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. . . . '. '. . . . . . . . . . . . . . dam spillway) and a six-hour duration storm (see Reference H). Differences in peak flow rates predicted by the 1961 study and by this present analysis result from basic differences in the unit hydrograph procedure applied and different rainfall durations and di stribut ions used to est imate 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. II 1. HYDROL.9_GIC_ ANAL YSI S 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 unit hydro- graphs utilized 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 util ized in this study were furnished by UDFCD (January 1978) and will be described below. The rainfall used in the CUHP method was based on a storm duration of two hours and the distribut ion 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, lOa-year rainfall as determined from the NOAA atlas (Ref. B). The I-hour depth was found to be 2.56 inches and thus the corresponding 2-hour depth of 2.95 inches is distributed 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 retention losses were taken as 0.1 inch for impervious 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., -1 h minimum rate of 0.5 in./hr., and decay rate of 0.0018 sec. . W en 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 unit hydrograph are found from the coefficients Cp and Ct, which in turn are computed from 4