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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 0 0 <br /> New + Replaced 780 1,622 2,402 <br /> Total Combined Site 780 1 ,622 2,402 <br /> Impervious <br /> Net New 780 1 ,622 2,402 <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 less than 5,Oxsf <br /> more than the existing conditions the developed flows are expected to only be slightly than <br /> in the existing conditions. During the site visit and review of the downstream system no <br /> portion of the system appeared to be inadequate. <br /> Chestnut North Lot 16-0213 <br /> February 2016 Page 5 <br />