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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />III. <br /> <br />SUMMARY OF HYDROLOGIC ANALYSIS <br /> <br />Floods occur whenever runoff from precipitation exceeds the conveyance capacity <br />of a stream or drainage channel and flood waters spill over the banks and inundate the <br />floodplain. Floods are a common, natural phenomenon. <br /> <br />Floods of varying probabilities can be predicted using measured flow data or de- <br />termined by synthesizing the precipitation runoff relationship of a watershed. While it is <br />desirable to correlate precipitation and runoff characteristics of a drainage basin, <br />precipitation and related flow records are scarce. <br /> <br />There is some such recorded information compiled recently for floods on DF A 0054, <br />but this data is insufficient to develop the complete runoff hydrographs required for <br />analysis. Reliable rainfall, soils and land use data are available for DF A 0054, and these <br />data have been applied in the derivation of synthetic flood hydrographs for selected design <br />points on the study reaches based on runoff from the 2-, 5-, 10-, 50-, and 100-year rainfall <br />events. All desIgn hydrographs are based on future basin development, assuming fully <br />developed conditions In addition, historic flood hydrographs have been developed.. <br /> <br />Derivation of Hydrographs <br /> <br />The procedure used in this study to derive the flood hydrographs is a modified test <br />version of the "Colorado Urban Hydrograph Procedure" (CUHP), developed by the Urban <br />Drainage and Flood Control District in 1979. The procedure is founded upon the synthetic <br />unit hydrograph method developed by Snyder in 1938 and adjusted to reflect data locally <br />collected and studies performed specifically for the Denver Metropolitan Area. The <br />original CUHP is outlined in detail in Volume I of the Urban Storm Drainage Criteria <br />Manual, and the modified test version parameters may be obtained from the Urban <br />Drainage and Flood Control District. <br /> <br />Input parameters required for the CUHP are rainfall hyetographs and watershed <br />parameters consisting of final and initial infiltration rates, decay rate, depression storage, <br />sub-basin area, length, slope, and centroidal distance, percentage of impervious area, and <br />peak flow coefficients which reflect time to peak and relate to the peak rate of runoff. <br /> <br />The drainage basin, subareas and channel configurations are shown on the Drainage <br />Basin Map, and Historic Drainage Basin Map, Sheets 9 and 10 of the Drawings, <br />respectively. Values for the DFA 0054 watershed parameters are listed in Table III-I. <br /> <br />Rainfall Data <br /> <br />One-hour rainfall depths for the 2-, 5-, 10-, 50-, and 100-year return periods were <br />derived from the "Precipitation-Frequency Atlas of the Western United States, Vol. III, <br />Colorado" published by the National Weather Service but were not aerally reduced due to <br />the small size of the basin. Temporal distributions of the storms were then determined <br />using recent experimental data accumulated by the Urban Drainage and Flood Control <br />District. Rainfall values are listed in Table 1II-2 and 100-year rainfall hyetograph used in <br />this analysis is presented in Figure III-I. <br /> <br />ill-I <br />