2020.63
2020.69
2020.84
23446.3
BA
2022.68
2022.87
2022.94
2023.08
23887.6
BB
2024.76
2025.10
2025.22
2025.48
24429.2
BC
2027.62
2028.00
2028.11
2028.33
24473.4
2027.82
2028.26
2028.38
2028.60
24501.5
BD
2027.93
2028.47
2028.64
2029.02
24573.9
BE
2027.93
2028.50
2028.69
2029.11
24966.5
BF
2031.33
2031.50
2031.58
2031.67
24998.8
2031.43
2031.60
2031.67
2031.77
25012.2
2031.63
2031.83
2031.84
2031.86
25049.9
BG
2031.80
2032.00
2032.06
2032.18
25292.5
BH
2032.91
2033.23
2033.28
2033.43
25783.0
BI
2035.65
2036.18
2036.23
2035.61
25880.9
2036.49
2036.99
2037.19
2037.67
25941.8
2041.31
2045.18
2045.68
2045.99
25989.6
BJ
2041.33
2045.19
2045.70
2046.00
26225.2
BK
2041.33
2045.19
2045.70
2046.00
26256.3
2041.33
2045.19
2045.70
2046.00
26281.7
2041.62
2045.19
2045.70
2046.00
26315.2
BL
2041.67
2045.19
2045.70
2046.00
26876.3
BM
2044.21
2045.19
2045.70
2046.00
27314.8
BN
2048.56
2048.94
2049.04
2049.38
27360.6
2048.88
2049.24
2049.34
2049.68
27392.3
2053.43
2054.65
2054.69
2054.81
Study Contractor:• WEST Consultants, Inc. Page 27
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
27409.9
BO
2053.45
2054.66
2054.70
2054.83
27594.4
BP
2053.46
2054.66
2054.71
2054.83
28139.2
BO
2056.29
2056.70
2056.85
2057.10
28173.9
2056.96
2057.44
2057.57
2057.86
28268.5
BR
2057.24
2057.78
2057.95
2058.35
28363.2
BS
2058.95
2059.23
2059.29
2059.47
28441.8
2059.91
2060.26
2060.39
2060.62
28488.5
BT
2061.78
2062.73
2063.07
2063.87
29140.9
BU
2068.94
2069.31
2069.43
2069.71
29681.6
BV
2079.32
2079.64
2079.75
2079.97
30242.2
BW
2088.28
2088.73
2089.01
2089.28
30497.0
2092.77
2093.37
2093.58
2093.99
30506.3
BX
2092.93
2093.55
2093.76
2094.17
30675.0
BY
2095.32
2095.88
2096.03
2096.38
30747.8
2096.68
2097.15
2097.28
2097.58
30797.0
2097.09
2097.71
2097.91
2098.39
30811.8
BZ
2097.09
2097.72
2097.93
2098.44
31471.5
CA
2104.91
2105.00
2105.04
2105.27
31927.4
CB
2111.18
2111.72
2111.88
2112.14
32539.9
CC
2125.85
2126.11
2126.22
2126.61
33402.9
CD
2141.05
2141.48
2141.61
2141.65
34088.5
CE
2150.64
2150.95
2151.06
2151.46
34604.2
CF
2158.70
2159.13
2159.26
2159.33
34625.5
2158.92
2159.35
2159.48
2159.67
34675.9
CG
2159.72
2160.36
2160.58
2161.09
34693.8
CH
2159.84
2160.55
2160.81
2161.40
35297.7
CI
2170.59
2170.69
2170.81
2171.09
35946.6
CJ
2185.85
2186.36
2186.45
2186.66
36421.2
CK
2195.43
2195.61
2195.74
2196.03
36928.8
CL
2206.57
2207.00
2207.12
2207.39
37454.6
CM
2216.32
2216.65
2216.78
2217.15
37835.3
CN
2223.99
2224.39
2224.55
2224.89
37906.5
2226.88
2227.23
2227.34
2227.64
37973.7
CO
2227.95
2228.51
2228.71
2229.19
38025.9
CP
2228.15
2228.73
2228.94
2229.43
Chester Creek - West of Borrow Pit
Study Contractor:• WEST Consultants, Inc. Page 28
Chester Creek Flood Insurance Study
Spokane County, Washington
DRAFT
11069.0
A
2000.26
2000.34
2000.35
2000.36
11616.0
B
2003.61
2003.66
2003.68
2003.71
11721.0
C
2003.66
2003.72
2003.74
2003.79
11942.0
D
2003.66
2003.72
2003.74
2003.79
12473.0
E
2003.66
2003.72
2003.75
2003.80
12974.0
F
2004.07
2004.19
2004.28
2004.55
13067.0
G
2004.35
2004.55
2004.63
2004.77
13166.0
H
2004.37
2004.58
2004.66
2004.81
13229.0
1
2004.40
2004.62
2004.70
2004.84
13317.0
J
2004.42
2004.65
2004.73
2004.87
13396.0
K
2004.42
2004.65
2004.73
2004.87
13616.0
L
2004.42
2004.65
2004.73
2004.87
14037.0
M
2004.42
2004.65
2004.73
2004.87
14344.0
N
2004.42
2004.65
2004.73
2004.87
14412.0
0
2005.90
2006.05
2006.13
2006.30
Chester Creek - Dredge Channel
16212.0
A
2004.68
2004.74
2004.77
2004.83
16684.0
B
2005.16
2005.28
2005.33
2005.41
17202.0
C
2005.19
2005.31
2005.37
2005.46
17443.0
D
2005.19
2005.31
2005.37
2005.46
Chester Creek - Golf Course Overflow
20779.0
A
2007.19
2007.80
2008.05
2007.80
21013.0
B
2007.65
2007.80
2008.05
2007.80
21128.0
C
2008.28
2008.52
2008.53
2008.52
21229.0
D
2008.63
2008.82
2008.86
2008.82
21385.0
E
2008.94
2009.08
2009.12
2009.08
21409.0
2008.98
2009.12
2009.17
2009.12
21431.0
2009.03
2009.18
2009.23
2009.18
21445.0
F
2009.09
2009.26
2009.31
2009.26
21456.0
2013.00
2013.11
2013.13
2013.11
21481.0
2013.10
2013.22
2013.25
2013.22
21515.0
2013.10
2013.22
2013.25
2013.22
21548.0
G
2013.10
2013.22
2013.26
2013.22
21924.0
H
2013.11
2013.23
2013.27
2013.23
22423.0
1
2013.25
2013.44
2013.51
2013.44
22972.0
J
2014.64
2014.76
2014.82
2014.76
23005.0
2015.01
2015.19
2015.25
2015.19
23050.0
K
2015.15
2015.32
2015.38
2015.32
23090.0
L
2015.45
2015.60
2015.65
2015.60
23446.0
M
2017.97
2018.07
2018.10
2018.07
23887.0
N
2022.83
2022.95
2022.99
2022.95
Study Contractor: WEST Consultants, Inc. Page 29
Chester Creek Flood Insurance Study
Spokane County, Washington
DRAFT
Chester Creek - Without Levee DS Bowdish Road
16684.6
AA
2005.5
2005.73
2005.87
2006.1
17202.3
AB
2005.54
2005.77
2005.91
2006.13
17443.6
AC
2005.9
2006.06
2006.15
2006.31
17655.0
AD
2006.16
2006.32
2006.39
2006.52
18180.1
AE
2006.4
2006.58
2006.65
2006.78
18716.0
AF
2006.5
2006.68
2006.74
2006.88
Chester Creek - Without Levee US Bowdish Road
19395.1
Al
2007.77
2008.01
2008.11
2008.3
19427.6
2007.79
2008.03
2008.12
2008.32
19444.0
AJ
2007.8
2008.04
2008.13
2008.32
19460.2
AK
2007.82
2008.05
2008.14
2008.33
19493.4
AO
2007.82
2008.05
2008.14
2008.33
19887.6
AM
2007.83
2008.06
2008.15
2008.34
Unnamed Tributary to Chester Creek
-1303.0
Al
n/a
2007.80
2008.05
2008.64
-1019.0
B1
n/a
2008.31
2008.36
2008.64
-880.0
C1
n/a
2008.35
2008.42
2008.64
-466.0
D1
n/a
2008.35
2008.42
2008.64
-89.0
E1
n/a
2008.35
2008.42
2008.64
1.0
F
2005.00
2009.60
2009.70
2010.00
149.0
G
2005.00
2009.60
2009.70
2010.00
343.0
H
2005.00
2009.60
2009.70
2010.00
383.0
1
2005.00
2009.60
2009.70
2010.00
472.0
J
2005.00
2009.60
2009.70
2010.00
576.0
K
2009.74
2009.89
2009.95
2010.20
651.0
L
2010.74
2010.85
2010.95
2011.00
918.0
M
2011.17
2011.35
2011.45
2011.57
1472.0
N
2012.38
2012.69
2012.83
2013.11
1510.0
2012.95
2013.16
2013.26
2013.46
1528.0
0
2013.36
2013.97
2014.30
2014.60
1557.0
P
2013.39
2013.98
2014.32
2014.62
1963.0
Q
2019.48
2019.59
2019.66
2019.75
1989.0
2019.85
2020.01
2020.06
2020.14
2080.0
2020.19
2020.48
2020.61
2020.86
2100.0
R
2020.39
2020.70
2020.83
2021.08
2110.0
2020.42
2020.74
2020.88
2021.14
2120.0
S
2020.44
2020.78
2020.91
2021.18
2651.0
T
2022.48
2023.16
2023.40
2023.89
3126.0
U
2025.46
2025.96
2026.19
2026.66
3617.0
V
2030.11
2030.51
2030.67
2031.02
4127.0
W
2034.25
2034.85
2035.09
2035.61
4623.0
X
2036.97
2037.48
2037.69
2038.17
4665.0
2037.96
2038.47
2038.70
2039.25
4704.0
Y
2039.43
2040.38
2041.00
2041.30
4750.0
Z
2039.58
2040.41
2041.03
2041.30
5165.0
AA
2043.28
2043.32
2043.40
2043.49
Study Contractor: WEST Consultants, Inc. Page 30
Chester Creek Flood Insurance Study
Spokane County, Washington
DRAFT
5676.0
AB
2047.51
2047.98
2048.08
2048.37
6117.0
AC
2051.68
2051.83
2052.07
2052.49
6154.0
2052.27
2052.63
2052.76
2053.16
6239.0
AD
2053.54
2053.93
2054.11
2054.53
6274.0
AE
2055.35
2055.56
2055.65
2055.89
6553.0
AF
2064.09
2064.36
2064.48
2064.75
7086.0
AG
2083.02
2083.35
2083.50
2083.90
7121.0
2084.19
2084.50
2084.63
2084.93
7143.0
AH
2085.20
2085.74
2086.04
2087.67
7169.0
2089.93
2090.31
2090.51
2090.90
7186.0
Al
2090.59
2091.16
2091.45
2092.18
7220.0
AJ
2092.00
2092.26
2092.39
2092.67
Unnamed Tributary Without Levee DS of Pines Road
-1303.0
A
-1303
2007.19
2007.8
2008.05
-1019.0
B
-1019
2008.47
2008.51
2008.52
-880.0
C
-880
2008.55
2008.6
2008.63
-466.0
D
-466
2008.55
2008.6
2008.63
-89.0
E
-89
2008.55
2008.6
2008.63
1.0
F
1
2008.57
2008.6
2008.63
149.0
G
149
2008.62
2008.65
2008.66
343.0
H
343
2008.62
2008.65
2008.66
383.0
I
383
2008.62
2008.65
2008.66
472.0
J
472
2008.62
2008.65
2008.66
576.0
K
576
2008.62
2008.65
2008.66
651.0
L
651
2011.18
2011.2
2011.21
918.0
M
918
2011.22
2011.25
2011.27
1472.0
N
1472
2013.73
2013.76
2013.77
1510.0
1510
2014.06
2014.1
2014.12
1528.0
O
1528
2014.06
2014.1
2014.12
1557.0
p
1557
2014.21
2014.25
2014.27
Unnamed Tributary Without Levee US of Pines Road
1511.0
2018.3
2019.23
2019.47
2020.03
1782.0
2019.13
2019.23
2019.47
2020.03
2120.0
S
2019.75
2019.92
2019.93
2020.12
2651.0
T
2020.72
2020.8
2020.83
2020.92
3126.0
U
2024.16
2024.25
2024.28
2024.36
3617.0
V
2028.76
2028.91
2028.96
2029.05
4127.0
W
2033.31
2033.43
2033.47
2033.57
4623.0
X
2037.16
2037.66
2037.77
2038.02
1 - For the 0 year event there is no flow downstream of Storage Area 6/cross-section F
Study Contractor: WEST Consultants, Inc. Page 31
Chester Creek Flood Insurance Study DRAFT
Spokane County, Washington
References
CH2M Hill, Chester Creek Watershed Plan, March 1997.
Federal Emergency Management Agency (FEMA) (1992). Flood Insurance Study, Spokane
County, Washington, Unincorporated Areas, Revised February 15, 1984.
Federal Emergency Management Agency (FEMA) (2002), Guidelines and Specifications for
Flood Hazard Mapping Partners, Federal Emergency Management Agency, February 2002.
Humphries, Jack (HYDMET), Chester Creek Hydrologic Study, prepared for Spokane County
Department of Public Works, HYDMET, January 22, 1997.
Jackson, Tom, Michael Baker Jr., Inc., (MBJr.) Memorandum — Spokane County Hydrology,
Chester Creek, June 28, 2004.
United States Environmental Protection Agency (EPA), Hydrological Simulation Program -
Fortran, Release 11, EPA/600/R-97/030, August 1997.
WEST Consultants, Inc. (WEST 2004), Flood Insurance Study Hydrologic Analysis for Chester
Creek, Spokane County, Washington, prepared for FEMA Region X, February 11, 2004.
WEST Consultants, Inc. (2004). Flood Insurance Study Hydrologic Analysis for Chester Creek,
Spokane County, Washington, prepared for FEMA Region X, December 8, 2004.
WEST Consultants, Inc. (2008). Technical Memo Report Chester Creek Flood Insurance Study
Hydrology Re-evaluation, prepared for FEMA Region X, March 12, 2008.
Study Contractor • WEST Consultants, Inc. Page 32
CheSte
ct
Table 1. Flood Frequency Flows for the Chester Creek and Unnamed Tributary.
Location
10 -yr
50 -yr
100 -yr
500 -yr
38025 — Chester Creek
58
89
101
131
26315 — Chester Creek
87
135
155
201
23887 — Chester Creek
75
104
116
144
21515 — Chester Creek
118
139
147
167
24430 — Golf Course
30
54
64
88
20492 — Chester Creek
115
138
148
170
18180 — Chester Creek
113
137
147
169
Storage Area 2 Outlet
60
100
117
155
14412 — West of Borrow Pit
14
26
32
44
SA5 Outflow — Chester
Creek
30
40
44
53
SA 7 Outflow — Chester
Creek (24th Ave)
25
42
49
67
SA 8 Outflow — Chester
Creek (16th Ave)
9
25
32
48
SA 9 Outflow — Chester
Creek (8th Ave)
0
0
0
0
7220 - Tributary
7
14
18
28
6553 - Tributary
11
22
28
45
4704 - Tributary
12
24
30
46
472 - Tributary
10
14
16
20
Storage Area 6 Outflow
0
2
4
7
Storage Area 1 Outflow
0
0
0
0
This was extracted from page 14 of the Technical Support Data Notebook for Chester Creek
dated June 2008 and prepared by WEST Consultants for FEMA.
D:\Chester Creek download from County\Chester BFEs Extracted from TSDN.doc
Table 2. Storage area water surface elevation magnitude frequency estimates.
Location
50 -yr
100 -yr
500 -yr
Overflow
Elevation (ft)
Storage Area 1
2007.19
2007.80
2008.05
2008.64
2009.0
Storage Area 2
2000.81
2001.06
2001.16
2001.78
2000.81
Storage Area 3
1999.99
2000.74
2001.05
2001.78
1999.99
Storage Area 4
2018.30
2019.23
2019.47
2020.03
2025.0
Storage Area 5
1999.99
2000.74
2001.05
2001.78
2001.9
Storage Area 6
2005.00
2009.60
2009.70
2010.00
2009.5
Storage Area 7
1997.14
1997.19
1997.21
1997.26
1996.9
Storage Area 8
1993.54
1993.65
1993.70
1993.78
1993.4
Storage Area 9
1988.54
1990.75
1990.79
1990.86
1992.5
Table 3. Comparison between effective and proposed hydrology.
Flooding Sources and Location
Drainage Area
(Square Miles)
Peak Discharges
(cfs)
100 -Year,
Chester Creek
Thorpe Rd.*
Effective
405
Proposed
12.4
211
Difference
(%)
-48
Schafer Rd.
Effective
644
Proposed
18.4
178
Difference
(%)
-72
24th Ave.
Effective
722
Proposed
22.0
49
Difference
(%)
-93
* This value includes flow continuing downstream from Thorpe and flow diverted to golf course
This was extracted from page 15 of the Technical Support Data Notebook for Chester Creek
dated June 2008 and prepared by WEST Consultants for FEMA.
D:\Chester Creek download from County\Chester BFEs Extracted from TSDN.doc