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S 7 <br /> 3. OFF-SITE ANALYSIS AND MITIGATION <br /> This site proposes to construct approximately 3,706 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 (TBR) 0 0 0 <br /> New + Replaced 1,000 2,706 3,706 <br /> Total Site Improvements 1,000 2,706 3,706 <br /> Net New 1,000 2,706 3,706 <br /> The new impervious surfaces will be mitigated using stormwater management BMPs <br /> including dispersion trenches, sheet flow dispersion, and soil management. <br /> A. UPSTREAM ANALYSIS <br /> During the site visit it was observed that the area to the south of the property drained towards <br /> the 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 primarily to the <br /> north. Runoff exits the site over the north property line. This runoff is intercepted by a <br /> driveway and directed to the east, where it is collected by a slotted drain, which appears to <br /> connect to the storm drain in Ocean Ave. If this drain becomes clogged, the overflow would <br /> flow over landscaped surfaces and be collected by the same stormdrain in Ocean Ave. The <br /> stormdrain in Ocean Ave runs north and eventually combines with the stormdrain for W <br /> Mukilteo Blvd. At this point the system was lost. During the site visit it was not clear if the <br /> conveyance system directed the water north or east. The conveyance system in this area <br /> has been well maintained and has adequate capacity to accommodate runoff from the area. <br /> It is assumed that the very slight increase in runoff will not negatively affect the downstream <br /> system. <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 dispersed on-site, developed flows are <br /> expected to be slightly higher than in the existing conditions. See Figure 5 for a depiction <br /> of the project basin and downstream path. <br /> Ocean Ave SFR 17-0802 <br /> Revised: October 2017 Page 5 <br />