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3. OFF-SITE ANALYSIS AND MITIGATION <br /> This site proposes to construct approximately 6,523 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 (TBR) 1,106 3,952 5,058 <br /> New+ Replaced 1,751 4,772 6,523 <br /> Total Site Improvements 1,751 4,772 6,523 <br /> Net New 645 820 1,465 <br /> The new impervious surfaces will be mitigated using stormwater management BMPs <br /> including tightening and soil management. <br /> A. UPSTREAM ANALYSIS <br /> During the site visit it was observed that there was no area that drains toward the proposed <br /> project area. It was clear that all off-site flows will pass through.or around this site and <br /> remain that way after construction. <br /> B. DOWNSTREAM ANALYSIS <br /> Runoff from the proposed disturbance area of the existing site sheet flows primarily to the <br /> northwest and down a steep on-site slope. Due to the steepness of the slope, the <br /> downstream was not walked further. Based on aerial photos and information provided by <br /> the client, at the base of the steep slope there is a culvert that passes under the railroad <br /> tracks. Once beyond the railroad tracks runoff enters directly into the Puget Sound. <br /> Based on the fact that this project proposes to construct more than 5,000 sf of impervious <br /> surfaces and that the new roof surfaces will be tightlined to the base of the steep slope, <br /> developed flows are expected to be higher than in the existing conditions. See Figure 5 for <br /> a depiction of the project basin and downstream path. <br /> Kennedy SFR 17-1002 <br /> October 2017 Page 5 <br />