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3. OFFSITE ANALYSIS AND MITIGATION <br /> This entire site proposes to construct approximately 3,304 sf of new and replaced <br /> impervious surfaces. The following table summarizes the new PGIS and NPGIS proposed <br /> for the entire project. <br /> PGIS NPGIS Total <br /> (sf) (sf) (sf) <br /> Existing (To be removed) 778 0 778 <br /> Existing (To be replaced) 1,004 839 1,843 <br /> New+ Replaced 1,004 2,300 3,304 <br /> Total Site Impervious 1,004 2,300 3,304 <br /> Net New Impervious 1,004 1,461 3,304 <br /> The new impervious surfaces will be mitigated using on-site stormwater management <br /> BMPs including an infiltration trench, sheet flow dispersion and soil management. <br /> A. UPSTREAM ANALYSIS <br /> During the site visit it was observed that there was some off-site area from the parcel to <br /> the south that drained toward the proposed project area. It was clear that all off-siteflows <br /> will pass through or around this site and remain that way during construction. Based on <br /> the location of the proposed improvements the existing runoff from the upstream area will <br /> continue to flow unaffected around this site after the project has been constructed. <br /> B. DOWNSTREAM ANALYSIS <br /> Runoff from the proposed disturbance area of the existing site sheet flows to the north <br /> over vegetated, grass surfaces toward the northern property line. Runoff leaves the site <br /> over the northern property line. Flows continue in a northeasterly direction and is collected <br /> by Forgotten Creek approximately 250 feet from the project site. This creek flows in a <br /> northwesterly direction and enters Possession Sound approximately 1,600 feet for the <br /> project site. See Figure 4 for a depiction of the project basin and downstream path. <br /> Based on the fact that the new impervious surfaces will either be infiltrated or dispersed <br /> on-site and that this project proposes to increase the total impervious surfaces on the site <br /> by less than 5,000 sf, the developed flows are expected to be slightly more than in the <br /> existing conditions. During the site visit and review of the downstream system, no portion <br /> of the system appeared to be inadequate. <br /> Kromer SFR 18-0303 <br /> April 2018 Page 5 <br />