1728 100TH ST SW 2024-01-04 - Laserfiche WebLink

I Western Washington Hydrology Model (WWHM2012) was used to analyze the volume of stormwater entering the Dispersion Trench. Stormwater runoff from the proposed SFRs will have a peak flow rate (100-year return period) of 0.086 CFS (Refer to Section III: Dispersion Trench Calculations). This is less than the maximum allowable flow rate (0.5 CFS) through the influent pipe per SMMWW Volume V, Chapter 4, Figure V-4.5, therefore, using linear interpolation. the Dispersion Trench will be a minimum 10 ft length. The proposed asphalt driveways (f600 st) will take access from Dakota Way off the southwestern property line and extend to two concrete parking areas (f535 sf, see Figure 3). The proposed driveways and parking areas are classified as pollution generating impervious surface (PGIS). ' Stormwater runoff will sheet flow to the south through a 2 ft transition zone and a minimum of 10 ft vegetated flow path per every 20 feet of contributing surface, from the discharge point to the nearest property line, per SMMWW Volume V. Chapter 3, BMP T5.12 and Chapter 11, BMP ' T5.13. All on -site disturbed areas, not covered with impervious surface, will have amended soils installed ' as required per SMMWW Volume V, Chapter 11, BMP T5.13. Due to the type of topsoil and existing vegetation on -site, the proposed project will stock pile topsoil for final stabilization. A "Compost and Topsoil Calculation Worksheet for the Pre -Approved Amendment Rate" has been attached to this report for the convenience of the contractor and/or property owner (Refer to Section IV). ' OFF -SITE ANALYSIS Upstream Analysis: ' Based on on -site field observations and county contour maps, there is a small upstream basin area of approximately 1.5 acres (Figure 4: Off -Site Basin Map). Due to the crown in ' the road and the existing municipal stormwater drainage. surface stormwater is not expected to flow onto the site from beyond 100`h Street Southwest (Figure 5: Drainage Inventory Map). The upstream basin area consists of developed SFR lots with lawn and a forested area. The upstream basin area is not expected to have adverse impacts on the project site. Downstream Analysis: Stormwater runoff from the site will discharge along the southwestern property line (See ' Figure 4). Stormwater runoff collects in the existing grass -lined roadside ditch and is conveyed southeast along Dakota Way, through an existing culvert, to an existing catch basin approximately 55 ft southeast of the project site (See Figure 5). The existing municipal storm drainage system conveys Swamp Creek, a Non -fish habitat Seasonal ' Stream, to the south side of Dakota Way (Figure 6: Stream Inventory Map). Swamp Creek is conveyed south on the surface and through the storm drainage system beyond `/4 mile from the project site. The downstream flow path consists of developed SFR lots with lawn and landscaping, and greenbelts of native vegetation. The proposed project is not expected to have adverse impacts to downstream properties. 3