Laserfiche WebLink
CN <br />>H[n <br />'4 0 <br />CJTY OF ET%='�� ON -SITE DETEYfIOt7 RED avar <br />Hta H <br />The sizing of on -site detentions in the City of Everett is <br />based on a 24 hour design star= typical of long duration <br />z <br />winter storms in Everett. For sites larger than 5.0 acres, <br />90 <br />OH <br />on -site detentions systems will be sized by City of Everett <br />g <br />personnel with the use of EPA's computer based stormwater <br />a% a <br />management model (5'R:M). A copy of the commuter printout will <br />H <br />be provided to the applicant at his/her request. <br />For sites smaller than 5.0 acres, a series of tables shown <br />her -site <br />gHF3 <br />W H <br />below allow the applicant to determine his or on <br />0tj rA <br />detention requirement. These tables were developed based upon <br />tt <br />a series of computer simulations of stormwater runoff from a <br />drainage <br />yC4 <br />hypothetical 20 acre drainage basin. This 20 acre <br />size from 0.25 <br />o <br />basin included ten parcels varying in shape and <br />acres to 5.0 acres. <br />How to use t''e tables: <br />The first step is to determine the design storm criteria for <br />the design storm criteria <br />the site in question. Table 1 shows <br />for eight drainage basins/areas in Everett. The locations of <br />the drainage basins are shown in figure I. <br />After the design storm c=i eria are determined, the allowable <br />is dete:zi.ned by <br />release_ rate for the on -site detention system <br />1 through 4, the <br />using tables 1 through 4. To use tables <br />slope and ground cover conditions of the site in <br />existing <br />question need to be determined.' <br />f <br />Cai <br />The slope of the site must be dater=fined from a topographical <br />1996 aerial <br />survey of the site or by using the City's <br />t`Ie slope of overland <br />topographical mains. The slope refers to <br />of the channel or swale that drains <br />C <br />flow rather than the slope <br />the site. The existing ground cover can be classified as <br />bare <br />woods or forested; as lawns, grass or landscaping; as <br />:n impervious <br />compacted earth or gravel surfaced; or as <br />rlj <br />surface. <br />The minimum size on icallowed for cn-site detention systems <br />�cP coefficient of <br />is 2". Assuming a orifice (contraction <br />the flow rate through a <br />0.51) and a head of t:o feet, minimum <br />is 0.13 cfs. Therefore, allowable release rates of <br />i <br />2" orifice <br />be dcalculate an -site <br />less than .13 c!s to � <br />uld nexample <br />;2ot <br />detention requirements (see below <br />tw <br />�' 4z <br />' /2 <br />C, 0Io'. ( "e e <br />I <br />