Colorado River Probable Maximum Floods

Swain, Robert E.; ,

Colorado River Probable Maximum Floods

by Robert E. Swain, Bureau of Reclamation, Denver, United States,

Document Type: Proceeding Paper

Part of: Waterpower '91: A New View of Hydro Resources

Abstract:

Upper limit design rainstorms (ULDRS) were developed for three locations in the Colorado River drainage above Hoover Dam. For each location, the rainfall magnitudes, spatial and temporal distributions, and seasonal variations were determined from transposed and moisture-maximized historical events. The magnitude of ULDRS was estimated as averaging from 6.93 to 7.29 inches (176 to 185 mm) in depth over a 40,000-mi² (104,000-km²) storm area for 72-hour storms for the three locations. Historical storm data indicated the possibility of two large rain events occurring somewhere in the basin within a few days of each other; therefore, design storm sequences were developed that incorporated two ULDRS or near ULDRS events. A hydrologic model was developed to convert excess precipitation to runoff and to generate the flood hydrographs. Input to the model consisted of the system configuration, rainfall, snowmelt flood, lag times, loss rates, dimensionless unit hydrographs, and starting reservoir elevations. Release decisions were based on current flood control operating criteria for the Colorado River reservoir system. The most critical condition for the reservoirs occurred in August. The probable maximum floods (PMF) for Glen Canyon and Hoover Dams had peak inflows of 697,000 and 1,130,000 ft³/s (19,700 and 32,000 m³/s) and 60-day volumes of 5.8 and 9.3 million acre-feet (7.2 ? 10⁹ and 1.2 ? 10¹⁰ m³), respectively.

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