9931 19TH AVE SE 2018-01-02 MF Import

2.2 RUNOFF CURVE NUMBERS The site is partially developed and partially vegetated with grass and trees. The runof equals 86 for existing pervious conditions. The runoff CN equals 86 for developed pert conditions. The runoff CN equals 98 for pre- and post -developed impervious coedit Landscape areas along the perimeter of the site bordering SR-527 and 100th Street S.E. drain to the existing streets, and since the curve numbers for pre- and post -level conditions for this area remain the same, this area will not be considered bypass nor will it be retained for detention. Runoff from the landscaping areas on the remainder of the site will be collected and conveyed into the detention systems. 2.3 TIME OF CONCENTRATION The time of concentration for the pre -developed condition has been determined by identifying the path of runoff from the hydrological most distant point of the site to the natural point of discharge. Figure 2 identifies the travel distance as approximately 180 lineal feet at a slope of approximately 2 percent. Tc equals 22.53 minutes for pervious area and 0.88 minutes for impervious area. The time of concentration for developed areas can be found in the Basin Summary section of the Hydrological Calculations. 2.4 SOILS See Figure 4 for a copy of the Soils Map for this project. The map indicates that the site's { soils are classified as Alderwood. Alderwood soil falls under the hydrological soils Group "C." 2.5 DETENTION VOLUME CALCULATIONS See Appendix "A" for the czlculations performed to size the detention vault. As previously mentioned, the City of Everett requirements utilize the Department of Ecology's Stormwater Management Manual for the Puget Sound Basin criteria. The volume required equals 3,434 cubic feet. Storage volume was increased by 50 percent based on the percentage of impervious area under developed conditions, which was conservative. 3.0 STORMWATER QUALITY CONTROL Treatment of stormwater will be provided prior to its release to the downstream drainage course. Biofiluation of all stormwater from the entire site will occur within a grass lined swale situated along the southern end of the site. The biofiltration swale will be appropriately designed to assure conveyance of stormwater through a minimum of 100 feet of vegetated ground cover. The biofiltration Swale will be situated downstream of the stormwater detention vault. The biofrltration swale was designed and sized per City of Everett criteria, which requires the 6-montlt/24-hour design storm be utilized for treatment, with a minimum residence time of 5 minutes and a Manning's *No value of 0.20. This swale has a width of 1 foot at the bottom. -2- 2918.011 [JJ/sml