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3. OFF -SITE ANALYSIS AND MITIGATION <br />This site proposes to. construct approximately 3,178 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(SI. <br />Total <br />Existing To Be Removed <br />81 <br />0 <br />81 <br />Existing To Be Replaced) <br />834 <br />0 <br />834 <br />New+ Replaced <br />1,784 <br />1,394 <br />3,178 <br />Total Site Improvements <br />1 1,784 <br />1 1,394 <br />3,178 <br />Net New <br />1,784 <br />1, 394 <br />3,178 <br />The new impervious surfaces will be mitigated using stormwater management BMPs <br />including infiltration trenches and soil management. <br />A. UPSTREAM ANALYSIS <br />During the site visit it was observed that there was no area that drains toward the proposed <br />project area. It was clear that all off -site flows will pass through or around this site and <br />remain that way after construction. <br />B. DOWNSTREAM ANALYSIS <br />Runoff from the proposed disturbance area of the existing site sheet flows to the east across <br />the eastern property line. The runoff then sheet flows onto the pavement of 27th Dr SE and <br />is immediately captured by a catch basin within the roadway. A series of catch basins and <br />conveyance pipes carry the runoff approximately 175 feet to the east where another pipe <br />flows out of the last catch basin and into what appears to be a small wetland area. Runoff <br />flows approximately 300 feet to the southeast in this low area where it enters a culvert that <br />flows approximately 200 feet to the southeast into a catch basin within 103rd PI SE. From <br />this point, runoff flows approximately 130 feet to the northeast through conveyance pipes <br />and a catch basin before it is discharged into a freshwater forest/shrub wetland <br />approximately 450 feet west of Hilton Lake. Runoff continues to flow gradually in this wetland <br />area approximately 450 feet to the east before entering Hilton Lake. At this point the <br />downstream analysis was concluded. <br />Based on the fact that this project complies with the LID Performance Standard, developed <br />flows are expected to be less than existing flows. See Figure 5 for a depiction of the project <br />basin and downstream path. <br />27th Dr SE - Haack 19-0508 <br />May 2019 Page 5 <br />