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3. OFF-SITE ANALYSIS AND MITIGATION <br /> This site proposes to construct approximately 4,860 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 0 0 0 <br /> New+ Replaced 1,616 3,244 4,860 <br /> Total Site Improvements 1,616 3,244 4,860 <br /> Net New 1,616 3,244 4,860 <br /> The new impervious surfaces will be mitigated using stormwater management BMPs <br /> including infiltration trenches, sheet flow dispersion, and soil management. <br /> A. UPSTREAM ANALYSIS <br /> During the site visit it was observed that the area to the north 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 /> southwest and over the southern property line. Runoff sheet flows over a parcel to the south <br /> and into Jefferson PI. Runoff is collected by a catch basin in front of an apartment complex <br /> at 1908 Jefferson PI and runs south down the apartment complex's driveway via a 12"drain <br /> pipe. MapEverett GIS shows that this pipe turns east just beyond the apartment complex <br /> and outlets to a wetland approximately 400 feet southeast of the project site. Any water <br /> leaving the wetland would flow southeast in Woods Creek for approximately 300 feet before <br /> being piped under Broadway Ave and I-5 (approximately 500 feet). From this point <br /> MapEverett GIS shows that Woods Creek flows approximately a quarter mile in a generally <br /> southeastern direction, at which point the downstream analysis was concluded. <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 /> Pyatak Duplex 18-0622 <br /> August 2017 Page 5 <br />