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o a <br /> • The filterra system will capture and treat the water quality design storm volume, or 91% <br /> of the runoff volume predicted by a continuous runoff model (WWHM3), with an <br /> ' underdrain connected to the existing conveyance system. <br /> South Basin <br /> Due to the presence of type "C" soils,the stormwater generated by the building rooftop and other <br /> parking areas will be collected, and routed to a combined water quality and detention wetpond. <br /> A wetpond providing water quality treatment is consistent with the COE Manual for a <br /> ' commercial site. In accordance with Figure 1-3.1 Volume Correction Factor for Streambank <br /> Erosion Control BMP's (COE Manual)the pond has been designed with a correction factor of <br /> 1.44. <br /> ' The wetpond has been designed to be in compliance with the COE Manual 3-3.5.1 BMP RD.05 <br /> Wetpond with one exception. Fencing is not proposed. The entire Snohomish County PUD No. <br /> ' 1 Operations Facility is fully fenced with security fencing and monitored by remote cameras that <br /> prevent any public access to the PUD facility. Additionally the pond has a maximum side slope <br /> of 3:1. <br /> ' The existing oil/water separator treating the South Storage Yard will not be affected by any <br /> ' changes to the wetpond as drainage from the south storage yard is conveyed to the oil/water <br /> separator via a separate conveyance system. <br /> ' A small parking area totaling 7 parking spaces and 1,400 square feet of impervious surfacing will <br /> bypass the wetpond within the south basin. This parking area will be hydraulically isolated from <br /> the north basin and will sheet flow into the large grassy area east of the existing administration <br /> ' building. The existing lawn area will function as a filter strip providing effective water quality <br /> treatment. After passing through approximately 200 feet of existing lawn runoff enters the <br /> existing 24"conveyance system merging with runoff leaving the wetpond. The 1,400 square <br /> foot parking area has been included in the wetpond sizing as a bypass area to mitigate any <br /> potential streambank erosion control impacts. <br /> 1 <br /> 1 <br /> 1 <br /> DAVID EVANS AND ASSOCIATES,INC. <br /> OPERATIONS CENTER ADDITION JUNE,2010 <br /> STORMWATER SITE PLAN PAGE 3 <br />