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3. OFFSITE ANALYSIS AND MITIGATION. <br />This entire project proposes to construct approximately 3,313 sf of new and replaced <br />impervious surfaces. The following table summarizes the new PGIS and NPGIS proposed <br />for the entire project. <br />PGIS <br />s <br />NPGIS <br />s <br />'Total <br />s <br />—Existin To-tre removed <br />0 <br />1-04 <br />0 <br />Existin To remain <br />0 <br />0 <br />0 <br />-New+ Replaced <br />1,211 <br />2,102 <br />3,313 <br />-Total Site Impervious <br />1,211 <br />2 102 <br />3,313 <br />-Net New Impervious <br />1,211 <br />2,102 <br />3,313 <br />The new impervious surfaces will be mitigated using on -site stormwater management <br />BMPs including sheet flow dispersion, infiltration trenches,and soil management. <br />A. UPSTREAM ANALYSIS <br />During the site visit it was observed that no significant off -site area appears to drain <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 during construction. Based on the location of the proposed <br />improvements the existing' runoff from the upstream area will continue.to flow unaffected <br />around this site after the project has been constructed. <br />B. DOWNSTREAM ANALYSIS <br />Runoff -from the proposed Aistur-bance• area of.the existing site sheet flows .northeast..and <br />across the northern' property line. Runoff then flows towards 74th St SE and is collected <br />into _a catch basin, approximately 460 feet from the project area. From the catch basin flow <br />follows the stormwater_ drainage system east approximately 460 .feet until reaching, a <br />vegetated area under, which the stormwater system is diverted and crosses east <br />underneath of 1-5, going approximately 450 feet into a vegetated area on the west side of <br />the freeway. At this point the downstream analysis was concluded. <br />Based on the fact that the new impervious surfaces will either be infiltrated or dispersed <br />on=.site; :and-,that,this project,pro.posies:to-,increase the .total impervious.. surfaces on the site <br />by less than 5,000 sf, the developed flows.are expected to be slightly greater_than in the <br />existing conditions. During the site visit and review of the downstream system, no portion <br />of the system appeared to be inadequate. <br />Haack 75th SFR 20-0817 <br />September 2020 <br />Revised November 2020. Page 5 <br />