1515 132ND ST SE 2022-01-20 - Laserfiche WebLink

4.0 STORMWATER CONTROLS Existing Site Hydrology Stormwater runoff from the existing site is conveyed off site through a series of catch basins and HDPE pipes. One catch basin onsite near the gas pumps directs water to a oil - water separator then Once offsite, stormwater is conveyed East to a detention pond in the adjacent lot. See Appendix A for a pre -developed basin map. Developed Site Hydrology The proposed improvements will reutilize multiple on site catch basins to direct stormwater to the existing downstream conveyance system. The overall pattern of discharge will remain the same. Multiple new catch basins and conveyance pipes are proposed to direct water towards the existing downstream conveyance system. Implemented BMPs Because the project triggers minimum requirement #1-5 the project must consider BMPs in List #1 of the SWMM to mitigate stormwater runoff from lawn and landscaped areas, roofs, and other hard surfaces. This project proposes to meet the LID performance standard by collecting and conveying stormwater to existing stormwater facilities offsite. Lawn and landscaped areas: Roofs: BMP T5.13: Post -Construction Soil and Depth. Soil quality will be met onsite by amending all soils onsite and importing new soil. BMP T5.30: Full Dispersion. There is no native vegetation onsite to make full dispersion possible. BMP T5.10A: Downspout Full Infiltration. As described in the geotechnical report in Appendix B, the soils do not have the capacity for any infiltration type systems. BMP T5.14A: Rain Gardens and BMP T7.30: Bioretention Cells, Swales, and Planter Boxes. As most of the site is already covered in pavement and will remain paved, there is no space onsite suitable for any of these BMPs. BMP 5.1013: Downspout Dispersion System. The site does not have a sufficient amount of room for a dispersion flowpath. BMP T5.100 Perforated Stub -out Connections. The design criteria for perforated stub -out connections cannot be met. An adequate amount of pipe will not fit with the minimum setbacks for this BMP.