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<br />I <br /> <br />'I <br /> <br />The <br /> <br />mapping prepared for this study. <br /> <br />the <br /> <br />were measured directly from <br /> <br />watershed geometries <br /> <br />HYDROLOGIC ANALYSIS <br /> <br />III. <br /> <br />and <br /> <br />maps <br />imperviousness for each sub-watershed was <br /> <br />land use <br /> <br />impervious percentages used in this study were derived from City and County <br /> <br />I <br /> <br />development plans and aerial photography. The percent <br /> <br />GENERAL <br /> <br />A. <br /> <br />estimated as the composite value resulting from a weighted area average computation for the various <br />within the limits were examined <br /> <br />in order to <br /> <br />detai <br /> <br />111 <br /> <br />study <br /> <br />areas <br /> <br />Several <br /> <br />. . <br />ImpervIOusness zones <br /> <br />runoff peak flows and <br />The runoff information <br /> <br />The hydrologic analysis for the study area was performed to define the storm <br />volumes for the various sub-watersheds <br /> <br />I <br /> <br />more accurately determine the appropriate composite imperviousness values. <br /> <br />ofthe Big Dry Creek watershed. <br /> <br />drainage problems, <br /> <br />potential <br /> <br />ities, <br /> <br />the existing drainage faci <br /> <br />to evaluate <br /> <br />subsequently used <br /> <br />IS <br /> <br />I,' <br /> <br />group as <br />were then <br /> <br />soi <br /> <br />were assigned to each hydrologic <br /> <br />infiltration parameters <br /> <br />Detention, retention and <br /> <br />alternative drainage improvements, and in the development of the Flood Hazard Area Delineation <br /> <br />Parameters <br /> <br />and 2-2 ofthe Runoff Chapter of the USDCM. <br /> <br />in Tables 2- <br /> <br />recommended <br /> <br />(FHAD) and preliminary design of improvements <br /> <br />I <br /> <br />summary of the CUHP <br /> <br />A <br /> <br />zones. <br /> <br />the predominate soil <br /> <br />determined for each sub-watershed using <br /> <br />parameters used for the analysis are presented in Table ITT-2 <br /> <br />Peak flows at each sub-watershed <br /> <br />The study area was divided into a total of 162 sub-watersheds <br /> <br />The study <br /> <br />and I OO-year recurrence mtervals <br /> <br />50- <br /> <br />0-, <br /> <br />5-, <br /> <br />outlet point were calculated for the 2-, <br /> <br />I <br /> <br />STORM WATER MANAGEMENT MODEL <br /> <br />D. <br /> <br />C <br /> <br />thru <br /> <br />area and sub-watershed areas are shown in Figures 1II-IA <br /> <br />I <br /> <br />The personal computer version of the UD&FCD SWMM was used to route the CUHP-generated <br /> <br />DESIGN RAINFALL <br /> <br />B. <br /> <br />The routing element <br /> <br />UDSWMM3 <br /> <br />used was <br /> <br />mode <br /> <br />The version of the <br /> <br />runoff hydrographs. <br /> <br />I <br /> <br />slope, <br /> <br />pipe, etc.), length, <br /> <br />type (e.g. channel <br /> <br />parameters required for SWMM are: element number, <br /> <br />The modified 2-hour design storm distributions recommended by the Urban Storm Drainage Criteria <br /> <br />routing elements used were based on the existing <br /> <br />The type of SWMM <br /> <br />value <br /> <br />n <br /> <br />and Manning's <br /> <br />routing <br />The incremental <br /> <br />flow <br /> <br />for the <br /> <br />were used to produce runoff hydrographs <br /> <br />) <br /> <br />(USDCM)(Reference <br /> <br />Manual <br /> <br />I <br /> <br />drainageway configurations and shapes. <br /> <br />Model (SWMM)(Reference 2). <br /> <br />required in the UD&FCD Storm water Management <br />rainfall <br /> <br />the <br /> <br />For existing channels, <br />the SWMM <br /> <br />sewers, <br />was estimated using Equation 2 recommended in <br /> <br />used. <br /> <br />value of 0.020 was <br /> <br />"n' <br /> <br />a Manning's <br /> <br />and storm <br /> <br />F or streets <br /> <br />in the Colorado Urban Hydrograph Procedure (CUHP)(Reference 3) <br />were used for th listed <br /> <br />is study and are <br />distribution for <br /> <br />depths for 5-minute increments <br />isted <br /> <br />depth option was used <br />runoff module. Zone I rainfall <br /> <br />I <br /> <br />manual <br /> <br />users <br /> <br />Manning's "n" value <br /> <br />storm <br /> <br />are from the 2-hour design <br /> <br />les <br /> <br />watersheds with areas between 5 and 10 square m <br /> <br />II-I <br /> <br />in Table <br /> <br />The values <br /> <br />III-I <br /> <br />in Table <br /> <br />routing element parameters used for the baseline analysis are presented <br /> <br />The SWMM <br /> <br />(Reference 2). <br />in Table 111-3 <br /> <br />I <br /> <br />COLORADO URBAN HYDROGRAPH PROCEDURE PARAMETERS <br /> <br />C. <br /> <br />I <br /> <br />structures that are currently in <br /> <br />Existing systems for the baseline flow routing modeling included al <br /> <br />place. Structures that are expected to be constructed in the near or distant future as part of the Big <br /> <br />runoff hydrographs for the various sub- <br /> <br />The CURP computer program was used to compute storm <br /> <br />ities (i.e. alternative <br /> <br />These future faci <br /> <br />Dry Creek Master Plan were not included in the baseline run <br /> <br />The version used for the study was CUHPEl/PC. The sub-watershed input parameters <br /> <br />watersheds <br /> <br />I <br /> <br />The <br /> <br />were modelled as part of the alternatives evaluation phase of this project. <br /> <br />detention ponds) <br />baseline SWMM <br /> <br />length and centroidal <br /> <br />drainage area, <br />retention <br /> <br />sub-watershed identification number, <br />detention and <br /> <br />required for CURP are <br /> <br />routing elements are shown on Figures III-2A, Band C <br /> <br />losses, <br />of the Runoff <br /> <br />and <br /> <br />storage <br /> <br />percentage, <br /> <br />impervious <br /> <br />slope, <br /> <br />drainage way <br /> <br />length <br />infiltration loss rates <br /> <br />I <br /> <br />slopes were adjusted using Figure 4-1 <br /> <br />The weighted channel <br /> <br />Existing detention ponds which are publicly owned and maintained were included in the SWMM <br /> <br />e 2-hour design storm was <br />less than 90 acres <br /> <br />Since the 5 to 10 square m <br /> <br />into CUHP. <br /> <br />input <br />used for this study, the CUHP rational <br /> <br />Chapter ofthe USDCM for <br /> <br />I <br /> <br />were taken from drainage studies approved by Douglas <br />three new <br /> <br />routing. Storage-discharge relationships <br /> <br />for drainage sub-watersheds <br />model <br /> <br />method option <br /> <br />In addition, <br /> <br />Appendix D <br /> <br />III <br /> <br />tabulated <br /> <br />and are <br /> <br />6, 7, 8, 9) <br /> <br />4, 5 <br /> <br />County (References <br />detention facilities <br /> <br />enhanced information design for <br /> <br />parameters <br />pervious/impervious area connections and water quality capture volume were not used for the study <br />Sub- <br /> <br />to <br /> <br />optional <br /> <br />The <br /> <br />used <br /> <br />not <br /> <br />was <br /> <br />Size <br /> <br />ill <br /> <br />These detention <br /> <br />master plan <br /> <br />are proposed in Douglas County as part of this <br />ncluded in the SWMM model for this <br /> <br />I <br /> <br />111-1 <br /> <br />report. <br /> <br />Phase B <br /> <br />were also <br /> <br />facilities <br /> <br />method used for traditional drainage practices <br /> <br />areas and the program defaulted to the analysis <br /> <br />I <br /> <br />I <br />