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<br />I. SPRJNGGULCH BASIN <br /> <br />limited capacity of the culvert at 3rt! Avcnue, the main channel system call collvey no more than 2- <br />yearnows, <br /> <br />, <br /> <br />Within the MSA, Spring Gukh is the largest of the: major basins and covers an nrea of appro~imately <br />2.600 acres (see Drawings 3 and 4). This basin originates just south of Oligarchy Ditch. wulllea't <br />of Mdnlosh Lake, and outfall, to St. Vrain Creek ju,t we,t of ,he City's Wastewater Trea!mem <br />Facility. For modelling pUlJ'O"e. the basin was subdivided into 41 sub-basin.' ranging in siz<: from 7 <br />lo IWacres, with an avcrage size of 64 acres. Convey'needemems for /lows lhrough the ba,in <br />included existing stonn.ewers, stre<t,. and the Spring Gulch chatUlel thatJie, br:twccn 17th Avenue <br />amiSt. VrainCreek. <br /> <br />ll1c base hydrologic model used in this study wa,developed primarily for the evaluationofa!te'nmive <br />slt>rmwator management systems. The model is based on the assumption that Spring Golch Basin and <br />Oligarchy Oitch Basin. described below. will remain hydrologically independent of each mher during <br />the various ra,nfall events. H"wever. in reality, the Oligarchy Ditch falls well ,hort of the cnpacity <br />required for major runoff and extensive overflow into the Sp,ing Gulch Basin would occur during <br />larger rainfall event'. ll1c portrayal oflhe Oligarcby Ditch Basin as separate [rom Spring Gulch <br />Basin has been dOlleprimarily to provide a ba,is for the evaluation of ex hting and alternalivc minor <br />,torm systems. The ,eparalion waS used for major runoff only to maintain consislency with the minor <br />runoffsY'leffiSanalysis. <br /> <br />, <br /> <br />r <br />I <br /> <br />I <br /> <br />In general, the Spring Gu1ch Ba,in has a complete lackofsuffieicnldrainagcsy,rems for the control <br />of lOO-year runoff. Main lrunk system eapacilies are gener.ll~' at or below thc required 2-year <br />capacities. Wilh thc possiblc exceplion of portions of Spring Gulch ilself. lhere arc n o trunk system <br />oom'eyaoce stroelU",s that have capacity lo accommodate major Mo,m flows. Furlhennore. due to <br />extellsivelicvelopttentlilroughnuttheba.in,lhcreexi'lalmmtnounJcvelopedprope'tie'lhatcould <br />be use<! forthcdevelopmenlofmajorrunoffdelemionfac:iHlies, <br /> <br />In addilion to the analy,is descrl""d above. a eombined Spring Gulch. Oligarchy DilCh model WaS <br />developed .0 cvaluate potcntial impacts of OligaJchy DilclJ overflows on d,ainage facililies ah.mg <br />Spring Gukh, For lhis ,nalysis five overflow localions aloog the ditch were identified. At lhese <br />lneatiuns runoff in excess of eSl;mated diKh ClIpaeity WaS modelled as diversions to the Sprittg Gulch <br />Basin. <br /> <br />[ <br /> <br />I <br /> <br />I <br /> <br />Il is expected that during a loo-year storm. runoff will moSlly be channeled along streets and <br />concentrate in areas dd,ocd by the generaltopograpllyotlhc basin. Drawings 3 and 4, based on <br />USGS mapping and generally confirmed b~ the de!.Oiled mapping used in the SlIIdy. show that Spring <br />Gulch basin <:<lmains lWo drainage areas lllal converge !\Car 3m Avenue ~nd Martin Slreet, It is <br />anticipated lhal loo-year runoff will concentrate along the low points of these two drainage,. The <br />Western mO'1 of the two drainages generally consists of areas between the interseclions of 15th <br />Avenue at Hover Road and 3rd Avenue at Baker Street. Loomille, Lake and Roosevelt Park are both <br />localed near concentralion points wi,hin the ,ubjCCl drainage basin. The Loomiller Lakedetemion <br />facilily has linleerfccl ill allenuallngdown't",am loo.yearrunoffpeak,. The lOO-yealflows in this <br />,eclion of Spring Gulch range fromaboUlI.OOOto3,()(lOeubic feet per second (cfsl. These now <br />rates generally exceed the exi'ling ,tnrm sewer ,y.tem capacity and .tllowahle Slrect now depths. <br />The East drainage of Ihc Sp,ing Guleh Bas;neoma;n' the Spring Gulth main cha nnelandisgenerally <br />localed between the Oligarehy Ditch and I.t Avenue. Avaibble informalion and computer mudeHiug <br />results indicateth~t the Spring Gulch mrsinehannel fall, well short of lOO-yeareap.1eily. Detemion <br />facilities along the channel are i""de4ualC to "'gnifICamiy reduce loo-year !lows, and due to thc <br /> <br />2, OU('^~CHY DtTCH RAS1N <br /> <br />The Olig~l"Chy Ditch Basin i, the second large'l major basin drainin~ the MSA (See Drawings I and <br />3). In nddition III approximalcly 4,030 acres of drainage area within llte LPA.lhisdrainage,}'stem <br />;S;ISSUrned lO al,o carry rullOff from roughly 4.800 acre, lhat lie 10 the north ofth eLPA Theoffsitc <br />suh-basinseontain numerou, re,crvoirslhalarellOl modelled in the ha.", run forexisling systems. <br />Use ofthcse reservoirs fordctemionbenenl,i,notcurrently legally>>c<:u red and securing their use <br />for such purposes is an improvemcnt propo!ed through Ihe Master Plan, Assumptions made with <br />re'peCt to the capacity of Oligarehy Ditch and overflows;nlo the Spring Gulch Basin are discussed <br />above, Further discussion ofdilCh capaeity is preSenled in lhe Alternatives Evaluation Report, A, <br />noted abovc,lheOligarchy OilCh Bas,n has been nuxlellcd both indcpendenl of and comhincd with <br />the Sprillg Gukh Basin, <br /> <br />, <br /> <br />7 <br /> <br />< <br /> <br />\ <br /> <br />, <br />