C153629 Feasibility Study - Laserfiche WebLink

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I I I I I I I I I I I I I I I I I I I 3.4 FLOOD HYDROLOGY New dams within the State of Colorado are required to safely pass a design flood according to the policies and procedures established by the Colorado State Engineer's Office. A hazard analysis will eventu- ally be required for the proposed dam and reservoir which then deter- mines the criteria to be applied in calculating the magnitude of the design flood. Due to the downstream development in the McCoy area near the confluence of Rock Creek and the Colorado River, the dam is ex- pected to be classified as a "moderate" to "high" hazard dam. In either case, the design flood criteria requires that the probable maximum flood (PMFl be used in the spillway design. The PMF is assumed to occur when the reservoir is full to the spillway crest. For the proposed Rock Creek roller compacted concrete gravity dam, the spillway section would be formed by an overflow section notched in the center of the dam. A PMF was estimated to have a peak discharge of 50,900 cfs as a result of probable maximum precipitation (PMP) from a 6-hour thunderstorm. Large floods could also occur from a general storm over the basin or from rapid snowmelt, but for basins of the size of the 52-mi2 Rock Creek basin, the short duration, high intensity thunderstorm produces the most critical PMF from the standpoint of peak discharge. The PMP was determined from the Hydrometeoro1ogica1 Report No. 49 pub- lished by the National Oceanic and Atmospheric Administration to be 7.5 inches of rainfall in six hours. The total volume of the PMF was calculated to be 12,800 acre-feet. A simulated routing of the PMF through the reservoir and over the pro- posed spillway section was performed with the HEC-1 computer model for various spillway widths. A 100-foot wide spillway crest width was selected. The large reservoir volume compared to the flood inflow volume provided significant attenuation of the flood peak. The peak 3 - 14 6819W/502~