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I I I I A. General The SWMM model was calibrated to the results of a Colorado Urban Hydro- graph Procedure (CUHP) model at three design points within the study area, A detailed discussion of the calibration procedure can be found in the Phase-A study and also in the hydrology report (Reference-13). The input and results of the analysis are on file at the UD&FCD offices, CHAPTER III - HYDROLOGICAL ANALYSIS I The purpose of the hydrologic analysis in a major drainageway planning study is to define the storm runoff peak flows and volumes for the hydrologic points of interest along the drainageways. The first step in the hydrologic analysis is to define the sub-basin boundaries and characteristics and then determine the storm hydrographs. The runoff information is subsequently used to assess the existing drainage facilities, potential flood damages, and potential drainage improvements, 2. DESIGN RAINFALL I The design rainfall information was obtained from the UD&FCD as a percent time distribution of the hour NOAA Atlas depths (Reference-7), The percent distribution was converted to incremental rainfall depths, for input in the CUHP model, and incremental rainfall intensities for input in the SWMM model, The rainfall information is presented on Table 111-3. I I The overall study was divided into 13 sub-basins to adequately define the run- off peak flows and volumes using the current Urban Drainage and Flood Control Dis- trict (UD&FCD) procedures. The details of the analysis used to define the required hydrologic data are presented in this chapter, C. Storm Runoff arid Hydrographs I I B, Basin Hydrology A sample output from the SWMM program is presented on Table 111-2, The out- put is arranged to give a depth and discharge for each hydrologic design point (listed on the top of the page) at 5 minute intervals, Storm hydrographs for three hydrologic design locations were plotted and are presented on Figures 111-2 through 111-4, As shown On Figure 111-4, it was estimated that the 2-year flood from the upper portion of the basin is intercepted by the Denver Highline Canal, This is discussed further in Section-D below. I ] . RUNOFF MODEL I I Peak flows and volumes were determined for the existing and future develop- ment conditions using the Storm Water Management Model (SWMM) (Reference-J4). These design flows were subsequently used to define the various frequency flood- plains and determine flood damages, I Input for the SWMM model is shown on Figure 111-) and Table 111-), The model describes the study area mathematically as a system of hydraulic elements, Sub-basins are idealized as rectangular subcatchment areas with a length, width, slope, imperviousness, and infiltration rate. The streets, storm sewers, and channelways are described in terms of gutters and pipes. The hydraulic proper- ties of each element are defined by size, slope, and roughness coefficient. The SWMM program automatically routes and combines the individual sub-basin storm hydrographs to determine peak flows and volumes at selected hydrologic design points by using a step-by-step accounting of rainfall, infiltration, detention, overland flow, and gutter flow. D. Flood Routing Through the Denver Highline Canal I The Denver Highline Canal intercepts altering the downstream drainage pattern. hydrologic impact are defined as follows: a significant portion of the upper basin, The baseline conditions for defining the I ], The upper basin 2-year flood is considered to be intercepted by the Denver Highline Canal and diverted to the Big Dry Creek control structure, The basis for the assumption is the information presented in the report "High- line Canal Master Plan", (Reference-8), The report defines the base flow of the canal as 200 cfs and the canal capacity as 600 cfs, "Then.. .overflow I I I -]]- -]2-