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0 <br />n <br />3. OFF -SITE ANALYSIS AND MITIGATION <br />This site proposes to construct approximately 8,277 sf of new plus replaced impervious <br />surfaces. The following table summarizes the new PGIS and NPGIS proposed. <br />PGIS <br />s <br />NPGIS <br />s <br />Total <br />s <br />Existing To Be Removed <br />. 0 <br />0 <br />0 <br />.Existing To Be Replaced) <br />0 <br />0 <br />0 <br />New + Replaced <br />3,896 <br />4,381 <br />8,277 <br />Total Site Improvements <br />3,896 <br />4,381 <br />8,277 <br />Net New <br />3, 896 <br />4,381 <br />8,277 <br />The final impervious roof and driveway surfaces for this project will be mitigated using on - <br />site stormwater management BMPs including an engineered conveyance system with a <br />discharge at the base on the bluff adjacent to the BNSF railroad embankment. Soil <br />management will also be used to mitigate the portions of the site that have been graded, <br />compacted or require soil amendments meeting BMP T5.13. <br />A. UPSTREAM ANALYSIS <br />During the site visit it was observed that due to the site grades and surrounding <br />development/improvements that the primary area contributing runoff to the project site was <br />limited to the roadway to the southeast (Madrona Ave). A slightly raised curb cut at the <br />driveway entrance will direct all road runoff southwest as in the existing conditions. It was <br />clear that all off -site flows will be allowed to pass through or around this site and will remain <br />that way after construction. <br />B. DOWNSTREAM ANALYSIS <br />Runoff from the area of disturbance sheet flows to the northwest over moderate to steep <br />landscaped slopes. Runoff flows northwest toward the steep slope that begins just within <br />the BNSF R/W and continues northwest down the steep vegetated slopes until the slope <br />levels out just southeast of the Great Northern Railway. The slope was not traversed due to <br />safety concerns. However, a ditch southeast of the railway was observed approximately 100 <br />feet northeast of the site runoff discharge at the diffuser. tee. Runoff likely flows gradually to <br />the northwest and into this ditch from which a culvert likely conveys runoff northwest under <br />the railway system and into Puget Sound. <br />Based on the fact that this project proposes to construct less than 10,000 sf of impervious <br />surfaces and .all proposed hard surface runoff will be collected and tightlined down the slope, <br />developed flows are expected to be slightly greater than the existing conditions. See Figure <br />5 for a depiction of the project basin and downstream path. <br />Mazza SFR 19-1014 <br />November 2019 <br />-Revised: 'April'2020 Page 5 <br />