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Provide your Evaluation belMd Show where these will take place ore map: <br />The project proposes to add new covered storage to the existing site, approximately 10,980 square feet <br />of new roof area. Roof runoff will be collected in downspouts. A 6-inch diameter PVC roof leader will <br />connect to each downspout. The 6" diameter roof leader will connect to an existing catch basin located <br />in the park area. Ultimately, runoff from the new roof area will discharge offsite in an existing 8" <br />diameter PVC storm line to the public storm drainage system in Hardeson Road. <br />Since the site is over 2,000 square feet and less than 5,000 of new plus replaced impervious surface the <br />implementation of stormwater BMPs were considered in the following order. First, roof water BMPs <br />were considered. The site is located over glacial till "hard pan." The USGS maps the site as Qvt (Glacial <br />Till), noting the following: "The till is a nonsorted mixture of mud, sand, pebbles, cobbles and boulders; <br />it resembles concrete mix.... Drainage is good in the uppermost weathered layer, but groundwater ponds <br />and moves laterally along the buried hardpan surface underlying the upper weathered layer." <br />Additionally, the geotech report provided three test pits indicating the presence of hardpan material <br />with high runoff potential over a portion of the site and fill with varying depths over a portion of the <br />site. <br />There is a fill slope installed during previous site improvements bordering the downhill (eastern) portion <br />of the site. Based on the site mapping from USDA and USGS, geotechnical report, and presence of a fill <br />slope infiltration BMPs have been rendered infeasible. Not only would native soils not provide an <br />infiltration rate but introducing roof runoff into the layers of fill could destabilize and undermine the <br />existing fill slope. <br />Next, rain gardens were considered. Rain gardens are a shallow depression made for stormwater to <br />soak into the subgrade, thereby maximizing onsite stormwater retention by promoting stormwater <br />infiltration. Due to the presence of existing fill over portions of the site and an existing fill slope, <br />saturating the soils could undermine the stability of the existing fill slope. Therefore, rain gardens are <br />considered infeasible. <br />Roof water dispersion BMPs were considered next. Dispersion BMPs require a gently sloping vegetated <br />flow path that are capable of safely conveying stormwater, allowing evapotranspiration to occur <br />through evaporation, infiltration, and plant uptake. This site has a fill slope located on the downhill side. <br />The slope is currently vegetated and stabile, however, the slope exceeds 15%, a recommended <br />maximum slope per BMP T5.10B. No other space for dispersion is available on the site without <br />removing existing buildings and infrastructure not planned for this project. Therefore, dispersion BMPs <br />are considered not feasible. <br />As a result if the presence of fill from previous grading activity, native soils consisting of glacial till with <br />high potential for runoff, and a fill slope infiltration and dispersion BMPs are not considered feasible on <br />this site. Roof water from the new roof area will be collected in a roof leader and connected to the <br />existing stormwater infrastructure. Runoff will be conveyed down the fill slope in an existing 8" <br />diameter storm line that discharges to the Hardeson Rd. storm drainage system. <br />(f Page 4 ofr11 <br />*-:�f :" ;: .`�fr f',Sz 1 3 f 00 C t A 982; 1i .;' 25- i 8 ; 3 E 4 �, � �"", <br />