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y <br />3. OFFSITE ANALYSIS AND MITIGATION <br />This entire project proposes to construct approximately 2,371 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 <br />sf <br />NPGIS <br />sf <br />Total <br />sf <br />Existing To be removed <br />0 <br />0 <br />0 <br />Existing To be replaced) <br />0 <br />0 <br />0 <br />Existing To remain <br />16 <br />.0 <br />16 <br />New +-Replaced <br />490 <br />1,881 <br />2,371 <br />Total Site Impervious <br />506. <br />1,881 <br />2,387 <br />Net New Impervious <br />490 <br />1,881 <br />2,371 <br />The new impervious surfaces will be mitigated using on -site stormwater management <br />BMPs including permeable pavement, perforated stubout connection ` and soil <br />management. <br />A, UPSTREAM ANALYSIS <br />During the site visit it was observed that there a small off -site area to the southeast that' <br />appeared to drain toward the proposed project area. It was clear that all off -site flows will <br />pass through or around this site and remain that way during construction. Based on the <br />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 north/northwest <br />and across the northwestern property lines. Runoff that flows northwest continues to flow <br />that direction and down the steep slopes approximately 100 feet and into an unnamed <br />stream at the bottom of the ravine. Runoff flows north/northeast, uninterrupted in this . <br />ravine approximately 1,300 feet before flowing into Pigeon Creek #1. <br />Most runoff from the site sheet flows north across the northern. property line, across the <br />pavement and into the catchbasin within. the existing shared driveway that flows into the <br />detention: auli,,--Runoff..out-of.-.the .detention vault-fi.ows. north approximately 100 feet and is <br />discharged at the bottom of the broad swale/ravine area. Runoff flows north/northwest in <br />this low area approximately 550 feet where it meets up with the rest of the site runoff in the <br />unnamed stream. At this point the downstream analysis was concluded. <br />Based on the fact that the .new impervious surfaces will either be infiltrated on -site or <br />Jightlined.,to..,the:,existing..drainage.system.,and :that this ,project proposes .to increase the <br />total impervious surfaces on the site by less than 5,000 sf, the developed flows are <br />expected to be slightly greater than in the existing conditions.. During the site visit and <br />review of the downstream system, no portion of the system appeared to be inadequate. <br />Moir SP Lot 6 20-0108 <br />February 2020 <br />Revised: October2020 Page 5 <br />