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7. ON-SITE STORMWATER MANAGEMENT <br /> To address the requirements for managing stormwater from roof surfaces, the applicability <br /> and feasibility of on-site infiltration areas were considered first. As previously mentioned, <br /> this site has already been developed and appears to have been graded. Soil logs were <br /> provided by R.W.Kramer Enterprises, Inc. The soils were found to be sandy loam and <br /> loamy sand with a depth to water/hardpan at 14-40 inches below grade. Two out of the <br /> four soil logs found water at 14 and 32 inches. Since the depth to restrictive layer/water <br /> table appears to be less than 3 feet below grade, infiltration is not feasible for this site. <br /> Rain gardens were considered next. However since the soils are considered sandy loam, <br /> and due to the layout of the site, the raingarden would need to cover the majority of the <br /> back yard. Since the back yard will consist of pervious lawn and the site already contains <br /> a vegetated buffer the existing vegetated surFaces will be utilized as much as possible to <br /> attenuate flows. <br /> This allowed for the selection of downspout dispersion as the stormwater management <br /> BMP of choice for application to the roof surfaces proposed for this project. The dispersal <br /> system will allow for roof runoff to disperse over vegetated surfaces for the maximum <br /> . length of flow path on-site before being collected by the existing off-site stormwater . <br /> system. <br /> To implement downspout dispersion, the roof downspouts of the proposed duplex will be <br /> dispersed via splash blocks. The proposed roof surfaces total approximately 3,012 sf. A <br /> maximum of 700 sf of roof area may be directed to each splash block. Therefore, a <br /> minimum of five splash blocks are required. Splash blocks must discharge to a vegetated <br /> flow path of at least 10 feet in width and 50 feet in length. For this project, six splash <br /> blocks are proposed. Four of the splash blocks will have a minimum of 50 feet before <br /> flows leave the site. Although the other two splash blocks will not have the minimum <br /> required flow path, less than 700 sf of roof runoff will be directed to each splash which will <br /> aide in attenuating flows. Runoff from the roof surfaces will sheet flow over vegetated <br /> surfaces and will be collected by the vegetated right-of-way of Oakes Ave before being <br /> directed toward the existing catch basin found on the west side of the roadway. This will <br /> ensure no runoff will leave the site and drain onto any neighboring parcels. Refer to sheet <br /> 2 of the drainage plans for more details. <br /> A total of approximately 905 sf of driveway surfaces are proposed for this project. The <br /> runoff generated by the impervious driveway surfaces will be dispersed as sheet flow to <br /> the north and south, per BMP 12.70. A 2' wide transition zone shall be installed along the <br /> downhill edge of each driveway to discourage channeling of flows. The dispersed flow will <br /> pass over existing lawn or amended soils prior to naturally infiltrating on-site or being <br /> collected by the existing stormwater system located within in the right-of-way of Oakes <br /> Ave. <br /> Finally, any area of the site not proposed to be covered with impervious surfaces but <br /> graded or compacted will be treated to meet the criteria for COE BMP 12.20. By <br /> amending the on-site soils in the disturbance areas this will aid in maximizing the amount <br /> of site runoff from the developed project to be absorbed and infiltrated into the existing on- <br /> site soils. <br /> Haack—Lexington Duplex 16-1112 <br /> January 2017 Page 9 <br />