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3. OFF-SITE ANALYSIS AND MITIGATION <br /> This site proposes to construct approximately 2,234 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) 19 578 597 <br /> Existing (To Be Replaced) 0 1,728 1,728 <br /> Existing (To Remain) 2,201 879 3,080 <br /> New + Replaced 0 2,234 2,234 <br /> Total Site Impervious 2,201 3,113 5,314 <br /> Net New -19 -72 -91 <br /> The new impervious surfaces will be mitigated using stormwater management BMPs <br /> including splash blocks and soil management. <br /> A. UPSTREAM ANALYSIS <br /> During the site visit it was observed that no offsite area drained toward the project area. It <br /> was clear that all off-site flows will pass through or around this site and remain that way after <br /> construction. <br /> B. DOWNSTREAM ANALYSIS <br /> Runoff from the proposed disturbance area of the existing site sheet flows to the southwest <br /> approximately 50 feet and across the southwestern property lines. Runoff continues to sheet <br /> flow southwest across private property where it flows into a season stream. Runoff in this <br /> stream flows southeast approximately.27.0 feet where it can be seen flowing .into a large <br /> concrete culvert that flows southeast into a storm drain manhole within Holly Drive. Runoff <br /> flows out of the manhole through another culvert to the southeast and is discharged on the <br /> south side of Holly Drive. This stream continues to flow south approximately 250 feet where <br /> it can be seen flowing into a small PVC culvert that flows south into a catch basin within <br /> Hollow Dale Place. Runoff flows south out of this CB and the stream continues to flow <br /> south/east where it flows into Swamp Creek, a fish bearing stream. Runoff in swamp creek <br /> flows in a generally southern direction approximately 1,000 feet, uninterrupted, at which <br /> point the downstream analysis was concluded. <br /> Based on the fact that this project proposes to construct less than 5,000 sf of impervious <br /> surfaces and that the new roof surfaces will be dispersed on-site, developed flows are <br /> expected to be approximately the same as in the existing conditions. See Figure 5 for a <br /> depiction of the project basin and downstream path. <br /> Lucas MFH 2O-0715 <br /> March'2021 Page 5 <br />