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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />l)I4 :1"11. 3 - IE3IQI srAlIIl\mE <br /> <br />3.1 J:aLtialllld Majlr DBI.gn StDn& <br /> <br />Every urban ares has t1<<l t1l'!JilI8te !lid d:lsI:ioct drainage systaI!l, 10bether or IDt tDey are <br />actually pl.amEd for am desi&red. ere is the initial systan cuu=pading to the initial (or <br />anlinary) stann recurring at regular intervals. '!be other is the DBjar' systan UAL=.....ding to <br />the DBjor (or extraardinary stann) which is 1D1like1.y to occur IIDI'e often than axe in 100 or <br />lIIJ['e years. Since the effects IIIld routing of stann waters for the DBjot' stann DBY lilt be the <br />SElIE as for the initial stann, all stann drainage plans su1mI.tt.ed for approval shall be <br />su1mI.tt.ed in detail in t1<<l seplI1lte}hlses. ere indirAt"iTJg the effects of the initial stann am <br />the other slDw1ng the effects of the DBjor storm. <br /> <br />a. Initial Storm ProvisI.cIIS. '!be initial stann drainage systan shall be !II des:Igned <br />as to provide protectim against regularly recurring damge, to redu:e street <br />DBi.ntenance costs, to }rOVide an (]["dedy urbm drainage systan, and to provide <br />convenience to the urbm residents. Initial stann drainage can be cmveyed in the <br />curb and gutter ares of the street (subje::t to street rl.....nfication md caJlldty <br />as dpfhlPrl herein) or by stann sewer, channel or other ~ facility. <br /> <br />b. lobjot' Storm Provisims. '!be DBjot' stann drainage systan shall be !II (I""; gTl"'I as <br />IDt to cause DBjor J'LUjICL Ly damge or loss of life fran stann runoff expected fran <br />the DBjot' stann. lobjar' storm flows can be cani.ed in the urbm street systan <br />(within acceptable depth criteria), channels, stann sewers or other facilities. The <br />effects of the DBjot' storm m the initial drainage systan shall be lilted. <br /> <br />3.2 DBI.gn S1mm Fh!quend.es <br /> <br />'lre initial and DBjot' design storDB shalllDt be less than the return period frequenc.i.es <br />listed in Table 1.1 <br /> <br />3.3 Rmaff (l..~..IMtion <br /> <br />Total. stann water runoff shall be lXI1l!l1ted in accordmx:e with the criteria set forth in <br />Sectim 2.2 above. Rumff caoputatioos far: both the initial stann am the DBjor stann shall be <br />su1mI.tted with the proposed stann drainage plan. CanputatiaIs shall be su1mI.tt.ed m fOIlll9 <br /><rlrnilAT to those inrl,ylP<l in these spec.ificatiaIs. <br /> <br />3.4 }1RH......l ~ <br /> <br />'lre jlm-i"",,1 ~ is a simplified stann runoff ll'itpling teclmique that has seen <br />widespread lIS'Ille in the front I'lII'Ige ares of Colorado. The DEtIJxi is acceptable for sizing <br />sewers, aJlverts. and SJBll drainagel.ays lbln the cont:riInting drainage 1Bsins are relati. vel y <br />SJBll. The Ratimal ~ shall not be used in storm runoff caoputatioos for 1Bsins larger <br />than :m a:res. <br /> <br />'lre Rat::l.a1al ~ Farnnla is given as follows: <br /> <br />Q=CIA <br /> <br />(1.1) <br /> <br />..mere Q = storm fl.ow (ds) <br />I = rainfall intensity (in/hr) obtaiIv:!d fran cIm1: 1.1. A = drainage area (a:res) <br />C = runoff coefficient obt:aired fran Table 1.2 <br /> <br />4 <br />