Painted_Hills_CLOMR_9-10-15
WEST Consultants, Inc. i CLOMR for Proposed Painted Hills Development
TABLE OF CONTENTS
INTRODUCTION ................................................................................................................ 1
BACKGROUND & RESEARCH ........................................................................................ 2
General ............................................................................................................................ 2
Levees ............................................................................................................................. 7
Infiltration Facilities ....................................................................................................... 9
Interior Drainage ........................................................................................................... 13
SITE INVESTIGATION .................................................................................................... 15
HYDROLOGY ................................................................................................................... 15
HYDRAULICS ................................................................................................................... 18
Duplicate Effective Model (DEM) ............................................................................... 18
Corrected Effective Model (CEM) ............................................................................... 20
Existing or Pre-Project Conditions Model .................................................................... 22
Proposed or Post-Project Conditions Models ............................................................... 24
FLOODPLAIN MAPPING ................................................................................................ 31
CERTIFICATION FORMS ................................................................................................ 31
SUMMARY ........................................................................................................................ 32
REFERENCES ................................................................................................................... 32
LIST OF TABLES
Table 1. Discharges Used in HEC-RAS Models ............................................................... 15
Table 2. Comparison of FIS and DEM model results for Golf Course Overflow Reach ... 19
Table 3. Comparison of FIS and DEM model results for Unnamed Tributary .................. 20
Table 4. DEM and CEM model results for the Golf Course Overflow .............................. 21
Table 5. DEM and CEM model results for the Unnamed Tributary .................................. 22
Table 6. CEM and ECM model results for the Golf Course Overflow .............................. 23
Table 7. CEM and ECM model results for the Unnamed Tributary ................................... 24
Table 8. ECM and PCM model results for the Golf Course Overflow............................... 27
Table 9. ECM and PCM model results for the Unnamed Tributary ................................... 28
Table 10. Change in Top Width for Golf Course Overflow ............................................... 29
Table 11. Change in Top Width for Unnamed Tributary ................................................... 30
Table 12. Change in Top Width for Unnamed Tributary without DS levee (left overbank
flowpath) ..................................................................................................................... 31
WEST Consultants, Inc. ii CLOMR for Proposed Painted Hills Development
LIST OF FIGURES
Figure 1. Chester Creek Location Map ................................................................................. 3
Figure 2. Effective FIS stream reaches and storage areas .................................................... 4
Figure 3. Detail of project area and levees ........................................................................... 6
Figure 4. FEMA Floodplain and levee south of Thorpe Road ............................................. 8
Figure 5. Golf Course Overflow facilities design drawing ................................................. 11
Figure 6. Unnamed Tributary facility design drawing ....................................................... 12
Figure 7. Interior drainage basin ......................................................................................... 14
Figure 8. Layout of HEC-RAS Cross Sections for Chester Creek Golf Course Overflow16
Figure 9. Layout of HEC-RAS Cross Sections for the Unnamed Tributary ..................... 17
WEST Consultants, Inc. iii CLOMR for Proposed Painted Hills Development
LIST OF EXHIBITS
Exhibit A. FEMA Forms
Exhibit B. Survey Maps and Plans of Proposed Project
Exhibit C. Duplicate Effective Models
Exhibit D. Corrective Effective HEC-RAS Models
Exhibit E. Existing Conditions HEC-RAS Models
Exhibit F. Post-Project Conditions HEC-RAS Models
Exhibit G. Effective Flood Insurance Rate Map (FIRM), Flood Profile, and Floodway
Data Table
Exhibit H. Revised Floodplain Boundaries, Flood Profile, and Floodway Data Table
Exhibit I. Floodplain Workmap for CLOMR
Exhibit J. Geotechnical Evaluation and Levee Certification Reports
Exhibit K. Infiltration Facilities Design Report
Exhibit L. Biological Opinion
Exhibit M. CD of Project Files
WEST Consultants, Inc. 1 CLOMR for Proposed Painted Hills Development
INTRODUCTION
A 99 acre mixed used development is proposed for the former Painted Hills Golf Course
property located in Spokane Valley, Washington. The development includes both
residential and commercial property, open space, and a small golf course. The property,
designated Storage Area 1 (SA1) within the effective FEMA Flood Insurance Study (FIS)
is designated a compensatory storage area. Within a compensatory storage area loss of
flood storage capacity due to placement of fill must be mitigated with an equivalent
compensatory volume of storage or through a reduction in flows such that the net
condition causes no adverse impact to the base flood or floodway elevations within the
storage area. In addition, loss of infiltration capacity due to placement of fill or
impervious surfaces must be mitigated such that the decrease in infiltration capacity will
causes no adverse impact to the base flood or floodway elevations within the storage
area. The overall purpose of the “compensatory” requirement is to ensure that
development activities do not cause an adverse impact on flood elevations within the
storage area, or downstream of the development (e.g. increasing downstream flows due to
reduced infiltration capacity within the storage area.)
Whipple Consulting Engineers, Inc., (WCE) proposes to address the compensatory
storage and infiltration requirements by intercepting floodwaters entering the storage area
and then storing and infiltrating flood flows and local storm water through the use of a
series of infiltration and storage facilities. The infiltration facilities will make use of dry
wells and gravel infiltration galleries. Due to the presence of glacially deposited sands
and gravels with high infiltration capacities, dry wells are currently in wide use
throughout the Chester Creek floodplain and are included in the effective FIS hydrologic
model. The inclusion of infiltration facilities within the proposed plan will create a net
benefit by significantly reducing flood elevations within and nearby the subject property.
Two existing levees are proposed to be certified, and one new certified levee is proposed
to be built to help protect the development property. The infiltration facilities and
certified levees will result in approximately 118 acres being removed from the 1% annual
chance floodplain, and the removal of 0.7 river miles of floodway.
Geotechnical analysis and levee design was conducted by Inland Pacific Engineering
Company (IPEC). Hydraulic analysis and design of the infiltration and storage facilities
was conducted by Whipple Consulting Engineers. WEST Consultants, Inc, conducted
the hydraulic and hydrologic analyses to evaluate the effects the proposed development
would have on base flood elevations (BFEs – water surface elevations associated with the
1% annual chance event), floodway elevations, floodplain boundaries for the 1% annual
chance event, and floodway limits of Chester Creek. This report, along with supporting
documentation, will be submitted to the Federal Emergency Management Agency
(FEMA) through the local communities (City of Spokane Valley, WA, and Spokane
County, WA) as a Conditional Letter of Map Revision (CLOMR).
Pertinent information about the request is provided as follows:
Identifier: Painted Hills Development
Flooding Source: Chester Creek and Unnamed Tributary
Community: Spokane Valley, WA, Spokane County, WA
WEST Consultants, Inc. 2 CLOMR for Proposed Painted Hills Development
Community Number: 530342, 430174
FIRM Panels Affected: 0751D
Unless otherwise stated, all elevations within this report are referenced to the North
American Vertical Datum of 1988 (NAVD88).
BACKGROUND & RESEARCH
General
The Chester Creek watershed is located in Spokane County and the City of Spokane
Valley. A location map of the watershed is shown in Figure 1. A map of the effective
stream reaches, storage areas, and CLOMR boundary is shown in Figure 2.
The watershed varies in elevation from 1,984 feet at 2nd Avenue (the downstream extent
of the effective study) to a high point of approximately 3,680 feet along the western
watershed boundary. The lower portions of the watershed are underlain by deep glacial
outwash deposits of high infiltration capacity. The upper basin is much steeper and
relatively undeveloped. Due to the high infiltration rates in the lower watershed, the
Chester Creek channel is distinct only in the upper reaches of the basin. Chester Creek
and its unnamed tributary have no outlet. Historically, both channels transitioned from
channel to pastures where no distinct channel is evident.
An FIS restudy for Chester Creek and its unnamed tributary was conducted in 2005. Due
to the unique infiltration characteristics of the Chester Creek watershed, it was
recognized that the prior effective FIS did not consider the effects of infiltration or
available storage in the watershed. The restudy included an extensive hydrologic
modeling effort that considered the effects of infiltration and several storage areas that
would serve to attenuate flood flows. The study resulted in significantly reduced flood
discharges. Six primary storage areas were identified, several of which were designated
by FEMA as ‘compensatory storage areas’ within which development must compensate
equally for reductions in storage and infiltration capacity.
The main channel of Chester Creek terminates at a large borrow pit (Storage Area 2)
which was developed as part of improvements to Dishman-Mica Road (D-M Road) in
1998 and is intended to act as a storm water retention and infiltration facility. The FEMA
regulatory floodplain continues north for approximately 1.5 miles beyond the physical
end of the channel.
WEST Consultants, Inc. 3 CLOMR for Proposed Painted Hills Development
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PROJECT LOCATION
NChester Creek Drainage Basin
2024Miles
Figure 1. Chester Creek Location Map
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Figure 2. Effective FIS stream reaches and storage areas
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The Unnamed Tributary channel does not physically connect to the main channel of
Chester Creek. The lower portion of the Unnamed Tributary was historically rerouted to
higher ground, and currently terminates in a large pit (Storage Area 6 per the effective
FIS) that is east of the proposed project site. Based on the effective FIS, the floodplain of
the Unnamed Tributary continues west from the Storage Area 6 (SA6) until it reaches
SA1, the site of the former Painted Hills Golf Course. For the with-levee condition, the
1% annual chance flow downstream of SA6 that continues to SA1 is 4 cfs. For the
without downstream levee condition in which the levee between Highway 27 and SA6 is
removed, the 1% annual chance flow that could continue to SA1 is 20 cfs.
The development project area is located within SA1 in the right overbank of Chester
Creek. SA1 is physically separated from the main channel of Chester Creek by a levee
along the right bank of the main channel between Thorpe Rd and Dishman-Mica Road
(Figure 3). Flood flows can enter the project site from two sources: The Golf Course
Overflow Reach, and the Unnamed Tributary. Due to the natural topography, the Golf
Course Levee, and D-M Road, there is no downstream exit for flows that enter SA1.
Flood flows that enter SA1 pond until they infiltrate.
Golf Course Overflow Reach - Flow escapes the Chester Creek channel approximately
3,000 ft upstream of the golf course due to limited channel capacity, and follows the right
overbank until it crosses Thorpe Road and enters the golf course (SA1). The flow
entering the golf course does not rejoin the main channel due to the topography of the
area and a small levee system along the right bank of the main channel. As the golf
course has no outlet, floodwaters are stored until they infiltrate.
Unnamed Tributary – Based on the effective FIS, flows from the Unnamed Tributary can
reach the project site via two paths. First, though SA6 has a noticeable impact on peak
discharge and serves to attenuate flood flows, flow from the 1%-annual-chance-flood
event will fill SA6 and then overflow (4 cfs) and continue to flow west via low ground,
overtopping driveways, and eventually Madison Road, at which point it would enter the
project site. Second, a levee is present along the left bank of the Unnamed Tributary
between SA6 and Highway 27. As this levee is not certified, a without levee analysis
was conducted in the effective FIS. Since the channel is perched at this location, failure
of the levee would result in all floodwaters (1% annual chance flow of 20 cfs) potentially
leaving the channel and flowing to the low ground of the left overbank where it would
flow west along low ground, bypassing SA6 before rejoining the regular flowpath
downstream of SA6, where it would continue until reaching SA1.
WEST Consultants, Inc. 6 CLOMR for Proposed Painted Hills Development
Figure 3. Detail of project area and levees
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Levees
Four non-accredited levees/non-levee embankments are present near the project area
based on the effective FIS that are of concern to the project site. They are described
below from south to north. A map denoting the levees is provided as Figure 3.
A non-accredited levee is located along the right bank and overbank of Chester Creek
between Thorpe Road and a private road approximately 1550 feet south of Thorpe Road.
The levee is located along the right bank of the original channel; however, based on the
2005 FIS, the channel was diverted to the left overbank and into a pond. Water exits the
pond via a rock spillway and returns to the original channel immediately south of Thorpe
Road. Due to the diversion, a large portion of the floodplain no longer abuts the levee
(Figure 4).
A non-accredited levee is located along the east bank of Chester Creek between Thorpe
Road and Dishman-Mica Road. This levee is approximately 1,000 feet in length and
protects the project site. A without-levee analysis for this levee was not conducted as
part of the original FIS since floodwaters of similar elevation are mapped on both sides of
the levee (the floodwaters on the landward side of the levee originating from the Golf
Course Overflow Reach).
Based on current FEMA policy as described in Procedure Memorandum 51 (FEMA,
2009) the portion of D-M Road that borders the northwest corner of the property and
divides the floodwaters of Chester Creek from those of SA1 (Figure 1) is considered a
‘non levee embankment’. Based on the data from the effective study, floodwaters up to 3
feet deep exist on the west side of Dishman-Mica Road. Because the effective FIS
predated PM51 and because floodwaters of similar elevations are mapped on both sides
of D-M Road, a without levee analysis was not conducted for the road during the 2005
study.
Another non-accredited levee embankment is located along the left bank of the Unnamed
Tributary between SA6 and Highway 27. In this area, the man-made channel is perched
and the levee protects the low ground to the south, in the left overbank. A without levee
analysis was conducted in the 2005 FIS.
In order to protect the proposed development and remove it from the 1%-annual change
floodplain, three of the four levees/embankments discussed above are proposed to be
improved and certified. Geotechnical analysis and levee design and certification is being
conducted by Inland Pacific Engineering Company (IPEC). The golf course levee
between Thorpe Road and D-M Road, as well as the levee along the Unnamed Tributary
are being improved to meet FEMA requirements for certification. Based on the effective
FIS, D-M Road is acting as a levee; however, as certification of roads to provide levee
protection is typically not possible, a new levee is proposed to be built immediately east
of and parallel to D-M Road which would tie into high ground to the north and the
existing (to be certified) levee to the south. Geotechnical evaluation and certification
reports for these three levees are provided in Exhibit J.
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Figure 4. FEMA Floodplain and levee south of Thorpe Road
WEST Consultants, Inc. 9 CLOMR for Proposed Painted Hills Development
The levee south of Thorpe Road is not proposed to be modified or certified. A review of
the effective FIS HEC-RAS model and additional modeling were conducted by WEST,
for the levee. The conclusions of that analysis area as follows:
1. Much of the effective floodplain no longer abuts the levee due to historic rerouting of
the channel to the nearby pond, in the west overbank of Chester Creek (Figure 4).
Based on the effective FIS, the base flood floodplain does not touch the levee along
the southern 1100 feet of its 1500 foot overall length.
2. In the areas where floodwaters do abut the levee, flood velocities are low
(approximately 0.45ft/s – 2.2 ft/s for the 1% annual chance flood) and an average of 1
foot or less in depth between the base flood elevation and the toe of the levee.
3. Any waters that were to escape the levee and merge with the Golf Course Overflow
in the right overbank would be intercepted by the proposed infiltration facility which
has a conservative design capacity that exceeds the 1%-annual chance-flood flood
peak discharge for the Golf Course Overflow Reach by 35 cfs (design capacity of 99
cfs is 54% greater than FIS 1% peak flow of 64 cfs). Further, the infiltration facility
also includes a ten acre overflow storage area to help store floodwaters that are not
included in the design capacity calculations.
Infiltration Facilities
The effective FIS included an extensive hydrologic modeling effort that considered the
effects of infiltration and several storage areas that would serve to attenuate flood flows.
Nine storage areas were identified and considered in the hydrologic analysis, six of which
have been designated by FEMA as compensatory storage areas within which
development must compensate equally for reductions in storage and infiltration such that
there is no adverse impact on water surface elevations within and downstream of the
storage areas. The proposed development is to occupy a large portion of Storage Area 1.
To mitigate for fill and reduced infiltration WCE proposes to construct two infiltration
facilities designed to intercept, store, and infiltrate flows from the Golf Course Overflow
and the Unnamed Tributary before they enter the Project site. This will result in the
entire storage area being removed from the 1% annual chance floodplain. The infiltration
facilities, designed and analyzed by WCE, have several components described below.
Further details regarding the facility design beyond what is described below can be found
in the technical memo, Painted Hills Flood Control Development Narrative (Storage
Area 1, SA1), by WCE (WCE, 2014). The report is provided in Exhibit K. A
geotechnical investigation was conducted by IPEC in order to help WCE determine the
design infiltration capacity of the proposed drywells. More information can be found in
the report, Preliminary Geotechnical Evaluation Phase I, located in Exhibit J
Golf Course Overflow - The largest of the two flood sources contributing to SA1 is the
Golf Course Overflow Reach. The peak 1%-annual-chance-flood discharge entering SA1
via this reach is 64 cfs based on the effective FIS. Flood flows for the 10-year and
greater events overtop the right bank of Chester Creek approximately 3,000 feet upstream
of Thorpe Road, and flow along low ground in the right overbank before entering the
property via three 18” culverts under Thorpe Road, and via overtopping of the roadway.
WEST Consultants, Inc. 10 CLOMR for Proposed Painted Hills Development
The proposed facility that will intercept and infiltrate this flow path includes collection
ponds, a gravel infiltration gallery, and a 10 acre park that serves as a final measure of
protection (Figure 5).
The first pond in the series is the Collection Pond which is located immediately south of
Thorpe Road. This pond is approximately 215 feet wide, 215 feet long, and 7 feet deep.
Flows can exit the Collection Pond via four 36” culverts under Thorpe Road which
connect to the Forebay (Figure 5). Flows exceeding the storage capacity of both ponds
can exit the Forebay via a 10 foot wide, 240 foot long broad crested weir which connects
the Forebay to the Discharge Pond. The Discharge Pond contains four manholes with
beehive grates which convey flood waters to the subsurface gravel infiltration gallery
with approximately 18 drywells as junction points which discharge to the aquifer. Based
on the design and modeling conducted by WCE the infiltration system has a design
infiltration rate capacity of approximately 99 cfs, which is 11 cfs greater than the 0.2%
annual chance flow, and 35 cfs greater than the 1% annual chance flow for the Golf
Course Overflow Reach. Further, the facility is surrounded by a 10 acre park set below
surrounding topography which would provide additional flood storage, if the capacity of
the system were exceeded, in order to provide for a conservative measure of protection.
Unnamed Tributary – The Unnamed Tributary currently terminates in SA6, a large pit.
Although no channel exists downstream of SA6, the FEMA floodplain extends
downstream of the pit and connects to SA1. Based on the effective FIS, the 1% annual
chance flow entering and leaving the pit is 16 cfs and 4 cfs, respectively. The pit was
purchased by the project owners so that it could be converted to a dedicated infiltration
facility. Proposed changes to the existing pit include regrading to increase overall
storage capacity, moving the entrance from the south side to the southeast corner of the
pit, construction of a rock spillway, enlargement of the channel immediately upstream of
the spillway, and construction of 18 double depth drywells (Figure 6). Based on the
effective FIS, SA6 attenuates the 1% peak flow exiting the pit by 75%, (16 -> 4 cfs). The
18 proposed drywells have a capacity of 18 cfs, which exceeds the 1% peak discharge of
16 cfs entering the pit. Further, the storage volume of the pit provides an additional level
of safety. Based on the proposed design for SA6 and the certification of the levee, the
revised 1% annual chance floodplain will terminate at SA6. Since the levee will not be
certified for the 0.2% annual chance flood, the floodplain south of the levee along the left
overbank, and downstream of SA6 will be mapped as Shaded X/0.2% based on the
hydraulic model output.
WEST Consultants, Inc. 11 CLOMR for Proposed Painted Hills Development Figure 5. Golf Course Overflow facilities design drawing
WEST Consultants, Inc. 12 CLOMR for Proposed Painted Hills Development Figure 6. Unnamed Tributary facility design drawing
WEST Consultants, Inc. 13 CLOMR for Proposed Painted Hills Development
Interior Drainage
Figure 7 shows the interior basin area that could drain to the project site and which would
not be intercepted by the two primary infiltration facilities. This basin area of 0.55
square miles is well under the 1 square mile threshold that FEMA requires for flood
inundation analysis. Storm water inflows from the largely undeveloped 0.33 square mile
area east of Madison Road (shown in yellow) will be addressed by drywells to be
installed at the existing culvert locations under the road which could convey minor flows
from east to west into the project site. During the road improvement associated with the
project, the existing culverts will be connected to multiple drywells to receive incoming
storm water. Drywell quantity per culvert was determined based on unit discharge and
drainage area ratios from the existing FIS hydrologic model output.
The remaining 0.22 square mile area west of Madison Road will be addressed by the
approximately 103 additional drywells being proposed as part of the project storm water
design. These drywells are not part of the proposed infiltration facilities meant to address
the Golf Course Overflow and the Unnamed Tributary. As part of the effective FIS, a
drywell analysis was conducted on several highly developed subbasins within the Chester
Creek watershed in order to determine if the existing drywell system in highly developed
portions of the basin could address the 1%, and 0.2% annual chance flood events (i.e. do
these subbasins contribute to flood flows in Chester Creek?) (WEST 2008). Based on
HSPF hydrologic model analysis, the highly developed subbasins had a unit discharge of
approximately 150, and 180 cfs/sq mi for the 1% and 0.2% annual chance events,
respectively. The existing drywell network density was found to have capacity to address
large storm events, and the basins were assumed to contribute no flow to Chester Creek.
Using the 0.2% annual chance flood unit discharge of 180 cfs/sq mi (reasonable since
much of this basin will be developed for this project) and a basin area west of Madison
Road of 0.22 sq mi, a peak discharge of 40 cfs was estimated. At 1 cfs per drywell, the
estimated total of 103 proposed drywells would have approximately 2.6 times the
capacity needed to address the peak 0.2% annual chance flood event drainage, interior to
the project site.
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Figure 7. Interior drainage basin
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SITE INVESTIGATION
A site visit was conducted on 12/18/2015 by Ken Puhn of WEST Consultants, Inc.
(WEST) in order to determine site conditions and observe any changes that may have
occurred since the effective FIS was conducted.
Survey data for the site was provided in a xyz format by WCE. Survey data were
supplemented by 2003 LiDAR data collected for the effective FIS. Plan views showing
the location of the cross sections in the hydraulic models are shown in Figure 8 and
Figure 9.
HYDROLOGY
Hydrology for the effective FIS is based on a detailed hydrologic analysis using the
Hydrologic Simulation Program Fortran (HSPF). Design flows for the proposed
infiltration facility are based on the effective FEMA discharges. The 100-year discharge
was obtained directly from the effective FIS hydraulic model.
Table 1. Discharges Used in HEC-RAS Models
Location 1% Annual Chance Peak Flow (cfs)
Golf Course Overflow Channel 64
Unnamed Tributary 16/4 (upstream/downstream of SA6)
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Figure 8. Layout of HEC-RAS Cross Sections for Chester Creek Golf Course Overflow
WEST Consultants, Inc. 17 CLOMR for Proposed Painted Hills Development Figure 9. Layout of HEC-RAS Cross Sections for the Unnamed Tributary
WEST Consultants, Inc. 18 CLOMR for Proposed Painted Hills Development
HYDRAULICS
Information related to the development of the various hydraulic models required for the
CLOMR application is provided in the following paragraphs. In the effective FIS,
Chester Creek (which includes the Golf Course Overflow Reach) and the Unnamed
Tributary, were modeled separately. The CLOMR follows this preexisting methodology.
The RAS models for the Golf Course Overflow and the Unnamed Tributary are provided
in separate folders within the digital submittal materials (Exhibit M) and are named
CCMain.prj and CCTrib.prj, respectively.
Duplicate Effective Model (DEM)
The Duplicate Effective Model (DEM) is a copy of the hydraulic model used to create the
effective FIS. Creation of the DEM is required to ensure proper transfer of data from the
effective FIS. As the effective FIS model was developed by WEST, the model was
obtained from WEST archives. The hydraulic analysis for the effective FIS had been
completed using the Corps of Engineers River Analysis System (HEC-RAS) standard-
step backwater computer program version 3.1.3.
The DEM model was run using HEC-RAS Version 4.1.0. and model output compared to
the effective Floodway Data Tables (FDT). A comparison of water surface elevations
(WSEs) for the 1% annual chance flood (100-year flood) event and the floodway for the
FIS and DEM model output is provided in Table 2 and Table 3. As shown in Table 2 and
Table 3, the water surface elevations computed using the DEM model are nearly identical
to elevations published in the effective FISs. For the golf course overflow reach the
model reports water surface elevations at cross section B and C that differ from the
published FIS elevations. This is due to computational changes between RAS version
3.1.3 and 4.1. The water surface elevation at cross section B is fixed in the RAS model
based on the static water elevation reported by the HSPF hydrologic model for SA1.
RAS 4.1 has difficulty converging on a subcritical solution at this cross section and
defaults to a critical depth solution, ignoring the fixed elevation. In this case, since the
floodplain at this location would reflect the ponded conditions expected within the
storage area, the reported critical depth solution is erroneous and the fixed elevation of
2008.05 (rounded to 2008.1) is the correct elevation. The erroneous solution at cross
section B results in a slight calculated increase of 0.1 feet at cross section C.
No modifications were made to the DEM because the noted differences were within the
±0.50 ft tolerance required by FEMA Guidelines and Specification for Flood Mapping
Partners (G&S) (FEMA, 2003). HEC-RAS DEM model results are provided in Exhibit C,
and an electronic version of the DEM model is included on the CD provided in Exhibit
M.
WEST Consultants, Inc. 19 CLOMR for Proposed Painted Hills Development
Table 2. Comparison of FIS and DEM model results for Golf Course Overflow Reach
XS
Letter
Effective FIS WSE (ft) DEM WSE (ft) Difference (ft)
1% Annual 1% Annual 1% Annual
Chance Flood Floodway Chance Flood Floodway Chance Flood Floodway
Event Event Event Event Event Event
A 2008.1 ‐‐ 2008.1 2009.1 0.0 ‐‐
B 2008.1 ‐‐ 2007.8 2009.1 ‐0.3 ‐‐
C 2008.5 ‐‐ 2008.6 2009.1 0.1 ‐‐
D 2008.9 2009.6 2008.9 2009.6 0.0 0.0
E 2009.1 2010.0 2009.1 2010.0 0.0 0.0
F 2009.3 2010.3 2009.3 2010.3 0.0 0.0
G 2013.3 2014.1 2013.3 2014.1 0.0 0.0
H 2013.3 2014.2 2013.3 2014.2 0.0 0.0
I 2013.5 2014.5 2013.5 2014.5 0.0 0.0
J 2014.8 2015.3 2014.8 2015.3 0.0 0.0
K 2015.4 2016.2 2015.4 2016.2 0.0 0.0
L 2015.7 2016.6 2015.7 2016.6 0.0 0.0
M 2018.1 2019.0 2018.1 2019.0 0.0 0.0
N 2023.0 2023.8 2023.0 2023.8 0.0 0.0
WEST Consultants, Inc. 20 CLOMR for Proposed Painted Hills Development
Table 3. Comparison of FIS and DEM model results for Unnamed Tributary
XS
Letter
Effective FIS WSE (ft) DEM WSE (ft) Difference (ft)
1% Annual 1% Annual 1% Annual
Chance Flood Floodway Chance Flood Floodway Chance Flood Floodway
Event Event Event Event Event Event
A 2008.1 2009.1 2008.1 2009.1 0.0 0.0
B 2008.4 2009.1 2008.4 2009.1 0.0 0.0
C 2008.4 2009.1 2008.4 2009.1 0.0 0.0
D 2008.4 2009.1 2008.4 2009.1 0.0 0.0
E 2008.4 2009.1 2008.4 2009.1 0.0 0.0
F 2009.7 2010.7 2009.7 2010.7 0.0 0.0
G 2009.7 2010.7 2009.7 2010.7 0.0 0.0
H 2009.7 2010.7 2009.7 2010.7 0.0 0.0
I 2009.7 2010.7 2009.7 2010.7 0.0 0.0
J 2009.7 2010.7 2009.7 2010.7 0.0 0.0
K 2010.0 2010.7 2010.0 2010.7 0.0 0.0
L 2011.0 2011.1 2011.0 2011.1 0.0 0.0
M 2011.5 2011.6 2011.5 2011.6 0.0 0.0
N 2012.8 2012.8 2012.8 2012.7 0.0 ‐0.1
O 2014.3 2014.3 2014.3 2014.3 0.0 0.0
P 2014.3 2014.3 2014.3 2014.3 0.0 0.0
Q 2019.7 2019.7 2019.7 2019.6 0.0 ‐0.1
R 2020.8 2020.8 2020.8 2020.8 0.0 0.0
Corrected Effective Model (CEM)
The Corrected Effective Model (CEM) is the model that corrects any errors that occur in
the DEM, adds any additional cross sections, and/or incorporates more detailed
topographic information than that used in the DEM. The DEM model review for both the
Unnamed Tributary and the Golf Course Overflow Reach found that the models have
reasonable cross section spacing and contain detailed topographic data based on channel
survey and LiDAR. For the Unnamed Tributary the topography and ‘n’ values within the
effective models are considered to be reasonable and representative of site conditions.
The CEM model is identical to the DEM with the exception of ineffective flow stations
which were added to cross sections 2,028 and 2,849 to better reflect local topography.
For the Golf Course Overflow, two cross sections were added to the model. It was
determined that one additional cross section (RS 21498) was needed to better define the
influence of Thorpe Road and a second cross section (RS 21983) was needed
approximately 550 feet upstream of Thorpe Road to better define local topography.
Mannings ‘n’ values and topographic data in other portions of the model were considered
reasonable; therefore, no other changes were made. A comparison of DEM and CEM
results are provided in
WEST Consultants, Inc. 21 CLOMR for Proposed Painted Hills Development
Table 4 and Table 5.
Table 4. DEM and CEM model results for the Golf Course Overflow
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
DEM CEM Difference DEM CEM Difference
20779 0 A 2008.05 2008.05 0.00 2009.05 2009.05 0.00
21013 773 B 2007.79 2007.79 0.00 2009.05 2009.05 0.00
21128 961 C 2008.60 2008.60 0.00 2009.05 2009.05 0.00
21229 1145 D 2008.87 2008.87 0.00 2009.59 2009.59 0.00
21385 1425 E 2009.12 2009.12 0.00 2010.03 2010.03 0.00
21409 ‐‐ ‐‐ 2009.17 2009.17 0.00 2010.12 2010.12 0.00
21431 ‐‐ ‐‐ 2009.23 2009.23 0.00 2010.23 2010.23 0.00
21445 1600 F 2009.31 2009.31 0.00 2010.29 2010.29 0.00
21456 ‐‐ ‐‐ 2013.13 2013.14 0.01 2013.79 2013.79 0.00
21481 ‐‐ ‐‐ 2013.25 2013.25 0.00 2014.10 2014.10 0.00
21498 1 n/a n/a n/a 2013.25 n/a n/a 2014.10 n/a
21515 ‐‐ ‐‐ 2013.25 2013.26 0.01 2014.11 2014.11 0.00
21548 1800 G 2013.26 2013.26 0.00 2014.13 2014.13 0.00
21924 2123 H 2013.27 2013.27 0.00 2014.23 2014.21 ‐0.02
21983 1 n/a n/a n/a 2013.32 n/a n/a 2014.26 n/a
22423 2704 I 2013.51 2013.64 0.13 2014.48 2014.47 ‐0.01
22972 3144 J 2014.82 2014.60 ‐0.22 2015.26 2015.26 0.00
23005 ‐‐ ‐‐ 2015.25 2015.28 0.03 2016.10 2016.10 0.00
23050 3287 K 2015.38 2015.39 0.01 2016.21 2016.21 0.00
23090 3387 L 2015.65 2015.65 0.00 2016.61 2016.61 0.00
23446 3721 M 2018.10 2018.10 0.00 2019.02 2019.02 0.00
23887 4318 N 2022.99 2022.99 0.00 2023.79 2023.79 0.00
24430 ‐‐ ‐‐ 2029.56 2029.56 0.00 2029.72 2029.72 0.00
1 New cross section
WEST Consultants, Inc. 22 CLOMR for Proposed Painted Hills Development
Table 5. DEM and CEM model results for the Unnamed Tributary
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
DEM CEM Difference DEM CEM Difference
‐1303 0 A 2008.05 2008.05 0.00 2009.05 2009.05 0.00
‐1019 283 B 2008.36 2008.36 0.00 2009.05 2009.05 0.00
‐880 422 C 2008.42 2008.42 0.00 2009.05 2009.05 0.00
‐466 836 D 2008.42 2008.42 0.00 2009.05 2009.05 0.00
‐89 910 E 2008.42 2008.42 0.00 2009.05 2009.05 0.00
1 1,378 F 2009.70 2009.70 0.00 2010.70 2010.70 0.00
149 1,525 G 2009.70 2009.70 0.00 2010.70 2010.70 0.00
343 1,720 H 2009.70 2009.70 0.00 2010.70 2010.70 0.00
383 1,760 I 2009.70 2009.70 0.00 2010.70 2010.70 0.00
472 1,849 J 2009.70 2009.70 0.00 2010.70 2010.70 0.00
576 1,952 K 2009.95 2009.95 0.00 2010.67 2010.67 0.00
651 2,028 L 2010.95 2011.13 0.18 2011.07 2011.07 0.00
918 2,295 M 2011.45 2011.63 0.18 2011.63 2011.63 0.00
1472 2,849 N 2012.83 2012.72 ‐0.11 2012.71 2012.71 0.00
1510 2013.26 2013.26 0.00 2013.26 2013.26 0.00
1528 2,905 O 2014.30 2014.30 0.00 2014.31 2014.31 0.00
1557 2,933 P 2014.32 2014.32 0.00 2014.32 2014.32 0.00
1963 3,339 Q 2019.66 2019.66 0.00 2019.63 2019.63 0.00
1989 2020.06 2020.06 0.00 2020.08 2020.08 0.00
2080 2020.61 2020.61 0.00 2020.61 2020.61 0.00
2100 3,485 R 2020.83 2020.83 0.00 2020.83 2020.83 0.00
2110 2020.88 2020.88 0.00 2020.88 2020.88 0.00
2120 3,509 S 2020.91 2020.91 0.00 2020.92 2020.92 0.00
2651 4,040 T 2023.40 2023.40 0.00 2023.40 2023.40 0.00
3126 4,515 U 2026.19 2026.19 0.00 2026.18 2026.18 0.00
Existing or Pre-Project Conditions Model
The Existing or Pre-Project Conditions Model is a modification of the CEM to reflect any
modifications that have occurred within the floodplain since the date of the effective
model but prior to the construction of the project for which the revision is being
requested. For the unnamed tributary, no significant changes have occurred to the
channel or existing floodplain since the time of the effective study; therefore, the Existing
Conditions model (ECM) is a duplicate of the CEM.
For the Golf Course Overflow Reach, the only significant modifications known to have
occurred with the floodplain are the addition of three, 18” corrugated metal culverts
WEST Consultants, Inc. 23 CLOMR for Proposed Painted Hills Development
under Thorpe Road. The Existing Conditions model was modified to include the
culverts. No changes are known to have occurred on the unnamed tributary within the
study area; therefore, the ECM is identical to the CEM. Comparisons of CEM and ECM
model results are summarized in Table 6 and Table 7.
Table 6. CEM and ECM model results for the Golf Course Overflow
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
CEM ECM Difference CEM ECM Difference
20779 0 A 2008.05 2008.05 0.00 2009.05 2009.05 0.00
21013 773 B 2007.79 2007.79 0.00 2009.05 2009.05 0.00
21128 961 C 2008.60 2008.60 0.00 2009.05 2009.05 0.00
21229 1145 D 2008.87 2008.87 0.00 2009.59 2009.59 0.00
21385 1425 E 2009.12 2009.12 0.00 2010.03 2010.03 0.00
21409 ‐‐ 2009.17 2009.17 0.00 2010.12 2010.12 0.00
21431 ‐‐ 2009.23 2009.23 0.00 2010.23 2010.23 0.00
21445 1600 F 2009.31 2009.31 0.00 2010.29 2010.29 0.00
21456 ‐‐ 2013.14 2013.14 0.00 2013.79 2013.79 0.00
21481 ‐‐ 2013.25 2013.25 0.00 2014.10 2014.09 ‐0.01
21498 1 n/a n/a 2013.25 n/a n/a 2014.10 n/a n/a
21515 ‐‐ 2013.26 2013.25 ‐0.01 2014.11 2014.09 ‐0.02
21548 1800 G 2013.26 2013.25 ‐0.01 2014.13 2014.10 ‐0.03
21924 2123 H 2013.27 2013.27 0.00 2014.21 2014.18 ‐0.03
21983 2 n/a n/a 2013.32 2013.31 n/a 2014.26 2014.24 n/a
22423 2704 I 2013.64 2013.64 0.00 2014.47 2014.52 0.05
22972 3144 J 2014.60 2014.60 0.00 2015.26 2015.26 0.00
23005 ‐‐ 2015.28 2015.28 0.00 2016.10 2016.10 0.00
23050 3287 K 2015.39 2015.39 0.00 2016.21 2016.21 0.00
23090 3387 L 2015.65 2015.65 0.00 2016.61 2016.61 0.00
23446 3721 M 2018.10 2018.10 0.00 2019.02 2019.02 0.00
23887 4318 N 2022.99 2022.99 0.00 2023.79 2023.79 0.00
24430 ‐‐ 2029.56 2029.56 0.00 2029.72 2029.72 0.00
1 XS converted to bridge with culverts
2 New cross section
WEST Consultants, Inc. 24 CLOMR for Proposed Painted Hills Development
Table 7. CEM and ECM model results for the Unnamed Tributary
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
CEM ECM Difference CEM ECM Difference
‐1303 0 A 2008.05 2008.05 0.00 2009.05 2009.05 0.00
‐1019 283 B 2008.36 2008.36 0.00 2009.05 2009.05 0.00
‐880 422 C 2008.42 2008.42 0.00 2009.05 2009.05 0.00
‐466 836 D 2008.42 2008.42 0.00 2009.05 2009.05 0.00
‐89 910 E 2008.42 2008.42 0.00 2009.05 2009.05 0.00
1 1,378 F 2009.70 2009.70 0.00 2010.70 2010.70 0.00
149 1,525 G 2009.70 2009.70 0.00 2010.70 2010.70 0.00
343 1,720 H 2009.70 2009.70 0.00 2010.70 2010.70 0.00
383 1,760 I 2009.70 2009.70 0.00 2010.70 2010.70 0.00
472 1,849 J 2009.70 2009.70 0.00 2010.70 2010.70 0.00
576 1,952 K 2009.95 2009.95 0.00 2010.67 2010.67 0.00
651 2,028 L 2011.13 2011.13 0.00 2011.07 2011.07 0.00
918 2,295 M 2011.63 2011.63 0.00 2011.63 2011.63 0.00
1472 2,849 N 2012.72 2012.72 0.00 2012.71 2012.71 0.00
1510 2013.26 2013.26 0.00 2013.26 2013.26 0.00
1528 2,905 O 2014.30 2014.30 0.00 2014.31 2014.31 0.00
1557 2,933 P 2014.32 2014.32 0.00 2014.32 2014.32 0.00
1963 3,339 Q 2019.66 2019.66 0.00 2019.63 2019.63 0.00
1989 2020.06 2020.06 0.00 2020.08 2020.08 0.00
2080 2020.61 2020.61 0.00 2020.61 2020.61 0.00
2100 3,485 R 2020.83 2020.83 0.00 2020.83 2020.83 0.00
2110 2020.88 2020.88 0.00 2020.88 2020.88 0.00
2120 3,509 S 2020.91 2020.91 0.00 2020.92 2020.92 0.00
2651 4,040 T 2023.40 2023.40 0.00 2023.40 2023.40 0.00
3126 4,515 U 2026.19 2026.19 0.00 2026.18 2026.18 0.00
Proposed or Post-Project Conditions Models
The Proposed Conditions models (PCM) reflect the construction of the infiltration
facilities along the Golf Course Overflow Reach and the Unnamed Tributary. The post-
project conditions model was developed by making the following modifications to the
Existing Conditions models:
WEST Consultants, Inc. 25 CLOMR for Proposed Painted Hills Development
Golf Course Overflow:
As the proposed infiltration facility will intercept all flow up through and
including the 0.2% annual-chance-flood, the model was shortened and assigned
new stationing based on Thorpe Road as the new downstream limit reference
point (Station 0). Existing cross sections north (downstream) of Thorpe Road
were removed. This includes FEMA XS A (RS 0) through F (RS 1600). This
corresponds with effective RAS cross sections 20779 through 21481. Upstream
of Thorpe Road, Effective cross sections G (RA 1800) and H (RS 2120) were
removed (corresponds with RAS stations 21515 through 21924).
Ten new cross sections were added to the model to accurately reflect the
proposed infiltration facility and new hydraulic conditions caused by reducing
the effective BFEs and slight redirection of flow lines. Seven cross sections are
located upstream of Thorpe Road (XS 0 through 557, based on revised
stationing), and three downstream (XS -160 through -80). Note that these cross
sections could not be added to the Existing Conditions model as the proposed
facilities will alter flow dynamics in the floodplain. Accordingly, the Proposed
Conditions cross section alignments in the vicinity of the project cannot
appropriately represent existing conditions.
The model geometry was modified to include the replacement of existing
culverts under Thorpe Road with 36” CMPs
Downstream boundary conditions were assigned to known water surface
elevations based on the output from the Hydro flow modeling of the infiltration
facility conducted by WCE (WCE, 2015). The known water surface elevations
used are the peak water surface elevations for the distribution pond, as reported
by WCE.
Floodway stations were maintained at a similar width to the effective FIS,
however, the width was increased for cross sections 0 through 298 such that the
floodway encompasses the entire collection pond of the infiltration facility.
Unnamed Tributary:
For the with levee condition, the model was truncated at SA6. Three cross sections were
added at the downstream end of the model to define the rock spillway (RS 546, 566,
586). The private culvert at RS 1,520 was removed as specified by the levee certification
design. For the without downstream levee condition, no cross section or geometry
changes were made as all flow is assumed to leave the perched channel and flow to the
low ground of the left overbank; therefore, changes made to the main channel in the
vicinity of SA6 do not impact results. The without levee model is only used for the 0.2%
annual chance flood. Since the model flows and geometry remain unchanged, the results
for the 0.2% flood elevations only change at near the downstream terminus of the model
where the flow enters SA1. At this location, the fixed elevation 0.2% boundary condition
in the model (based on HSPF model output) was changed from a fixed elevation of
WEST Consultants, Inc. 26 CLOMR for Proposed Painted Hills Development
2008.64 to 2007.19 to reflect the proposed conditions in which 0.2% water surface
elevations from flooding in SA1 are reduced by the interception of the Golf Course
Overflow Reach.
Post-Project Conditions model output is provided in Exhibit F, and an electronic version
of this model is included on the CD provided in Exhibit M.
The Proposed Conditions model results for the 1% annual chance flood event and the
floodway are summarized in Table 8 and Table 9. A comparison of the floodplain widths
as determined from the CEM and the Post-Project models is provided in Table 10, Table
11 and Table 12. The proposed infiltration facilities and levee certification would have
the following impacts:
SA1 would be completely removed from the 1% annual chance floodplain. This
includes an area of approximately 94 acres. BFEs upstream of Thorpe Road
would be reduced by as much as 3 feet.
The portion of the floodway north of Thorpe Rd along the Golf Course Overflow
reach would be removed from the mapping.
A large portion of the 1% annual chance floodplain (24 acres) will be removed
from the Unnamed Tributary reach. This includes all of the left overbank
flowpath (without DS levee condition) as the floodplain downstream of SA6.
Along the Unnamed Tributary the floodway downstream of SA6 and along the
left overbank would be removed from the mapping.
Along the unnamed tributary water surface elevations decrease as much as 1.5
feet. A slight increase in water surface elevations (0.01 to 0.02 feet) will occur at
five cross sections between RS 2,080 and RS 3,126. The increase only occurs for
the ‘with upstream levee’ condition and will have inconsequential changes to the
floodplain between Highway 27 and the levee along the right bank of the channel.
WEST Consultants, Inc. 27 CLOMR for Proposed Painted Hills Development
Table 8. ECM and PCM model results for the Golf Course Overflow
Effective Revised 1% Annual Chance Flood Event Floodway
RAS
Station
FEMA
Station
XS
Letter
RAS
Station
FEMA
Station
XS
Letter ECM PCM Difference ECM PCM Difference
20779 2 0 A ‐‐ ‐‐ ‐‐ 2008.05 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
21013 2 773 B ‐‐ ‐‐ ‐‐ 2007.79 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
21128 2 961 C ‐‐ ‐‐ ‐‐ 2008.60 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
21229 2 1145 D ‐‐ ‐‐ ‐‐ 2008.87 ‐‐ ‐‐ 2009.59 ‐‐ ‐‐
21385 2 1425 E ‐‐ ‐‐ ‐‐ 2009.12 ‐‐ ‐‐ 2010.03 ‐‐ ‐‐
21409 2 ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ 2009.17 ‐‐ ‐‐ 2010.12 ‐‐ ‐‐
21431 2 ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ 2009.23 ‐‐ ‐‐ 2010.23 ‐‐ ‐‐
21445 2 1600 F ‐‐ ‐‐ ‐‐ 2009.31 ‐‐ ‐‐ 2010.29 ‐‐ ‐‐
21456 2 ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ 2013.14 ‐‐ ‐‐ 2013.79 ‐‐ ‐‐
21481 2 ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ 2013.25 ‐‐ ‐‐ 2014.09 ‐‐ ‐‐
21515 2 ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ 2013.25 ‐‐ ‐‐ 2014.09 ‐‐ ‐‐
‐‐ ‐‐ ‐‐ ‐160 1 ‐‐ ‐‐ ‐‐ 2007.81 ‐‐ ‐‐ 2007.81 ‐‐
‐‐ ‐‐ ‐‐ ‐105 1 ‐‐ ‐‐ ‐‐ 2009.96 ‐‐ ‐‐ 2009.96 ‐‐
‐‐ ‐‐ ‐‐ ‐80 1 ‐‐ ‐‐ ‐‐ 2009.95 ‐‐ ‐‐ 2009.95 ‐‐
21548 1800 G 54 54 A 2013.25 2010.27 ‐2.98 2014.10 2010.27 ‐3.83
‐‐ ‐‐ ‐‐ 143 1 143 B ‐‐ 2010.27 ‐‐ ‐‐ 2010.27 ‐‐
‐‐ ‐‐ ‐‐ 276 1 276 C ‐‐ 2012.06 ‐‐ ‐‐ 2012.10 ‐‐
‐‐ ‐‐ ‐‐ 287 1 287 D ‐‐ 2012.28 ‐‐ ‐‐ 2012.45 ‐‐
‐‐ ‐‐ ‐‐ 298 1 298 E ‐‐ 2012.35 ‐‐ ‐‐ 2012.53 ‐‐
21924 2123 H 431 431 F 2013.27 2012.85 ‐0.42 2014.18 2013.03 ‐1.15
21983 ‐‐ ‐‐ 557 557 G 2013.31 2013.09 ‐0.22 2014.24 2013.52 ‐0.72
22423 2704 I 997 997 H 2013.64 2013.62 ‐0.02 2014.52 2014.22 ‐0.30
22972 3144 J 1437 1437 I 2014.60 2014.60 0.00 2015.26 2015.26 0.00
23005 ‐‐ ‐‐ 1522 ‐‐ ‐‐ 2015.28 2015.28 0.00 2016.10 2016.10 0.00
23050 3287 K 1580 1580 J 2015.39 2015.39 0.00 2016.21 2016.21 0.00
23090 3387 L 1680 1680 K 2015.65 2015.65 0.00 2016.61 2016.61 0.00
23446 3721 M 2014 2014 L 2018.10 2018.10 0.00 2019.02 2019.02 0.00
23887 4318 N 2611 2611 M 2022.99 2022.99 0.00 2023.79 2023.79 0.00
24430 ‐‐ ‐‐ 3121 ‐‐ ‐‐ 2029.56 2029.56 0.00 2029.72 2029.72 0.00
1 New XS
2 Removed XS from PCM
WEST Consultants, Inc. 28 CLOMR for Proposed Painted Hills Development
Table 9. ECM and PCM model results for the Unnamed Tributary
Effective Revised 1% Annual Chance Flood Event Floodway
RAS
Station
FEMA
Station
XS
Letter
RAS
Station
FEMA
Station
XS
Letter ECM PCM Difference ECM PCM Difference
‐1303 0 A ‐1303 0 A 2008.05 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
‐1019 283 B ‐1019 283 B 2008.36 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
‐880 422 C ‐880 422 C 2008.42 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
‐466 836 D ‐466 836 D 2008.42 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
‐89 910 E ‐89 910 E 2008.42 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
1 1,378 F 1 1,378 F 2009.70 ‐‐ ‐‐ 2010.70 ‐‐ ‐‐
149 1,525 G 149 1,525 G 2009.70 ‐‐ ‐‐ 2010.70 ‐‐ ‐‐
343 1,720 H 343 1,720 H 2009.70 ‐‐ ‐‐ 2010.70 ‐‐ ‐‐
383 1,760 I 383 1,760 I 2009.70 ‐‐ ‐‐ 2010.70 ‐‐ ‐‐
472 1,849 J 472 1,849 J 2009.70 ‐‐ ‐‐ 2010.70 ‐‐ ‐‐
‐‐ ‐‐ ‐‐ 546 1 ‐‐ ‐‐ ‐‐ 1991.10 ‐‐ ‐‐ 1991.10 ‐‐
‐‐ ‐‐ ‐‐ 566 1 ‐‐ ‐‐ ‐‐ 2000.35 ‐‐ ‐‐ 2000.35 ‐‐
576 1,952 K 586 1 1,952 K 2009.95 2008.85 ‐1.10 2010.67 2008.85 ‐1.82
651 2,028 L 651 2,028 L 2011.13 2010.44 ‐0.69 2011.07 2010.44 ‐0.63
918 2,295 M 918 2,295 M 2011.63 2011.00 ‐0.63 2011.63 2011.00 ‐0.63
1472 2,849 N 1472 2,849 N 2012.72 2012.71 ‐0.01 2012.71 2012.71 0.00
1510 ‐‐ ‐‐ 1510 ‐‐ ‐‐ 2013.26 2012.81 ‐0.45 2013.26 2012.81 ‐0.45
1528 2,905 O 1528 2,905 O 2014.30 2012.84 ‐1.46 2014.31 2012.84 ‐1.47
1557 2,933 P 1557 2,933 P 2014.32 2012.81 ‐1.51 2014.32 2012.81 ‐1.51
1963 3,339 Q 1963 3,339 Q 2019.66 2019.35 ‐0.31 2019.63 2019.35 ‐0.28
1989 ‐‐ ‐‐ 1989 ‐‐ ‐‐ 2020.06 2019.72 ‐0.34 2020.08 2019.72 ‐0.36
2080 ‐‐ ‐‐ 2080 ‐‐ ‐‐ 2020.61 2020.63 0.02 2020.61 2020.63 0.02
2100 3,485 R 2100 3,485 R 2020.83 2020.84 0.01 2020.83 2020.84 0.01
2110 ‐‐ ‐‐ 2110 ‐‐ ‐‐ 2020.88 2020.89 0.01 2020.88 2020.89 0.01
2120 3,509 S 2120 3,509 S 2020.91 2020.92 0.01 2020.92 2020.92 0.00
2651 4,040 T 2651 4,040 T 2023.40 2023.39 ‐0.01 2023.40 2023.40 0.00
3126 4,515 U 3126 4,515 U 2026.19 2026.19 0.00 2026.18 2026.19 0.01
1 New or revised XS in PCM
WEST Consultants, Inc. 29 CLOMR for Proposed Painted Hills Development
Table 10. Change in Top Width for Golf Course Overflow
Effective Revised Top Width Base Flood (ft) Top Width Floodway (ft)
RAS
Station
FEMA
Station
XS
Letter
RAS
Station
FEMA
Station
XS
Lette
r
ECM PCM Difference ECM PCM Difference
20779 0 A ‐‐ ‐‐ ‐‐ 1985 0 ‐1985 1344 0 ‐1344
21013 773 B ‐‐ ‐‐ ‐‐ 395 0 ‐395 173 0 ‐173
21128 961 C ‐‐ ‐‐ ‐‐ 443 0 ‐443 25 0 ‐25
21229 1145 D ‐‐ ‐‐ ‐‐ 546 0 ‐546 35 0 ‐35
21385 1425 E ‐‐ ‐‐ ‐‐ 286 0 ‐286 30 0 ‐30
21409 ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ 284 0 ‐284 25 0 ‐25
21431 ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ 223 0 ‐223 25 0 ‐25
21445 1600 F ‐‐ ‐‐ ‐‐ 143 0 ‐143 20 0 ‐20
21456 ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ 115 0 ‐115 30 0 ‐30
21481 ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ 634 0 ‐634 37 0 ‐37
21515 ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ 623 0 ‐623 44 0 ‐44
‐‐ ‐‐ ‐‐ ‐160 ‐‐ ‐‐ ‐‐ 221 ‐‐ ‐‐ ‐‐ ‐‐
‐‐ ‐‐ ‐‐ ‐105 ‐‐ ‐‐ ‐‐ 271 ‐‐ ‐‐ ‐‐ ‐‐
‐‐ ‐‐ ‐‐ ‐80 ‐‐ ‐‐ ‐‐ 271 ‐‐ ‐‐ ‐‐ ‐‐
21548 1800 G 54 54 A 600 164 ‐436 45 164 118
‐‐ ‐‐ ‐‐ 143 143 B ‐‐ 113 ‐‐ ‐‐ 113 ‐‐
‐‐ ‐‐ ‐‐ 276 276 C ‐‐ 126 ‐‐ ‐‐ 74 ‐‐
‐‐ ‐‐ ‐‐ 287 287 D ‐‐ 285 ‐‐ ‐‐ 90 ‐‐
‐‐ ‐‐ ‐‐ 298 298 E ‐‐ 197 ‐‐ ‐‐ 80 ‐‐
21924 2123 H 431 431 F 260 207 ‐53 43 51 8
‐‐ ‐‐ ‐‐ 557 557 G ‐‐ 130 ‐‐ 28 26 ‐2
22423 2704 I 997 997 H 203 202 ‐1 40 40 0
22972 3144 J 1437 1437 I 54 54 0 14 14 0
23005 ‐‐ ‐‐ 1522 ‐‐ ‐‐ 135 135 0 27 27 0
23050 3287 K 1580 1580 J 152 152 0 22 22 0
23090 3387 L 1680 1680 K 155 155 0 20 20 0
23446 3721 M 2014 2014 L 208 208 0 20 20 0
23887 4318 N 2611 2611 M 160 160 0 20 20 0
24430 ‐‐ ‐‐ 3121 ‐‐ ‐‐ 37 37 0 20 20 0
WEST Consultants, Inc. 30 CLOMR for Proposed Painted Hills Development
Table 11. Change in Top Width for Unnamed Tributary
Effective Revised Top Width Base Flood (ft) Top Width Floodway (ft)
RAS
Station
FEMA
Station
XS
Letter
RAS
Station
FEMA
Station
XS
Letter ECM PCM Difference ECM PCM Difference
‐1303 0 A ‐1303 0 A 104 0 ‐104.00 31 0 ‐31.00
‐1019 283 B ‐1019 283 B 48 0 ‐48.00 21 0 ‐21.00
‐880 422 C ‐880 422 C 362 0 ‐362.00 40 0 ‐40.00
‐466 836 D ‐466 836 D 389 0 ‐389.00 46 0 ‐46.00
‐89 910 E ‐89 910 E 346 0 ‐346.00 40 0 ‐40.00
1 1,378 F 1 1,378 F 102 ‐‐
2 ‐‐ ‐‐
1 ‐‐
2 ‐‐
149 1,525 G 149 1,525 G 176 ‐‐
2 ‐‐ ‐‐
1 ‐‐
2 ‐‐
343 1,720 H 343 1,720 H 267 ‐‐
2 ‐‐ ‐‐
1 ‐‐
2 ‐‐
383 1,760 I 383 1,760 I 280 ‐‐
2 ‐‐ ‐‐
1 ‐‐
2 ‐‐
472 1,849 J 472 1,849 J 193 ‐‐
2 #VALUE! ‐‐
1 ‐‐
2 ‐‐
‐‐ ‐‐ ‐‐ 546 ‐‐ ‐‐ ‐‐ 13 ‐‐ ‐‐ 13 ‐‐
‐‐ ‐‐ ‐‐ 566 ‐‐ ‐‐ ‐‐ 10 ‐‐ ‐‐ 10 ‐‐
576 1,952 K 586 1,952 K 6 10 4 5 10 5
651 2,028 L 651 2,028 L 64 9 ‐55 16 9 ‐7
918 2,295 M 918 2,295 M 43 14 ‐29 8 14 6
1472 2,849 N 1472 2,849 N 5 11 6 5 11 6
1510 ‐‐ ‐‐ 1510 ‐‐ ‐‐ 6 13 7 6 13 7
1528 2,905 O 1528 2,905 O 10 15 5 10 15 5
1557 2,933 P 1557 2,933 P 8 9 1 8 9 1
1963 3,339 Q 1963 3,339 Q 15 7 ‐8 9 7 ‐2
1989 ‐‐ ‐‐ 1989 ‐‐ ‐‐ 48 15 ‐33 9 15 6
2080 ‐‐ ‐‐ 2080 ‐‐ ‐‐ 15 15 0 15 15 0
2100 3,485 R 2100 3,485 R 15 15 0 15 15 0
2110 ‐‐ ‐‐ 2110 ‐‐ ‐‐ 16 16 0 15 15 0
2120 3,509 S 2120 3,509 S 16 16 0 15 15 0
2651 4,040 T 2651 4,040 T 8 7 ‐1 8 8 0
3126 4,515 U 3126 4,515 U 10 10 0 10 10 0
1 No floodway in SA6 in effective FIS
2 Inside infiltration facility
WEST Consultants, Inc. 31 CLOMR for Proposed Painted Hills Development
Table 12. Change in Top Width for Unnamed Tributary without DS levee (left overbank
flowpath)
Effective Revised Top Width Base Flood (ft) Top Width Floodway (ft)
RAS
Station
FEMA
Station
XS
Letter
RAS
Station
FEMA
Station
XS
Lette
r
ECM PCM Difference ECM PCM Difference
‐1303 0 A ‐1303 0 A 104 0 ‐104 31 0 ‐31
‐1019 283 B ‐1019 283 B 72 0 ‐72 21 0 ‐21
‐880 422 C ‐880 422 C 368 0 ‐368 40 0 ‐40
‐466 836 D ‐466 836 D 403 0 ‐403 46 0 ‐46
‐89 910 E ‐89 910 E 388 0 ‐388 40 0 ‐40
1 1,378 F 1 1,378 F 113 0 ‐113 15 0 ‐15
149 1,525 G 149 1,525 G 270 0 ‐270 15 0 ‐15
343 1,720 H 343 1,720 H 282 0 ‐282 15 0 ‐15
383 1,760 I 383 1,760 I 215 0 ‐215 15 0 ‐15
472 1,849 J 472 1,849 J 215 0 ‐215 15 0 ‐15
576 1,952 K 586 1,952 K 155 0 ‐155 15 0 ‐15
651 2,028 L 651 2,028 L 96 0 ‐96 15 0 ‐15
918 2,295 M 918 2,295 M 231 0 ‐231 15 0 ‐15
1472 2,849 N 1472 2,849 N 68 0 ‐68 15 0 ‐15
1510 ‐‐ ‐‐ 1510 ‐‐ ‐‐ 129 0 ‐129 15 0 ‐15
1528 2,905 O 1528 2,905 O 119 0 ‐119 3 0 ‐3
1557 2,933 P 1557 2,933 P 140 0 ‐140 15 0 ‐15
FLOODPLAIN MAPPING
An annotated FIRM showing existing and proposed floodplain mapping is provided in
Exhibit H along with revised flood profiles and floodway data tables. The proposed
floodplain mapping reflects the proposed infiltration facilities and levee certifications. In
the effective FIS, the 0.2% annual chance floodplain within SA1 was based on model
output for the Golf Course Overflow reach. Since the infiltration facility will be
intercepting up to the 0.2% annual change flood, and since the D-M Road levee, and the
golf course levee will not be certified for the 0.2% annual chance flood event the
mapping for that flood in SA1 is based on projection of the water surface elevations from
the riverward side of the levee. The downstream end of the Unnamed Tributary
(mapping and profile) reflect this condition.
CERTIFICATION FORMS
Completed FEMA certification forms are included in Exhibit A. Exhibit A contains
Forms 1, 2 and 3. Supporting documentation that includes a copy of public notices and
the Biological Opinion are provided in Exhibit L.
WEST Consultants, Inc. 32 CLOMR for Proposed Painted Hills Development
SUMMARY
A hydraulic analysis was conducted for a proposed development within the Chester
Creek floodplain. The hydraulic analysis was completed to support a Conditional Letter
of Map Revision (CLOMR) application for the proposed development per requirements
of the City of Spokane Valley and FEMA. The revised hydraulic models and mapping
products reflect the proposed certification of 2 levees, construction of one additional
levee, and construction of two large infiltration and storage facilities. The results of the
analysis indicate that with limited exceptions, the proposed development will
significantly reduce the floodplain extent and water surface elevations for both the base
flood and floodway conditions within the extents of the CLOMR.
REFERENCES
Federal Emergency Management Agency, Procedure Memorandum No. 51, Guidance for
Mapping of Non-Levee Embankments, February 27, 2009
Federal Emergency Management Agency, Flood Insurance Study, Spokane County,
Washington and Incorporated Areas, July 6, 2010.
Federal Emergency Management Agency, Guidelines and Specifications for Flood
Hazard Partners, July 6, 2003.
U.S. Army Corps of Engineers, HEC-RAS River Analysis System User’s Manual, Version
4.1, January 2010.
Whipple Consulting Engineers, Inc., Painted Hills Flood Control Development
Narrative, September 26, 2014.
WEST Consultants. Inc., Technical Memorandum Chester Creek Flood Insurance Study
Hydrology Re-evaluation, January 14, 2008.
WEST Consultants. Inc., Flood Insurance Study Hydrologic Analysis for Chester Creek,
December 8, 2004.
Inland Pacific Engineering Company, Preliminary Geotechnical Evaluation Phase I
Painted Hills Golf Course Property, December 31, 2013.
Inland Pacific Engineering Company, Geotechnical Evaluation Levee Evaluation and
Certification 4403 South Dishman-Mica Road Spokane County, Washington,
February 12, 2015
Biology Soil & Water, Inc, Critical Areas Assessment, Buffer Averaging, and Habitat
Management Plan for the Painted Hills PRD, May 14, 2015