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Preliminary Stormwater Site Plan Everett, WA <br /> Conveyance <br /> IAn on-site conveyance analysis will be performed for construction document design for <br /> the site infrastructure. <br /> 1 100-Year Flood/Overflow Condition <br /> The stormwater system for this project has been designed to address all storm events in <br /> accordance with design criteria described previously. In the event of a larger storm, the <br /> primary system may fail. In that event the following series of overflows would convey the <br /> stormwater to the outfall: <br /> 1. Primary Overflow If the control structure orifices fail or become plugged, the <br /> water would overflow into the control structure through the top of the manhole, <br /> bypassing the orifices and discharging directly to the outfall pipe. <br /> 2. Secondary Emergency Overflow — This is a 48" that bypasses the control structure <br /> to discharge directly to the outfall pipe. The emergency overflow elevation is set at <br /> the water surface elevation achieved when the undetained 100-year storm is <br /> flowing into the primary overflow. <br /> IWater Surface Elevation Calculations <br /> I Design 100-year water surface elevation = 405.14' <br /> Post Development 100 year, undetained flow = 21.31 cfs <br /> Per Figure 1-5.16 Riser Inflow Curves (see Miscellaneous Forms and Worksheets) 0.70 feet <br /> of head is required for 21.31 cfs into a 30-inch riser. <br /> Therefore the 100-year overflow elevation = 405.84' <br /> Thisprovides 1,16' of freeboard above the primaryoverflow water surface elevation to the <br /> pond berm. <br /> Based on our review, we anticipate the additional overflow of the stormwater detention <br /> system would allow surface water to discharge directly to the wetlands in the northern <br /> ' portion of the site and eventually to Edgewater Creek and Powdermill Gulch Creek. <br /> 1 <br /> PACLAND Job# 50175001 Page 15 <br />