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WATERSHED SCIENCE & ENGINEERING · 506 2nd Ave, Suite 2700, Seattle, WA 98104 · 206-521-3000
Memorandum
To: Amanda Pierce, City of Renton
From: Kaleb Madsen, Watershed Science & Engineering
Mark Ewbank, P.E., Herrera Environmental Consultants, Inc.
Date: March 10, 2021
Re: Madsen Creek Flooding Improvement Project—Summary of Hydraulic Modeling for
Design and Permitting of Flood Control Improvements and Evaluation of Floodplain Fill
Mitigation
INTRODUCTION
Watershed Science & Engineering and their subconsultant Herrera Environmental Consultants (Herrera)
was retained by the City of Renton to investigate causes of flooding and propose design solutions to
reduce flooding along the lower Madsen Creek network. An existing conditions report was developed in
2019 identifying the sources of flooding and provided several potential solutions to mitigate 100-year
flooding. The City chose multiple solutions at four sites for design and construction. The improvements
include constructing berms and a small floodwall along the low flow channel, constructing a berm along
the high flow bypass channel, and the removal of sediment accumulation in the high flow bypass channel
to improve flood conveyance capacity.
The proposed improvements are within regulated critical areas and will require mitigation and permitting.
Hydraulic modeling was done to support the design and permitting of the modifications to the Madsen
Creek network. The modeling and existing conditions also determined the design approach for providing
compensatory flood storage to offset displaced flood storage, due to placement of berm fill in regulated
floodplain areas.
PROPOSED DESIGN PLANS AND INFORMATION NEEDED FOR PERMITTING
The project proposes several berms and a small floodwall adjacent to existing stream channels to contain
flood flows in three areas (Sites 2, 3, and 4 in the project design plans). WSE utilized the existing hydraulic
model to determine the necessary design for flood control improvements to contain the Madsen Creek
100-year peak flood flow. The model and proposed improvements are not designed to protect against
extreme flooding within the Cedar River.
The flood containment berms and wall are within the Cedar River floodplain and require mitigation. The
berm and floodwall at Site 3, as well as a berm at Site 4, will displace existing flood storage in a regulated
floodplain. The proposed flood containment facilities at Sites 2 and 3 are entirely located within city limits.
The proposed berm at Site 4 will be partially in King County jurisdiction. Therefore, the displaced flood
storage and required mitigation at Site 4 needs to address King County code requirements in addition to
City requirements. The City plans to provide compensatory flood storage (as required by City and County
codes) for all of the berm fill and floodwall impacts in mapped floodplain areas via widening a portion of
the Madsen Creek high flow bypass channel (which extends from the Madsen Creek sediment basin at the
upstream end to the Cedar River at the downstream end). The City also plans to redefine the originally
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constructed (in 1976) high flow bypass channel bed in conjunction with improvements at Site 2, providing
necessary capacity for flood protection. The design plans need to satisfy City engineering standards for
freeboard in the 100-year flood event within the Madsen Creek network, while also serving as a basis for
defining impacts to regulated critical areas for permitting purposes and mitigating those impacts.
The rationale for the proposed design plans is presented below, followed by a discussion of project
impacts and mitigation in regulated floodplain areas. This memo also documents analysis of the area
inundated by the peak 2-year flood flow through the high flow bypass channel in existing conditions as a
surrogate for the ordinary high water mark along that channel, which is essential information needed for
permitting of impacts to wetland and stream buffers.
SEDIMENT REMOVAL NEEDED TO RESTORE ORIGINAL HIGH FLOW BYPASS CHANNEL GEOMETRY
The high flow bypass channel was constructed in 1976 to convey high flows in Madsen Creek directly to
the Cedar River. Figure 1 shows the high flow bypass channel elevation profile with the bottom surface
elevation as surveyed in 2018 and the original (1976) channel bottom elevation profile based upon best
available data. The 1976 design consists of a channel profile and one typical cross-section applicable to
the length of the bypass channel upstream of State Route (SR) 169. In the absence of “as-built” detail, the
City necessarily assumes that this information reflects what was built in 1976. The original design profile
shows the channel tied into the top of the low flow channel (LFC) concrete box culvert (at elevation 96.4),
graded upstream at a slope of 0.28 percent for 395 feet, then at a slope of 2.52 percent from there
upstream to the sediment basin outlet. The LFC culvert was rebuilt by WSDOT in the early 1990s and is
shown in as-built drawings prepared by WSDOT as being 0.1 foot higher (elevation 96.5) than the original
top-of-box elevation but in the same location. The WSDOT as-built drawings also match well between the
original and new location of the high flow bypass culvert under SR 169, as well as in comparison to recent
survey data for the height of the concrete wall on the upstream end of the low flow channel culvert.
Given that the 1976 bypass channel design tied directly into the top of the LFC box culvert, and the top of
the rebuilt LFC culvert was 0.1 foot higher, this comparison assumed that the original design profile of the
high flow bypass channel should be assumed to be 0.1 foot below the top of the existing LFC box culvert
(which is definitively surveyed) and then extended upstream per the original design profile slopes and
typical channel cross-section. A topographic surface model was constructed using these parameters to
define the extents of sediment removal in the bypass channel to restore the 1976 channel design
geometry. This volume of sediment removal is estimated to be 200 cubic yards (54 cubic yards below
100-year floodplain and 146 cubic yards above), and is considered to be maintenance of the channel to
restore it to historical conditions for the purposes of the City’s Hydraulic Project Approval issued by the
Washington Department of Fish and Wildlife specific to the high flow bypass system.
Figure 2 shows a typical cross-section in the midst of Site 2 with the originally constructed (and as
proposed to be restored) channel geometry. It is assumed that the finished grade of the original channel
bed and banks was at the top of a 1-foot-thick layer of riprap shown in the original design cross-section.
ADDITIONAL BYPASS CHANNEL WIDENING FOR COMPENSATORY FLOOD STORAGE
The mapped floodplain area that overlaps with Sites 2 and 3 does not have a defined 100-year flood peak
water surface elevation (it is a “Zone A” flood hazard area on the preliminary Flood Insurance Rate Map
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[FEMA 2017]). It is assumed that the water surface elevation in those areas during the 100-year flood
would be controlled by backwater from the Cedar River floodplain to the north of State Route (SR) 169
(Renton-Maple Valley Road), and thus the mapped 100-year flood elevation (also called the base flood
elevation [BFE]) on the north side of the highway (102.5 feet [NAVD88]) is a reasonable basis for
determining the peak 100-year flood level at Sites 2 and 3.
Mitigation for displaced flood storage associated with berm and floodwall construction at Site 3, and berm
construction at Site 4, requires an additional 64 cubic yards of flood storage below elevation 102.5 feet
and contiguous with the existing Cedar River floodplain straddling SR 169. The high flow bypass channel
is within the same flow conveyance network as Sites 3 and 4, while also being directly connected with the
existing Cedar River backwater floodplain area upstream of SR 169. Adjusting the high flow bypass channel
geometry beyond channel maintenance allows for the creation of additional flood storage capacity to
effectively mitigate for displaced flood storage caused by the project. Deepening of the channel beyond
the 1976 channel bed and banks would result in major changes to the hydraulics of the channel, adjacent
structures, and the City’s existing easement for channel maintenance. Thus, the City’s design team chose
to widen the bypass channel to create the necessary flood storage, reduce impacts, and retain the channel
integrity. The high flow bypass channel will be widened 2.25 feet to the east – while retaining the 1976
channel bed surface elevation profile and re-creating the 1976 side slopes of the channel on both sides. A
total of 125 cubic yards of excavation will occur below elevation 102.5 feet, of which 71 cubic yards is
added flood storage and the remaining 54 cubic yards are maintenance to restore the historic channel
capacity. The 71 cubic yards of added flood storage will be used to compensate for 71 cubic yards of
displaced flood storage at Sites 3 and 4 as documented later in this memorandum.
SITE 2 BERM DESIGN FOR 100-YEAR FLOOD CONTAINMENT
The proposed berm along the east side of the widened high flow bypass channel at Site 2 will be raised to
contain the 100-year peak flood flow with a minimum of 6 inches of freeboard per the City’s design
standards. The 6 inches of freeboard is based on the modeled 100-year Madsen Creek flood flows and not
the 100-year flood inundation depicted on Cedar River FEMA flood insurance rate maps, as the Madsen
Creek flood elevations are higher. The berm is designed to have a low-permeability core of compacted
soil with topsoil for riparian plantings surrounding the core. The top elevation of the berm core is set to
be equivalent to 6 inches above the simulated 100-year peak water surface elevation profile through the
length of Site 2, with the proposed channel geometry modifications accounted for. This will assure that
floodwaters cannot erode the portion of the berm that provides flood containment. The topsoil on the
berm will promote lush vegetation growth, which is needed to offset project impacts to wetland and
stream buffers.
SIMULATED 2-YEAR FLOOD INUNDATION IN THE HIGH FLOW BYPASS CHANNEL
Permitting of the proposed improvements also requires defining the ordinary high water mark (OHWM)
of the stream channels in relation to stream buffer impacts. Herrera delineated the OHWM of the Madsen
Creek low flow channel at Sites 3 and 4 as part of field work to document existing environmental critical
areas in 2018. The OHWM was not delineated in the high flow bypass channel since there is no “ordinary”
flow within channel that create visual indicators of the OWHM on the channel banks. However, permitting
of the work proposed at Site 2 requires analysis of stream buffer impacts and the OHWM in a stream
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channel is the City’s standard basis for assessing such impacts. The hydraulic model was also used as a
basis for defining the OHWM at Site 2, using the simulated 2-year flood inundation area as a surrogate for
the OHWM. The 2-year flood elevation throughout the bypass channel was used as an acceptable
determination of the OHWM based upon the Department of Ecology’s Determining the Ordinary High
Water Mark for Shoreline Management Act Compliance in Washington State (2016). Figure 3 presents
existing conditions model results for the 2-year flood event as a surrogate for the OHWM, used for
calculating stream buffer impacts.
HYDRAULIC MODELING SUMMARY FOR DESIGN OF SITES 2 AND 3
The Madsen Creek hydraulic model is a 2-dimensional, unsteady flow, HEC-RAS model. It was constructed
using LiDAR and survey topography. Hydrology was developed using precipitation records, a stream gage
record, and a numerical hydrologic simulation. The original hydraulic model was based on the existing
conditions survey, but it was modified to simulate various design improvements throughout the system.
The model was used to determine the height and length of the proposed flood containment berm on the
east side of the high flow bypass channel at Site 2 and the dimensions of a floodwall and flood
containment berm on the south side of the low flow channel at Site 3. The flood improvements at Sites 2
and 3 are designed to contain 100-year flood flows with sufficient freeboard to satisfy City of Renton
design requirements for flood containment performance.
FLOOD STORAGE DISPLACEMENT AND MITIGATION ASSOCIATED WITH SITE 2
A proposed design surface, representing the high flow bypass channel after sediment removal, and
adjacent berm installation at Site 2, was input into the hydraulic model to evaluate performance. Iterative
adjustments in the design were tested with the model to yield the optimum design configuration. The
final design surface was then modeled for the 100-year recurrence flood event peak flow to yield a water
surface elevation profile for use in confirming the top elevation of the low permeability berm core fill
needed for sufficient freeboard. The model also confirmed that berming the east side of the bypass
channel will reduce the probability that 100-year flood flows will overtop the existing maintenance access
driveway on the west side of the channel.
As stated previously, a peak 100-year flood elevation of 102.5 feet (NAVD88) was used to calculate the
volume of fill in the regulated floodplain at Site 2. The berm fill to be placed above existing ground at
Site 2 is all above elevation 102.5 feet, and therefore berm construction at this site will not displace any
existing flood storage.
FLOOD STORAGE DISPLACEMENT AND MITIGATION ASSOCIATED WITH SITE 3
The design for Site 3 includes a small berm on the Wonderland Estates property adjacent to the Madsen
Creek low flow channel, and a small floodwall extending to the east across City-owned land in front of
(north of) a single-family residential property with the same top elevation as the berm. The berm will tie
into high ground at the west end where the low flow channel enters a culvert under SR 169. The west end
of the floodwall will tie into the east end of the berm. The east end of the floodwall will tie into high
ground near the outlet end of the culvert that conveys the Madsen Creek low flow channel beneath the
high flow bypass channel.
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Figure 4 shows a cross-section of the Madsen Creek low flow channel and the proposed berm on the left
bank side of the channel at Site 3. Calculating the volume of flood storage displaced by the floodwall and
the berm fill below the 100-year flood elevation used the same backwater base flood elevation of
102.5 feet (NAVD88) as at Site 2. This is because the high flow bypass and low flow channels are currently
hydraulically linked in the 100-year flood backwater pool that forms upstream of SR 169. The volume of
proposed berm fill above existing ground level (which ranges in elevation from 101 to 102 feet) and below
elevation 102.5 feet will be 30.5 cubic yards at Site 3. The volume of flood storage displaced by a small
concrete masonry unit floodwall equates to 8 inches wide in plan view and 90 feet in total length. This
type of thin concrete wall will displace approximately 2.5 cubic yards of existing flood storage below
elevation 102.5 feet. Thus, the total floodplain fill at Site 3 is estimated to be 33 cubic yards. Mitigation
for that displaced flood storage will occur via widening beyond the as-built high flow bypass channel
geometry at Site 2 as described above.
FLOOD STORAGE DISPLACEMENT AND MITIGATION ASSOCIATED WITH SITE 4
Figure 5 shows a cross-section of the Madsen Creek low flow channel and the proposed berm on the right
bank side of the channel at Site 4. The 100-year flood elevation used to calculate fill proposed in the
floodplain at this site is 102.0 feet (NAVD88), taken from FEMA (2017). The berm at Site 4 is not meant to
provide flood protection for the private residence north of the Madsen Creek low flow channel when the
Cedar River is in a 100-year flood condition, as that residence and others to the north of it are within the
river’s floodplain. The top of the berm is designed at an elevation that would be completely submerged
in a 100-year flood event in the river. The berm is meant to reduce the risk of flooding during a 100-year
event or less within Madsen Creek. The net volume of berm fill placement, minus the removed volume of
existing concrete blocks and soil below elevation 102.0 feet, is estimated to be approximately 38 cubic
yards. The average existing ground elevation where the berm will be placed ranges from 99 to 101 feet.
Mitigation for that displaced flood storage will occur via widening the high flow bypass channel at Site 2
as described above.
SUMMARY OF FLOOD STORAGE DISPLACEMENT AND MITIGATION
Table 1 lists the amount of displaced flood storage at each of Sites 2, 3, and 4, and the volume of created
flood storage at Site 2 within the high flow bypass channel.
Table 1. Flood Storage Displacement and Mitigation.
Displaced Flood Storage Flood Storage Created
Site 2: 0 cubic yards Site 2: 71 cubic yards
Site 3: 33 cubic yards Site 3: none
Site 4: 38 cubic yards Site 4: none
Total: 71 cubic yards Total: 71 cubic yards
FLOOD HAZARD CERTIFICATION
The portion of the project that is within King County jurisdiction requires a flood hazard certification,
specifically for berm construction within the mapped floodplain at Site 4. King County requires a flood
hazard certification be prepared by a licensed professional engineer as a condition of floodplain permit
approval. Attachment A to this memorandum contains the flood hazard certification form, based upon
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the information presented in the body of this memorandum and the floodplain shown on Figure 3 in the
Mitigation Plan report to which this memorandum is appended.
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REFERENCES
Anderson, P.S.; S. Meyer; P. Olson; and E. Stockdale. 2016. Determining the Ordinary High Water Mark for
Shoreline Management Act Compliance in Washington State. Ecology. Publication 1606029. Washington
State Department of Ecology.
FEMA. 2017. Preliminary Flood Insurance Rate Map Number 53033C0984G. US Department of Homeland
Security, Federal Emergency Management Agency. September 15.
WSDOT. 1993. SR 169 196th Ave. S.E. / Jones Road to Maplewood as-built design plans. Washington State
Department of Transportation.
WSE. 2019. Final Lower Madsen Creek Existing Conditions Flood & Sediment Assessment. Prepared for
the City of Renton Public Works. Watershed Science & Engineering, Seattle, Washington. March 20.
Page | 9
ATTACHMENT A–
FLOOD HAZARD CERTIFICATION
Project Name: ______________________________________________________________
Parcel Number(s): ______________________ DPER Permit Number:__________________
This form is current as of August 29, 2013.
Section A.1 (to be completed by applicant or applicant’s engineer)
Site Location Within Floodplain
The proposed development site lies at least partially within the King County regulatory floodplain based on review and determination
from any of the following sources:
FEMA Flood Insurance Rate Map (FIRM): Panel #____________________ Panel date:____________
Special Study as required by section 4.4.2 of the King County Surface Water Design Manual
Other: (please note source)________________________________________________________________
Zero-Rise Analysis
Based on section 21A.24.250 of the King County Code, and section 4.4.2 of the King County Surface Water Design Manual,
the proposed development cannot create a measurable change to the water surface elevation or energy grade line for the 100-
year flood event (base flood elevation). This is to be determined and certified by a registered professional engineer using
standard methods and practices accepted by the King County Department of Natural Resources and Parks (DNRP) and will
be referred to as a “zero-rise analysis”.
Based on a review of the potential impacts of this project, a “zero-rise analysis”:
Is required. Completion of Section B of this form by a professional engineer licensed in the State of Washington is a
condition of the issuance of this permit.
Is not required for the following reasons:
Elevating or improvement to an existing structure without increasing the foundation footprint of the structure.
Post and pier foundation system with no significant impedance to flow.
Coastal “A”, “VE”, “AE” zone.
Shallow flooding area (AO/AH zone) not adjacent to a riverine system. (Explain)
__________________________________________________________________________________
Ineffective flow area. (Explain)
__________________________________________________________________________________
Proposed project lies within a hydraulic shadow. (explain)
__________________________________________________________________________________
Other (explain)
__________________________________________________________________________________
King County Flood Hazard Certification
The goal of Section A.1 is to identify the type and location of the flood hazards on the project parcel and identify study
requirements. If the proposed project does not meet the exemptions listed under Zero-Rise Analysis, Compensatory Storage
Analysis, or Base Flood Depth and Base Flood Velocity Analysis, then you may need to do the specific analysis. If there are
flooding issues on the project parcel, but they are not mapped, then you may need to do the specific analysis.
Find FEMA Map information at http://www.kingcounty.gov/environment/waterandland/flooding/maps.aspx or go directly to
FEMA’s site https://msc.fema.gov
Madsen Creek Flooding Improvements Project
2323059070
X 0984G 9/15/2017
X
Compensatory Storage Analysis:
Based on section 21A.24.240 of the King County Code, the proposed development cannot reduce the effective base flood
storage volume of the floodplain, and must provide compensatory storage if grading or other activity displaces any effective
flood storage volume. This is to be determined and certified by a registered professional engineer using standard methods and
practices accepted by the King County Department of Natural Resources and Parks (DNRP) and will be referred to as a
“compensatory storage analysis”.
Based on a review of the potential impacts of this project, a “compensatory storage analysis”:
Is required. Completion of Section B of this form by a professional engineer licensed in the State of Washington is a
condition of the issuance of this permit.
Is not required for the following reasons:
Elevating or improvement to an existing structure without increasing the foundation footprint of the structure.
Post and pier foundation system with no significant reduction in flood storage.
Grading or fill placed within the foundation of an existing residential structure to bri ng the interior foundation grade to
the same level as the lowest adjacent exterior grade.
Other (explain)
__________________________________________________________________________________
Base Flood Depth and Base Flood Velocity Analysis
Based on section 21A.24.240 of the King County Code, development proposals and alterations are not allowed if the base flood
depth exceeds three feet and the base flood velocity exceeds three feet per second. This is to be determined and certified by a
registered professional engineer using standard methods and practices accepted by the King County Department of Natural
Resources and Parks (DNRP) and will be referred to as a “base flood depth and base flood velocity analysis ”.
Based on a review of the potential impacts of this project, a “base flood depth and base flood velocity analysis”:
Is required. Completion of Section B of this form by a professional engineer licensed in the State of Washington is a
condition of the issuance of this permit.
Is not required for the following reasons:
The structure is an agricultural structure and will not be used for human habitation .
Elevating or improvement an existing structure without increasing the foundation footprint of the structure.
Other (explain)
__________________________________________________________________________________
Submitted by: _________________________________________ Date: ____________________________
Applicant or Applicant’s Engineer
Section A.2 (to be completed by the DPER Drainage Engineer when applicable)
DPER Drainage Engineer Certification
(For sites in unmapped flood hazard areas) The development proposal site is not within the unmapped flood hazard area
based on inspection of the site, and therefore further flood hazard review is not required.
The development proposal does not involve any site disturbance, clearing, or grading, and therefore only requires a
permit or approval under K.C.C. chapter 16.04 or 17.04. Further flood hazard review is not required.
Reviewed by: _________________________________________ Date: ____________________________
DPER Drainage Engineer
X
X
X
This is a flood hazard mitigation project that will improve flooding conditions for nearby land
Mark Ewbank, PE 11/12/2020
Section B (to be completed by the applicant’s engineer)
I have considered the hazards represented on Panel __________________ of the Flood Insurance Study for King County, dated
_________________, _______, and the supporting documentation for DPER Permit Number_______________. I have also searched
for and considered all other available information including: Preliminary Flood Insurance Rate Maps (P-FIRMs); Preliminary Flood
Insurance Studies; Draft flood boundary work maps and associated technical reports; Critical areas reports prepared in accordance
with FEMA standards set forth at 44 C.F.R. Part 65 and consistent with the King County Surface Water Design Manual provisions for
floodplain analysis set forth at section 4.4.2; Letter of Map Amendments (LOMAs); Letter of Map Revisions (LOMRs); Channel
migration zone maps and studies; Historical flood hazard information; and Site topography and ground elevations. All sources are
clearly identified in the attached report. In addition, I have created new data where existing sources are not sufficient to assure
compliance, and the attached report clearly documents my methods and assumptions.
I certify that the attached technical data supports the fact that this submitted design will meet requirements for protection of floodplain
storage and floodplain conveyance, as well as base flood depth and base flood velocity requirements, as set forth in King County
Code, Title 21A. Compliance is achieved as described below.
Code
Requirement
Analytical Methodology
(check one or more)
Engineering
Certification
Required?
No impact to 100-year flood
elevations, floodway elevations and
floodway widths (no encroachments
or obstruction of floodwaters). No
reduction in floodplain conveyance
both onsite and on adjacent
properties, during 100-year flood
event (“zero-rise” floodplain).
Hand calculations showing that flood conveyance (K=1.49/n AR2/3) will equal
or exceed existing values at every location.
Yes
HEC-RAS analysis showing that neither the water surface nor the energy grade
will rise by even 0.01 feet at any location when proposed conditions are
compared to existing conditions.
Yes
Other. See attached information.Yes
Compensatory floodplain storage
provided (no net fill).
Volumetric calculations to show that compensatory storage provides equivalent
volume at equivalent elevations to that being displaced, and is hydraulically
connected to the source of flooding. For this purpose, equivalent elevations
means having similar relationship to ordinary high water and to the best
available ten-year, fifty-year and one-hundred-year water surface profiles;
Yes
Other. See attached information.Yes
Base flood depth does not exceed 3
feet or base flood velocity does not
exceed 3 feet per second.
Base flood depth and base flood velocity mapping and data show less than 3
feet depth or less than a velocity of 3 feet per second at the project location.
Yes
Other. See attached information.Yes
Attached are all support data and calculations.
_____________________________________________
Signature
_____________________________________________
Date
_____________________________________________
Name and Title
_____________________________________________
Company
_____________________________________________
Address
_____________________________________________
City, State, Zip
Professional Engineer’s stamp, if methodology requires
certification.
The goal of Section B is to identify and present which analytical methodologies were used to demonstrate compliance with the
King County Code. This section shall be completed by an engineer licensed in the State of Washington when an analysis is
required per Section A.
0984G
X
November 12, 2020
Mark Ewbank, Principal Engineer
Herrera Environmental Consultants
2200 Sixth Avenue, Suite 1100
Seattle, WA 98121-1820
May 16 1995
Section C (to be completed by the DNRP, RFMS engineer)
Based on a review of the subject development proposal, the River and Floodplain Management Section of the Department of Natural
Resources and Parks determines the following:
No flood hazard analysis is required.
A flood hazard analysis is required and the development proposal meets the zero rise, compensatory storage, and base
flood depth and base flood velocity requirements of King County Code 21A.24.24.240, 21A.24.250, 21A.24.260 and the
King County Surface Water Design Manual Section 4.4.2. This determination does not include a review of the other
flood hazard areas standards in King County Code 21A.24.240, 21A.24.250, 21A.24.260, 21A.24.270, and 21A.24.272.
A flood hazard analysis is required and the development proposal meets the zero rise, compensatory storage, and base
flood depth and base flood velocity requirements of King County Code 21A.24.24.240, 21A.24.250, 21A.24.260 and the
King County Surface Water Design Manual Section 4.4.2; however this approval is with additional comments or
conditions (DNRP, RFMS shall provide comments in an e-mail or another written format to DPER).
A flood hazard analysis is required and the development proposal does not meet the zero rise, compensatory storage, and
base flood depth and base flood velocity requirements of King County Code 21A.24.24.240, 21A.24.250, 21A.24.260
and the King County Surface Water Design Manual Section 4.4.2.
Reason(s) not approved:
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
Reviewed by: _________________________________________ Date: ____________________________
DNRP, RFMS Engineer
Page | 11
FIGURES
STATION1160+500+77ELEVATION IN FEET
STATION96100104108112961001041081120+000+501+001+502+002+50ELEVATION IN FEET
STATION96100104108112961001041081122+503+003+504+004+505+00ELEVATION IN FEET
STATION96100104108112961001041081125+005+506+006+507+007+25O:\proj\Y2018\18-06779-000\CAD\Exhibits\HydraulicReport\Figures.dwgFigure 1.High Flow Bypass ChannelBottom Elevation Profile UnderExisting and ProposedConditions.EXISTING CHANNEL BOTTOMPROPOSED CHANNEL BOTTOMSEDIMENT REMOVAL100-YEAR FLOOD (MODELED, EXISTING CONDITIONS)100-YEAR FLOOD (MODELED, PROPOSED CONDITIONS)
ELEVATION IN FEET96100104108 112961001041081120+000+400+70EASEMENTROAD 8.6'EXISTINGPROPOSED1976 CHANNEL DESIGNCUT BELOW BFEBFE (102.5 FT)100-YR FLOOD (MODEL)COMPACTED FILLTOPSOILO:\proj\Y2018\18-06779-000\CAD\Exhibits\HydraulicReport\Figures.dwgFigure 2.Proposed High Flow BypassDesign Cross-section at Site 2LEGENDEXISTING GROUND SURFACEPROPOSED GROUND SURFACE1976 CHANNEL DESIGN TYP SECTION100-YR FLOODPLAIN ELEVATION
0 80 16040Feet
K:\Projects\Y2018\18-06779-001\Project\GISWorking\HydraulicModelingReprt\Fig3-2-yearHFB.mxd
Figure 3.
Existing Conditions 2-Year Modeling
Results for the Upper High Flow Bypass.E
King County (2019)
Legend
2-Year Water Surface Elevation
High : 110
Low : 100
Cut Grade
Fill Grade
ELEVATION IN FEET
STATION96100104961001040+000+40O:\proj\Y2018\18-06779-000\CAD\Exhibits\HydraulicReport\Figures.dwgFigure 4.Proposed Design Cross Sectionat Site 3.LEGENDEXISTING GROUND SURFACEMAPPED 100-YEAR FLOOD BACKWATERPROPOSED BERM
ELEVATION IN FEET
STATION96100104961001040+000+40O:\proj\Y2018\18-06779-000\CAD\Exhibits\HydraulicReport\Figures.dwgFigure 5.Proposed Design Cross Sectionat Site 4.LEGENDEXISTING GROUND SURFACEMAPPED 100-YEAR FLOOD ELEVATION, 102.0 FTPROPOSED BERM