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Les Schwab Wetpond Design <br /> Wetpond sizing follows the design criteria found in the 2001 Western Washington <br /> Stormwater Management Manual. Runoff treatment must treat the water quality design <br /> storm, which is defined as the 6-month, 24-hour storm event. Since there are no 6-month <br /> isopluvial data, the Department of Ecology further defines the 6-month storm event as <br /> 72% of the 2-year, 24-hour storm event. For the Les Schwab location this is 1-inch of <br /> precipitation in a 24-hour period. <br /> The wetpond volume must equal the post development runoff volume from this 6-month, <br /> 24-hour storm event. Roof runoff does not need to be treated and can be bypassed. The <br /> Les Schwab site is 0.29 acres, 0.11 acres is roof area. Therefore, the remaining 0.18 acres <br /> of area is subject runoff treatment. <br /> This remaining pervious and impervious area was evaluated by the SBUH method within <br /> the StormShed hydrologic software. Peak runoff volume is 388 cf(see attached runoff <br /> data). The wetpond floor elevation is specified as 33.0 feet; the outlet invert elevation <br /> will be 35.3 feet, yielding a volume of 430 cf(see attached spreadsheet). The pond will <br /> be lined with clay or bentonite or a 30 mil impermeable liner. If liner is used pond will be <br /> over excavated and 8-12 inches of soil will be placed over the liner. Because this pond is <br /> relatively small, only a single cell is required. <br /> The plan includes the addition of two Type 1 catch basins (CB), approximately 130 feet <br /> of ductile iron storm drain pipe. Ductile iron was specified because of the limited cover <br /> over the pipe. PVC pipe can be used under the concrete slab, but water-tight connections <br /> with Fernco couplings will be needed. The inlet pipe is routed to the back of the pond to <br /> minimize short-circuiting. The pond will be lined to the depth of the permanent pool (2.3 <br /> feet). The pond side slopes fronting the parking area and sidewalk will be 3:1, 2:1 on the <br /> backside. There will be separate 4-inch PVC pipes to bypass roof runoff directly to the <br /> outlet CB. The outlet CB has a PVC tee device to retain floatables within the CB. <br /> Additionally, there has been minor site grading changes to direct surface runoff to the CB <br /> located adjacent to the alley. <br />