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

Stormwater Site Plan January 9, 2018 Catholic Housing Services—Everett Safe Streets Supportive Housing E. Stormwater Treatment Plan Following the guidelines presented in the DOE Stormwater Management Manual Basic treatment for the Site runoff is needed. A bioretention cell shall be constructed per Volume V,Chapter V-7, near the proposed parking lot edges. Since,infiltration will not be used as a form of final dispersion,water shall infiltrate the bioretention soil mix,and drain to the underground detention via an underdrain pipe(8- inch minimum diameter). Runoff treatment of at least 91%of the entire runoff volume is achieved per section 1-2.5.6, Minimum Requirement#6, in the SWMM. MGS Flood model results are provided in Appendix D. F. Conveyance Analysis The proposed roof runoff shall be collected and directed to downspouts along the building exterior, which shall connect directly to the detention vault via tight-lines. Tight-lines shall be rigid 6-inch pipe. Catch basins shall be placed in the proposed curbs at the locations shown on the Site Storm Drainage and Utility Plan (Appendix A). Runoff from the parking lot will be collected and diverted through a bio- retention cell. Six(6)-inch underdrain pipes in the bio-retention will convey the captured runoff to a type 1 catch basin before flowing via 8-inch storm drain into the underground detention tanks for controlled discharge from the Site. The site west of the building shall be collected and drain via area drains,grated inlets into Type 1 catch basins to be conveyed directly to the underground detention vault beneath the parking lot. G. Maintenance and Operation Plan The design life of common materials used for storm drain pipe(eg HDPE, PVC) is 50 to 100 years based on studies performed by the Plastic Pipe Institute. Assuming the 100-year event as the base flood for the Final Design Phase,the exceedance probability of the proposed system will be within the range of 39 to 63 percent. Coterra recommends that the COE implements the Snohomish County Drainage Manual (2016)Volume V,Chapter 4, Section 4.6 for an operation and maintenance(0& M) plan of all existing and proposed drainage structures at the ESSSH Site. For each drainage structure or component Table 5.3 within Section 4.6 lists possible defects,conditions signifying when maintenance is needed,and the expected results once maintenance is performed. Snohomish County Code Chapter 7.54, and Chapter 30.63A.575 through 605 are other possible guidelines for an 0& M plan. More detail can be found in Appendix F. Wage