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

Stormwater Site Plan January.9, 2018 Catholic Housing Services—Everett Safe Streets Supportive Housing PRE POST (cfs) (cfs) 2-year 0.02506 0.02204 50-year 0.08904 0.06206 100-year 0.09649 0.07668 Table 1:Runoff Flow Results D. Stormwater Flow Control Plan Post-developed runoff was mitigated to be less than pre-development conditions for 50%of the pre- developed 2-year through the 50-year runoff as per Minimum Requirement#7 from the DOE. The proximity of the Landslide Hazard Critical Area does not allow for the application of most of the traditional best management practices(BMPs)to be within a 200-foot buffer. An underground detention vault is proposed beneath the parking lot constructed of precast concrete sections, 100 ft long,38 ft wide,and 5.5 ft deep,with an invert of 410.0. This provides just over 0.4 acre-feet of storage,and receives runoff collected from the roof and proposed parking lot,after first being run through the bio-retention cell located near the south end of the parking lot. Any Emergency overflow of the detention will be directed to Berkshire Drive curb and gutter. The detention facility shall be constructed as shown in the site plan details(Appendix A),with and internal flow restrictor assembly at the downstream end. The detention vault shall have a standard flow control restrictor outlet structure as shown in COE standard drawings 415. The restrictor assembly shall consist of an 18-inch vertical pipe,open at the top, and with a 0.5-inch diameter orifice at invert 410.50,and additional 0.85-inch orifice with invert 413.50. Effluent will be discharged into the 12-inch storm drain pipe along Berkshire drive. All proposed pipes are Corrugated Polyethylene Pipe(CPEP).A roughness coefficient of 0.014 was used in the design of all pipe sizes to ensure the 2 foot per second minimum velocity.A minimum slope of 1.5 percent for 6 pipes,and 1 percent for 8 and 12 inch pipe was used.Table 2 below,shows pipe capacities, and the highest 25-yr peak flow(computed using Rational Method)expected on site.All pipe crowns are more than 1 ft below adjacent manhole covers and/or grates and are not expected to run full,thus HGL5 should be low enough to prevent any overflow of the system. Rational Method parameters were chosen using the COE DCSS,Section 4-3.1(1). Runoff coefficients were averaged using area weighting per drainage sub-basin. Coefficients used from COE Table 4-3.1(1)-2 were 0.90 for roofs and pavement,and 0.25 for lawns.The time of concentration was assumed at a minimum of 6 minutes(NRCS,TR-55)to compute a conservative rainfall intensity. Composite Intensity Area Q25 Pipe Min. Manning's Velocity Capacity C (in/hr) (ac) (cfs) Dia. Slope 'n' (fps) (cfs) Notes (in) (%) Entire Roof 0.90 2.76 0.254 0.631 6 1.5 0.014 3.26 0.640 West Tight-Line "To Detention" West Storm 0.60 2.76 0.659 1.097 8 1.0 0.014 3.23 1.140 Drainage Area Drain Pipes North Yard 0.33 2.76 0.081 0.074 8 0.9 0.014 3.07 1.082 East Storm Drain Pipes Table 2:Rational Method and Pipe Capacities 7IPage