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• <br /> Verizon Communications-Stormwater Site Plan Everett,WA <br /> V. Ar►alysi and Design of Required Runoff Control RMPs <br /> Flow Control <br /> The proposed site improvements will consist of new parking stalls along the northern <br /> frontage, new paving over existing gravel, two (2) new buildings and associated <br /> landscaping, as shown on Exhibit B — Post Developed Conditions. <br /> Table 2— Proposed Land Coverage Here is the mention of the <br /> Proposed oversized flow control as <br /> SF AC mentioned in the response <br /> Pervious 78,745 1.81 letter. <br /> PGIS 240,440 5.52 <br /> Roof top 78,064 1.79 <br /> TOTAL 397,249 9.12 <br /> Since the site is site is currently developed, flow control is only required to be provided for <br /> the net increase in impervious area as shown in Table 3. However, the flow control <br /> facility has been conservatively over-sized for 20,000 SF. <br /> Table 3 <br /> Flow Control <br /> Only areas affected by development included in calculations. <br /> Existing Proposed Difference <br /> SF AC SF AC SF <br /> impervious* 308,490 7.08 318,504 7.31 10,014 <br /> Pervious 88,759 2.04 78,745 1.81 -10,014 <br /> *Existing gravel areas, roof area, PGIS <br /> Flow control will be provided by bioretention areas along the northern property frontage <br /> between the parking lot and the existing sidewalk. These bioretention areas were sized <br /> utilizing the Western Washington Hydrology Model 3 (WWHM) and criteria set forth by <br /> the 2005 Department of Ecology Stormwater Manual for Western Washington(2005 DOE) <br /> and the Low Impact Development Technical Guidance Manual for Puget Sound (LID <br /> Manual). <br /> Bioretention areas closely mimic natural conditions, where healthy soil structure and <br /> vegetation promote the infiltration, storage and slow release of stormwater flows. Since <br /> the bioretention areas are the only flow control measures provided for the increase in <br /> impervious area, the bioretention areas will be designed to infiltrate all of the runoff that is <br /> directed to them. An overflow structure is provided above the maximum water surface. <br /> Bioretention Sizing Criteria <br /> • Per Volume III, Section 7.7.5 of 2005 DOE, the bioretention areas are modeled as <br /> an infiltration pond in WWHM. <br /> PACLAND Job#50183001 Page 12 <br />