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3. OFFSITE ANALYSIS AND MITIGATION <br />This site proposes to construct approximately 7,814 sf of new plus replaced impervious. <br />The following table summarizes the net new impervious surfaces proposed. <br />New and Replaced <br />7,814 <br />Existing (TBR) <br />671 <br />Net New <br />3,804 <br />The new impervious surfaces will be mitigated using on -site stormwater management <br />BMPs including downspout splash blocks, level spreader trench and soil management. <br />A. UPSTREAM ANALYSIS <br />During the site visit it was observed that an asphalt berm was restricting runoff from <br />entering the site therefore the existing roadway to the south does not contribute runoff to <br />the parcel. However, a ditch was found entering the parcel near the southeast property <br />corner from the roadway. The beginning of the ditch could not be located. This ditch was <br />lined with rock and drained northwest passing beneath the existing driveway then drained <br />west leaving the parcel over the western property line. The ditch continued to drain west <br />to a concrete lined manmade pond. As previously mentioned the roadway has an asphalt <br />lip within the right-of-way which will remain that way after construction. It was clear that all <br />off -site flows will pass through or around this site and will remain that way after <br />construction. Based on the location of the proposed improvements the existing runoff <br />from the upstream area will continue to flow unaffected around this site after the project <br />has been constructed. <br />B. DOWNSTREAM ANALYSIS <br />Runoff from the existing improvements appeared to be collected by yard drains, catch <br />basins, French drains, and roof downspouts. Runoff collected by these stormwater <br />catchments appeared to be tightlined to the previously mentioned, existing concrete lined <br />manmade pond. Some runoff from the southern existing improvements appeared to be <br />tightlined to catch basins found at the southern driveway access near Olympic Blvd. <br />These catch basins appeared to pipe runoff in the westerly direction and connect to the <br />existing storm system within the right-of-way of Olympic Blvd. The remainder of the <br />downstream consisted of a series of catch basins and ditches along the eastern side of <br />Olympic Blvd and eventually was collected by Pigeon Creek #2. This creek drains in the <br />northerly direction to the Puget Sound. See Figure 5 for a depiction of the project basin <br />and downstream path. <br />Based on the fact that all of the new NPGIS runoff will be dispersed on -site the developed <br />flows are expected to slightly greater than in the existing conditions. During the site visit <br />and review of the downstream system no portion of the system appeared to be <br />inadequate. <br />Pignataro SFR 15-1008 <br />March 2016 Page 5 <br />Revised June 2016 <br />