10725 19TH AVE SE 2023-04-07 - Laserfiche WebLink

MR#4 -Preservation of Natural Drainage Systems and Outfall The project will maintain and preserve the existing downstream pipe conveyance system.A detention pipe facility has been sized to maintain the storm water runoff to predeveloped site conditions. Tight lining of downspouts will be utilized to route storm-water to the storm-water detention pipe facility and control structure of the site prior to discharge without causing erosion or drainage impacts to the downstream neighboring properties. UPSTREAM DRAINAGE The upstream site area consists of developed both multi-family and single-family homes. Considering the site and surrounding area topography and through field observation,there are nominal amount of tributary runoff entering the site if any.No identifiable watercourses were observed crossing the site. DOWNSTREAM DRAINAGE Stormwater runoff from the development area of the site collects onsite and sheet flows to the onsite existing catch basins and conveyance system along the west boundary of the site. Storm runoff is conveyed to the existing storm drainage conveyance system along 19th Ave SE flowing south along the Everett-Bothell HWY right-of-way to Silver Lake.No impacts to the downstream drainage system are anticipated with the development of the Zerzura Townhouses site. MR#5 - On-site Stormwater Management The total new plus replaced hard impervious surface is 12,632 sq. ft. The total effective, pollution-generating impervious surface(PGIS) is 4,795 sq. ft. Stormwater management requires implementation of on-site stormwater BMPs in accordance with project thresholds, standards, and lists to infiltrate, disperse, and retain stormwater runoff on site to the extent feasible. The following BMPs should be considered in the order as listed in "List#2" of the 2014 Stormwater Management Manual for Western Washington Manual in Volume I, Chapter 2, section 1-2.5.5. Lawn and landscaped areas: The project will implement Post-Construction Soil Quality and Depth in accordance with BMP T5.13. Roofs: 1. Full Dispersion in accordance with BMP T5.30—This BMP is considered not feasible on this project due to site limitation, soils conditions and ground water. 2. Bioretention -This BMP was considered not feasible due the the measured native soil infiltration rate of less than 0.3 in/hour, site constrains and limited space availability to construct such a facility.