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Stormwater Management Report Everett, WA <br /> t <br /> B. Soil Stabilization and Sediment Trapping <br /> Structural control measures will be used to reduce erosion and retain sediment on the <br /> construction site. The control measures will be selected to fit the specific site and seasonal <br /> conditions. For instance, the soils on the site will significantly affect the erosion potential. <br /> On-site soils are Alderwood Gravelly Sandy Loam and Everett Gravelly Sandy Loam, <br /> described as having slight erosion potential. The general site area is relatively gradual in <br /> ' slope (avg. 6%). In consideration of the area topography and site soils, minimal erosion <br /> control and sediment trapping measures will be needed during construction. <br /> The following structural items will be used to control erosion and sedimentation processes: <br /> • Stabilized construction entrance <br /> • Filter fabric fences <br /> • Cover measures <br /> • Haul Road and Staging areas <br /> • Temporary Interceptor Swales/Rock Check Dams <br /> • Sediment Ponds <br /> Sediment Pond Sizing <br /> The sediment ponds were sized according to the City of Everett Standards to accommodate <br /> ' the peak discharge from the 10-year 24-hours storm, based on post development site <br /> conditions. StormShed was used to obtain the 10-year peak discharges for Narbeck Creek <br /> (east) and Powdermill Gulch Creek (west). The required surface areas were calculated <br /> using the following equation from Section 2-5.7 in the Everett Stormwater Management <br /> Manual: <br /> (SA) = 1.2Qio/Vsed <br /> Vsed was given as 0.00096 ft/sec. The sediment ponds will have a 3' sediment storage <br /> depth and a 3' settling depth along with 1 ' of free board for a total depth of 7'. The <br /> required surface area is at the bottom of the settling depth. The pond side slopes are 3:1 . <br /> Pond design and geometry are noted in the following calculations and table: <br /> East Sediment Pond (Narbeck Creek) <br /> Q=1.52cfs per StormShed Calculation for 10-year post developed flow. <br /> SA =1.2x Q = 1,900sf minimum <br /> 0.00096 <br /> 3:1 Length to width Ratio 25.5' x 76.5' = 1,912 sf <br /> West Sediment Pond (Powdermill Gulch Creek) <br /> ' Q=1.21cfs per StormShed Calculation for 10-year post developed flow. <br /> SA =1.2x / 1,512sf minimum <br /> 0.00096 <br /> 3:1 Length to width Ratio 22.5' x 67.5' = 1,518 sf <br /> PACLAND Job# 50108001 Page 1 1 <br />