Painted Hills FEIS Appendix M - MAY 2023APPENDIX M:
Technical Memorandum Report –
Chester Creek
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Technical Memorandum for Record WEST Consultants, Inc. 2601 25th St. SE Suite 450 Salem, OR 97302-1286
(503) 485 5490 (503) 485-5491 Fax www.westconsultants.com
Date: January 15, 2008 Revised March 12, 2008 Subject: Chester Creek Flood Insurance Study
Hydrology Re-evaluation
Introduction
In 2004, WEST Consultants, Inc. (WEST) completed a detailed hydrologic analysis of Chester Creek in
the City of Spokane Valley, WA, as part of the Federal Emergency Management Agency (FEMA) Flood
Insurance Study (FIS) of Chester Creek. The analysis was completed using the Hydrological Simulation Program-FORTRAN (HSPF) software developed by the U.S. Environmental Protection Agency (EPA, 2005). Detailed information about the development of the HSPF model for the Chester Creek FIS is documented in Flood Insurance Study Hydrologic Analysis for Chester Creek, Spokane County,
Washington (WEST, 2004), and it will be referred to as the FEMA FIS HSPF model in the remainder of
this memorandum. During the appeal period following the release of the preliminary FIS results, the City of Spokane Valley, WA, sent an appeal letter (City Spokane Valley, October 2006) to FEMA requesting that additional
hydrologic analyses be done to evaluate the impact of 43 drywells located within the lower Chester Creek
floodplain. As a result, WEST was contracted by FEMA to re-evaluate the hydrology of lower Chester Creek. Detailed information about the re-evaluation is documented in this memorandum. This memorandum also documents additional hydrologic analysis performed on Subbasin C2. Subbasin
C2 is located in the lower northeastern portion of the watershed. It is the most densely developed area of
the watershed, but it is underlain by highly porous glacial flood deposits. There are also several stormwater infiltration drywell systems that capture and infiltrate the runoff from this area. It was assumed for the FIS that runoff from this subbasin would not contribute any flood flows to Chester Creek due to the presence of such a large number dry wells within the subbasin. As Subbasin C2 contains 22 of the 43
drywells located within the Chester Creek floodplain, additional analyses were required to determine the
validity of using these 22 drywells to reduce peak flood discharges, given that they were previously assumed to be part of the dry well system responsible for preventing Subbasin C2 flood discharge from reaching Chester Creek.
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Hydrologic Analysis of Subbasin C2 The runoff from this subbasin typically flows in a southwest direction to one of four low spots along the
east side of Dishman-Mica Road. The delineation of this subbasin is shown in Figure 1. Storage Area 3 (SA3) is located within Subbasin C2 and contains 22 of the 43 drywells located in the Chester Creek floodplain. In the FEMA FIS it was assumed that no flow from Subbasin C2 contributes to Chester Creek and that local flow within the basin is infiltrated by the drywells. Some upstream flood flows from Chester Creek overtop Dishman Mica road just downstream of 28th Avenue and flow east to Storage Area 3 where
they pond and infiltrate. A hydrologic analysis of Subbasin C2 was conducted to determine the validity of including the 22 drywell located in SA3 in the reduction of flood discharge in addition to local flow from Subbasin C2. An HSPF model was developed for subbasin C2 using information from the HSPF model developed for
the FEMA FIS (WEST, 2004), and topographic, land-use, and geologic/soil type information for the subbasin. Information utilized from the FEMA FIS HSPF model (WEST, 2004) included the meteorological data and calibrated HSPF parameters. The meteorological data consist of precipitation, temperature, solar radiation, evaporation, dew point temperatures, and wind speed measured at Spokane Airport. The HSPF parameters were calibrated using a two-step procedure. The parameters controlling
snow accumulation and melt were first calibrated to observed snow depth data collected at Spokane Airport for water years 1948 through 2002. Then, the parameters controlling runoff were calibrated to the Chester Creek streamflow record. The calibrated HSPF parameters are shown in Table 1.
Table 1. Calibrated HSPF Parameters for Pervious Areas
Pervious Land Type FOREST LZSN INFILT LSUR SLSUR KVARY AGWRC
Bedrock
Undeveloped 0.800 5.50 0.20 400 0.10 0.50 0.975
Bedrock Developed 0.100 5.50 0.10 400 0.10 0.50 0.975
Outwash Undeveloped 0.800 5.50 2.00 400 0.05 0.50 0.975
Outwash Developed 0.100 5.50 0.80 400 0.05 0.50 0.975
Pervious Land Type PETMAX PETMIN INFEXP INFILD DEEPFR BASETP AGWETP
Bedrock Undeveloped 0.0 0.0 2.0 2.0 0.0 0.0 0.0
Bedrock Developed 0.0 0.0 2.0 2.0 0.0 0.0 0.0
Outwash Undeveloped 0.0 0.0 2.0 2.0 0.0 0.0 0.0
Outwash Developed 0.0 0.0 2.0 2.0 0.0 0.0 0.0
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Pervious Land Type CEPSC UZSN NSUR INTFW IRC LZETP
Bedrock
Undeveloped 0.20 0.20 0.35 6.0 0.60 0.60
Bedrock Developed 0.10 0.10 0.05 6.0 0.60 0.20
Outwash Undeveloped 0.20 0.20 0.35 0.0 0.60 0.60
Outwash Developed 0.10 0.10 0.05 0.0 0.60 0.20
The area within each subbasin was classified into areas of common land-use and geologic/soil type called
“PERLNDS” (short for pervious land segments), and the same PERLNDS types used in the FEMA FIS HSPF model were used. The total surface area for each PERLNDS was determined using the GIS software (ArcGIS Version 9) and shapefiles of the land-use and geology, and they are summarized in Table 2.
Table 2. Summary of Existing Land-Use for Subbasin C2
Subbasin
Land-Use (acres)
Impervious Urban Outwash Undeveloped Outwash Urban Bedrock Undeveloped Bedrock Total Area
C2A 312.7 1104.7 0.0 0.0 0.0 1417.4
C2B 44.6 149.5 0.0 0.0 0.0 194.1
C2C 23.9 80.0 0.0 0.0 0.0 103.9
C2D 22.0 73.6 0.0 0.0 0.0 95.6
A statistical analysis of the HSPF results was conducted to determine the discharge-frequency relationship
for each of the subbasins of Subbasin C2. The discharge-frequency relationship was determined using the same methodology used in the FIS. The relationship was determined using a probability-plot regression approach since many of the annual maximum values were zero or near zero. The discharge-frequency relationships for each of the subbasins of Subbasin C2 are summarized in Table 3.
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Table 3. Discharge-Frequency Relationship for Subbasin C2
Subbasin Discharge (cfs) Unit Discharge (cfs/acre) 10-yr 50-yr 100-yr 500-yr 10-yr 50-yr 100-yr 500-yr
C2A 216 290 322 394 0.152 0.205 0.227 0.278
C2B 32 41 45 53 0.164 0.210 0.230 0.275
C2C 17 22 24 29 0.163 0.211 0.231 0.278
C2D 15 20 22 27 0.159 0.211 0.233 0.285
A GIS coverage, consisting of the surveyed location of all drywells within Subbasin C2, was provided by the City of Spokane Valley. Based on this coverage, 1059 dry wells are located within Subbasin C2, of which 709 are double depth, 332 are single depth and 18 are unknown. The drywells are evenly distributed throughout the subbasin (Figure 2). For the purpose of this analysis and to be conservative, the unknown
drywells were considered to be single depth. Single depth drywells are typically 8 feet deep with an
approximately 4-foot high perforated section, and double depth drywells are typically 12 feet deep with an approximately 8-foot high perforated section. Based on field measurement tests, the design outflow rate of a single drywell is nominally 0.3 cubic feet per second (cfs) and 1.0 cfs for a double depth drywell. In 2006, the City of Spokane Valley conducted infiltration tests on 7 drywells to determine their normalized
outflow rates. The tests resulted in normalized rates that were 2.7 to 9.1 times greater than the design flows
(City Spokane Valley, October 2006). A comparison of total dry well outflow capacity and basin discharge is shown in Table 4. Subbasins C2A, C2B and C2C have 1.8 to 2.0 times more nominal drywell capacity than the 100-yr basin discharge and 1.6
to 1.7 times more nominal drywell capacity than the 500-yr basin discharge. The drywell outflow for
Subbasin C2D exceeds the basin discharge for the 100-yr flood. Although the 500-yr basin discharge for Subbasin C2D exceeds the nominal drywell outflow capacity by 1 cfs, the topography in this area is relatively flat and during large floods that exceed the drywell capacity, water would be stored at the dry well inlets and depressions and would eventually infiltrate as the flood subsided. We note that for
Subbasin C2A (which contains SA3) the nominal drywell capacity exceeds the basin discharge by a factor
of 2. Taking this in conjunction with the drywell design flow safety factor, and the even distribution of drywells throughout the entire basin, we believe it valid to conclude that no flow from Subbasin C2A will flow to SA3 and that the drywells within SA3 can be assumed to help reduce peak flood discharge from Chester Creek and be included in the hydrology re-evaluation.
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Table 4. Comparison of Drywell Outflow to Basin Discharge
Subbasin
Number
of Double Wells
Number of Single Wells
Total Well
Outflow Capacity (cfs)
100-yr Event 500-yr Event
Basin Discharge (cfs)
Outflow to
Basin Discharge Ratio
Basin Discharge (cfs)
Outflow to
Basin Discharge Ratio
2A 574 284 659 322 2.0 394 1.7
2B 73 33 83 45 1.8 53 1.6
2C 41 16 46 24 1.9 29 1.6
2D 21 17 26 22 1.2 27 0.96
Re-evaluation of the Hydrology for Lower Chester Creek
The hydrology for lower Chester Creek was re-evaluated using the HSPF computer program. The re-evaluation involved making three revisions to the FEMA FIS HSPF model. The first revision involved
dividing the single reach downstream of 24th Avenue (Storage Area 5) into five smaller reaches: (1) Reach 1 is from 24th Avenue to about 1,400 feet downstream, (2) Reach 2 is from the downstream end of Reach 1 to 16th Avenue, (3) Reach 3 is from 16th Avenue to 8th Avenue, (4) Reach 4 is from 8th Avenue to about 600 feet downstream, and (5) Reach 5 is from the downstream end of Reach 4 to 2nd Avenue. This
revision was made to include several storages areas within the reach and to obtain discharge values at more
locations than considered in the FEMA FIS. As a result of this revision, Subbasin C1 and C3 had to be re-delineated to determine the portion of these subbasins that contribute flows to each of the reaches and the total surface area of PERLNDS had to be calculated for the each of new subbasins. The five reaches and the re-delineation of Subbasins C1 and C3 are shown in Figure 3. The total surface area of the PERLNDS
determined for all of the subbasins within the watershed is summarized in Table 5.
The second revision involved adding storage areas (SA) immediately upstream of the street crossings at 2nd Avenue (SA 10), 8th Avenue (SA 9), 16th Avenue (SA 8), and 24th Avenue (SA 7). The added storage areas are also shown in Figure 3. The surface area-volume-elevation relationship for each storage area was determined using ArcGIS software and the Triangulation Irregular Network (TIN) created from 2-foot
contours developed from LiDAR data collected in 2003. The stage-discharge relationship for each storage area was determined from the HEC-RAS model developed for the FIS. The discharge associated with infiltration for each basin was assumed to be equal to the rate (2 inches per hour) considered for Subbasin C3 in the FEMA FIS HSPF model.
The last revision included the influences of the drywells located within the floodplain. Information about the drywells was provided by the City of Spokane Valley, WA. Table 6 provides the number of wells, the City’s Well Number, well type (“single” or “double” depth drywell), and rim elevation within each of the storage areas. HSPF models were developed with and without the influences of the drywells. The outflow of the drywells was simulated in the HSPF model using a discharge rating curve defined using the design
outflow rate at the surveyed rim elevation. As an example, the drywell outflow rating curve for a storage area that has a bottom elevation of 96 feet, a single drywell with an rim elevation of 100 feet, and a double drywell with an rim elevation of 102 feet would be 0 cfs from elevation 96 to 100 feet, 0.3 cfs from
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elevation 100 to 102 feet, and 1.3 cfs for elevations greater than 102 feet.
Table 5. Summary of Existing Land-Use for Chester Creek
Subbasin
Land-Use (acres)
Impervious Urban Outwash Undeveloped Outwash Urban Bedrock Undeveloped Bedrock Total Area
C1A 0.0 0.0 39.4 0.0 69.1 108.5
C1B 0.0 0.0 19.4 0.0 9.9 29.3
C1C 0.0 0.0 21.2 0.0 197.5 218.7
C1D 0.0 0.0 30.4 0.0 141.0 171.4
C1E 0.0 0.0 10.7 0.0 1.9 12.6
C1F 0.0 0.0 29.7 0.0 6.1 35.9
C21 403.2 1810.9 0.0 0.0 0.0 1810.9
C3A 13.2 117.9 0.0 0.0 0.0 131.0
C3B 6.0 54.3 0.0 0.0 0.0 60.4
C3C 1.8 16.5 0.0 0.0 0.0 18.3
C3D 1.4 12.8 0.0 0.0 0.0 14.2
C3E 4.4 40.0 0.0 0.0 0.0 44.5
C3F 0.4 4.0 0.0 0.0 0.0 4.4
C3G 2.6 22.8 0.0 0.0 0.0 25.4
C4 0.0 0.0 185.1 0.0 1367.4 1552.5
C5 63.9 296.3 0.0 279.0 0.0 639.3
C6 10.5 0.0 195.3 0.0 57.2 263.0
C7 25.9 0.0 218.0 0.0 403.9 647.8
C7A 10.2 0.0 205.0 0.0 39.2 254.4
C8 0.0 0.0 50.2 0.0 246.8 297.0
C9 0.0 0.0 158.9 0.0 197.3 356.1
C10 0.0 0.0 111.8 0.0 1279.2 1391.0
C11 0.0 0.0 15.5 0.0 588.9 604.3
C12 0.0 0.0 220.6 0.0 1223.2 1443.8
C13 0.0 0.0 86.1 0.0 724.1 810.2
C14 0.0 0.0 6.3 0.0 430.7 437.0
C15 0.0 0.0 248.1 0.0 2068.8 2317.0
C16 0.0 0.0 92.2 0.0 1272.9 1365.2
Runoff from Subbasin C2 infiltrates into the ground through various drywells and does not contribute flow to Chester Creek.
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Table 6. Information of Drywells within the Storage Areas of Chester Creek
Storage Area Number of Wells Well Number Well Type Rim Elevation (ft)
1 0 n.a. n.a. n.a. 2 0 n.a. n.a. n.a.
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DW – 2 Single 2000.97
DW – 3 Double 1999.08
DW – 4 Double 1999.09
DW – 5 Double 1999.54
DW – 6 Double 1999.30
DW – 7 Double 1999.48
DW – 8 Double 1999.27
DW – 9 Double 2002.35
DW – 10 Double 2002.37
DW – 11 Double 2000.52
DW – 12 Single 1999.96
DW – 13 Single 1999.65
DW – 14 Double 1998.72
DW – 15 Double 1998.07
DW – 16 Single 1998.26
4 0 n.a. n.a. n.a.
5 3 DW – 17 Double 1999.72 DW - 18 Double 1998.11 DW - 19 Double 1998.30 6 0 n.a. n.a. n.a. 7 0 n.a. n.a. n.a.
8 0 n.a. n.a. n.a.
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DW - 23 Single 1990.19 DW - 24 Double 1988.80 DW - 25 Double 1987.61 DW - 26 Single 1989.14 DW - 27 Double 1989.03 DW - 28 Double 1988.32 DW - 29 Double 1988.32 DW - 30 Double 1988.32 DW - 31 Single 1986.54 DW - 32 Single 1986.62 DW - 33 Double 1986.80 DW - 34 Double 1988.36 DW - 35 Single 1988.39 DW - 36 Single 1991.49 DW - 37 Single 1991.74
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DW - 38 Double 1985.36 DW - 39 Single 1984.15 DW - 40 Single 1983.29 DW - 41 Double 1981.80 DW - 42 Double 1985.77 DW - 43 Double 1985.77
The HSPF results were analyzed using the same statistical methodology as in the FIS to determine the
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discharge-frequency relationship for the lower Chester Creek. The resulting relationships are provided in Table 7 and Table 8. For comparison, Table 7 provides the discharge-frequency relationship at various
locations downstream of 28th Ave for the original 2004 discharges along with the 2008 revised results for the with and without the drywell influences conditions. The ‘without drywells’ condition considers the changes that occur based on the inclusion of the 4 new storage areas in the lower reaches and the ‘with drywells’ condition considers the changes based on the new storage areas and the addition of the drywells. Table 8 provides the stage-frequency relationship for the storage areas in the lower reach of the Chester
Creek. The results indicate that there will be no flow downstream of 8th Avenue (SA 9). However, there will be minor ponding within SA 10 due to local runoff to this area. This ponding would average less than 1 foot in depth. Table 9 and Table 10 provide a summary of revised proposed discharges for Chester Creek.
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Table 7. Comparison of Discharge-Frequency Relationships for Lower Chester Creek
Location
Original 2004 Results Revised 2007 Discharges
Discharge (cfs) Discharge (cfs) without Drywells Discharge (cfs) with Drywells
10-yr 50-yr 100-yr 500-yr 10-yr 50-yr 100-yr 500-yr 10-yr 50-yr 100-yr 500-yr
Storage 5 Outflow (Culvert under
RR between 24th and 28th Ave)
n/a* n/a* n/a* n/a* 35 45 49 58 30 40 44 53
Cross Section K (24th Ave) 36 49 55 68 29 46 53 69 25 42 49 67
SA 8 Outflow (16th Avenue) n/a n/a n/a n/a 12 30 37 54 9 25 32 48
SA 9 Outflow (8th Avenue) n/a n/a n/a n/a 0 0 0 0 0 0 0 0
SA 10 Outflow/Cross Section L (2nd Avenue)
2 3 4 5 0 0 0 0 0 0 0 0
*Though discharges for this area were not listed in the 2004 study results they are the essentially the same as the “without drywells” scenario as no changes were made to the model upstream of this location aside from the addition of drywells in the ‘with drywell’ scenario.
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Table 8. Water Surface Elevation-Frequency Relationship for Storage Areas of Lower Chester Creek
Location
Overflow
Elevation
(ft)
Original 2004 Results Revised 2007 Discharges
WS Elevation (ft) WS Elevation (ft) without Drywells WS Elevation (ft) with Drywells
10-yr 50-yr 100-yr 500-yr 10-yr 50-yr 100-yr 500-yr 10-yr 50-yr 100-yr 500-yr
Storage Area 2 2000.30 2000.82 2001.18 2001.48 2001.90 2000.81 2001.15 2001.45 2001.90 2000.81 2001.06 2001.16 2001.78
Storage Area 3 1999.80 2000.45 2001.18 2001.48 2001.90 2000.44 2001.15 2001.45 2001.90 1999.99 2000.74 2001.05 2001.78
Storage Area 5 2001.90 2000.45 2001.18 2001.48 2001.90 2000.44 2001.15 2001.45 2001.90 1999.99 2000.74 2001.05 2001.78
Storage Area 7 1996.85 n/a n/a n/a n/a 1997.15 1997.20 1997.22 1997.26 1997.14 1997.19 1997.21 1997.26
Storage Area 8 1993.35 n/a n/a n/a n/a 1993.56 1993.68 1993.73 1993.80 1993.54 1993.65 1993.70 1993.78
Storage Area 9 1992.50 n/a n/a n/a n/a 1989.96 1991.75 1991.81 1991.93 1988.54 1990.75 1990.79 1990.86
Storage Area 10 1986.00 n/a n/a n/a n/a 1983.28 1983.44 1983.51 1983.66 1983.04 1983.19 1983.25 1983.39
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Table 9. Proposed Flood Magnitude Frequency Estimates for Lower Chester Creek Watershed
Location Discharge (cfs)
10-yr 50-yr 100-yr 500-yr
Storage 5
Outflow (Culvert under RR between
24th and 28th Ave)
30 40 44 53
Cross Section K (24th Ave) 25 42 49 67
SA 8 Outflow
(16th Avenue) 9 25 32 48
SA 9 Outflow
(8th Avenue) 0 0 0 0
SA 10
Outflow/Cross Section L (2nd Avenue)
0 0 0 0
Table 10. Proposed Water Surface Elevation Magnitude Frequency Estimates for Lower Chester Creek Watershed (ft)
Location 10-yr 50-yr 100-yr 500-yr Overflow Elevation (ft)
Storage Area 2 2000.81 2001.06 2001.16 2001.78 2000.3
Storage Area 3 1999.99 2000.74 2001.05 2001.78 1999.8
Storage Area 5 1999.99 2000.74 2001.05 2001.78 2001.9
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
Storage Area 10 1983.04 1983.19 1983.25 1983.39 1986.0
Conclusions
WEST completed two hydrologic analyses of Chester Creek using HSPF. The first analysis analyzed Subbasin C2 to determine the validity of using the 22 drywells located in SA3 to reduce peak flood discharges for Chester Creek. The discharge-frequency relationship for the four subbasins of Subbasin C2 is provided in Table 3. A comparison of drywell capacity and basin discharge is provided in Table 4. The
runoff from the subbasin was compared to the infiltration potential of the existing drywells in the subbasin,
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and it was determined that the runoff from this subbasin will not contribute to either SA3 or Chester Creek and therefore the drywells located within SA3 can be considered to help reduce peak flood discharge from
Chester Creek in addition to the drywells located further downstream. The second analysis involved re-evaluating the hydrology for lower Chester Creek. The re-evaluation included additional outflow locations downstream of 24th Avenue, additional storage areas at the downstream end of the watershed, and the effects of drywells within the floodplain. The results of the re-
evaluation are provided in Table 7 and Table 8, and indicate that there will be no flow downstream of 8th Avenue (SA 9), but there will be minor ponding within SA 10 due to local runoff to this area. This ponding would average less than 1 foot in depth. References
City of Spokane Valley, 2006 (July), Data for Drainage Structures in Lower Chester Creek Floodplain.
City of Spokane Valley, 2006 (October), Letter from Neil Kersten, City of Spokane Valley, to Ryan Ike, U.S. Department of Homeland Security, Re: Chester Creek Floodplain Revisions.
City of Spokane Valley, 2006 (November), Letter from Henry Allen, City of Spokane Valley, to Joseph T.
Weber, Jr., U.S. Department of Homeland Security, Re: Chester Creek Floodplain Revisions.
WEST Consultants, Inc., 2004 (December). Flood Insurance Study Hydrologic Analysis for Chester Creek, Spokane County, Washington, prepared for FEMA Region X. United States Environmental Protection Agency (EPA), 2005 (July), Hydrological Simulation Program-
FORTRAN, Release 12.2.
ATTACHMENT 1
FIGURES
Figure 1. Subbasin delineations for Subbasin C2
Figure 2. Drywell distribution for Subbasin C2
Figure 3. Subbasin delineations for Lower Chester Creek
Subbasin C2 does not contribute flows to the Chester Creek