Laserfiche WebLink
III <br /> Goldfinch Brothers <br /> Drainage Study <br /> IAugust 27,2008 <br /> 5. WATER QUALITY DESIGN CRITERIA <br /> 1 For a commercial with more than 10,000 SF of proposed pavement, the City of <br /> Everett Water Quality BMP must be addressed as a combination open/closed system. In <br /> this case, it is not practical for reasons of topography, soil conditions, Site Plan density, <br /> i etc. to dispose of the design volume of runoff by infiltration or to treat with open pond <br /> structures. A series of treatment processes are proposed to accomplish the required water <br /> quality treatment. Proposed Water Quality Treatment will be provided by dead storage in <br /> Ithe bottom of the Detention piping system exceeding the volume of the 6-month storm,to <br /> be followed by Storm Filters at the downstream of the restrictor, to be followed by a man <br /> Imade bio-swale/wetland cell between the pipe discharge point and the public storm drain. <br /> Non-polluting roof drains will be diverted up gradient as far as practical to provide a <br /> Irefreshing flow to the wetland at the highest elevation possible <br /> WATER QUALITY <br /> 10 0.009 1320 412 CF 1.20 UNDEVELOPED <br /> 11 0.192 480 3170 CF 1.20 DEVELOPED <br /> Dead Storage required is greater than or equal to 3,170 CF. Average Dead Storage <br /> 1 Depth will be 3.0 feet across the proposed vault for this project to be followed by the <br /> Cartridge Storm filter facility. The actual Dead Storage volume will be 6,240 CF or <br /> I200%of the minimum requirement. <br /> Contec hasprovided Storm Filter mass loadingcalculations for this project. The reports <br /> o p <br /> I are provided in the Appendices. <br /> TABLE 2 <br /> EXISTING PEAK STORM FLOWS FROM <br /> DEVELOPED PARKING TO <br /> STORM FILTER AFTER DETENTION <br /> ii <br /> II Event Existing Qd(CFS) Design(CFS) <br /> 6-month 0.009 (4.04 GPM) <br /> I 2-Year 0.027 10-Year 0.071 0.015 (6.73 GPM) <br /> 0.085 <br /> 100-Year 0.163 0.156 <br /> IAccording to a Contec Mass Loading calculation, a 72" storm filter cartridge <br /> manhole with a 6-18" cartridges (treating 7.5 gpm) will provided water quality <br /> I improvement to the project and will comply with DOE regulations. The <br /> manufacturer has addressed the particular pollutants of this site and has specified <br /> the Filter cartridge media at detailed design. It is anticipated that oil, grease and <br /> silt will be the primary targets and that the media will be Perlite. <br /> I <br /> I 8 <br />