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3. OFFSITE ANALYSIS AND MITIGATION <br />This entire project proposes, to construct approximately 3,429 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 />s <br />NPGIS <br />s <br />Total <br />s <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 />0 <br />0 <br />0 <br />New + Replaced, <br />1,115 - <br />2,314 <br />3,429 <br />Total Site Impervious <br />1,115 <br />2,314 <br />3,429 <br />Net New Impervious <br />1,115 <br />2,314 <br />3,429 <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 northwest/north <br />and across the northwestern property lines. Runoff that flows northwest continues to sheet <br />flow west down the steep slopes approximately 100 feet and into an unnamed stream at <br />the bottom of the ravine. Runoff flows north/northeast, uninterrupted in this ravine <br />approximately 1,300 feet before flowing into Pigeon Creek #1. <br />The majority of runoff from the site sheet flows north/northeast and into the . catchbasin <br />within the existing shared driveway that flows into the detention vault. Runoff out of the <br />detention vault..flows.north.ap.proximately-1.00,.feet,.andas .discharged at the bottom of the <br />broad swale/ravine area. Runoff flows north/northwest in this low area approximately 550 <br />feet where it meets up with the rest of the site runoff in the unnamed stream. At this point <br />- the downstream analysis was concluded, <br />Based. on the fact that the new impervious surfaces will either' be infiltrated on -site or <br />tightlined 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 tot 4 20-0230 <br />February'2020 <br />Revised. October 2020 Page 5 <br />