The URL can be used to link to this page

Home My WebLink About21-102193-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