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

Drainage Report—Revision 1 Boeing Everett—Rail Spur Parking Lot September 26,2013 runoff from the north and south basin since the bioretention swales that currently provide stormwater treatment will be removed as a part of this project. The water quality facilities for the north and south subbasins are addressed in the following sections. 5.1 Developed Hydrology The developed site conditions, shown in the project plans submitted under separate cover, will include an additional 104,823 SF of pavement, converting pervious grass field to impervious surface. Onsite drainage facilities must mitigate runoff from new impervious surfaces and provide runoff treatment from new PGIS surfaces from all three drainage sub-basins,which are described below: • The central subbasin is the proposed 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 which was previously routed to a bioretention swale north of the proposed pavement area. • The south subbasin consists of 12,035 SF of impervious surface which was previously routed to a bioretention swale along the southern portion of the proposed pavement area. See FIGURE 2 for basin delineations. The swales to the north and south of the new pavement area will be paved under proposed conditions and proposed stormwater facilities will provide necessary runoff treatment. Catch basins with spill control elbows will serve as source control and 12" ductile iron pipe will convey the stormwater to the detention and runoff treatment facilities. TABLE 1 summarizes the developed site conditions and land cover. 5.2 Stormwater Control Scenario Per Volume I, Chapter 1-4, Section 1-4.3.1 of the Stormwater Manual, the project is classified as Stormwater Control Scenario 2, requiring BMPs for source control, conventional runoff treatment, and streambank erosion control. As described previously in this section, additional streambank erosion control is not required for the new and replaced impervious surface. Source control will be provided with the use of source control BMPs selected from Volume IV, Chapter IV-4 of the Stormwater Manual, which are discussed in detail in the PSQC Plan. Runoff treatment for the central subbasin will be provided by an existing water quality swale before runoff reaches the detention pond (see APPENDIX F for sizing calculations for the water quality swale). Runoff from the north subbasin will be treated by an 8'xl 1' StormFilter vault in the southwest corner of the proposed pavement area. Runoff from the south basin will be treated in a 3-cartridge StormFilter catch basin. Section 5.3, below, explains the modeling of the water quality facilities for the north and south subbasins. 5.3 Water Quality Modeling The water quality facilities for the north and south subbasins were sized using the StormShed2G single event modeling program. The design storm, as described in Section III-4.2.1 of the Stormwater Manual, is the 6-month, 24-hour event. The 6-month, 24-hour event is determined by multiplying the 2-year, 24-hour event by a factor of 0:64 for water quality facilities not DCG, Inc. Page 6 of 11