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

MAGNUSSON KLEMENCIC ___ ASSOCIATES II sewer system is a closed conveyance system currently with no known capacity or flooding problems. There are no receiving waters within a quarter mile downstream of the project site. The project will meet the duration standard for the flow control design, and is therefore mitigating any increase in flow that would detrimentally affect the downstream or offsite conditions. No erosion is visible around the perimeter of the site, so side slopes are considered to be stable. PRE-DEVELOPED HYDROLOGY In compliance with the Interim Policy, the North Site project will use the existing site conditions to assess the pre-developed hydrology instead of using forested land cover. During the project's Pre-Application meeting with COE reviewers, direction was provided to use the post-CUP project condition, rather than the current, post-CUP Parking project condition, as the existing condition. Because the detention pond that was installed with the CUP Parking project will be replaced during the North Site project, the design is in effect returning to what would have been considered the existing conditions for that project. As shown in Figure 4, Existing Pervious and Impervious Surfaces,the pre-developed site is broken into detained (captured and routed to the flow control facility) and undetained (discharged off-site without flow control) basins. The pervious and impervious areas for each have been summarized on this figure, and the totals for both were then input into the continuous runoff modeling software, MGS Flood Version 4.38. In MGS Flood the pervious area was assumed to be Till Grass land cover, while the impervious area was simply input as Impervious land cover. For the North Site pre-developed conditions, the detained and undetained flows can be input into the same MGS Flood basin node because there are no existing flow control structures prior to the point of compliance. DEVELOPED HYDROLOGY As shown in Figure 5, similar to the pre-developed conditions, the Proposed Pervious and Impervious Surfaces for the developed site is broken into detained and undetained basins with summarized pervious and impervious areas provided for each. The areas were input into MGS Flood with similar assumptions as the pre-developed conditions, where the pervious was assumed to be Till Grass land cover and the impervious was input as Impervious land cover. The undetained areas were modelled as bypass, so that they do not route through the flow control facility but are included at the point of compliance for comparing final discharge flow durations. STORMWATER TREATMENT PLAN In accordance with the Interim Policy, Minimum Requirement#6: Runoff Treatment is not applicable. STORMWATER FLOW CONTROL PLAN Minimum Requirement#7: Flow Control requires stormwater discharge to "match developed discharge durations to predeveloped durations for the range of predeveloped discharge rates from 50 percent of the 2-year recurrence interval peak flow up to the full 50-year peak flow." As noted above,the continuous modeling software MGS Flood was used to determine the design durations and flow rates for the pre-developed and developed conditions. Calculation results are included in Appendix F. In order to comply with Minimum Requirement#7, a detention volume of 25,552 cubic feet of storage is required with a storage depth of 5.5 feet (the riser crest elevation will be Storm Drainage Report 9 Providence Regional Medical Center Everett North Site, Everett,Washington