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t 7 <br /> 3. OFF-SITE ANALYSIS AND MITIGATION <br /> This site proposes to construct approximately 3,616 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) 1,561 0 1,561 <br /> New+ Replaced 910 2,706 3,616 <br /> Total Site Improvements 910 2,706 3,616 <br /> Net New -651 2,706 2,055 <br /> The new impervious surfaces will be mitigated using stormwater management BMPs <br /> including infiltration trenches, bioretention areas, and soil management. <br /> A. UPSTREAM ANALYSIS <br /> During the site visit it was observed that the area to the southwest of the property drained <br /> towards the project area. It was clear that all off-site flows will pass through or around this <br /> site and 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 /> northeast. Runoff exits the site over the north and east property lines. This runoff is <br /> intercepted by a driveway and directed to the east, where it is collected by a slotted drain, <br /> which appears to connect to the storm drain in Ocean Ave. If this drain becomes clogged, <br /> the overflow would flow over landscaped surfaces and be collected by the same stormdrain <br /> in Ocean Ave. The stormdrain in Ocean Ave runs north and eventually combines with the <br /> stormdrain for W Mukilteo Blvd. At this point the system was lost. During the site visit it was <br /> not clear if the conveyance system directed the water north or east.The conveyance system <br /> in this area has been well maintained and has adequate capacity to accommodate runoff <br /> from the area. It is assumed that the very slight increase in runoff will not negatively affect <br /> the downstream 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 infiltrated on-site, developed flows are <br /> expected to be slightly lower than in the existing conditions. See Figure 5 for a depiction of <br /> the project basin and downstream path. <br /> Chinook Dr SFR 17-0802 <br /> Revised: October 2017 Page 5 <br />