6200 12TH DR SE 2020-10-22 - Laserfiche WebLink

Stormwater Site Plan January 9, 2018 Catholic Housing Services—Everett Safe Streets Supportive Housing "Ns"Stream (COE,Critical Area) before emerging on the west side of Evergreen way as an "F"Stream. The COE describes"Ns"classified streams as seasonal in which surface flow is not present for at least some portion of a year of normal rainfall that are not located downstream from any Type"Np"stream segments. Care and consideration of these critical areas was held paramount during the design of the project site. A 24-inch polyethylene (PE)storm drain currently runs along the east property line of the site, beneath the existing pavement,and serves the fire training site. This line reduces to a 6-inch diameter at the site entrance and conveys flow to the south to a combined sewer manhole on an 8-inch line beneath Berkshire Dr. Additionally,a 12-inch polyvinyl chloride(PVC)storm drain runs along the north side of Berkshire Dr., parallel to the south property line, but does not serve the site. Finally,a 36-inch Alderwood Water District transmission water main runs roughly parallel the 24-inch sewer pipe, mentioned above,on the east property line. Caution to not damage these lines during construction shall be taken. III. Site Development Plan with On-Site Stormwater Management The existing ground shown in the Grading Plan (Appendix A)as the"limits of disturbance"will be approximately 79,520 sf and will need to be stripped,cleared and grubbed of all vegetation,gravel and existing pavement,with the exception of the existing slope to the west and beyond. Grading will be performed that shall not alter existing drainage paths and outfall locations. The site shall continue to drain to Pigeon Creek to the west and south following construction. See Site Storm Drainage and Utility Plan (Appendix A). The new multi-family residential building shall be constructed with a footing drain surrounding the entire perimeter of the building. The footing drain is to be constructed of 4-inch perforated pipe surrounded by 1-inch minus washed rock that is wrapped in a non-woven geotextile,and will empty to the existing 12-inch storm drain beneath Berkshire Dr. The project site triggers Minimum Requirements 1 through 9 as described by the DOE SWMM. Therefore, List#2 per Table 1-2.5.1, in Volume I, Page 56 for On-site Stormwater Management BMPs selection was followed for Redevelopment on any parcel inside the Urban Growth Area (UGA). The ground shall be graded to slope away from the base of the building. Some of the surface drainage, not collected on the roof, will runoff as sheet flow on its historic path to the west and south. The rest will be collected in an area drain,grated catch basin in the western lawn and conveyed directly to the underground detention tank. All proposed lawn areas shall have soils remediated in accordance with DOE BMP 15.13,Post-Construction Soil Quality and Depth. Non-roof surface drainage on the east side of the proposed building will sheet flow to the proposed parking lot drainage system of bioretention to be treated prior to detention and controlled release. Rainfall runoff from the parking lot shall also be conveyed to bioretention before proposed detention tanks for water quality treatment and controlled discharge. The roof downspouts shall connect via tight-line directly to the detention system. Full dispersion and/or infiltration are infeasible as Site soil quality is low,and thus shall have a controlled release into the public storm drain. Poor soils also make permeable pavement non-beneficial and thus infeasible. Parking lot runoff shall be collected via a curb opening and will enter an under-drained, bioretention cell for treatment prior to being collected in 2 ! Page Page