8920 EVERGREEN WAY THE RESERVE 2025-07-25

Stormwater Site Plan Report CG Project#13008.20 Chapter III — Site Development Plan with On-Site Stormwater Management Chapter 111 Summary: Narrative City BMP 12:20 Post Construction Soil Quality and Depth Text The proposed area of work is to be collected in catch basins,tying into the site's existing storm system. The proposed work adds approximately 1.82 acres of new/replaced impervious area, which will be primarily mitigated using a large detention vault located underneath the upper parking lot for the site. It is first necessary however,to evaluate the stormwater management BMPs located in Volume III, Chapter 12 Several design strategies will be used to assist in on-site stormwater management and are listed in the following paragraphs. Several suggested design strategies that involve more infiltration and dispersion type techniques were not incorporated due to site restrictions. The site is being developed to a degree to which there are no areas for flowpaths for any sort of dispersion system. The geotechnical report included in Section VI of this report indicates that infiltration techniques should also not be used on the site. City BMP 12.20 is required for areas that are disturbed during construction and will be pervious after construction.To the maximum extent possible, the duff layer, native topsoil and natural vegetation on the site will be left undisturbed.This can only occur on the north side of the property, in the 15' buffer. Stormwater runoff from proposed impervious surfaces will be kept as close to existing conveyance systems as possible to minimize site impact. This will be done by tying the outfall from the detention and water quality system into the main on Evergreen Way. Prior to this,the runoff will be collected in site catch basins and routed into the existing conveyance lines. Stormwater runoff below the detention vault will need to be pumped toward the vault. Lastly, the proposed site grading is designed in an attempt to limit the cut and fill quantities as much as practicable. The parking area is relatively flat in the area of the existing asphalt, then steepens to 10% (fire department maximum allowable slope) in the middle of the site,then the parking area flattens out again to roughly match the existing shape of the site.The building also steps accordingly. Several of the catch basins will be installed with floatable material separator/gas trap tees on their outlets, in order to provide a degree of water quality to site runoff prior to its detention. o C G 250 4th Ave South, Suite 200 ENGINEERING Edmonds, WA 98020