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6. STORMWATER TREATMENT PLAN <br /> A. Western Basin Analysis <br /> The existing western basin contains asphalt pavement. The current basin area <br /> tributary to the collection, conveyance and treatment system totals 8370 sf of <br /> PGIS. After the proposed addition and site grading/paving the tributary basin area <br /> will be 7,190 sf which is a reduction of 1,180 sf or 14 %. Therefore, the treatment <br /> system for this basin will receive 14% less pollution generating runoff after the <br /> project is complete and will function at or better then current levels. <br /> B. Northern Basin Analysis <br /> The existing north basin contains gravel surfacing used for parking. Much of the <br /> existing basin will be converted to NPGIS in the form of the roof over the proposed <br /> addition. The current basin drains to the east and is tributary to the existing <br /> collection, conveyance and treatment system located north of the existing <br /> building. By collecting, and treating this basin prior to release from the site <br /> downstream of the existing treatment system to the east this design will remove a <br /> total of 20,000 sf of basin from the current system. This will improve the function <br /> of the existing system by reducing the peak flows and volumes into the system for <br /> treatment. <br /> The proposed PGIS in the northern basin will total 8,246 sf or 0.19 acres after <br /> construction. This basin has been analyzed using WWHM3 to determine the <br /> Water Quality BMP flow rate for an on-line system adjusted for 15 min. For this <br /> basin the WQ flow rate was calculated to be 0.0321 cfs, refer to Appendix A for a <br /> complete printout of the WWHM3 data. <br /> Working with Contech Stormwater Inc. staff it was determined that a 2 filter catch <br /> basin would be required to meet the treatment standards for this basin. This <br /> system will use their standard height cartridges, details have been added to the <br /> plans. <br /> B. PERFORMANCE STANDARDS <br /> The City of Everett 2010 Stormwater Management Manual as well as the DOE <br /> 2005 Stormwater Management Manual for Western Washington were used in the <br /> design of the stormwater mitigation plan for this project. <br /> C. FLOW CONTROL SYSTEM <br /> Flow control for this project is not required as outlined previously based on the <br /> City of Everett 2010 Stormwater Management Manual. <br /> Rodland Toyota 12-0412 <br /> May 2012 Page-13 <br />