320 S CABOT RD 2023-01-26 - Laserfiche WebLink

3. OFF-SITE ANALYSIS AND MITIGATION This site proposes to construct approximately 6,021 sf of new plus replaced impervious surfaces. The following table summarizes the new PGIS and NPGIS proposed. PGIS NPGIS Total (sf) (sf) (sf) Existing (To Be Removed) 0 41 41 Existing (To Be Replaced) 1,051 759 1,810 New+ Replaced 2,348 3,673 6,021 Total Site Improvements 2,348 3,673 6,021 Net New 2,348 3,673 6,021 The new impervious surfaces will be mitigated using stormwater management BMPs including infiltration trenches, permeable pavement (turf stone), sheet flow dispersion and soil management. A. UPSTREAM ANALYSIS During the site visit it was observed that there was no area that drains toward the proposed project area. It was clear that all off-site flows will pass through or around this site and remain that way after construction. B. DOWNSTREAM ANALYSIS Runoff from the northern portion of the lot in the area of the proposed SFR sheet flows to the northeast and into the existing catch basin within South Cabot Rd. From this catch basin, runoff flows approximately 275 feet west through a series of catch basins at which point it enters the main stormwater conveyance system within Beverly Lane. The runoff in this conveyance system flows south approximately 1,000 feet through a series of pipes and catch basins at which point it flows east through 24" pipe approximately 250 feet before • flowing into Beverly Lake. Runoff from the southern portion of the lot in the area of the proposed garage sheet flows to the southwest and across the southern property line. Due to private property and cross fencing, the downstream path could not be followed from this point. However, topographic contour lines on the City of Everett GIS Map portal indicate that runoff flows west approximately 250 feet through private property before naturally infiltrating or being captured by a catch basin just east of Beverly Lane that flows into the same conveyance system within Beverly Lane that has the flow path described in the previous paragraph.At this point, the downstream analysis was concluded. Based on the fact that this project proposes to construct less than 5,000 sf of effective impervious surfaces and that the new roof surfaces will be infiltrated, developed flows are expected to be slightly less than in the existing conditions. See Figure 5 for a depiction of the project basin and downstream path. Fogg-LEXAR SLDS 19-0506 July 2019 Revised:September 2019 Page 5