1815 13TH ST SEATTLE CHILDRENS NORTH CLINIC 2019-07-08

MAGNUSSON KLEMENCIC ,. __, ASSOCIATES ■ As noted in the calculations, a portion of the existing storm drainage system is being reused around the north side of the CUP. This approach minimizes the disturbance around the existing fuel tanks located in the indicated area, underneath the portion of existing concrete pavement. To check the feasibility of this approach, a backwater analysis has been performed using StormCAD with the tailwater elevation of the outfall set to the riser crest elevation of the flow control structure. The modeling results show that reusing these existing storm drainage pipes and structures still allows the proposed system to meet the COESWMM requirement of a minimum of six inches between the water surface and rim of any structure without any pipe needing to flow more than full. These backwater results are shown in Appendix H, where the resulting headwater surface or hydraulic grade line (HGL) is reported in comparison to the rim elevations. In addition to conveyance calculations for the on-site dedicated storm system, a conveyance calculation for the re-routed portion of the public combined sewer has been included in Appendix I. As noted on the COE record drawings for the existing combined sewer system, the wet weather flow is 1,000 gallons per minute. The re-routed system is designed at a slope slightly greater than the existing pipes' slopes, and will be able to accommodate a discharge flow well above the required wet weather flow as shown in the calculation results. MAINTENANCE AND OPERATION PLAN A Maintenance and Operations Plan is included in Appendix J which addresses Minimum Requirement #9: Operation and Maintenance. Storm Drainage Report 11 Providence Regional Medical Center Everett North Site, Everett, Washington