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

1 ' Drainage Report Boeing Everett-E-6 Parking Lot Expansion July 16,2013 1.6 Site Map The Site Plan has been attached in APPENDIX D and complies with the requirements set forth in the Stormwater Manual. I 2. SITE ASSESSMENT 2.1 Topography The topography of the project site generally slopes from east to west at approximately 3-4%. A ' small portion located north of the site slopes from southeast to northwest at approximately 3-7% and southernmost portion of the site slopes from southeast to northwest at 5-11%. There is an existing grass swale to the west of the existing parking lot which will be removed and graded to ' allow for the new parking lot. The developed topography of the site can be seen on the Grading Plan submitted with the Project Plans submitted under separate cover, but generally mimics the existing site grades. New parking areas will be graded so as to prevent new surface flows to the existing parking area. 2.2 Drainage ' The existing impervious areas drain to Type 1 and Type 2 catch basins and runoff is routed to the large detention/water quality pond to the north of the Boeing campus. The detention pond was sized to accommodate the runoff from the new parking areas. The pond drains to Powder Mill Gulch at an approved release rate. Runoff from lawn areas surrounding the parking area sheet flows to low points in the northwest, and southwest portions of the site and infiltrates or is dispersed. ' 2.3 Soils A geotechnical analysis of the site soils was performed on July 1st. The site analysis included ' infiltration tests in several locations to determine an infiltration rate of the native soils for the feasibility of porous asphalt as the parking lot surfacing and/or other infiltrating facilities. The results of the tests showed several feet of fill, more in the areas with existing utilities, underlain by glacial till of very low permeability. The design infiltration rate was less than 0.1 inches per hour, which does not fall within the recommended design infiltration rates for porous asphalt per DOE standards. The soils will provide adequate subgrade support for the anticipated loading given the design asphalt pavement section shown in the Project Plans submitted under separate cover. 2.4 Critical Areas There are no critical areas on the site such as high erosion risk areas, wetlands, streams, or steep slopes (potential landslide areas). 2.5 Adjacent Areas Bordering the project area to the west is a larger Boeing employee parking lot and one way road used by traffic exiting the large parking lot to the east. This road will also serve as the access to the new parking lot and exit point as well. To the south is a landscaped area bordering SR 526, to the east is a landscaped area bordering Seaway Blvd., and to the north is an electrical substation. I 20130715_Drainage Report_Boeing Everett_Gate E-6 Parking Lot.doc Page 4 1