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<br />PreViOllsStudies
<br />Four previous hydrologic studies have been prepared forDiny Woman and Crystal
<br />Creeks. These studies are Fwod Hazard Identification Report, Crystal Creek and Dirty Woman
<br />Creek, Monument, Coloruoo prepared by the Colorado Water Conservation Board (CWCB) in
<br />February 1987, the FEMA Flood Insurance Study, prepared in 1986. a study by Tn-Consultants
<br />on Crystal Creek through Casey's Subdivision, and an ongoing study by Resource Consultants,
<br />loc. which included overall hydrology forDiny Woman and CryslaI Creeks.
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<br />Hydrologic data for each sub.basin was developed using the Soil ConversalionService
<br />(SCS) Dimensionless Hydrograph Model within HEC-l. Basin characteristics required for the
<br />SCS Dimensionless Hydrograph Melhoo using HEC-l are area. curve number, and SCS lag time
<br />(Tiag). Basin areas were planimelercd to determine their area in square miles. Curve numbers
<br />were determined for each sub-basin utilizing the hydrologic soil type, ground cover (both
<br />existing and proposed), and Tables 5-4 and 5-5 of the City/Coullly Criteria Manual. The
<br />calculation of the SCS lag time was hased upon its relationship to time of concentration (tc).
<br />The time of concentration for each suh"basin was detennined by adding !ravel times for overland
<br />flow, channel flow, and pipe flow from the hydrologically most distalll poilll in the basin to the
<br />outfall poilll. The parameters used in these calculations were determined from available
<br />topographic maps, soils maps, aerial phOlogrnphy, land use maps, and field investigation, Figure
<br />2-2 shows the hydrologic soil types and Figure 2-3 shows the land use types within the Dirty
<br />Woman and Crystal Creek drainage basins,
<br />Sub-basin flows were routed and/or combined with other sub-basin flows to establish
<br />discharges at various poillls throughout the drainage basin, Routing off1ows was accomplished
<br />using the kinematic wave method. The ldnemalic wave method is based upon characteristics of
<br />each reach including length, slope, Manning's roughness, type of channel, bottom width of
<br />channel,andchannelsideslope. Rowsfromupstrenm sub-basins or design poinl$ (points of
<br />combined flow) wereroured through thc channel reach derennining the channelst orage and lag
<br />time for the routing. At design points, two or more hydrogrdphs were combined to determine the
<br />outflow hydrogr:l.ph at that particular point (in the inpll! to the HEC.] computer model design
<br />points are designed with the prdix "DP" and routing elements are deSignated wllh the prefix
<br />"RT").
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<br />III. HYDROLOGIC ANALYS!S
<br />A hydrologic analysis was conducted in order to determine peak discharges and nmoff
<br />volumes for various storm types, and basin development conditions. This data was used in the
<br />evaluation of existing flooding problems and in the evaluation of alternative plans, A Technical
<br />Addendum has been prepared under a separate cover. Detailed information with respect to the
<br />hydrologic analysis is contained 'Within the Addendum along with hydraulic information on the
<br />basin.
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<br />RllnoffModel
<br />The runoff mo:iel used to detennine the peak flows and volumes within the study area is
<br />the HEC-I computer program developed by the COE Hydrologic Engmeering Center_ The use
<br />of this hydrological model is in conformance with the Gty of Colorado SpringsfEl Paso Counl)'
<br />Drainage Criteria Manual. The HEC.I hydrologic modeling W3S approved by El Paso County
<br />for use in lhlS Dnunage Basin Planning Study.
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<br />BasinCh3racteristics
<br />The study area subject to this hydrologic evaluation is the Diny Woman and Crystal
<br />Creek Drainage Baslns_ The Dirty Woman and Crystal Creek Basins were divided into seven
<br />major regional basins for analysis purposes. Diny Woman Creek includes the main stem, Lake
<br />Fork, Middle Fork, Nonh Fork, South Fork, and Upper Diny Woman Creeks. Crystal Creek is a
<br />regional sub-basin itself. The major regional basins arc shown in Exhibit 1 in the map pocket.
<br />The regional basins have been fUMher subdivided into sub-basins, Each sub-basin hilS
<br />beenassigncdanalphanumeri~dcsignation. This designation is bcrween four and six characteT1i
<br />in length. The idphabctical characters in thedcsignation rcpresent the regional basin in which
<br />the sub,basin is immediately tributary, The numbers in the sub,hasin designation are unique to
<br />each sub-basin, Eaeh sub-basin has a unique number identifying it. Howe\'er, with many
<br />different sub-hasins the adilition of the alphabetieal characters allow foraq uickdeterminationof
<br />where within the drainage basin that particular sub-basin lies,
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<br />ImnerviollsLandDensitv
<br />Land use for existing and future basin conditions were determined using a combioarion
<br />of ;wning maps, City/County Comprehensive Plan(s), aerial photographs, lnl.nspollation planes),
<br />and other related land use documents. Land use density and corresponding curve number~ were
<br />determined in accordiln~"e with the City/County Dnlinage Criteria ManuaL Soil Conservation
<br />Service (SCS) curve numbers for both the existing and future conditions for the both Diny
<br />Woman and Crystal Creeks are presented in the Hydrology Report Tcchnical Addendum,
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<br />Desi~n Rainfall
<br />The CilylCounly Dral,wge Criteria Manual identifies a number of procedures to be used
<br />in developing storm rainfall for lflput into hydrologic mooch, The criteria manual stipulates that
<br />two stonn durations (2-hour and 24.hour) be checked to determine the critical design stonn (the
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