9512 EVERGREEN WAY 2018-01-02 MF Import

�_• CTTY OF E�IERETT ON-SITE DETENTS�N REOUIREMENTS The sizinq af on-site detentions in the City ot EVerett is based on a 2a hour design storm typical o! long duration winter ston.s in Everett. For sites larqer than 5.0 acres, on-site detentions systems will be sized by City oE Everett personnel with the use of EPA's computer based stornwater management model (SWi�44) . A copy of the computer printout vrill be provided to the applicant at his/her requesr. For sites smaller than 5.0 acres, a series of tables shown below allow the applicant to determine his or her on-site det=ntion requirement. These tables were developed based unon a series of comnuter si�ulations of stormwater runoff from a hypothetical 20 acre drainage basin. This 20 acre drainage basin included ten parcels varying in shape and size from o.25 acres to 5.0 acres. How to use the tables: The first step is to determine the desiqn storm criteria for the site in question. Table 1 shows the design stox� criteria for eight drainage basins/areas in Ewerett. The locatians of the drainage basins are shown in figure l. A�ter the design stor.a criteria are determined, the allow:,nle release rate for t�e on-site detention system is deteanined by using tables 1 througa 4. To use tables i. through 4, the existing slope and ground cover conditions oi the site in question need to be determined. The slope of the site must be determined from a topoqraohical sur�ey of the site or by using the City's 1986 aerial topographical maps. The slope refers to the slope of overland flow rather than the slope of the channel or swale that drai.ns the site. The existing ground caver can be classified as woods or forested; as lawns, grass or landscaping; as bare comoacted earth or gravel sur�aced; or as an i.mpervious surface. The minimum size orifice allowed for on-site detention systems is 2". Assuming a borda orifice (contraction coefficient of 0.51) and a head of two feet, the minimum flow rate through a 2" ori£ice is 0.13 cfs. Therefore, allowable release rates of less than .13 cfs should not be used to calculate on-site detention requirements (see example $2 below)- �