21-102193-Stormwater Memorandum-06-03-2021-V1
MEMORANDUM
TO: KEVIN PETERSON, CITY OF FEDERAL WAY
FROM: LAURA BARTENHAGEN, PE
RE: REASONABLE USE 359TH NORTH - PARCEL A STORMWATER
DRAINAGE MEMORANDUM
JOB #: 2120-001-020
DATE: JUNE 3, 2021
The purpose of this memorandum is to provide the necessary storm drainage analysis for
the proposed site improvements as associated with the Reasonable Use 359th North –
Parcel A project (King County Parcel No. 2921049091).
The proposed development consists of a single-family residence which requires a
reasonable use permit due to the wetlands present on the property.
The existing project site was regularly managed as a garden and livestock pasture
associated with an adjacent homesite to the east for several years. The project site slopes
from the northeast to the southwest with slopes ranging from 3 to 12 percent. Based on
the attached soils documentation, soils on the project site consist of silt loam which are
not suitable for infiltration
The proposed project consists of adding a building with 1,562 square feet of impervious
area as well as 1,140 square feet of new concrete pavement. Stormwater from the
proposed building roof is proposed to disperse using three splash bocks at a maximum of
700 square feet of area draining to each splash block. Stormwater from the proposed
concrete pavement driveway and walkways will disperse into existing vegetation and
continue to drain in the same direction as pre-developed conditions.
Based on the King County 2016 Surface Water Design Manual as adopted by the City of
Federal Way, this project qualifies for Simplified Drainage Review because it has more
than 2,000 square feet but less than 5,000 square feet of new and replaced impervious
surface and less than 7,000 square feet of land disturbing activity. Under the Simplified
Drainage Review requirements, the project shall apply flow control Best Management
Practices (BMPs) to the developed surfaces to the maximum extent feasible and apply
temporary erosion and sediment control (TESC) measures to the disturbed areas during
construction.
Regarding flow control Best Management Practices (BMPs), due to the soil conditions –
infiltration, bioretention, and permeable pavement are not feasible for the project site. Due
to the already limited footprint of the project as associated with the reasonable use
2
application, reduced impervious surface is also not feasible. Since the existing project site
was used as a garden and pasture, the native growth retention BMP is also not feasible.
The proposed flow control BMP to be used for the project site is dispersion via discharge
from proposed splash blocks as well as sheet flow from proposed concrete pavement.
Furthermore, the proposed site will apply TESC measures to the disturbed area including
marking of clearing limits and installing inlet protection to the existing catch basins.
Additional design details for storm drainage compliance will be provided with the final
engineering plans.
\\esm8\engr\esm-jobs\2120\001\020\document\memo-001.docx
Hydrologic Soil Group—King County Area, Washington
(Parcel A Soils )
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
6/3/2021
Page 1 of 45236545523655452365635236572523658152365905236545523655452365635236572523658152365905236599551515551524551533551542551551551560551569551578551587551596
551515 551524 551533 551542 551551 551560 551569 551578 551587 551596
47° 16' 51'' N 122° 19' 7'' W47° 16' 51'' N122° 19' 3'' W47° 16' 49'' N
122° 19' 7'' W47° 16' 49'' N
122° 19' 3'' WN
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84
0 15 30 60 90
Feet
0 5 10 20 30
Meters
Map Scale: 1:386 if printed on A landscape (11" x 8.5") sheet.
Soil Map may not be valid at this scale.
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Rating Polygons
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Lines
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Points
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: King County Area, Washington
Survey Area Data: Version 16, Jun 4, 2020
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Jul 6, 2020—Jul 20,
2020
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
Hydrologic Soil Group—King County Area, Washington
(Parcel A Soils )
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
6/3/2021
Page 2 of 4
Hydrologic Soil Group
Map unit symbol Map unit name Rating Acres in AOI Percent of AOI
Bh Bellingham silt loam C/D 0.6 76.9%
KpB Kitsap silt loam, 2 to 8
percent slopes
C 0.2 23.1%
Totals for Area of Interest 0.8 100.0%
Description
Hydrologic soil groups are based on estimates of runoff potential. Soils are
assigned to one of four groups according to the rate of water infiltration when the
soils are not protected by vegetation, are thoroughly wet, and receive
precipitation from long-duration storms.
The soils in the United States are assigned to four groups (A, B, C, and D) and
three dual classes (A/D, B/D, and C/D). The groups are defined as follows:
Group A. Soils having a high infiltration rate (low runoff potential) when
thoroughly wet. These consist mainly of deep, well drained to excessively
drained sands or gravelly sands. These soils have a high rate of water
transmission.
Group B. Soils having a moderate infiltration rate when thoroughly wet. These
consist chiefly of moderately deep or deep, moderately well drained or well
drained soils that have moderately fine texture to moderately coarse texture.
These soils have a moderate rate of water transmission.
Group C. Soils having a slow infiltration rate when thoroughly wet. These consist
chiefly of soils having a layer that impedes the downward movement of water or
soils of moderately fine texture or fine texture. These soils have a slow rate of
water transmission.
Group D. Soils having a very slow infiltration rate (high runoff potential) when
thoroughly wet. These consist chiefly of clays that have a high shrink-swell
potential, soils that have a high water table, soils that have a claypan or clay
layer at or near the surface, and soils that are shallow over nearly impervious
material. These soils have a very slow rate of water transmission.
If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is
for drained areas and the second is for undrained areas. Only the soils that in
their natural condition are in group D are assigned to dual classes.
Rating Options
Aggregation Method: Dominant Condition
Hydrologic Soil Group—King County Area, Washington Parcel A Soils
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
6/3/2021
Page 3 of 4
Component Percent Cutoff: None Specified
Tie-break Rule: Higher
Hydrologic Soil Group—King County Area, Washington Parcel A Soils
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
6/3/2021
Page 4 of 4