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1 <br /> Preliminary Stormwater Site Plan Everett, WA <br /> II <br /> Analysis and Design of Required Runoff Control BMPs <br /> I <br /> Proposed Development <br /> 1 The proposed Intermec Lot 2 development would consist of five buildings totaling <br /> approximately 360,000 S.F. with associated parking lot and landscape areas. For the <br /> I design of the detention pond, considering landscape requirements and onsite slopes, Lot 2 <br /> is assumed to be 80% impervious. For streambank erosion control, the detention system is <br /> sized to detain the post-developed peak flow rates from the 2-yr, 24 hour storm and <br /> 1 release at % the pre-developed peak flow 2-yr, 24 hour storm. Also, the 1 0-yr, 24 hour and <br /> 100-yr, 24 hour post-developed flows are detained to release at the pre-developed 10-yr <br /> and 100-yr peak flow rates. A correction factor is applied to the detention volume to <br /> I further mitigate the increased durations of flows. Figure 1-3.1 from the City of Everett's <br /> Stormwater Management Manual was used to calculate the correction factor. For a site <br /> with 80% impervious area, a correction factor of 0.43 is required. <br /> ISub-Basin Description <br /> I For the purpose of this report, Lot 2 has been split into four basins, they are as follows: the <br /> developable portion of the site that drains to Edgewater; the developable portion of the site <br /> that drains to Powdermill Gulch Creek; 10 acres south of the site that runs on to the <br /> IEdgewater Creek basin; and the steep slopes and buffers in the eastern portion of the site. <br /> The entire developed site will be drained to a single pond. The pond will have two <br /> Icontrol structures to release the appropriate amount of runoff to each basin. <br /> Since the upstream run-on in the Edgewater Creek basin is not on the property it will be <br /> Iconsidered undeveloped in the pre and post condition. Since it is not part of the project, <br /> the flows are matched at 100% of the predeveloped flow for the 2-, 10- and 100-year <br /> storms, while the developed onsite basin is matched at 50% of the 2-year storm and 100% <br /> Iof the 10- and 100-year storms. The increased flow will be factored into the water quality <br /> calculations in the design of the permanent pool portion of the wetpond. <br /> IThe steep slopes and buffers will also not be analyzed since that portion of the property <br /> will remain untouched. <br /> I <br /> 1 <br /> I <br /> I <br /> PACLAND Job# 50175001 Page 7 <br />