Laserfiche WebLink
r r <br /> • <br /> 3. OFF-SITE ANALYSIS AND MITIGATION <br /> This site proposes to construct approximately 3,978 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 (To Be Removed) 0 967 967 <br /> Existing (To Be Replaced) 0 1,228 1,228 <br /> Existing (To Remain) 3,507 1,599 5,106 <br /> New+ Replaced 1,216 3,005 4,221 <br /> Total Site Improvements 4,723 4,238 8,961 <br /> Net New 1,218 1,777 2,998 <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 no significant offsite area drained toward the project <br /> area. It was clear that all off-site flows will pass through or around this site and remain that <br /> way after construction. <br /> B. DOWNSTREAM ANALYSIS <br /> Runoff from the area of disturbance sheet flows to the north-overmoderate landscaped <br /> slopes. Runoff flows north toward the steep slope that begins approximately 90 feet south <br /> of the BNSF Right of Way and continues north down the steep, vegetated slopes into the <br /> BNSF Right of Way until the slope levels out just south of the Great Northern Railway. The <br /> slope was not traversed due to safety concerns and vegetation obscured the view of the <br /> bottom of the slope. However, it is likely that a ditch system on the south side of the railway <br /> collects runoff and a culvert likely conveys runoff north under the railway system and into <br /> Puget Sound. <br /> Based on the fact that this project proposes to construct less than 5,000 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 /> Haack-Taylor Drive 19-1112 <br /> January 2020 <br /> Revised:.October2020 Page 5 <br />