3003 W CASINO RD BLDG 40-YD 2019-11-27

Drainage Report—Revision 1 Boeing Everett—Rail Spur Parking Lot September 26, 2013 3. PRELIMINARY CONDITIONS SUMMARY The existing site consists of 104,823 SF of grass fields. There are also two existing grass swales that run along the south, west and north borders of the project site and serve as existing runoff treatment BMPs for pollution-generating impervious surfaces in the adjacent north and south impervious areas. These adjacent impervious areas (hereafter referred to as North Subbasin and South Subbasin) include 92,650 SF of impervious surface and will have minimal disturbance during this project. The site also includes several small trees which will be removed, along with abandoned concrete pads for old utilities. The south side of the site is bordered by train tracks which deliver parts and equipment to the Boeing facility. To the north and east of the project site there is an asphalt access road, and to the west is additional grassed area with a water quality bioswale. In total, the site is comprised of three drainage subbasins, as described below and illustrated in FIGURE 2: • The central subbasin is the proposed concrete pavement surface, which will add 104,823 SF of impervious surface to the site. • The north subbasin consists of 80,615 SF of impervious surface. Under existing conditions, this subbasin routes to a bioretention swale north of the proposed pavement area. Runoff will be rerouted through the project site to a water quality facility and then to the outfall west of the site. • The south subbasin consists of 12,035 SF of impervious surface. Under existing conditions, this subbasin routes to a bioretention swale along the south portion of the proposed pavement area. Runoff will be rerouted through a water quality facility and tightlined to the outfall west of the site. North Bioretention Swale(paved . - ^ u under proposed conditions) -" NORTH SUBBASIN j CENTRAL SUBBASIN- PROPOSED PAVEMENT AREA South Biorete oti on Swale - 'i:k. 11 - SOUTH SUBBASIN West Bloretention Swale FIGURE 2 Project Basin Delineation DCG, Inc. Page 2 of 11