31 SW EVERETT MALL WAY 2018-01-02 MF Import

CT_TY OF SV-E".RETT off-SrTr_ D=r_%m"rox RrnnTVrxrti S The sizing of on -site detentions in the city of Everett is based on a 24 hour design star= typical of long dur tff6n winter storms in Everett. For sites larger than 5.0 acres, on -site detentions systems will be sized by City of Everett personnel with the use of EPA's computer based star --water management model (S*40(). A copy of the computer printout will be provided to the applicant at his/her request. For sites smaller tb! .0 acres, a series of tables shown below allow the applicant to determine his or her on -site detention requirement. These tables were developed based upon a series of computer simulations of stor=water runoff from a hypothetical 20 acre draihace basin. This 20 acre drainage basin included ten parcels varying in shame and size from 0.25 acres to 5.0 acres. icw to use the tables: The first stem is to determine the design staa criteria for the site in question. Table 1 shows the design stoa criteria far eight drainage basins/areas in Everett. The locations of t.:e drainage basins are shown in figure 1. yftar the design star` criteria are determined, the allcurable release rate for t=e:On-site detention system is dete^?zed by using tables 1 :thrauch 4. To use tables 1 tarouct 4, the existing slope and around cover ccnditians of the site in question need to be dete_*-mined. The slope of t:.e site must be deter -?Zed 'ram a topographical surrey of the site or by using the City's 1986 aerial topographical mans. The slave refers to the slave of overland flow rather than the slope of the channel or Swale that drains the site. The existing- ground cover can be classified as woods or forested; as lawns, grass or landscaping; as bare compacted eax-..'i or gravel surfaced; or as an =per7icus surface. *116 The minimum size, orifice allowed fbon-site detention syste=s is 211. Assuming a borda orifice (contraction coefficient of 0.51) and a head of t7o feet, the minimum flow rate through a 2" orifice is 0.13 cfs. Therefore, allowable release rates of less than .13 cfs should not he used to calculate on -site detention requirements (see example -2 below).