1517 132ND ST SE 2024-12-09 - Laserfiche WebLink

3.0 OFFSITE ANALYSIS Qualitative Analysis A qualitative downstream analysis was performed for the project site. Due to the development surrounding the subject property there is no upstream basin which discharges stormwater runoff to the project site. All available information regarding existing and potential water quality, runoff volumes and rates, flooding and stream bank erosion problems within the study area have been reviewed. Reviewed material included NRSC soil information, City of Everett maps and assessment reports prepared for other projects in the area. See Appendix A for appropriate maps and information. With the proposed development, stormwater will be collected on-site and conveyed East through a 12" pipe to a pond about 100 feet east of the property. Overflow from the pond is conveyed south through 18" and 24" pipes to a drainage ditch. The upstream downstream flow path was followed and observed for 1/4 mile using Snohomish County GIS. Pipe information is displayed in order from upstream to downstream: 1) 50 feet of 12" 0 corrugated metal pipe @ 0.40% 2) 87 feet of 12" 0 corrugated metal pipe @ 1.78% 3) 134 feet of 12" 0 corrugated metal pipe @ 0.12% 4) 145 feet of 18" 0 corrugated metal pipe @ 0.81% 5) 200 feet of 24" 0 corrugated metal pipe @ 0.57% 6) 70 feet of 24" 0 corrugated metal pipe @ 0.96% The five pipe runs mentioned above total a flow path of approximately 679 feet. There were no apparent signs of capacity issues or flooding issues identified in this downstream system. See Appendix A for the downstream map. 4.0 STORMWATER CONTROLS Existing Site Hydrology Stormwater runoff from the existing site is conveyed off site through a series of catch basins and HDPE pipes. Once offsite, stormwater is conveyed East to a detention pond in the adjacent lot. See Appendix A for a pre-developed basin map. Developed Site Hydrology The proposed improvements will reutilize multiple on site catch basins to direct stormwater to the existing downstream conveyance system. The overall pattern of discharge will remain the same. Multiple new catch basins and conveyance pipes are proposed to direct water towards the existing downstream conveyance system.