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1 <br /> Drainage Report <br /> Boeing Everett-E-6 Parking Lot Expansion July 16, 2013 <br /> The following sections address and summarize site specifics and requirements. <br /> 8.1 Off-site Analysis <br /> Stormwater runoff from the developed site, along with a large majority of the developed plant <br /> site, will be conveyed to the Powder Mill Gulch detention/water quality basin by an extensive <br /> network of interconnected storm sewer pipelines. The drainage system consists of two storm <br /> ' branches (east and west) that join near the south end of the Powder Mill Gulch detention/water <br /> quality basin. Most pipes are concrete, along with a limited number of corrugated metal pipes <br /> and newer ductile iron pipe. The pipes range in size from 12 to 66 inches in diameter. <br /> Stormwater from the south complex (plant area south of SR-526) discharges through a triple- <br /> barrel inverted siphon under the SR-526 corridor and connects to the west branch of the north <br /> complex drainage system. <br /> The Powder Mill Gulch detention/water quality basin consists of an asphalt-lined storage basin, <br /> dam embankment and an outlet discharge-control facility. The basin was constructed in 1966 to <br /> provide runoff control of stormwatet discharged from the plant site to the downstream Powder <br /> Mill Gulch. <br /> A formal off-site analysis was not required for this project since the project site has been vested <br /> in a previously approved basin plan prepared as part of the original Boeing Master Plan. There <br /> are no known problems with the Boeing stormwater conveyance system or the existing <br /> detention/water quality pond. No stormwater from the site flows to the city's conveyance system <br /> or detention facilities. <br /> 8.2 Pre-Developed Hydrology <br /> iThe predeveloped site contains an existing 54,320 SF (1.25 AC)parking lot and 58,949 SF (1.35 <br /> AC) of lawn and landscaping to the north, west, and south. The existing asphalt parking lot <br /> contains two catch basins which capture stormwater runoff and convey it southwesterly to an <br /> existing Type 2 catch basin, then westerly through the privately owned and maintained Boeing <br /> stormwater conveyance system to the existing detention pond. In the lawn and landscaped areas, <br /> it appears stormwater is infiltrated or sheet flows until fully dispersed or infiltrated. There is an <br /> 1 existing grass swale on the west side of the existing parking lot which does not detain or treat <br /> any runoff from any existing impervious surfaces according to Boeing personnel. It will be <br /> removed as part of this project. <br /> 8.3 Developed Hydrology <br /> The developed site conditions, shown in the project plans submitted under separate cover, will <br /> include an additional 51,141 SF (1.17 AC) of asphalt pavement surrounding the existing asphalt <br /> parking lot on three sides (north, south and west sides). The north expansion area consists of <br /> 16,888 SF (0.39 AC), the west expansion consists of 18,886 SF (0.43 AC), and the south <br /> expansion adds 15,367 SF (0.35 AC) of new pavement. Drainage for the developed site will be <br /> achieved by capturing runoff in catch basins and conveying it via 12" diameter storm pipe to the <br /> existing stormwater conveyance system. The catch basins will have spill control elbows to help <br /> prevent oils and floatables from entering the storm system. As detailed in the Offsite Analysis, <br /> the existing conveyance system extends from the project site to the existing Powder Mill Gulch <br /> detention/water quality basin north of the Boeing campus.New pavement surfaces will be graded <br /> 1 <br /> 20130715_Drainage Report_Boeing Everett_Gate E-6 Parking Lot.doc Page 15 <br />