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from impervious surfaces will be routed to bio-retention facilities (rain gardens), which will use <br /> compost amended soils to provide treatment and stormwater runoff retention. <br /> 4.2 Flow Control System <br /> This project is exempt from flow control by the City of Everett, as it is located within the <br /> combined storm and sewer system. However, detention/infiltration pipe is proposed at the <br /> Building site,which, in combination with the reduced impervious surfaces and rain gardens in the <br /> proposed condition, reduces the 2-year, 24-hour storm peak flow rate by more than 25 percent <br /> from the existing. The infiltration rate was determined by the project geotechnical engineer to <br /> be 0.1 inches per hour.-Calculations for the detention/infiltration pipe can be found in Appendix <br /> B, Exhibit B-1. <br /> Flow control is not required at the 10th Street Parking lot; however, the reduction of impervious <br /> surface results in a 2-year, 24-hour developed peak flow rate reduction of 34 percent. <br /> Calculations can be found in Appendix B, Exhibit B-2. <br /> Table 1: 2-Year,24-Hour Peak Flow Reduction <br /> Site ' Existing 2-yr,24-hr peak j Developed 2-yr,24-hr peak Reduction <br /> flow rate flow rate <br /> Building 0.735 cfs 0.464 cfs r 37% <br /> 10th Street 0.140 cfs 0.093 cfs 34% <br /> Parking <br /> 4.2.1 LEED Requirements <br /> The project team is pursuing LEED v.3 2009 for New Construction and Major Renovation <br /> certification for this project. One goal is the Stormwater Quantity Control credit, as summarized, <br /> below: <br /> SS Credit 6.1: Stormwater Design—Quantity Control <br /> Case 2. Sites with Existing Imperviousness Greater than 50 percent <br /> Implement a stormwater management plan that results in a 25 percent decrease in the volume <br /> of stormwater runoff from the 2-year, 24-hour design storm. <br /> The proposed site represents a significant reduction in impervious area when compared to the <br /> existing site, as indicated in Table 2. Furthermore, rain gardens and detention/infiltration pipes <br /> are proposed, which use compost-amended soil and iow ievels of infiltration. Combined, this <br /> results in a reduction of volume during the 2-year, 24-hour storm event of more than 25 percent. <br /> The volume calculations are summarized in Table 3 and can be found in Appendix B, Exhibits B-4 <br /> and B-5. <br /> Stormwater Site Plan Report Q©Q O <br /> Everett Commmunity.College <br /> Index Hall Replacement&Surface Parking Lot"M" 5 <br /> 209459.10/209459.11 <br />