9620 19TH AVE SE 2018-01-02 MF Import

PROJECT OVERVIEW The proposed development of the Alfy's Pizza site is located on a parcel of approximately 1.20 acres in south Everett. It lies west of 19th Avenue and just north of the existing Skate Deck. (See Exhibits "A" & "B".) Currently, the site is undeveloped with extensive existing fill. It is vegetated by sparse grass with much of the site exposed bare earth. The parcel slopes from east to west at approximately 3% and outfalls to an existing drainage way within an existing 20' easement. The drainage course continues westerly to a larger northerly course at the Interstate 5 right of way. Development of the site :s proposed to consist of approximately 8,000 s.f. of restaurant space with 68 parking spaces and asphalt pavement drive to provide access and circulation within the site. This report addresses the collection, conveyance, biofiltration and detention of drainage runoff from the developed site. COLLECTION AND CONVEYANCE Runoff from the developed site will be collected and conveyed primarily as sheet flow to the northwest comer of the parking lot. The paved surfaces and landscaped areas will be collected by catch basins constructed adjacent to the north curb line of the entrance driveway. Flows from the roofs will be discharged directly to the most easterly catch basin. The lower end of the conveyance system shall be oversized pipes to accorr, modate detention requirements as an underground tank. The detention facility will discharge to the biofiltration swale for low flows and through the high flow bypass for larger storm events. Both the biofiltration swale and the high flow bypass will discharge to a new piping system constructed within the 30 foot easement. Final outfall from the system will be into an energy dissipater of 6 to 8 inch quarry spalls at the edge of the existing northerly drainage course to the west of the site. BIOFILTRATION In accordance with City of Everett design criteria of 100 feet plus 1 foot for every 400 s.f. over 10,000 s.f. of asphalt paving surface, the project requires a minimum of 160 lineal feet of biofiltration Swale. The design proposes to construct the 160 lineal feet of swale adjacent to the west boundary of the site. DETENTION SYSTEM According to City of Everett design criteria, the proposed project will require 1,526 cubic feet of detention to attenuate runoff flows from the developed site prior to discharge to the existing drainage course. (For Detention Calculations, see Appendix "A'.) The detention system is designed to consist of 48 inch diameter CMP with a capacity of 1,530 cubic feet.