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I I I I I I I I I I I I I I I I I I I III. HYDROLOGIC ANALYSIS dam spillway) and a six-hour duration storm (see Reference H). Differences in peak flow rates pred icted by the 1961 st udy and by th is present an al ys is resul t from basic differences in the un it hydrograph procedure appl ied and different rainfall durations and distributions used to estimate excess rainfall. In addition, the present analysis covers only about 85% of the drainage basin considered in the 1961 study; that is, the present analysis considers only that portion of the Dad Clark Gulch drainage basin upstream of McLellan Reservoir and is not concerned with those downstream areas which drain directly into the reservoir. Peak discharges (Qs) were determined at various locations in the watershed by employing the Colorado Urban Hydrograph Procedure (CUHP). This hydrologic model is described in the UDFCD drainage manual (Ref. C) and in the user manual for the CUHP computer program (Ref. D). The un it hydro- graphs util i,zed in the CUHP method are determined from empirical relation- ships between parameters of the unit hydrograph and physical character- istics of the watershed. These relationships are based on field data from developing watersheds in the Denver area. The relationships utilized in th i s study were furn i shed by UDFCD ( January 1978) and wi 11 be descr ibed below. The rainfall used in the CUHP method was based on a storm duration of two hours and the distributiDn shown in Table 1. This duration and distribu- tion were recommended by UDFCD (January 1979) as a typical 100-year flood- producing storm for the Denver area. The distribution is given as a percentage of the I-hour, 100-year rainfall as determined from the NOAA atlas (Ref. B). The l~hour depth was found to be 2.56 inches and thus the correspond ing 2-hour depth of 2.95 inches is d i str ibuted as shown in Table 1. The rainfall was assumed to be uniformly distributed throughout the drainage basin. No areal correct ion factors were appl ied since the reduction would only be a few percent for a basin of this size (7.75 mi. 2). Surface retent ion losses were taken as 0.1 inch for imperv ious surfaces and 0.4 inches for pervious areas. Infiltration rates were determined from Horton's equation with a maximum rate of 3.0 in./hr., minimum rate of 0.5 in./hr., and decay rate of 0.0018 sec. -1 When these losses are appl ied to the total rainfall, the result ing excess rainfall is 1.87 inches for the entire watershed. In the CUHP method, the time to peak and peak Q of the un it hydrograph are found from the coefficients C and Ct, which in turn are computed from p 4