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3. OFF-SITE ANALYSIS AND MITIGATION <br /> This site proposes to construct approximately 3,049 sf of new plus replaced impervious <br /> surfaces. The following table summarizes the new PGIS and NPGIS proposed. <br /> PGIS NPGIS Total <br /> (sf) •(sf) (sf) <br /> Existing 2,630 0 2,630 <br /> Replaced 0 268 268 <br /> New+ Replaced 0 3,049 3,049 <br /> Total Site Improvements 2,362 3,049 5,411 <br /> Net New 0 2,781 2,781 <br /> The new impervious surfaces will be mitigated using stormwater management BMPs <br /> including a perforated stub-out connection and soil management. <br /> A. UPSTREAM ANALYSIS <br /> During the site visit it was observed that there was no area that drains toward the proposed <br /> project area. It was clear that all off-site flows will pass through or around this site and <br /> remain that way after construction. <br /> B. DOWNSTREAM ANALYSIS <br /> Runoff from the proposed disturbance area of the existing site sheet flows to the east toward <br /> the eastern property line. Runoff sheet flows over vegetated steep slopes on the east side <br /> of the project site. Runoff drains in the easterly direction to the right-of-way of Grand Ave. <br /> Runoff appears to be collected by an existing catch basin within the right-of-way. This catch <br /> basin directs runoff east to an existing stormwater system. The remainder of the <br /> downstream consists of a series of catch basins and underground pipes. <br /> Based on the fact that this project proposes to construct less than 5,000 sf of impervious <br /> surfaces and that the new roof surfaces will be tightlined to the base of the steep slope, <br /> developed flows are expected to be slightly higher than in the existing conditions. See <br /> Figure 4 for a depiction of the project basin and downstream path. <br /> Grandview Ave Duplex 18-1001 <br /> January 2019 <br /> 715 <br />