HomeMy WebLinkAboutRS_Babu BFE DeterminationWEST Consultants, Inc. 1 Babu BFE Determination
Technical Memorandum
WEST Consultants, Inc.
11400 SE 8th St
Ste 410
Bellevue, WA 98004
(425) 646-8806
www.westconsultants.com
To: Ferose Babu
CC: Erynn Sullivan, Harmsen Engineering
Date: February 2, 2025
From: J.Paul Rinehimer, PhD; Andreas Kammereck, PE
Subject: BFE Determination for LOMR-F Shoreline Exemption Minor Fill
Mr. Babu is applying for a LOMR-F and Shoreline Exemption for Minor Fill for his property at 15022 135th
Ave SE, Renton, WA 98058. The project site is located within the FEMA Special Flood Hazard Area
(SFHA) in both Zone AE (riverine flooding) and Zone AO (1 ft ponding) (Figure 1). As the site straddles two
different SFHA Zones, there is inconsistency between the Base Flood Elevations (BFEs) for each zone.
Harmsen Engineering determined that the controlling BFE for the site should be based off the Zone AE
BFE in a memo dated August 19, 2025. After reviewing the effective Flood Insurance Study (FIS) model
this memo confirms the findings by Harmsen that the Zone AE BFE is the controlling water surface
elevation.
The project site lies within two different mapped flood zones: Zone AO and Zone AE. Zone AE delineates
flooding associated with the Cedar River with a 1% Annual Chance of Exceedance (ACE) (i.e. 100-year)
and BFEs determined from detailed modeling. The majority of the structural area on the property is located
in Zone AE. By interpolating the Zone AE cross sections BFEs from the FIRM, the Harmsen memo
determined a BFE of 77.9 ft NAVD88 at the north side of the Garage.
The west face of the garage is located in Zone AO, however. Zone AO delineates shallow sheet flow or
ponding during the 1% ACE event. The Zone AO zone for this site is mapped with 1 ft depths. Because
Zone AO is not mapped with detailed BFEs, the standard practice to determine a BFE is to add the flood
depth (1 ft) to the highest natural elevation adjacent to the structure, called the Highest Adjacent Grade
(HAG), which would mean the BFE at the western face of the garage is 79 ft NAVD88 according to the
property survey.
Therefore, there is a 1.1 ft discrepancy in the BFE determination on the property. To resolve this
discrepancy, we reviewed the effective FIS model. WEST obtained the effective HEC-RAS model from the
King County River and Floodplain Management Section. The model cross sections and reaches in the
vicinity of the project site are shown in Figure 2. The property is located between the FIRM labeled cross
sections AN (RS 427.3) and AM (RS 414.5). The summary profile table showing the water surface
elevation results for the model is shown in Figure 3. The water surface elevations from the HEC-RAS
model match the BFE elevations listed on the FIRM.
WEST Consultants, Inc. 2 Babu BFE Determination
Reviewing the model geometry shows that the mapped Zone AO to the west of the property lies in a
overflow reach (MW Overflow) of the Cedar River. At this location, the river overflows the right bank and
proceeds through an overflow path to reconnect downstream. When the FIS was developed the main
portion of the river was mapped as a Zone AE, while the overflow path was mapped as a Zone AO likely
due to uncertainty with the actual amount of overflow and flood depths in this area. Note also that the Zone
AO designation is less precise than the Zone AE designation.
The model results list a water surface elevation of 78.4 ft at the upstream end of the overflow path at MW
Overflow RS 16.63. This water surface elevation, however, should not be taken as an accurate
representation of flooding or the BFE at this location. The model uses a lateral structure to connect the
main Cedar River Reach (labeled Cedar Upper Mid) to the MW Overflow path with a 1D cross section at
MW Overflow RS 16.63. As the model is 1D, it is limited to only a single water surface elevation for each
cross section. This forces the modeled water surface elevation at MW Overflow RS 16.63 to be the same
across the entire section and defined by the upstream-most connection to the main channel reach, hence
overpredicting the BFE. This inaccuracy at this location is potentially an additional reason it was mapped as
Zone AO and not Zone AE.
This single water surface elevation does not accurately represent the real-world physical flood conditions.
Physically, the water surface elevations at the crest of the overbank would match the main channel
elevations so as to create one continuous surface from the main channel through to the overflow path. The
main channel elevations are represented by the more accurate Zone AE BFEs and therefore they should
be the controlling elevations for this MT-1 application, as developed in the Harmsen memo.
WEST Consultants, Inc. 3 Babu BFE Determination
Figure 1. FEMA FIRMette of property Pin shows site location.
WEST Consultants, Inc. 4 Babu BFE Determination
Figure 2. RASMapper Geometry and results for the 100-year water surface elevation.
Figure 3. HEC-RAS Summary Output Table Results
FIRM Section AN 78.9’
FIRM Section AM 77.6’
FIRM Section AN 74.4’
Project
Location