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MAGNUSSON <br /> KLEMENCEC <br /> ASSOCIATES ■ <br /> event is occurring AND thhro osed 8" storm drain gravity pipes become entirely blocked AFTER the <br /> storm event peak flow has titled the ponds to the 50-year event level and surface runoff then continues <br /> to drain into the ponds. In that extremely unlikely condition no flow would be able to backwater release <br /> through any of the grated inlets and the water level within the ponds would reach the lowest pond top of <br /> slope at elevation = 118. No overtopping is likely to occur but slope erosion and sediment movement <br /> are likely that could block the ponds' detention release system for the storm event and would require a <br /> difficult maintenance and cleanup operation. <br /> STORMW\TER TREATMENT PLAN <br /> Stormwater treatment is not a requirement of the project since the project conveys runoff to the existing <br /> combined system. The vegetated ponds, swale, and rain garden will in fact provide some measure of <br /> treatment, but the pollutant load removals are not analyzed herein. <br /> STORMW\TER FLOW CONTROL PLAN <br /> The City Stormwater Manual requires that the proposed project match the pre-developed site's <br /> discharge durations from 50 percent of the 2-year recurrence interval peak flow up to the peak flow of <br /> the 50-year recurrence event. Calculations of these event storms are included in Figures 4 and 5 for <br /> the pre-developed conditions. Calculations of the developed conditions are included as Figures 6 <br /> and 7. <br /> The table below summarizes the results of the calculations: <br /> Table 2. <br /> Condition 2-Year Storm Event Discharge(cfs) 50-Year Storm Event Discharge(cfs) <br /> Pre-Developed 0.9 2.1 <br /> Developed 3.9 (0.9 al I owabl e) 9.2 (2.1 al I owebl e) <br /> It is critical to note that the pre-developed condition does not actually discharge flow into a conventional <br /> drainage system. Therefore,the proposed condition is designed to match the existing condition to the <br /> greatest extent feasible,which is to detain the runoff in the detention ponds and slowly release it back <br /> into the combined system. <br /> The detention pond design has been performed in accordance with Volume 4, Chapter 3 of the <br /> Stormwater Management Manual. <br /> CONVEYANCE ANALYSIS <br /> Storm Drainage Design <br /> 7 <br /> Utility Plant Expanded Parking Facility, Everett, Washington <br />