3003 W CASINO RD BOEING PARKING LOTS - E 6 2025-07-25

I I Drainage Report Boeing Everett-E-6 Parking Lot Expansion July 16, 2013 I TABLE 1 Project Site Area and Size of Improvements Project Site Areas I Existing Developed SF Acres SF Acres Impervious Areas: I Existing Parking North Expansion 54,320 1.25 54,320 1.25 16,888 0.39 South Expansion 15,367 0.35 West Expansion 18,886 0.43 I Total Impervious Surface: 54,320 1.25 105,461 2.42 Total New Impervious Surface: 51,141 1.17 Total Pollution Generating I Impervious Surface: 54,320 1.25 105,461 2.42 Total New Pollution Generating Impervious Surface: 51,141 1.17 I Pervious Areas: Lawn 58,949 1.35 7,808 0.18 Total Pervious Surface 58,949 1.35 7,808 0.18 I Total Project Site Area 113,269 2.60 113,269 2.60 The areas in TABLE 1 were determined by area measurements in AutoCAD from a topographic I survey. As shown in TABLE 1, the project adds 51,141 SF of new impervious surface. There is 51,141 SF of new PGIS on the proposed site comprised of the three asphalt parking expansion areas. These areas represent the drainage basin requiring mitigation. I1.4 Predeveloped Stormwater Runoff Conditions Runoff from the existing parking area flows to Type 1 and Type 2 catch basins and is conveyed I to the west before being sent to the Powder Mill Gulch detention/water quality pond north of the Boeing campus. No offsite flow enters the site from any direction. Stormwater falling on existing lawn areas to the north, west, and south of the existing parking area infiltrates onsite or is Idispersed via sheet flow. 1.5 Postdeveloped Stormwater Runoff Conditions I Stormwater runoff from the developed site will be collected in new Type 1 and Type 2 catch basins and conveyed to the existing asphalt parking lot collection system via 8" SDR 35 storm pipe. No runoff from the existing parking lot or the surrounding vegetation will flow onto the I proposed asphalt. There will be down-turned elbows within the proposed catch basins to serve as spill control and oil/water separation. Flow control and water quality facilities are not required for the site since the area was vested as tributary area to the Powder Mill Gulch detention/water I quality pond north of the Boeing campus in the 1991 Boeing Master Plan. This inclusion of the site as part of the Master Plan was confirmed by the City of Everett prior to the design of the project. See APPENDIX F for the City of Everett's approval to the flow control and water 1 quality exemption. I 20130715_Drainage Report Boeing Everett_Gate E-6 Parking Lot.doc Page 3 I