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3. OFFSITE ANALYSIS AND MITIGATION <br />This short• plat proposes to construct approximately 3,380 sf of new plus replaced <br />impervious. surfaces. The following table summarizes the new PGIS and NPGIS proposed <br />for the entire short plat. <br />PGIS s <br />NPGIS s <br />Total s <br />Existing <br />1,714 <br />2,241 <br />3,955 <br />Replaced <br />0 <br />0 <br />0 <br />New <br />880 <br />2,500 <br />3,380 <br />New + Replaced <br />880 <br />2,500 <br />3,380 <br />Total .Impervious 12,594 <br />4,741 <br />7,335 <br />The new impervious surfaces will be mitigated using on -site stormwater management <br />BMPs including perforated stub -out connection and soil management. <br />A. UPSTREAM ANALYSIS <br />During the site visit it was observed that due to the site grades and surrounding <br />development/improvements, there is no area that potentially contributes runoff to this short <br />plat from the adjacent roadway to the east. Runoff from the roadway has a vegetated <br />buffer within the right-of-way which will remain that way after construction. There were <br />also a couple yard drains found at the east side of the existing driveway and it was <br />observed that any runoff that could potentially drain from the existing roadway to the east <br />would be collected by the existing stormwater system on Lot 2. It was clear that all off -site <br />flows will pass through or around this short plat and will remain that way after construction. <br />Based on the location of the proposed improvements the existing runoff from the upstream <br />area will continue to flow unaffected around this site after the project has been <br />constructed. <br />B. DOWNSTREAM ANALYSIS <br />Runoff from the proposed disturbance area of the existing short plat sheet flows to the <br />northwest over lawn and leaves the parcel over the western property line. Runoff drains to <br />the southeastern .side of Olympic Blvd and drains north. Runoff drains in the northerly <br />direction along the southeastern side of Olympic Blvd for approximately 400 feet before <br />the road bends. Runoff continues along the side of Olympic Blvd draining in the <br />southeasterly direction. Runoff appears to combine with Pigeon Creek #2, passing <br />beneath Olympic Blvd via a, large diameter culvert. Pigeon Creek #2 drains in the <br />northwesterly direction to Puget Sound. See Figure 5 for a depiction of the project basin <br />and downstream path. <br />The area east of Olympic Blvd where the existing flows drain north is a shallow swale area <br />which has not been maintained. In addition, since the original site visit a new gravel <br />shoulder and concrete driveway has been constructed on the parcel to the north of this <br />project. No culver or other conveyance improvement was constructed within the R/W for <br />that project. In addition, the other existing driveway locations to the north along the east <br />side of Olympic Blvd also do not contain culverts. Therefore, the existing flows in this area <br />Reid SP 15-0714 <br />July 2017 Page 5 <br />n <br />