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3. OFF-SITE ANALYSIS AND MITIGATION <br /> This site proposes to construct approximately 7,943 sf of new plus replaced impervious <br /> surfaces. The following table summarizes the new PGIS and NPGIS proposed. <br /> PGIS NPGIS Total <br /> (sf) (sf) (sf) <br /> Existing (To Be Removed) 0 0 0 <br /> Existing (To Be Replaced) 0 0 0 <br /> Existing (To Remain) 0 0 0 <br /> New + Replaced 3,401 4,542 7,943 <br /> Total Site Improvements 3,401 4,542 7,943 <br /> Net New 3,401 4,542 7,943 <br /> The new impervious surfaces will be mitigated using stormwater management BMPs <br /> including an infiltration trench, tightlining, permeable pavement and soil management. <br /> A. UPSTREAM ANALYSIS <br /> During the site visit it was observed that there was some off-site area to the south that <br /> appears to drain toward the proposed project area. As long as the contractor grades the <br /> landscaping in the southern portion of the lot to flow around the garage, all off-site flows will <br /> pass through or around this site and remain that way after construction. <br /> B. DOWNSTREAM ANALYSIS <br /> All proposed hard surface runoff for this project will be infiltrated and this project has shown <br /> to result in a small net increase in site runoff. Runoff that leaves the site over the northern <br /> property line will sheet flow across the sidewalk and enter the curb gutter on Viewcrest Ave <br /> that flows east and into the catch basin approximately 80 feet east of the property. Runoff <br /> flows east in the stormwater main approximately 450 feet before flowing northeast out of a <br /> catch basin through a 12-inch plastic culvert and into a small seasonal stream area(Narbeck <br /> Creek). Runoff flows northwest approximately 150 feet in this stream and turns to flow <br /> northeast approximately 300 feet before flowing northeast through a culvert under a private <br /> driveway. Runoff flows out of the culvert and into a ravine (Narbeck Creek) that flows north <br /> an additional 1,500 feet at which point the downstream analysis was concluded. <br /> Based on the fact that this project proposes to construct less than 5,000 sf of effective <br /> impervious surfaces and that the new roof surfaces will be infiltrated, developed flows are <br /> expected to be approximately the same as in the existing conditions. <br /> McCann SFR 19-0913 <br /> September 2019 <br /> -Revised:January 2020 Page 5 <br />