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3. OFF-SITE ANALYSIS AND MITIGATION <br />This site proposes to construct approximately 2,402 sf of new impervious surfaces, <br />including the north and south parcels. The following table summarizes the new PGIS and <br />NPGIS proposed for the entire site. <br />PGIS (sf) NPGIS (sf) Total (sf) <br />Existing (TBR) 0 1,921 1,921 <br />New + Replaced 780 1,622 2,402 <br />Total Combined Site 780 1,622 2,402 <br />Impervious <br />Net New 780 -299 481 <br />The new impervious surfaces will be mitigated using on-site stormwater management <br />BMPs including downspout splash blocks, shallow swales 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 site. <br />It was clear that all off-site flows will pass through or around this site and will remain that <br />way after construction. Based on the location of the proposed improvements the existing <br />runoff from the upstream area will continue to flow unaffected around this combined site <br />after the project has been constructed. <br />B. DOWNSTREAM ANALYSIS <br />Runoff from the proposed disturbance area of the existing site sheet flows to the <br />north/northeast toward the northern property line. Runoff sheet flows toward the northern <br />property line and northeast corner and leaves the site over the northeast property corner. <br />Runoff continues to drain from the site over vegetated lawn within the right-of-way of <br />Chestnut St. Runoff drains over the lawn and sidewalk to Chestnut St and drains north <br />along the curb on the west side of the existing roadway. Runoff continues to drain north <br />along the curb to a catch basin found at the intersection of Chestnut St and 18th St. The <br />catch basin connects to an underground storm pipe system and drains west down to the <br />alley then drains north. See Figure 5 for a depiction of the project basin and downstream <br />path. <br />Based on the fact that the net new impervious surfaces for this site are only 481 sf more <br />than the existing conditions the developed flows are expected to only be slightly than in <br />the existing conditions. During the site visit and review of the downstream system no <br />portion of the system appeared to be inadequate. <br />Haack Chestnut SFR 15-0118 <br />October 2015 Page 5 <br />