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t r <br /> 3. OFF-SITE ANALYSIS AND MITIGATION <br /> This project proposes to construct approximately 17,502 sf of new plus replaced <br /> impervious surfaces. The following table summarizes the new PGIS and NPGIS proposed <br /> for the project site. <br /> PGIS (sf) NPGIS (sf) Total (sf) <br /> Existing 14,283 3,007 17,290 <br /> Existing (TBR) 14,283 3,007 17,290 <br /> New+ Replaced 7,348 10,154 17,502 <br /> Total Site Impervious 7,348 10,154 17,502 <br /> Net New -6,935 7,147 .212 <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 little area to the west that potentially contributes <br /> runoff to this project site. Runoff from the existing lots to the west is mostly collected as <br /> part of those previous developments. There is an existing underground storm conveyance <br /> system running along the south property line,just north of the existing building to the south <br /> of the project site. This 8-inch pipe appears to direct runoff from the alley, across the <br /> property and connects to the sewer main within the right-of-way of McDougall Ave. It was <br /> clear that all off-site flows will pass through or around this project 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 site after the <br /> project has been constructed. <br /> B. DOWNSTREAM ANALYSIS <br /> The runoff in the existing conditions will spread over the pervious and impervious surfaces <br /> on-site and drain toward the eastern property line. The runoff is then collected within the <br /> conveyance system in the McDougall Ave R/W. The downstream system for this project is <br /> comprised of surface flow within the roadway curb/gutter and then the flows are collected <br /> in a catch basin near the intersection of 33rd St and McDougall Ave to the north of this <br /> project. That conveyance system appears to connect to the sewer and flows to the east <br /> within underground pipes and catch basins to Smith Ave. The downstream flows appear <br /> to be fully within existing road RNV and engineered conveyance systems to the % mile <br /> downstream point for this project. <br /> Based on the fact that the existing site is currently developed and that this project only <br /> proposes a net new impervious surface total of 212 sf, the developed flows are expected <br /> to be only slightly more than in the existing conditions. <br /> McDougall Warehouse . 17-0212 <br /> April 2017 Page 5 <br />