Painted_Hills_CLOMR_DRAFT_01-28-2019
WEST Consultants, Inc. i CLOMR for Proposed Painted Hills Development
(DRAFT) CLOMR APPLICATION
FOR THE
PROPOSED PAINTED HILLS DEVELOPMENT
CITY OF SPOKANE VALLEY & SPOKANE COUNTY,
WASHINGTON
Prepared for:
Whipple Consulting Engineers, Inc.
21 S. Pines Road
Spokane Valley, WA 99206
Prepared by:
WEST Consultants, Inc.
2601 25th St. SE, Suite 450
Salem, OR 97302
(503) 485-5490
January 28, 2019
WEST Consultants, Inc. ii CLOMR for Proposed Painted Hills Development
TABLE OF CONTENTS
INTRODUCTION ................................................................................................................ 1
BACKGROUND & RESEARCH ........................................................................................ 2
General ............................................................................................................................ 2
Levees ............................................................................................................................. 7
Infiltration Facilities ....................................................................................................... 9
Interior Drainage ........................................................................................................... 14
SITE INVESTIGATION .................................................................................................... 17
HYDROLOGY ................................................................................................................... 17
HYDRAULICS ................................................................................................................... 20
Duplicate Effective Model (DEM) ............................................................................... 22
Corrected Effective Model (CEM) ............................................................................... 25
Existing (Pre-Project) Conditions Model ..................................................................... 28
Proposed or Post-Project Conditions Models ............................................................... 30
FLOODPLAIN MAPPING ................................................................................................ 38
CERTIFICATION FORMS ................................................................................................ 38
SUMMARY ........................................................................................................................ 39
REFERENCES ................................................................................................................... 39
LIST OF TABLES
Table 1. Thorpe Levee Breach Discharges ........................................................................... 9
Table 2. Assumed runoff for Adjacent Subbasins Draining to Project Site. ..................... 14
Table 3. Discharges Used in HEC-RAS Models ............................................................... 17
Table 4. Summary of HEC-RAS Model Plans .................................................................. 20
Table 5. Summary of Model Reaches ................................................................................. 22
Table 6. Summary of 1% Annual Chance Flood Flows for CLOMR Reaches ................. 22
Table 7. Comparison of FIS and DEM model results for Golf Course Overflow Reach ... 24
Table 8. Comparison of FIS and DEM model results for Unnamed Tributary .................. 25
Table 9. DEM and CEM model results for the Golf Course Overflow .............................. 27
Table 10. DEM and CEM model results for the Unnamed Tributary ................................ 28
Table 11. CEM and ECM model results for the Golf Course Overflow ............................ 29
Table 12. CEM and ECM model results for the Unnamed Tributary ................................. 30
Table 13. ECM and PCM model results for the Golf Course Overflow............................. 34
Table 14. ECM and PCM model results for the Unnamed Tributary ................................. 35
Table 15. Change in Top Width for Golf Course Overflow ............................................... 36
Table 16. Change in top width for Unnamed Tributary ..................................................... 37
WEST Consultants, Inc. iii CLOMR for Proposed Painted Hills Development
Table 17. PCM model with- and without-levee results comparison for Golf Course Reach
..................................................................................................................................... 38
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. Effective FEMA Floodplain and levee south of Thorpe Road .............................. 8
Figure 5. Golf Course Overflow facilities overview. (Full plan set located in Appendix J)
..................................................................................................................................... 12
Figure 6. Unnamed Tributary facility overview. (Full plan set located in Appendix J) ..... 13
Figure 7. Interior drainage basins ....................................................................................... 16
Figure 8. Layout of HEC-RAS Cross Sections for Chester Creek Golf Course Overflow 18
Figure 9. Layout of HEC-RAS Cross Sections for the Unnamed Tributary ...................... 19
WEST Consultants, Inc. iv CLOMR for Proposed Painted Hills Development
LIST OF EXHIBITS
Appendix A. FEMA Forms
Appendix B. Duplicate Effective Models
Appendix C. Corrective Effective HEC-RAS Models
Appendix D. Existing Conditions HEC-RAS Models
Appendix E. Post-Project Conditions HEC-RAS Models
Appendix F. Effective Flood Insurance Rate Map (FIRM), Flood Profile, and Floodway
Data Table
Appendix G. Revised Floodplain Boundaries, Flood Profile, and Floodway Data Table
Appendix H. Floodplain Work map for CLOMR
Appendix I. Geotechnical Evaluation/Groundwater Analysis
Appendix J. Infiltration Facilities Design Report, Plans, O&M Plans
Appendix K. Supporting Information
WEST Consultants, Inc. 1 CLOMR for Proposed Painted Hills Development
INTRODUCTION
A 91 acre mixed use development is proposed for the former Painted Hills Golf Course
property located in Spokane Valley, Washington. The development includes both
residential and commercial property, and open space. Construction of flood control
facilities as well as grading and fill are proposed to address floodplain requirements. The
property is located within the area known as Storage Area 1 (SA1) in the effective FEMA
Flood Insurance Study (FIS). SA1 is designated as 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, drywells are currently in wide use
throughout the Chester Creek floodplain and are included in the effective FIS hydrologic
model and have the effect of reducing flood elevations and volumes. The inclusion of
infiltration facilities within the proposed development plan will create a net benefit by
significantly reducing flood elevations within and nearby the subject property. The
infiltration facilities and proposed changes will result in approximately 126 acres being
removed from the 1% annual chance floodplain, and the removal of 0.7 river miles of
floodway.
Geotechnical analysis was conducted by Inland Pacific Engineering Company (IPEC).
Design of infiltration and storage facilities was conducted by Whipple Consulting
Engineers. WEST Consultants, Inc, conducted the hydraulic analyses to evaluate the
effects the proposed development would have on base flood elevations (BFEs – water
surface elevations associated with the 1% annual chance event), floodway elevations,
floodplain boundaries, and floodway limits of Chester Creek. This report, along with
supporting documentation, will be submitted to the Federal Emergency Management
Agency (FEMA) through the local communities (City of Spokane Valley, WA, and
Spokane County, WA) as a Conditional Letter of Map Revision (CLOMR).
Pertinent information about the request is provided as follows:
Identifier: Painted Hills Development
Flooding Source: Chester Creek and Unnamed Tributary
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 HSPF 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. Nine 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
hydrology went through multiple rounds of peer review and revision which included FEMA,
FEMA’s review consultant Baker, and the USGS. Details of the original hydrologic analysis can
be found in the original FIS hydrology report.
The main channel of Chester Creek currently terminates at a large borrow pit (Storage Area 4)
which was developed as part of improvements to Dishman-Mica Road (D-M Road) in 1998 and
is intended to act as a storm water retention and infiltration facility. The FEMA regulatory
floodplain continues north for approximately 1.5 miles beyond the physical end of the channel.
WEST Consultants, Inc. 3 CLOMR for Proposed Painted Hills Development
#
.-,90
(/195
"!27
SPOKANE
Chester
C
re
e
k
H
a
n
g
m
a
n
Chester Creek
L it tle S p o k a n e R iv e r
W A S H I N G T O N
#
PROJECT LOCATION
NChester Creek Drainage Basin
2024Miles
Figure 1. Chester Creek Location Map
WEST Consultants, Inc. 4 CLOMR for Proposed Painted Hills Development
Figure 2. Effective FIS stream reaches and storage areas
WEST Consultants, Inc. 5 CLOMR for Proposed Painted Hills Development
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. A small levee is located along the left bank between Highway 27 and SA6.
For the with-levee condition, the 1% annual chance flow downstream of SA6 that continues to
SA1 (project site) is 4 cfs. For the without downstream levee condition in which the levee
between Highway 27 and SA6 is removed, the floodplain bypasses SA6 and the 1% annual
chance flow that could continue to SA1 is 16 cfs.
The proposed 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 shows the
location of reaches and levees within and near the CLOMR project area.. 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 and flood flows that enter the storage area pond until
they infiltrate. Descriptions of the two primary flow paths affected by this CLOMR are below.
Golf Course Overflow Reach - Flow escapes the Chester Creek channel approximately
3,000 ft upstream of the golf course due to limited channel capacity, and follows the right
overbank until it crosses Thorpe Road and enters the golf course (SA1). The flow entering
the golf course does not rejoin the main channel due to the topography of the area, a small
levee system along the right bank of the main channel, and the raised embankment of
Dishman-Mica Road. As the golf course has no outlet, floodwaters up to the 0.2% annual
chance flood (500-yr flood) are stored until they infiltrate. This flow path generally does
not contain a physical channel.
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 assumed all floodwaters (1% annual chance flow of 16 cfs) potentially leave the
channel. Flood waters would then flow along the low ground of the left overbank,
bypassing SA6 and continue until reaching SA1.
WEST Consultants, Inc. 6 CLOMR for Proposed Painted Hills Development
Figure 3. Detail of project area and levees
WEST Consultants, Inc. 7 CLOMR for Proposed Painted Hills Development
Levees
Three non-certified levees are present near the project area based on the effective FIS, that are of
concern to the project site. They are described below from south to north. A map denoting the
levees is provided as Figure 3.
A non-certified levee, referred to in this report as the “levee south of Thorpe”, is located along
the right bank and overbank of Chester Creek between Thorpe Road and a private road
approximately 1550 feet south of Thorpe Road. The levee is located along the right bank of the
original channel; however, based on the 2005 FIS, the channel was diverted to the left overbank
and into a pond. Water exits the pond via a rock spillway and returns to the original channel
immediately south of Thorpe Road. Due to the diversion, a large portion of the floodplain no
longer abuts the levee (Figure 4).
A non-certified levee, referred to in this report as the “golf course levee”, is located along the
east bank of Chester Creek between Thorpe Road and D-M Road. This levee is approximately
1,000 feet in length and protects the project site. A without levee analysis for this levee was not
conducted as part of the original FIS since floodwaters 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).
A third non-certified levee, referred to as the “Gustin Ditch levee”, is located along the left bank
of the Unnamed Tributary between SA6 and Highway 27. In this area, the man-made channel is
perched and the levee protects the low ground to the south, in the left overbank. A without levee
analysis was conducted in the 2005 FIS.
WEST Consultants, Inc. 8 CLOMR for Proposed Painted Hills Development
Figure 4. Effective FEMA Floodplain and levee south of Thorpe Road
WEST Consultants, Inc. 9 CLOMR for Proposed Painted Hills Development
A review of the effective FIS HEC-RAS model and additional RAS modeling were conducted by
WEST, for the levee South of Thorpe Road. The conclusions of that analysis are as follows:
Based on the effective FIS. much of the 1% annual chance floodplain no longer abuts
the levee due to historic rerouting of the channel to the nearby pond, in the west
overbank of Chester Creek Based on the effective FIS, the base flood floodplain does
not touch the levee along the southern 1,100 feet of its 1,500 foot overall length
(Figure 4).
In the areas where floodwaters do abut the levee, flood velocities are low;
approximately 0.5ft/s – 2.2 ft/s for the 1% annual chance flood. Velocities are likely
higher immediately downstream of the rock spillway from the pond and it is
recommended the levee bank across from the spillway be protected by rock, if
possible. Additionally, where floodwaters do abut the levee the 1% annual chance
flood generally averages 1 foot or less in depth between the base flood elevation and
the toe of the levee.
Although the flood depths and velocities adjacent to the levee are relatively small, it is
recognized that a failure of this levee could cause some additional floodwaters to leave the main
channel of Chester Creek and join the Golf Couse Overflow channel and hence the proposed
infiltration facility. Accordingly, a without-levee scenario was modeled in HEC-RAS to
determine the amount of additional flow that could join the Golf Course Overflow Reach in the
event of a levee failure so that the infiltration facility could be designed to accommodate this
scenario. In this scenario, the approximately 400-feet long section of the levee that abuts the
effective floodplain is removed (to the levee toe) in the hydraulic model and flow is allowed to
move between the main channel and the Golf Course Overflow Reach. Under this scenario, an
additional 27 cfs and 35 cfs can join the Golf Course Oveflow Reach for the 100-yr and 500-yr
events, respectively (increases of 42% and 40%) (Table 1). Accordingly, the infiltration facility
being used to intercept the Golf Course Overflow Reach has been designed by WCE with an
ultimate infiltration outflow of 162 cfs, which exceeds the 500-year without-levee scenario. Per
FEMA guidelines, the revised CLOMR mapping shown in the work maps and annotated FIRM
reflect the without-levee scenario. The FDT includes both with- and without-levee data and the
flood profiles include a fifth profile indicating the 1% annual chance without-levee water surface
elevations for the Golf Course Overflow Reach.
Table 1. Thorpe Levee Breach Discharges
Event With
Levee
Without
Levee
Difference
(cfs)
Difference
(%)
100‐yr 64 91 27 0.42
500‐yr 88 123 35 0.40
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
WEST Consultants, Inc. 10 CLOMR for Proposed Painted Hills Development
designated by FEMA as compensatory storage areas within which development must
compensate equally for reductions in storage and infiltration such that there is no adverse impact
on water surface elevations within and downstream of the storage areas. The proposed
development is to occupy a large portion of Storage Area 1.
To mitigate for fill and reduced infiltration WCE proposes to construct two infiltration facilities
designed to intercept and infiltrate the flows from the Golf Course Overflow and the Unnamed
Tributary before they enter the project site. This will result in the entire storage area (and areas
outside SA1) being removed from the 1% and 0.2% annual chance floodplains. The infiltration
facilities, designed and analyzed by WCE, have several components described below. Further
details regarding the facility design beyond the general description below can be found in the
technical memo, Painted Hills Flood Control Development Narrative (Storage Area 1, SA1), by
WCE (WCE, 2018). The design narrative and full design plans are provided in Appendix J. A
geotechnical investigation was conducted by IPEC in order to help WCE determine the design
infiltration capacity of the proposed drywells and gravel gallery. More information can be found
in the two reports, Preliminary Geotechnical Evaluation Phase I and Full-Scale Drywell Testing,
located in Appendix I.
The largest of the two flood sources contributing to SA1 is the Golf Course Overflow Reach.
The peak 1%-annual-chance-flood discharge entering SA1 via this reach is 64 cfs based on the
effective FIS. Flood flows for the 10-year and greater events overtop the right bank of Chester
Creek approximately 3,000 feet upstream of Thorpe Road, and flow along low ground in the
right overbank before entering the property through three 15” culverts under Thorpe Road, and
via overtopping of the roadway when flows exceed the capacity of the culverts. The proposed
facility that will intercept this flow path includes a replacement box culvert under Thorpe Road,
a short section of open concrete channel, a series of culverts, a bioswale, a settling pond, weir,
and a series of gravel infiltration trenches containing 48 drywells (Figure 5).
The roadside ditch along the south side of Thorpe will be regraded to drain flows to a 3-foot-tall
by 30-foot-wide box culvert under Thorpe Road which will replace the three existing 15”
culverts. On the north side of Thorpe Road, a short section of 24-foot-wide rectangular concrete
channel will transition flows to a set of two 48” RCP culverts that will convey flow north along
the west side of S Madison Road to the northeast corner of the project site where flow exits the
pipe into a prismatic open bioswale which will be planted to help filter sediment. At the
downstream end of the bioswale, flows will enter a bio-infiltration settling pond. Once flows
exceed 1’ in depth they will overtop a rock lined weir and flow into a second bio-infiltration
pond which contains the gravel gallery infiltration system. Once flood waters reach a depth of 1’
water will crest over the rims of the drywell and enter into the gravel gallery infiltration system.
Based on the design report from WCE the infiltration system has a design infiltration rate
capacity of approximately 162 cfs, which is 56 cfs greater than the 0.2% annual chance flow of
88 cfs (or 106 cfs when including local drainage from the hills to the east of Madison St), and 98
cfs greater than the 1% annual chance flow of 64 cfs (or 79 cfs including local drainage) for the
Golf Course Overflow Reach. It is recognized however, that the twin 48” RCPs, while having
the capacity to convey the 0.2% without-levee flows of 123 cfs, would surcharge and overtop
before reaching the 162 cfs capacity of the downstream infiltration gallery. Although that
scenario is outside the scope of this analysis, topography indicates that overtopping flows would
likely first enter a detention facility located immediately north of Thorpe Road, which contains
an outflow pipe into the western RCP. If overflows were to fill the detention facility, water
WEST Consultants, Inc. 11 CLOMR for Proposed Painted Hills Development
would likely discharge northeast along local low ground, flow across a walking path, and then
north along proposed roadside swales, eventually discharging into catch basins which lead to the
Madison culverts and the bio infiltration facility. A more complete description of the facilities is
available in the WCE design report and plan sheets provided in Appendix J.
The Unnamed Tributary (locally known as Gustin Ditch) 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 privately owned pit has a County easement
and is currently used for flood control and storage. Proposed changes to the existing pit include
regrading to increase overall storage capacity, moving the channel entrance from the south side
to the southeast corner of the pit, and construction of 18 double depth drywells (Figure 6). The
ditch between Highway 27 and SA6 will be replaced with a 3’ diameter pipe which has the
capacity to convey the 0.2% annual chance flood per the effective FIS. Based on the effective
FIS, SA6 attenuates the 1% peak flow exiting the pit by 75%, (16 cfs to 4 cfs) and the 0.2% peak
flow by 65% (20 cfs to 7 cfs). The 18 proposed drywells have a capacity of 18 cfs, which
exceeds the 1% peak discharge of 16 cfs entering the pit. Further, the storage volume of the pit
provides an additional factor of safety and can storage up to the 0.2% flood. The effective FIS
includes an overflow path in the left overbank of the unnamed tributary downstream of Highway
27 which is based on a levee failure scenario assuming the levee on the left bank of the channel
will fail. Since this potion of the watercourse will be piped from the highway to SA6 under the
proposed design, there is no longer a need for a without levee scenario. This As proposed, the
modification to the channel and pit will provide capacity to store and infiltrate events up to the
0.2% annual chance event, and the proposed FEMA floodplain for this reach will terminate at
SA6.
WEST Consultants, Inc. 12 CLOMR for Proposed Painted Hills Development Figure 5. Golf Course Overflow facilities overview. (Full plan set located in Appendix J)
WEST Consultants, Inc. 13 CLOMR for Proposed Painted Hills Development Figure 6. Unnamed Tributary facility overview. (Full plan set located in Appendix J)
WEST Consultants, Inc. 14 CLOMR for Proposed Painted Hills Development
Interior Drainage
It is recognized that there are additional local drainage areas that can also contribute flows to the
project (Figure 7). This basin area of 0.6 square miles is well under the 1 square mile threshold
that FEMA requires for flood inundation analysis; however, the area was considered in order to
make sure the proposed infiltration and storm drain facilities can accommodate nearby storm
runoff that naturally drains to the project site. The pink and yellow areas in Figure 9 currently
drain to the lower portion of the Unnamed Tributary Without Downstream Levee flow path.
These areas could potentially contribute flow to the project site that would not be intercepted by
the SA6 infiltration facility. Runoff from the largely undeveloped 0.34 square mile area east of
Madison Road (shown in pink) will be conveyed to the SA1 infiltration facility via the four
existing culverts under S Madison Rd. The four culverts are to be connected to the eastern of the
two proposed north/south trending RCP culverts immediately west of S Madison Rd which
conveys flood flows to the infiltration facility. Discharges were calculated based the unit
discharge and drainage area ratios from similar watersheds reported by the HSPF hydrologic
model in the existing FIS (Table 2). The small portion of the school property (in yellow)
identified as ‘school west’ has a calculated runoff of 3 cfs; however, five drywells are located at
the lower end of the basin which would reduce flow downstream. Single depth drywells in the
area typically have a design flow of 0.3 cfs; therefore, it is expected that less than 2 cfs would
flow downstream of this portion of the basin.
Table 2. Assumed runoff for Adjacent Subbasins Draining to Project Site.
Subbasin
Assumed 1% annual
chance event discharge
(cfs/sq mi)
Calculated runoff
from unit discharge
Painted Hills 150 21
School West 100 3
Madison East 44 15
The remaining 0.22 square mile area west of Madison Road (purple) 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 have
capacity to infiltrate 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 1% annual chance flood unit discharge of 150
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 33 cfs was estimated. At 0.3 cfs per
drywell (conservatively assuming all single depth), the total of 103 proposed drywells would
WEST Consultants, Inc. 15 CLOMR for Proposed Painted Hills Development
have approximately the same capacity needed to address the peak runoff, assuming no detention
facilities were present. The WCE development plan does however contain several large
detention facilities as well as pervious open space.
WEST Consultants, Inc. 16 CLOMR for Proposed Painted Hills Development
Figure 7. Interior drainage basins
WEST Consultants, Inc. 17 CLOMR for Proposed Painted Hills Development
SITE INVESTIGATION
A site visit was conducted on 12/18/2014 by Ken Puhn of WEST Consultants, Inc. (WEST) in
order to determine site conditions and observe any changes that may have occurred since the
effective FIS was conducted.
Survey data for the site was provided in a xyz format by WCE. Survey data were supplemented
by 2003 LiDAR data collected for the effective FIS. The 2003 LiDAR data is considered
sufficient since areas where it is used have not been modified or changed since 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 and 500-year discharges were obtained
directly from the effective FIS hydraulic model. No modifications were made to the HSPF model
for this CLOMR.
Table 3. Discharges Used in HEC-RAS Models
Location 1% Annual Chance Peak Flow
(cfs)
0.2% Annual Chance Peak Flow
(cfs)
Golf Course Overflow
Channel 64 88
Unnamed Tributary 16/4 (upstream/downstream of
SA6)
20/7 (upstream/downstream of
SA6)
WEST Consultants, Inc. 18 CLOMR for Proposed Painted Hills Development
Figure 8. Layout of HEC-RAS Cross Sections for Chester Creek Golf Course Overflow
WEST Consultants, Inc. 19 CLOMR for Proposed Painted Hills Development Figure 9. Layout of HEC-RAS Cross Sections for the Unnamed Tributary
WEST Consultants, Inc. 20 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. A
summary list of model plans is provided in Table 4. A summary of model reaches is provided in
Table 5. A summary of modeled existing and proposed conditions 1% annual-chance flood flows
within the CLOMR reaches is provided in Table 6.
Table 4. Summary of HEC-RAS Model Plans
Model Plan Flow File Geometry File Description
CCMain.prj DEM Upper Reach
US reach w/levee
(DEM)
Duplicate Effective Model ‐ Upper Reach of
Chester Creek (above SA 5 Borrow Pit), and Golf
Course Overflow
CCMain.prj DEM FW
Upper Reach,
floodway
US reach w/levee
(DEM)
Duplicate Effective Model for Enchroachment
Analysis ‐ Upper Reach of Chester Creek (above
SA 5 Borrow Pit), and Golf Course Overflow
CCMain.prj CEM Upper Reach
US reach w/levee
(CEM)
Corrected Effective Model ‐ Upper Reach of
Chester Creek (above SA 5 Borrow Pit), and Golf
Course Overflow
CCMain.prj CEM FW
Upper Reach,
floodway
US reach w/levee
(CEM)
Corrected Effective Model for Enchroachment
Analysis ‐ Upper Reach of Chester Creek (above
SA 5 Borrow Pit), and Golf Course Overflow
CCMain.prj ECM Upper Reach
US reach w/levee
(ECM)
Existing Conditions Model ‐ Upper Reach of
Chester Creek (above SA 5 Borrow Pit), and Golf
Course Overflow
CCMain.prj ECM FW
Upper Reach,
floodway
US reach w/levee
(ECM)
Existing Conditions Model for Enchroachment
Analysis ‐ Upper Reach of Chester Creek (above
SA 5 Borrow Pit), and Golf Course Overflow
CCMain.prj PCM
Upper Reach
Proposed
US reach w/levee
(PCM)
Proposed Conditions Model ‐ Upper Reach of
Chester Creek (above SA 5 Borrow Pit), and Golf
Course Overflow
CCMain.prj PCM FW
Upper Reach
Proposed,
Floodway
US reach w/levee
(PCM)
Proposed Conditions Model for Enchroachment
Analysis ‐ Upper Reach of Chester Creek (above
SA 5 Borrow Pit), and Golf Course Overflow
CCMain.prj PCM
w/breach
Upper Reach
Proposed,
without Thorpe
Levee
US reach wo/Thorpe
levee (PCM)
Proposed Conditions Model ‐ Upper Reach of
Chester Creek (above SA 5 Borrow Pit), and Golf
Course Overflow, without Thorpe levee
WEST Consultants, Inc. 21 CLOMR for Proposed Painted Hills Development
CCTrib.prj DEM With levee (DEM)
With levee flows and
boundary
Duplicate Effective Model ‐ Unnamed Tributary
with levee conditions (with both levees US and
DS of Hwy 27)
CCTrib.prj DEM FW With levee (DEM)
With levee flows,
floodway
Duplicate Effective Model for Encroachment
Analysis ‐ Unnamed Tributary with levee
conditions (with both levees US and DS of Hwy
27)
CCTrib.prj DEM w/o DS
lev
Without DS levee
(DEM)
Without ds levee flows
and boundary
Duplicate Effective Model ‐ Unnamed Tributary
without downstream levee conditions (with
levee US of Hwy 27 and without levee DS of Hwy
27)
CCTrib.prj DEM w/o DS
lev FW
Without DS levee
(DEM)
Without ds levee flows
and boundary, fw
Duplicate Effective Model for Encroachment
Analysis ‐ Unnamed Tributary with levee
conditions(with levee US of Hwy 27 and without
levee DS of Hwy 27)
CCTrib.prj CEM With levee (CEM)
With levee flows and
boundary
Corrected Effective Model ‐ Unnamed Tributary
with levee conditions (with both levees US and
DS of Hwy 27)
CCTrib.prj CEM FW With levee (CEM)
With levee flows,
floodway
Corrected Effective Model for Encroachment
Analysis ‐ Unnamed Tributary with levee
conditions (with both levees US and DS of Hwy
27)
CCTrib.prj CEM w/o DS
lev
Without DS levee
(CEM)
Without ds levee flows
and boundary
Corrected Effective Model ‐ Unnamed Tributary
without downstream levee conditions (with
levee US of Hwy 27 and without levee DS of Hwy
27)
CCTrib.prj CEM w/o DS
lev FW
Without DS levee
(CEM)
Without ds levee flows
and boundary, fw
Corrected Effective Model for Encroachment
Analysis ‐ Unnamed Tributary with levee
conditions(with levee US of Hwy 27 and without
levee DS of Hwy 27)
CCTrib.prj ECM With levee (ECM)
With levee flows and
boundary
Existing Conditions Model ‐ Unnamed Tributary
with levee conditions (with both levees US and
DS of Hwy 27)
CCTrib.prj ECM FW With levee (ECM)
With levee flows,
floodway
Existing Conditions Model for Encroachment
Analysis ‐ Unnamed Tributary with levee
conditions (with both levees US and DS of Hwy
27)
CCTrib.prj ECM w/o DS
lev
Without DS levee
(ECM)
Without ds levee flows
and boundary
Existing Conditions Model ‐ Unnamed Tributary
without downstream levee conditions (with
levee US of Hwy 27 and without levee DS of Hwy
27)
CCTrib.prj ECM w/o DS
lev FW
Without DS levee
(ECM)
Without ds levee flows
and boundary, fw
Existing Conditions Model for Encroachment
Analysis ‐ Unnamed Tributary with levee
conditions(with levee US of Hwy 27 and without
levee DS of Hwy 27)
CCTrib.prj PCM With levee (PCM)
With levee flows and
boundary (proposed)
Proposed Conditions Model ‐ Unnamed Tributary
with levee conditions (with both levees US and
DS of Hwy 27)
CCTrib.prj PCM FW With levee (PCM)
With levee flows,
floodway (proposed)
Proposed Conditions Model for Encroachment
Analysis ‐ Unnamed Tributary with levee
conditions (with both levees US and DS of Hwy
27)
WEST Consultants, Inc. 22 CLOMR for Proposed Painted Hills Development
Table 5. Summary of Model Reaches
Model Reach Description
Part of
CLOMR
CCMain.prj Upper
Chester Creek from upstream end of FIS to
FEMA XS AO (culvert under RR tracks
between Bowdish Road and Schafer Rd)
No
CCMain.prj Dredge Channel
Parallel channel of Chester Creek on east
side of RR tracks between Bowdish Road and
Schafer
No
CCMain.prj DS Dredge Channel
Chester Creek from FEMA XS AO to FEMA XS
AD (upstream end of SA 5, Dishman‐Mica Rd
Borrow Pit)
No
CCMain.prj Golf Course Golf Course Overflow reach of Chester Creek Yes
CCTrib.prj Trib Main channel of unnamed tributary Yes
CCTrib.prj LOB Overflow
Left overbank flowpath; flowpath is modeled
only for without DS levee conditions Yes
Table 6. Summary of 1% Annual Chance Flood Flows for CLOMR Reaches
Watercourse Location
RAS Station
(Existing FIS)
RAS Station
(Revised
CLOMR)
1% Annual Chance Flood (cfs)
Effective
Model (DEM,
CEM, ECM)
Revised
Model (PCM)
Unnamed
Tributary Highway 27 to SA6 2080 ‐ 149 2080 ‐ 548 16 16
Unnamed
Tributary Outflow from SA6 1 ‐ (‐1303) n/a 4 0
Unnamed
Tributary
Left Overbank Flowpath
(without levee scenario 1989 ‐ (‐1303) n/a 16 0
Chester
Creek
Golf Course Overflow
Reach 23887 ‐ 20779 2863 ‐ (‐2737) 64 64
Duplicate Effective Model (DEM)
The Duplicate Effective Model (DEM) is a copy of the hydraulic model used to create the
effective FIS. Creation of the DEM is required to ensure proper transfer of data from the
effective FIS. As the effective FIS model was developed by WEST, the model was obtained
from WEST archives. The hydraulic analysis for the effective FIS had been completed using the
WEST Consultants, Inc. 23 CLOMR for Proposed Painted Hills Development
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 7 and Table 8. As shown in Table 7 and Table 8, the water
surface elevations computed using the DEM model are nearly identical to elevations published in
the effective FISs. For the golf course overflow reach the model reports water surface elevations
at RAS cross section 21,013 and 21,128 that differ from the published FIS elevations. This is
due to computational changes between RAS version 3.1.3 and 4.1. The water surface elevation
at cross section 21,013 is fixed in the RAS model based on the static water elevation reported by
the HSPF hydrologic model for SA1. RAS 4.1 has difficulty converging on a subcritical solution
at this cross section and defaults to a critical depth solution, ignoring the fixed elevation. In this
case, since the floodplain at this location would reflect the ponded conditions expected within
the storage area, the reported critical depth solution is erroneous and the fixed elevation of
2008.05 (rounded to 2008.1) is the correct elevation. The erroneous solution at cross section
21,013 results in a slight calculated increase of 0.1 feet at cross section 21,128.
For the Unnamed Tributary, RAS cross sections 1,472 and 1,963 are 0.1 foot lower than the FIS.
This again is likely due to small computational differences between RAS 3.1.3 and 4.1. No
modifications were made to the DEM because the noted differences were within the ±0.50 ft
tolerance required by FEMA Guidelines and Specification for Flood Mapping Partners (G&S)
(FEMA). HEC-RAS DEM model output data are provided in Exhibit C.
WEST Consultants, Inc. 24 CLOMR for Proposed Painted Hills Development
Table 7. Comparison of FIS and DEM model results for Golf Course Overflow Reach
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
Effective
FIS
(FT NAVD)
DEM
(FT NAVD)
Difference
(FT)
Effective
FIS
(FT NAVD)
DEM
(FT NAVD)
Difference
(FT)
20779 0 A 2008.1 2008.1 0.0 ‐‐ 2009.1 ‐‐
21013 773 B 2008.1 2007.8 ‐0.3 ‐‐ 2009.1 ‐‐
21128 961 C 2008.5 2008.6 0.1 ‐‐ 2009.1 ‐‐
21229 1145 D 2008.9 2008.9 0.0 2009.6 2009.6 0.0
21385 1425 E 2009.1 2009.1 0.0 2010.0 2010.0 0.0
21445 1600 F 2009.3 2009.3 0.0 2010.3 2010.3 0.0
21548 1800 G 2013.3 2013.3 0.0 2014.1 2014.1 0.0
21924 2123 H 2013.3 2013.3 0.0 2014.2 2014.2 0.0
22423 2704 I 2013.5 2013.5 0.0 2014.5 2014.5 0.0
22972 3144 J 2014.8 2014.8 0.0 2015.3 2015.3 0.0
23050 3287 K 2015.4 2015.4 0.0 2016.2 2016.2 0.0
23090 3387 L 2015.7 2015.7 0.0 2016.6 2016.6 0.0
23446 3721 M 2018.1 2018.1 0.0 2019.0 2019.0 0.0
23887 4318 N 2023.0 2023.0 0.0 2023.8 2023.8 0.0
WEST Consultants, Inc. 25 CLOMR for Proposed Painted Hills Development
Table 8. Comparison of FIS and DEM model results for Unnamed Tributary
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
Effective
FIS
(FT NAVD)
DEM
(FT NAVD)
Difference
(FT)
Effective
FIS
(FT NAVD)
DEM
(FT NAVD)
Difference
(FT)
‐1303 0 A 2008.1 2008.1 0.0 2009.1 2009.1 0.0
‐1019 283 B 2008.4 2008.4 0.0 2009.1 2009.1 0.0
‐880 422 C 2008.4 2008.4 0.0 2009.1 2009.1 0.0
‐466 836 D 2008.4 2008.4 0.0 2009.1 2009.1 0.0
‐89 910 E 2008.4 2008.4 0.0 2009.1 2009.1 0.0
1 1,378 F 2009.7 2009.7 0.0 2010.7 2010.7 0.0
149 1,525 G 2009.7 2009.7 0.0 2010.7 2010.7 0.0
343 1,720 H 2009.7 2009.7 0.0 2010.7 2010.7 0.0
383 1,760 I 2009.7 2009.7 0.0 2010.7 2010.7 0.0
472 1,849 J 2009.7 2009.7 0.0 2010.7 2010.7 0.0
576 1,952 K 2010.0 2010.0 0.0 2010.7 2010.7 0.0
651 2,028 L 2011.0 2011.0 0.0 2011.1 2011.1 0.0
918 2,295 M 2011.5 2011.5 0.0 2011.6 2011.6 0.0
1472 2,849 N 2012.8 2012.8 0.0 2012.8 2012.7 ‐0.1
1528 2,905 O 2014.3 2014.3 0.0 2014.3 2014.3 0.0
1557 2,933 P 2014.3 2014.3 0.0 2014.3 2014.3 0.0
1963 3,339 Q 2019.7 2019.7 0.0 2019.7 2019.6 ‐0.1
2100 3,485 R 2020.8 2020.8 0.0 2020.8 2020.8 0.0
Corrected Effective Model (CEM)
The Corrected Effective Model (CEM) is the model that corrects any errors that occur in the
DEM, adds any additional cross sections needed, and/or incorporates more detailed topographic
information than that used in the DEM. The DEM model review for both the Unnamed
Tributary and the Golf Course Overflow Reach found that the models have reasonable cross
section spacing and contain detailed topographic data based on channel survey and LiDAR
capable of supporting 2-foot contours. Further, the floodplain areas within the CLOMR
boundary have remained essentially unchanged since the effective FIS and the effective model
topography is representative of current conditions.
For the Unnamed Tributary, the topography and ‘n’ values within the effective models are
considered to be reasonable and representative of site conditions based on engineering
judgement and guidance provided in the HEC-RAS Hydraulic Reference Manual; therefore, no
changes were made. The CEM model is identical to the DEM except for the following two
changes. First, ineffective flow stations which were added to RAS cross sections 651 and 1,742
to better reflect local topography. Second, new survey collected by WCE shows that the
WEST Consultants, Inc. 26 CLOMR for Proposed Painted Hills Development
downstream invert of the Highway 27 culvert is at elevation 2017.4, rather than elevation
2017.83 as shown in the effective model. The change in the culvert invert results in very minor
changes to water surface elevations, (ranging from -0.02 to 0.04 feet) at the next five cross
sections upstream of the culvert.
For the Golf Course Overflow, in order to more accurately assess the impacts of the proposed
project, cross sections 22,423, 21,924, and 21,548 were removed and replaced with five new
cross-sections (21,525, 21,609, 21,726, 21,857, and 21,983), the spacing and alignment of which
better represent the area immediately upstream of Thorpe Road. Cross section 21,498 was also
added to better define the influence of Thorpe Road. Mannings ‘n’ values for the new cross
sections are identical to the ‘n’ values for the cross sections they were replacing, and were
considered reasonable based on engineering judgement and the RAS Hydraulic Reference
Manual. Mannings ‘n’ values and topographic data in other portions of the model were
considered reasonable; therefore, no other changes were made. A comparison of DEM and CEM
results are provided in Table 9 and Table 10.
WEST Consultants, Inc. 27 CLOMR for Proposed Painted Hills Development
Table 9. DEM and CEM model results for the Golf Course Overflow
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
DEM
(FT NAVD)
CEM
(FT NAVD)
Difference
(FT)
DEM
(FT NAVD)
CEM
(FT NAVD)
Difference
(FT)
20779 0 A 2008.05 2008.05 0.00 2009.05 2009.05 0.00
21013 773 B 2007.79 2007.79 0.00 2009.05 2009.05 0.00
21128 961 C 2008.60 2008.60 0.00 2009.05 2009.05 0.00
21229 1145 D 2008.87 2008.87 0.00 2009.59 2009.59 0.00
21385 1425 E 2009.12 2009.12 0.00 2010.03 2010.03 0.00
21409 ‐‐ ‐‐ 2009.17 2009.17 0.00 2010.12 2010.12 0.00
21431 ‐‐ ‐‐ 2009.23 2009.23 0.00 2010.23 2010.23 0.00
21445 1600 F 2009.31 2009.31 0.00 2010.29 2010.29 0.00
21456 ‐‐ ‐‐ 2013.13 2013.13 0.00 2013.79 2013.79 0.00
21481 ‐‐ ‐‐ 2013.25 2013.25 0.00 2014.10 2014.10 0.00
21485 1 ‐‐ ‐‐ ‐‐ 2013.25 ‐‐ ‐‐ 2014.10 ‐‐
21515 ‐‐ ‐‐ 2013.25 2013.26 0.01 2014.11 2014.11 0.00
21525 1 ‐‐ ‐‐ ‐‐ 2013.26 ‐‐ ‐‐ 2014.12 ‐‐
21548 2 1800 G 2013.26 ‐‐ ‐‐ 2014.13 ‐‐
21609 1 ‐‐ ‐‐ ‐‐ 2013.26 ‐‐ ‐‐ 2014.14 ‐‐
21726 1 ‐‐ ‐‐ ‐‐ 2013.26 ‐‐ ‐‐ 2014.16 ‐‐
21924 3 2123 H 2013.27 2013.27 0.00 2014.23 2014.24 0.01
21983 1 ‐‐ n/a ‐‐ 2013.31 ‐‐ ‐‐ 2014.28 ‐‐
22423 2704 I 2013.51 2013.64 0.13 2014.48 2014.45 ‐0.03
22972 3144 J 2014.82 2014.60 ‐0.22 2015.26 2015.26 0.00
23005 ‐‐ ‐‐ 2015.25 2015.28 0.03 2016.10 2016.10 0.00
23050 3287 K 2015.38 2015.39 0.01 2016.21 2016.21 0.00
23090 3387 L 2015.65 2015.65 0.00 2016.61 2016.61 0.00
23446 3721 M 2018.10 2018.10 0.00 2019.02 2019.02 0.00
23887 4318 N 2022.99 2022.99 0.00 2023.79 2023.79 0.00
24430 ‐‐ ‐‐ 2029.56 2029.56 0.00 2029.72 2029.72 0.00
WEST Consultants, Inc. 28 CLOMR for Proposed Painted Hills Development
Table 10. DEM and CEM model results for the Unnamed Tributary
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
DEM
(FT NAVD)
CEM
(FT NAVD)
Difference
(FT)
DEM
(FT NAVD)
CEM
(FT NAVD)
Difference
(FT)
‐1303 0 A 2008.05 2008.05 0.00 2009.05 2009.05 0.00
‐1019 283 B 2008.36 2008.36 0.00 2009.05 2009.05 0.00
‐880 422 C 2008.42 2008.42 0.00 2009.05 2009.05 0.00
‐466 836 D 2008.42 2008.42 0.00 2009.05 2009.05 0.00
‐89 910 E 2008.42 2008.42 0.00 2009.05 2009.05 0.00
1 1,378 F 2009.70 2009.70 0.00 2010.70 2010.70 0.00
149 1,525 G 2009.70 2009.70 0.00 2010.70 2010.70 0.00
343 1,720 H 2009.70 2009.70 0.00 2010.70 2010.70 0.00
383 1,760 I 2009.70 2009.70 0.00 2010.70 2010.70 0.00
472 1,849 J 2009.70 2009.70 0.00 2010.70 2010.70 0.00
576 1,952 K 2009.95 2009.95 0.00 2010.67 2010.67 0.00
651 2,028 L 2010.95 2011.13 0.18 2011.07 2011.07 0.00
918 2,295 M 2011.45 2011.63 0.18 2011.63 2011.63 0.00
1472 2,849 N 2012.83 2012.72 ‐0.11 2012.71 2012.71 0.00
1510 ‐‐ ‐‐ 2013.26 2013.26 0.00 2013.26 2013.26 0.00
1528 2,905 O 2014.30 2014.30 0.00 2014.31 2014.31 0.00
1557 2,933 P 2014.32 2014.32 0.00 2014.32 2014.32 0.00
1963 3,339 Q 2019.66 2019.66 0.00 2019.63 2019.63 0.00
1989 ‐‐ ‐‐ 2020.06 2020.06 0.00 2020.08 2020.08 0.00
2080 ‐‐ ‐‐ 2020.61 2020.65 0.04 2020.61 2020.65 0.04
2100 3,485 R 2020.83 2020.86 0.03 2020.83 2020.86 0.03
2110 ‐‐ ‐‐ 2020.88 2020.90 0.02 2020.88 2020.90 0.02
2120 3,509 S 2020.91 2020.93 0.02 2020.92 2020.93 0.01
2651 4,040 T 2023.40 2023.37 ‐0.03 2023.40 2023.38 ‐0.02
3126 4,515 U 2026.19 2026.19 0.00 2026.18 2026.19 0.01
Existing (Pre-Project) Conditions Model
The Existing or Pre-Project Conditions Model is a modification of the CEM to reflect any
modifications that have occurred within the floodplain since the date of the effective model but
prior to the construction of the project for which the revision is being requested. For the
Unnamed Tributary, no significant changes have occurred to the channel or existing floodplain
since the time of the effective study; therefore, the Existing Conditions model (ECM) is a
duplicate of the CEM.
For the Golf Course Overflow Reach, the only significant modifications known to have occurred
WEST Consultants, Inc. 29 CLOMR for Proposed Painted Hills Development
are the addition of three, 15” corrugated metal culverts under Thorpe Road. The Existing
Conditions model was modified to include the culverts. Comparisons of CEM and ECM model
results are summarized in Table 11 and Table 12.
Table 11. CEM and ECM model results for the Golf Course Overflow
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
CEM
(FT NAVD)
ECM
(FT NAVD)
Difference
(FT)
CEM
(FT NAVD)
ECM
(FT NAVD)
Difference
(FT)
20779 0 A 2008.05 2008.05 0.00 2009.05 2009.05 0.00
21013 773 B 2007.79 2007.79 0.00 2009.05 2009.05 0.00
21128 961 C 2008.60 2008.60 0.00 2009.05 2009.05 0.00
21229 1145 D 2008.87 2008.87 0.00 2009.59 2009.59 0.00
21385 1425 E 2009.12 2009.12 0.00 2010.03 2010.03 0.00
21409 ‐‐ ‐‐ 2009.17 2009.17 0.00 2010.12 2010.12 0.00
21431 ‐‐ ‐‐ 2009.23 2009.23 0.00 2010.23 2010.23 0.00
21445 1600 F 2009.31 2009.31 0.00 2010.29 2010.29 0.00
21456 ‐‐ ‐‐ 2013.13 2013.14 0.01 2013.79 2013.79 0.00
21481 ‐‐ ‐‐ 2013.25 2013.27 0.02 2014.10 2014.14 0.04
21485 1 ‐‐ ‐‐ 2013.25 ‐‐ ‐‐ 2014.10 ‐‐ ‐‐
21515 ‐‐ 2013.26 2013.27 0.02 2014.11 2014.15 0.04
21525 ‐‐ ‐‐
2013.26 2013.28 0.02 2014.12 2014.15
0.03
21609 1800 G 2013.26 2013.28 0.02 2014.14 2014.19 0.05
21726 ‐‐ ‐‐ 2013.26 2013.28 0.03 2014.16 2014.21 0.05
21857 2123 H 2013.27 2013.30 0.03 2014.24 2014.24 0.00
21983 n/a n/a 2013.31 2013.33 n/a 2014.28 2014.27 n/a
22423 2704 I 2013.64 2013.64 0.00 2014.45 2014.45 0.00
22972 3144 J 2014.60 2014.60 0.00 2015.26 2015.27 0.01
23005 ‐‐ ‐‐ 2015.28 2015.28 0.00 2016.10 2016.10 0.00
23050 3287 K 2015.39 2015.39 0.00 2016.21 2016.21 0.00
23090 3387 L 2015.65 2015.65 0.00 2016.61 2016.61 0.00
23446 3721 M 2018.10 2018.10 0.00 2019.02 2019.02 0.00
23887 4318 N 2022.99 2022.99 0.00 2023.79 2023.79 0.00
24430 ‐‐ ‐‐ 2029.56 2029.56 0.00 2029.72 2029.72 0.00
1 XS converted to bridge with culverts in model
WEST Consultants, Inc. 30 CLOMR for Proposed Painted Hills Development
Table 12. CEM and ECM model results for the Unnamed Tributary
RAS
Station
FEMA
Station
XS
Letter
1% Annual Chance Flood Event Floodway
CEM
(FT NAVD)
ECM
(FT NAVD)
Difference
(FT)
CEM
(FT NAVD)
ECM
(FT NAVD)
Difference
(FT)
‐1303 0 A 2008.05 2008.05 0.00 2009.05 2009.05 0.00
‐1019 283 B 2008.36 2008.36 0.00 2009.05 2009.05 0.00
‐880 422 C 2008.42 2008.42 0.00 2009.05 2009.05 0.00
‐466 836 D 2008.42 2008.42 0.00 2009.05 2009.05 0.00
‐89 910 E 2008.42 2008.42 0.00 2009.05 2009.05 0.00
1 1,378 F 2009.70 2009.70 0.00 2010.70 2010.70 0.00
149 1,525 G 2009.70 2009.70 0.00 2010.70 2010.70 0.00
343 1,720 H 2009.70 2009.70 0.00 2010.70 2010.70 0.00
383 1,760 I 2009.70 2009.70 0.00 2010.70 2010.70 0.00
472 1,849 J 2009.70 2009.70 0.00 2010.70 2010.70 0.00
576 1,952 K 2009.95 2009.95 0.00 2010.67 2010.67 0.00
651 2,028 L 2011.13 2011.13 0.00 2011.07 2011.07 0.00
918 2,295 M 2011.63 2011.63 0.00 2011.63 2011.63 0.00
1472 2,849 N 2012.72 2012.72 0.00 2012.71 2012.71 0.00
1510 2013.26 2013.26 0.00 2013.26 2013.26 0.00
1528 2,905 O 2014.30 2014.30 0.00 2014.31 2014.31 0.00
1557 2,933 P 2014.32 2014.32 0.00 2014.32 2014.32 0.00
1963 3,339 Q 2019.66 2019.66 0.00 2019.63 2019.63 0.00
1989 2020.06 2020.06 0.00 2020.08 2020.08 0.00
2080 2020.65 2020.65 0.00 2020.65 2020.65 0.00
2100 3,485 R 2020.86 2020.86 0.00 2020.86 2020.86 0.00
2110 2020.90 2020.90 0.00 2020.90 2020.90 0.00
2120 3,509 S 2020.93 2020.93 0.00 2020.93 2020.93 0.00
2651 4,040 T 2023.37 2023.37 0.00 2023.38 2023.38 0.00
3126 4,515 U 2026.19 2026.19 0.00 2026.19 2026.19 0.00
Proposed or Post-Project Conditions Models
The Proposed Conditions models (PCM) reflect the construction of the infiltration facilities
along the Golf Course Overflow Reach and the Unnamed Tributary. The post-project conditions
model was developed by making the following modifications to the Existing Conditions models:
Golf Course Overflow:
As the proposed infiltration facility will intercept all flow up through and including the
0.2% annual-chance-flood, the model was shortened and assigned new stationing based
WEST Consultants, Inc. 31 CLOMR for Proposed Painted Hills Development
on Thorpe Road as the new downstream limit reference point (Station 0). Thorpe Road
is the location at which the flow path is considered to enter the flood control facility.
Table 13 provides a comparison of existing and revised model and FEMA stationing.
Existing cross sections north (downstream) of Thorpe Road were removed. This
includes FEMA XS A (RS 0) through F (RS 1600). This corresponds with effective
RAS cross sections 20,779 through 21,481. Two cross sections were added downstream
of Thorpe Road in order to represent the proposed concrete open channel between
Thorpe Road and the entrance to the culverts that carry north along the west side of S
Madison Rd to the proposed bioswale and infiltration facility. It should be noted that
the RAS model changes were not used to design the open channel, culverts, or any other
components of the flood control facility (anything north of Thorpe Road). These
components were added to the RAS model based on design plans provided by WCE.
The model geometry was modified to include the replacement of existing culverts under
Thorpe Road with a concrete box culvert with a 3 foot rise and 25 foot effective span,
perpendicular to flow.
Downstream boundary conditions at the entrance to the Madison Road culverts were
assigned as known water surface elevations based on an XP SWMM model used to
model the Madison Road culverts.
Floodway stations upstream of Thorpe Rd were maintained at a generally similar width
to the effective FIS; however, since the current/proposed culvert location differs in
location from the existing overtopping point of Thorpe Road in the effective FIS, the
floodway location was shifted for cross sections 964 through -41 to align with the
current centerline of the overflow path.
Unnamed Tributary:
The Proposed Conditions models (PCM) reflect the construction of the infiltration facility within
SA6 and the conversion of Gustin Ditch to a culvert. The post-project conditions model was
developed by making the following modifications to the ECM:
With downstream levee condition
The model was truncated at SA6. Since the infiltration facility will contain all flows up to
and including the 0.2% annual-chance flood, the main channel of the tributary no longer
continues to Painted Hills. Cross sections between SA6 and Highway 27 were removed
and replaced with the cross sections needed to model the proposed culvert. The new
downstream most river station in the RAS model is is 548. It should be noted that it
appears FEMA shifted the stationing of the FIS maps products on this reach, so they do
not correspond directly to the original RAS model stationing. Table 14 provides a
comparison of existing and revised model and FEMA stationing.
Gustin Ditch between the current Highway 27 outlet and the entrance to SA1 was
converted to a culvert and the cross sections along this reach were removed.
WEST Consultants, Inc. 32 CLOMR for Proposed Painted Hills Development
No PCM was developed for the Without Downstream Levee condition since that scenario
no longer exists as the leveed reach will be within a culvert and a without-levee scenario
is no longer required.
Existing vs Proposed Conditions model results for the 1% annual chance flood event and the
floodway are summarized in Table 13 and Table 14. A comparison of the floodplain widths is
provided in Table 15 and Table 16. As seen in Table 13 and Table 14 the proposed project will
not cause any increase in water surface elevations for the 1% annual chance base flood or
floodway elevations. Although a levee failure scenario was not conducted for the levee south of
Thorpe for the effective FIS, current FEMA standards require that the failure scenario be
considered for analysis and remapping of this area. Accordingly, the final revised BFEs and
mapped floodplain downstream of XS 528 on the Golf Course Overflow depict this scenario. A
comparison of with- and without-levee scenario base flood elevations is provided in Table 17.
Although the levee breach scenario contributes significantly more flow to this reach, the
proposed water surface elevations are still lower than the effective FIS due to the flood control
improvements, with the exception of revised cross section 964, F (effective XS I). At this cross
section the CLOMR will result in the published based flood elevation increasing by 0.1 feet.
This is not due to the proposed project (which only serves to lower flood elevations), but is
strictly artifact of the revised hydraulic model having additional cross sections and thus more
detail than the FIS model. Additionally, the published base flood elevation for XS 2100 on the
Unnamed Tributary will increase by 0.1 feet. Again, this is not due to the proposed project but is
a function of the correction made to the Highway 27 culvert outlet elevation, in the revised
hydraulic model. Post-Project Conditions model output is provided in Appendix E.
The proposed flood control facilities would have the following impacts:
Golf Course Overflow
There will be no increase in base flood or floodway elevations due to the proposed
project. Proposed conditions flood elevations will either remain unchanged or be
reduced when compared to existing conditions. Reductions will be between 0.0 and 1.1
feet.
SA1 would be completely removed from the 1% annual chance floodplain. This includes
an area of approximately 94 acres.
The portion of the floodway north of Thorpe Rd along the Golf Course Overflow reach
would be removed from the floodplain mapping.
The floodway would be 3 feet wider at cross section 964 on the revised FIRM panel. This
change is not due to the proposed project but is an artifact of the revised hydraulic
model having additional cross sections and thus more detail than the FIS model.
The BFE at XS 964 will be increased by 0.1 feet on the revised FIRM panel. This change
is not due to the proposed project. The proposed project will cause no adverse impact to
water surface elevations.
WEST Consultants, Inc. 33 CLOMR for Proposed Painted Hills Development
Unnamed Tributary
The 1% annual chance floodplain will effectively be removed for the areas downstream
of Highway 27. This includes Gustin Ditch, all of the left overbank flow path (without
DS levee condition) and the floodplain downstream of SA6 between the pre-project SA6
outlet and the Left Overbank Flow path.
The floodway downstream of SA6 and along the left overbank would be removed from
the mapping.
WEST Consultants, Inc. 34 CLOMR for Proposed Painted Hills Development
Table 13. ECM and PCM model results for the Golf Course Overflow
Effective Revised 1% Annual Chance Flood Event Floodway
RAS
Station
FEMA
Station
XS
Letter
RAS
Station
FEMA
Station
XS
Letter
ECM
(FT
NAVD)
PCM
(FT
NAVD)
Differen
ce (FT)
ECM
(FT
NAVD)
PCM
(FT
NAVD)
Differen
ce (FT)
20779 0 A
2 ‐‐ ‐‐ ‐‐ 2008.05 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
21013 773 B
2 ‐‐ ‐‐ ‐‐ 2007.79 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
21128 961 C
2 ‐‐ ‐‐ ‐‐ 2008.60 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
21229 1145 D
2 ‐‐ ‐‐ ‐‐ 2008.87 ‐‐ ‐‐ 2009.59 ‐‐ ‐‐
21385 1425 E
2 ‐‐ ‐‐ ‐‐ 2009.12 ‐‐ ‐‐ 2010.03 ‐‐ ‐‐
21409 ‐‐ ‐‐
2 ‐‐ ‐‐ ‐‐ 2009.17 ‐‐ ‐‐ 2010.12 ‐‐ ‐‐
21431 ‐‐ ‐‐
2 ‐‐ ‐‐ ‐‐ 2009.23 ‐‐ ‐‐ 2010.23 ‐‐ ‐‐
21445 1600 F
2 ‐‐ ‐‐ ‐‐ 2009.31 ‐‐ ‐‐ 2010.29 ‐‐ ‐‐
21456 ‐‐ ‐‐
2 ‐‐ ‐‐ ‐‐ 2013.14 ‐‐ ‐‐ 2013.79 ‐‐ ‐‐
21481 ‐‐ ‐‐
2 ‐‐ ‐‐ ‐‐ 2013.27 ‐‐ ‐‐ 2014.14 ‐‐ ‐‐
‐‐ ‐‐ ‐‐ ‐41 ‐41 ‐‐
1 ‐‐ 2009.49 ‐‐ ‐‐ 2011.05 ‐‐
‐‐ ‐‐ ‐‐ ‐32 ‐32 ‐‐
1 ‐‐ 2009.49 ‐‐ ‐‐ 2011.05 ‐‐
21515 ‐‐ ‐‐ 23 23 ‐‐
1 2013.27 2009.51 ‐3.76 2014.15 2011.06 ‐3.09
21525 ‐‐ ‐‐ 36 36 A 2013.28 2012.20 ‐1.08 2014.15 2012.24 ‐1.91
21609 1800 G 178 178 B 2013.28 2012.63 ‐0.65 2014.19 2013.09 ‐1.10
21726 ‐‐ ‐‐ 276 276 C 2013.28 2012.67 ‐0.62 2014.21 2013.20 ‐1.01
21857 2123 H 402 402 D 2013.30 2012.82 ‐0.48 2014.24 2013.39 ‐0.84
21983 ‐‐ ‐‐ 528 528 E 2013.33 2013.09 ‐0.24 2014.27 2013.59 ‐0.68
22423 2704 I 964 964 F 2013.64 2013.62 ‐0.02 2014.45 2014.11 ‐0.34
22972 3144 J 1404 1404 G 2014.60 2014.60 0.00 2015.27 2015.27 0.00
23005 ‐‐ ‐‐ 1489 1489 H 2015.28 2015.28 0.00 2016.10 2016.10 0.00
23050 3287 K 1547 1547 I 2015.39 2015.39 0.00 2016.21 2016.21 0.00
23090 3387 L 1647 1647 J 2015.65 2015.65 0.00 2016.61 2016.61 0.00
23446 3721 M 1981 1981 K 2018.10 2018.10 0.00 2019.02 2019.02 0.00
23887 4318 N 2578 2578 L 2022.99 2022.99 0.00 2023.79 2023.79 0.00
24430 ‐‐ ‐‐ 2863 2863 ‐‐ 2029.56 2029.56 0.00 2029.72 2029.72 0.00
1 New XS added to PCM
2 Removed XS from PCM
WEST Consultants, Inc. 35 CLOMR for Proposed Painted Hills Development
Table 14. ECM and PCM model results for the Unnamed Tributary
RAS Station FEMA
Station XS Letter
1% Annual Chance Flood Event Floodway
ECM
(FT NAVD)
PCM
(FT NAVD) Difference (FT) ECM
(FT NAVD)
PCM
(FT NAVD) Difference (FT)
‐1303 0 A 2 2008.05 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
‐1019 283 B
2 2008.36 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
‐880 422 C
2 2008.42 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
‐466 836 D
2 2008.42 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
‐89 910 E 2 2008.42 ‐‐ ‐‐ 2009.05 ‐‐ ‐‐
1 1,378 F
2 2009.70 ‐‐ ‐‐ 2010.70 ‐‐ ‐‐
149 1,525 G
2 2009.70 ‐‐ ‐‐ 2010.70 ‐‐ ‐‐
343 1,720 H
2 2009.70 ‐‐ ‐‐ 2010.70 ‐‐ ‐‐
383 1,760 I
2 2009.70 ‐‐ ‐‐ 2010.70 ‐‐ ‐‐
472 1,849 J
2 2009.70 ‐‐ ‐‐ 2010.70 ‐‐ ‐‐
548 ‐‐ ‐‐
1 ‐‐ 1991.95 ‐‐ ‐‐ 1992.10 ‐‐
576 1,952 K 2009.95 ‐‐ ‐‐ 2010.67 ‐‐ ‐‐
651 2,028 L 2011.13 ‐‐ ‐‐ 2011.07 ‐‐ ‐‐
665 ‐‐ ‐‐
1 ‐‐ 1999.23 ‐‐ ‐‐ 1999.23 ‐‐
918 2,295 M
2 2011.63 ‐‐ ‐‐ 2011.63 ‐‐ ‐‐
1472 2,849 N
2 2012.72 ‐‐ ‐‐ 2012.71 ‐‐ ‐‐
1510 ‐‐ ‐‐
2 2013.26 ‐‐ ‐‐ 2013.26 ‐‐ ‐‐
1528 2,905 O
2 2014.30 ‐‐ ‐‐ 2014.31 ‐‐ ‐‐
1557 2,933 P
2 2014.32 ‐‐ ‐‐ 2014.32 ‐‐ ‐‐
1963 3,339 Q
2 2019.66 ‐‐ ‐‐ 2019.63 ‐‐ ‐‐
1989 ‐‐ ‐‐ 2020.06 2019.11 ‐0.95 2020.08 2019.11 ‐0.97
2080 ‐‐ ‐‐ 2020.65 2020.65 0.00 2020.65 2020.65 0.00
2100 3,485 R 2020.86 2020.86 0.00 2020.86 2020.86 0.00
2110 ‐‐ ‐‐ 2020.90 2020.90 0.00 2020.90 2020.90 0.00
2120 3,509 S 2020.93 2020.93 0.00 2020.93 2020.93 0.00
2651 4,040 T 2023.37 2023.37 0.00 2023.38 2023.38 0.00
3126 4,515 U 2026.19 2026.19 0.00 2026.19 2026.19 0.00
1 New XS added to PCM
2 Removed XS from PCM
WEST Consultants, Inc. 36 CLOMR for Proposed Painted Hills Development
Table 15. Change in Top Width for Golf Course Overflow
Effective Revised Top Width Base Flood (ft) Top Width Floodway (ft)
RAS
Station
FEMA
Station
XS
Letter
RAS
Station
FEMA
Station
XS
Letter
ECM
(FT)
PCM
(FT)
Differen
ce (FT)
ECM
(FT)
PCM
(FT)
Differen
ce (FT)
20779 0 A
2 ‐‐ ‐‐ ‐‐ 1985 0 ‐1985 ‐‐ ‐‐ ‐‐
21013 773 B
2 ‐‐ ‐‐ ‐‐ 395 0 ‐395 ‐‐ ‐‐ ‐‐
21128 961 C
2 ‐‐ ‐‐ ‐‐ 443 0 ‐443 ‐‐ ‐‐ ‐‐
21229 1145 D
2 ‐‐ ‐‐ ‐‐ 546 0 ‐546 35 0 ‐35
21385 1425 E
2 ‐‐ ‐‐ ‐‐ 286 0 ‐286 30 0 ‐30
21409 ‐‐ ‐‐
2 ‐‐ ‐‐ ‐‐ 284 0 ‐284 25 0 ‐25
21431 ‐‐ ‐‐
2 ‐‐ ‐‐ ‐‐ 223 0 ‐223 25 0 ‐25
21445 1600 F
2 ‐‐ ‐‐ ‐‐ 143 0 ‐143 20 0 ‐20
21456 ‐‐ ‐‐
2 ‐‐ ‐‐ ‐‐ 115 0 ‐115 30 0 ‐30
21481 ‐‐ ‐‐
2 ‐‐ ‐‐ ‐‐ 640 0 ‐640 45 0 ‐45
‐‐ ‐‐ ‐‐ ‐41 ‐41 ‐‐
1 ‐‐ 24 ‐‐ ‐‐ 24 ‐‐
‐‐ ‐‐ ‐‐ ‐32 ‐32 ‐‐
1 ‐‐ 24 ‐‐ ‐‐ 24 ‐‐
21515 ‐‐ ‐‐ 23 23 ‐‐
1 521 29 ‐492 29 29 0
21525 1 ‐‐ ‐‐ 36 36 A 521 133 ‐388 29 29 0
21609 1800 G 178 178 B 484 439 ‐45 44 44 0
21726 ‐‐ ‐‐ 276 276 C 364 245 ‐119 44 44 0
21857 2123 H 402 402 D 250 204 ‐46 43 43 0
21983 ‐‐ ‐‐ 528 528 E 180 130 ‐50 43 43 0
22423 2704 I 964 964 F 203 202 ‐1 40 43 3
22972 3144 J 1404 1404 G 54 54 0 14 14 0
23005 ‐‐ ‐‐ 1489 1489 H 136 136 0 27 27 0
23050 3287 K 1547 1547 I 152 152 0 22 22 0
23090 3387 L 1647 1647 J 155 155 0 20 20 0
23446 3721 M 1981 1981 K 208 208 0 20 20 0
23887 4318 N 2578 2578 L 160 160 0 20 20 0
24430 ‐‐ ‐‐ 2863 2863 ‐‐ 37 37 0 20 20 0
1 XS 21609 in DEM model
2 Removed XS from PCM
WEST Consultants, Inc. 37 CLOMR for Proposed Painted Hills Development
Table 16. Change in top width for Unnamed Tributary
RAS Station FEMA
Station XS Letter
Top Width Base Flood (ft) Top Width Floodway (ft)
ECM (FT) PCM (FT)
Difference
(FT) ECM (FT) PCM (FT)
Difference
(FT)
‐1303 0 A
2 104 0 ‐104 31 0 ‐31
‐1019 283 B
2 48 0 ‐48 21 0 ‐21
‐880 422 C
2 362 0 ‐362 40 0 ‐40
‐466 836 D
2 389 0 ‐389 46 0 ‐46
‐89 910 E
2 346 0 ‐346 40 0 ‐40
1 1,378 F
2 102 0 ‐102 117 0 ‐117
149 1,525 G
2 176 0 ‐176 180 0 ‐180
343 1,720 H
2 267 0 ‐267 271 0 ‐271
383 1,760 I
2 280 0 ‐280 283 0 ‐283
472 1,849 J
2 193 0 ‐193 9 0 ‐9
576 1,952 K 6 0 ‐6 5 0 ‐5
651 2,028 L 64 0 ‐64 16 0 ‐16
918 2,295 M
2 43 0 ‐43 8 0 ‐8
1472 2,849 N
2 5 0 ‐5 5 0 ‐5
1528 2,905 O
2 10 0 ‐10 10 0 ‐10
1557 2,933 P
2 8 0 ‐8 8 0 ‐8
1963 3,339 Q
2 15 0 ‐15 9 0 ‐9
1989 ‐‐ ‐‐ 48 0 ‐48 9 0 ‐9
2080 ‐‐ ‐‐ 16 16 0 15 15 0
2100 3,485 R 15 15 0 15 15 0
2110 ‐‐ ‐‐ 16 16 0 15 15 0
2120 3,509 S 16 16 0 15 15 0
2651 4,040 T 7 7 0 7 7 0
3126 4,515 U 10 10 0 10 10 0
WEST Consultants, Inc. 38 CLOMR for Proposed Painted Hills Development
Table 17. PCM model with- and without-levee results comparison for Golf Course Reach
Effective Revised 1% Annual Chance Flood Event
RAS Station FEMA
Station XS Letter RAS Station
FEMA
Station XS Letter PCM With
Levee
PCM
Without
Levee
Difference
(FT)
‐‐ ‐‐ ‐‐ ‐41 ‐41 ‐‐
1 2009.49 2010.12 0.63
‐‐ ‐‐ ‐‐ ‐32 ‐32 ‐‐
1 2009.49 2010.12 0.63
21515 ‐‐ ‐‐ 23 23 ‐‐
1 2009.51 2010.14 0.63
21525 ‐‐ ‐‐ 36 36 A 2012.20 2012.31 0.11
21609 1800 G 178 178 B 2012.63 2012.80 0.17
21726 ‐‐ ‐‐ 276 276 C 2012.67 2012.86 0.19
21857 2123 H 402 402 D 2012.82 2013.11 0.29
21983 ‐‐ ‐‐ 528 528 E 2013.09 2013.33 0.24
22423 2704 I 964 964 F 2013.62 2013.64 0.02
22972 3144 J 1404 1404 G 2014.60 2014.60 0.00
23005 ‐‐ ‐‐ 1489 1489 H 2015.28 2015.28 0.00
23050 3287 K 1547 1547 I 2015.39 2015.39 0.00
23090 3387 L 1647 1647 J 2015.65 2015.65 0.00
23446 3721 M 1981 1981 K 2018.10 2018.10 0.00
23887 4318 N 2578 2578 L 2022.99 2022.99 0.00
24430 ‐‐ ‐‐ 2863 2863 ‐‐ 2029.56 2029.56 0.00
FLOODPLAIN MAPPING
Work maps showing existing and proposed floodplain mapping is provided in Appendix H. An
annotated FIRM showing existing and proposed floodplain mapping is provided in Appendix G
along with revised flood profiles and floodway data tables. The proposed floodplain mapping
reflects the proposed flood control improvements. Since portions of the main channel of Chester
Creek abut an uncertified levee within the project area, and since Dishman Mica Road is acting
as a non-levee embankment for a short stretch of channel under the without-downstream levee
(Trailer Park levee) scenario downstream of Dishman-Mica Road, some portions of the project
site between the fill and the main channel of Chester Creek have been mapped as 1% annual
chance floodplain per the BFE’s in the effective FIS.
CERTIFICATION FORMS
Completed FEMA MT-2 forms are included in Appendix A. Supporting documentation that
includes a copy of public notices and the Biological Opinion are provided in Appendix K.
WEST Consultants, Inc. 39 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, Spokane County, and FEMA. The revised hydraulic models and mapping
products reflect the proposed fill and construction of two infiltration and storage facilities and
associated infrastructure. The results of the analysis indicate the proposed development will
provide significant flood protection for the surround area, will not cause any increase in base
flood or floodway elevations when compared to existing conditions, and will significantly reduce
the floodplain extent 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
APPENDIX A
FEMA FORMS
APPENDIX B
DUPLICATE EFFECTIVE MODEL
APPENDIX C
CORRECTIVE EFFECTIVE HEC-RAS MODEL
APPENDIX D
EXISTING CONDITIONS HEC-RAS MODEL
APPENDIX E
POST-PROJECT CONDITIONS HEC-RAS MODEL
APPENDIX F
EFFECTIVE FLOOD INSURANCE RATE MAP (FIRM),
FLOOD PROFILE, AND FLOODWAY DATA TABLE
APPENDIX G
REVISED FLOOD PROFILES AND
FLOODWAY DATA TABLES
APPENDIX H
FLOODPLAIN WORK MAPS
APPENDIX I
GEOTECHNICAL EVALUATION/GROUNDWATER
ANALYSIS
REPORT 1: PRELIMINARY GEOTECHNICAL EVALUATION, PHASE 1
REPORT 2: GEOTECHNICAL EVALUATION, PROPOSED STORMWATER POND
REPORT 3: GEOTECHNICAL EVALUATION, PHASE 2
REPORT 4: SUPPLEMENTAL GEOTECHNICAL EVALUATION
REPORT 5: FULL-SCALE DRYWELL TESTING
APPENDIX J
INFILTRATION FACILITIES DESIGN REPORT, SITE
PLANS, O&M PLANS
NOTE: O&M PLANS AWAITING COMPLETION PER JURISDICTION COMMENTS
APPENDIX K
REQUIRED SUPPORTING DOCUMENTATION