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3. OFFSITE ANALYSIS AND MITIGATION <br />This site proposes to construct approximately 3,910 sf of new impervious surfaces. The <br />following table summari; es the new PGIS and NPGIS proposed. <br />PGIS sf <br />NPGIS sf Total sf <br />Existing <br />0 <br />0 0 <br />New <br />710 <br />3,200 3,910 <br />Total Site Impervious <br />710 <br />3,200 3,910 <br />The new impervious surfaces will be mitigated using on -site stormwater management <br />BMPs including downspout splash blocks, dispersion swales and soil management. <br />A. UPSTREAM ANALYSIS <br />During the site visit it is observed that due to the site grades and surrounding <br />development/improvements, there is some area, from the ally to the west, which <br />potentially contributes runoff to this site. It was clear that all off -site flogs will pass through <br />or around this site and will remain that way after construction. Based on the location of <br />the pnposed improvements the existing runoff from the upstream area will continue to <br />flow unaffected around this site after the project has been constructed <br />Runoff from the proposed disturbance area of the existing site sheet flows to the east and <br />leaves the site over the eastern property line. The runoff continues east from the eastern <br />property line over the vegetated shoulder and then over the sidewalk along the western <br />side of Walnut St. The runoff then drains south along the western side of Walnut St <br />Runoff continues south and enters a catch basin on the corner of Walnut St and 25"' St, <br />approximately 300 feet from the parcel. The catch basin drains south and connects to a <br />combined storm -sewer pipe system centrally located at the same junction and drains east. <br />See Figure 5 for a depiction of the project basin and downstream path. <br />Based on the fact that the new NPGIS runoff will be dispersed on -site with splash blocks <br />and two swales, and will drain runoff over the future amended soils and landscaping <br />before the runoff leaves the site, and that this project only proposes to increase the total <br />impervious surfaces on the site by less than 4,000 sf the developed flows are expected to <br />be slightly more than in the existing conditions. During the site visit and review of the <br />downstream system no portion of the system appeared to be inadequate. <br />Walnut SFR 14-0420 <br />May 2014 Page 5 <br />