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3. OFF-SITE ANALYSIS AND MITIGATION <br /> This project proposes to construct approximately 2,310 sf of impervious surfaces on-site, <br /> however the off-site improvements total approximately 4,998 sf of new plus replaced <br /> impervious surfaces. Therefore, this project proposes to construct approximately 7,218 sf <br /> of new plus replaced impervious surfaces, including off-site and on-site. The following <br /> table summarizes the new PGIS and NPGIS proposed for the entire project area, including <br /> frontage improvements. <br /> PGIS (sf) NPGIS (sf) Total (sf) <br /> Existing 4,174 0 4,174 <br /> Replaced 4,174 0 4,174 <br /> New 500 2,544 3,044 <br /> New + Replaced 4,674 2,544 7,218 <br /> Total Site Impervious 4,674 2,544 7,218 <br /> The new impervious surfaces will be mitigated using on-site stormwater management <br /> BMPs including infiltration trenches, permeable pavement, and soil management. <br /> A:.UPSTREAM ANALYSIS <br /> During the site visit it was observed that due to the site grades and surrounding <br /> development/improvements, the alley to the west drains onto the project site, as well as a <br /> portion of the two parcels directly to the west. To prevent upstream runoff from draining <br /> toward the proposed SFR's, the new alley will be graded as an inverted crown section. <br /> This will allow runoff from the alley and the adjacent parcels to be collected within the alley <br /> and directed south to the existing drainage system. See drainage plans for details. <br /> This layout will allow the large upstream area to-pass.around the..site to the downstream <br /> conveyance system. <br /> • <br /> • <br /> Zillah Lot 1 SFR 18-0605 <br /> Aug.2019 Page 5 <br />