Technical_Memo_Report-_Chester_Creek 1
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 14, 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 Spokane County, WA. The analysis was completed using the Hydrological
Simulation Program-FORTRAN (HSPF) software developed by the U.S. Department of Environmental
Protection Agency (EPA, 2005). Detailed information about the development of the HSPF model for
the FEMA 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 analysis be completed 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 completed 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 comprised of an underlain of highly infiltrative 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 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 was
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
2
flood discharge from reaching Chester Creek.
Hydrologic Analysis of Subbasin C2
The runoff from this subbasin typically drains in a southwest direction to one of four low spot locations
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 2 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.
The analysis was conducted using the HSPF computer program. An HSPF model was developed 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 consists of precipitation, temperature, solar radiation, evaporation, dew point
temperatures, and wind speed measured at the 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 the Spokane Airport for water years 1948 through
2002. Then, the parameters controlling runoff were calibrated to the Chester Creek stream flow record.
The calibrated HSPF parameters are included in this memorandum 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 INFEX
P 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
3
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 type of PERLNDS considered in the
FEMA FIS HSPF model were used. The total surface area for each PERLNDS was determined using
the Geographical Information System (GIS) computer software (ArcGIS Version 9) and GIS 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
Undevelope
d 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 utilized in the FIS. Briefly, 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 relationship for each of the subbasins of Subbasin C2 is
summarized in Table 3.
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
4
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 purposes of this analyses 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 test, the design outflow
rate of a single drywell is 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
2A, 2B and 2C have 1.8 to 2.0 times more drywell capacity than 100-yr basin discharge and 1.6 to 1.7
times more drywell capacity than 500-yr basin discharge. The well outflow for Subbasin 2D exceeds
the basin discharge for the 100-yr flood. Though the 500-yr basin discharge for Subbasin 2D exceeds
the 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. It was noted for basin 2A (which contains SA3)
the well capacity exceeds the basin discharge by a factor of 2. Taking this in conjunction with the well
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 2A will flow to SA3 and that the dry wells within SA3
can be assumed to help reduce peak flood discharge from Chester Creek and be included in the
hydrology re-evaluation.
Table 4. Comparison of drywell outflow to basin discharge
Subbasin Doubles Singles
Total Well
Outflow
Capacity (cfs)
100-yr Basin
Discharge
(cfs)
500-yr Basin
Discharge
(cfs)
2A 574 284 659 322 394
2B 73 33 83 45 53
2C 41 16 46 24 29
2D 21 17 26 22 27
5
Re-evaluation of the Hydrology for Lower Chester Creek
The hydrology for the 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 immediately upstream of the street crossings at
2nd Avenue, 8th Avenue, 16th Avenue, and 24th Avenue. 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 provided by the City of Spokane Valley, WA is summarized in Table 6. Table 6
provides the number of wells, the City’s Well Number, well type, and rim elevation within each of the
storage areas. As indicated in this table, there are single and double depth drywells. 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 test, the design outflow rate of a single drywell is 0.3 cubic feet per second (cfs)
and 1.0 cfs for a double depth drywell. 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 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
6
Subbasin
Land-Use (acres)
Impervious
Urban
Outwash
Undevelope
d 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
Notes:
1. Runoff from Subbasin C2 infiltrates into the ground through various drywells and does not contribute flow to
Chester Creek.
7
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.
3 18
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.
9 15
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
10 6
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
8
The HSPF results were analyzed using the same statistical methodology as in the FIS to determine the
discharge-frequency relationship for the lower Chester Creek. The resulting relationships are provided
in Table 7 and Table 8. Table 7 provides the discharge-frequency relationship at various locations
along the lower reach of the Chester Creek for with and without the drywell influences, while 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 (Storage Area 9). However,
there will be minor ponding within Storage Area 10 due to local runoff to these areas. This ponding
would average less than 1 foot in depth.
Table 7. Discharge-Frequency Relationships for Chester Creek
Location
Discharge (cfs) without Drywells Discharge (cfs) with Drywells
10-yr 50-yr 100-yr 500-yr 10-yr 50-yr 100-yr 500-yr
Cross Section H 81 118 134 170 n.a. n.a. n.a. n.a.
Cross Section K 35 49 54 67 n.a. n.a. n.a. n.a.
Cross Section L 2 3 4 5 n.a. n.a. n.a. n.a.
Storage 5 Outflow 35 46 51 62 30 41 46 57
Storage 8 Outflow
(16th Avenue) 17 30 35 48 13 25 30 43
Storage 9 Outflow
(8th Avenue) 0 0 0 0 0 0 0 0
Storage 10 Outflow
(2nd Avenue) 0 0 0 0 0 0 0 0
Table 8. Water Surface Elevation-Frequency Relationship for Storage Areas of Chester
Creek
Location
Overflow
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
Storage Area 3 1999.80 2000.44 2001.15 2001.45 2001.9 1999.99 2000.74 2001.05 2001.78
Storage Area 5 2001.90 2000.44 2001.15 2001.45 2001.9 1999.99 2000.74 2001.05 2001.78
Storage Area 7 1996.85 1996.53 1997.05 1997.07 1997.10 n.a. n.a. n.a. n.a.
Storage Area 8 1993.35 1993.59 1993.69 1993.73 1993.83 1993.56 1993.66 1993.70 1993.79
Storage Area 9 1992.50 1990.65 1991.88 1992.00 1992.28 1989.15 1990.64 1990.73 1990.95
Storage Area 10 1986.00 1983.28 1983.44 1983.51 1983.66 1983.04 1983.19 1983.25 1983.39
9
Conclusions
WEST completed two hydrologic analyses of Chester Creek using HSPF. The first analysis involved
analyzing Subbasin C2 to determine 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 drywells
existing within the subbasin, and it was determined that the runoff from this subbasin will not
contribute to either SA3 or Chester Creek and therefore the dry wells located within SA3 can be
considered to help reduce peak flood discharge from Chester Creek in addition to the dry wells located
further downstream.
The second analysis involved re-evaluating the hydrology for the 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. The results indicate that there will be no flow
downstream of 8th Avenue (Storage Area 9), but there will be minor ponding with Storage Area 10 due
to local runoff to these areas. 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