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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />4. COLORADO URBAN HYDROGRAPH PROCEDURE <br />1982 Version <br /> <br />. <br /> <br />For basins that are larger than 160 acres it is recommended that the <br />design storm runoff be analyzed by deriving synthetic unit hydrographs. <br />However, if the engineer desires, the 5-minute unit hydrograph can be used for <br />basins as small as 90 acres. The unit hydrograph principle was originally <br />developed by Shennan in 1932 (12). The synthetic unit hydrograph, which is <br />used for analysis when there is no rainfall-runoff data for the basin under <br />study, as is often the case in the Denver region, was developed by Snyder in <br />1938 (13). The presentation given in this chapter is termed the Colorado <br />Urban Hydrograph Procedure (cUHP) because coefficients and the form of the <br />equation are based upon data collected in Colorado and on studies conducted or <br />financed by the Urban Drainage and Flood Control District. The data for use <br />in the development of the 1982 version of the cUHP were collected by the U.S. <br />Geological Survey between 1969 and 1981 under a cooperative agreement with the <br />Urban Drainage and Flood Control District. Data collection activities are <br />being continued under a similar cooperative agreement between the District and . <br />USGS; however, the number of stations have been reduced. The goal of the <br />currently ongoing data collection effort is to develop a long term data base <br />for further refinements to the hydrologic techniques in the Denver region. <br /> <br />4.1 Defi nition <br />A unit hydrograph is defined as the hydrograph of one inch of direct <br />runoff from the tributary area resulting from a unit storm. The unit <br />hydrograph thus represents the integrated effects of factors such as tributary <br />area, shape, street pattern, channel capacities, and street and land slopes <br />(14, 15, 16, 17, 18, 19). <br />The basic premise of the unit hydrograph is that individual hydrographs <br />resulting from the successive increments of rainfall excess that occur <br />throughout a storm period will be proportional in discharge throughout their <br />length. Thus, the hydrograph of total storm discharge is obtained by summing <br />the ordinates of the individual hydrographs. <br /> <br />. <br /> <br />5-1-84 <br />