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<br />II. STUJIY AREA D~;SCRlPTION
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<br />MAJOR !lASINS AND SUB-BASINS
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<br />A,
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<br />GENERAL
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<br />The ,'ludy area and Iriburory areas were divided into II major basins based On ropography and
<br />existing drainage syslems (SIX Drawings I IhrOURh 4), Fo, analysis purposes. the major ba.,in. were
<br />further subdivided imo ,manerbasins ba_",d on topography. Slreet runoff. nowvalhs. and existing
<br />drainaRe sy'lems. furthermore. the major ba.,ins were considered lO be composed of three regio",
<br />for which different le,cls ofdelllil ofhydTologicanalysis we'e prepared pUTsuaotlothcobjeetives
<br />of this MasterPlan'
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<br />The Master Plan study area is located in the oorrheasrern parr of Boulder Cnumy. Colorado (",e
<br />Figure-I), The arca lie, approximately 6 mile. cast of the from range fOOlhiIls and is generally
<br />characterized by gemly rolling terrain. The northemp:ut of lhe study areaslo pes generally 10 the east
<br />a,1(! ,outhe"lat approximalcly0.7 perccm, The sOUlhem part oflhe srudyarea gencral1y slopes 10
<br />thc cast at approximatolyO,5 rereem, Surface elevations in lhedelailed.tudyarcarange fromS.lOO
<br />to 4.910 fcct above sea levcl. Vegelation in lhc rural areas of the study area is primarily dryland
<br />brush and grasses along wilh eultivated tields,
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<br />1, The MSA represented the area requiring the most deroiled study for the estimation of runoff
<br />roresanddrainagemuetoreeapacitiesforexiSling and prop"'eddevelopmentconditiorn.
<br />Derailcdanalysesofpercemirnpel:\liousness.drainageflowpalhs. basin divide, and <;apaciry
<br />of existing conveyance ,U'uctun:s were performed in thi.areJ.
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<br />The City is travel"llcd by St. Vr"in Creek whieh drains an area of abom 350 "Iuarc miles from !he
<br />City weM 10 the Continental Divide;, Left Hand Creek al,o drains a relatively large mountainous
<br />watershffi. and c>ulfal\s 10 St, Vrain Creek ncar the downstream boundaryoflhesludyatea. Several
<br />well defined drainages p"" generally from west 10 east through the smdy area and include Lykins
<br />Gukh. Dry Creek No, I and Dry Creek. With lhe exception of Dry Creek. all of these drainages
<br />convergewilhlheSt. Vrain Creek UpStTeam of the eaStcm bOllndary of the srudy area. The srudy are a
<br />is transeetcd b,' numerous irription ditches, wirh lhe off.site basin' a IsoeontainingnumerOll,storage
<br />reservo;r.l.
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<br />The LPA was stodied in less derail. Wilh emphasis gi\'en to the enirnation Jf runoff under
<br />a projected full.dcvelopment scenario such lhar majorslru<:tural needs eo uldbe identified for
<br />Ihe alternalives evalll-"tion component of the Master Plan, Delention fac;lities and major
<br />channel. arc the only improvements pwposcdby lheMnsler Plan for the LPA
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<br />3.
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<br />Rninfall-runoffmodclling was performed for sub-basins omside the LPA thar conlrihule flow,
<br />1<> the Planning Area. Such now. were a"",med to be inrere"fltcd or coneentraHon points
<br />along lhe L?A boun<lar)i and these flows were rOUled lhruugh the cxisling and recommended
<br />allemative L?AiMSA drainage systems. For the purposes of this analysis. thi" area was
<br />eonsidered 10 be undeveloped or ro include detcntion fa<;ilirles tobee onstruetedthrough
<br />fmuredcvelopmcntandwhiehrelea.<eathislorieflow rates.
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<br />Through the guidelines set fonli in Ihe LACP adopted by the Ci'y. a lhree.t;er 'ystem hos been
<br />implemented hy the City", a mecha"i,m to ma""ge the e,pansion of the City into surrounding
<br />unincorpora'ed arCaS. The LACP de'ignate, incorpo",tc'd longmon! .g Tier One. rhe Munieip.,1
<br />Service Mea (MSA). in which the City provides or intend. to provide urban service" At the option
<br />of the Cily. unanne,ed areas may also be included 10 the MSA. Tier Two is known as the Longmom
<br />Planning Area (LPA) and is comJX'!'ed of unanncxed area. mat the City has indu<led in its plan f",
<br />cxpansionofscl:\liecs, TIie third tier is theSt. Vroin Valley Planning Area and ineludcs areasour,ide
<br />the LPA lhal have potential di,c<;!or indirecteffcc1-S on ,he City A, discussed helow. this stody
<br />primarily deJI. with stOrm drainage conditions and needs within the MSA and LPA, Areas within
<br />the MSA are mostly near their fully develored condition. while much of the LPA is sparsely
<br />dcvclop"d .ndundcrgoing rapid gTowth al lhepresent time
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<br />A dIscussion ot the major fe"rure. and delineation of sub.basin, w;tllin the major basins is presented below.
<br />Funher discussion of e,isting 'ystems and system modelling alsumplions nre included in Ihe Alternatives
<br />baluallonRepon,
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