10200 19TH AVE SE COSTCO FUEL STATION 2022-05-23

5.0. PERMANENT STORMWATER CONTROL PLAN A. EXISTING SITE HYDROLOGY The existing topography of the project site generally falls from north to south and drains to an existing detention/water quality vault, and a one-cell detention and water quality pond with a baffle. See Figure 8 for the Existing Conditions Map. B. DEVELOPED SITE HYDROLOGY The project does not propose changes to the exiting site hydrology. See Figure 9 for the Developed Conditions Map. • New Impervious Surface area: 300 s.f. • Replaced Impervious Surface area: 8,392 s.f. • Converted Pervious Surface area: 0 s.f. C. PERFORMANCE STANDARDS AND GOALS This project is subject to the 2014 SWMMWW flow control standard. The existing flow control systems satisfies this standard. The existing detention/water quality vault and a one-cell detention and water quality pond with a baffle have been identified on See Figure 8 for the Existing Condition Map. D. LOW IMPACT DEVELOPMENT(LID) FEATURES Due to the proposed project triggering Minimum Requirements Nos. 1 through 5, LID Features include BMPs selected from List No. 1. Applicable areas are limited to areas disturbed by the proposed fuel facility expansion project and do not include the areas disturbed in order to meet the requirement to install Fire Suppression (per the Exemption for Underground utility projects under section 1-2.2) Lawn and Landscape Areas: Post Construction Soil Quality and Depth shall be in accordance with BMP T5.13. Roofs: Dispersion (full or downspout infiltration) is not feasible due to the constraints of the project area. The site subsurface conditions can be classified as sandy loam for the design criteria for infiltration trenches. Per the Manual, a roof area of 7,800 square feet would require 975 linear feet of infiltration trenches to be provided. The project area is limited to an approximately 150 foot by 60 foot area; it is infeasible to install the required 975 linear feet of infiltration trench within the project. Rain Gardens are not feasible due the vertical constraints of the existing storm drainage system and the surface grade. The bottom elevation of a rain garden would be higher than the rim of the upstream structures. A perforated stub-out connection has been proposed in accordance with BMP T5.10C prior to connecting to the existing hardline system. The runoff from the existing and proposed canopy will be treated with the perforated stub-out BMP, resulting in the treatment of 7,800 square feet of impervious area (equivalent to the applicable treatment area). 6226.Oxx