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<br /> <br />S-l <br /> <br /> <br />SUMMARY <br /> <br />High runoff from the late season melt of the snowpack in the spring <br />and summer of 1983 resulted in the first nontest operation of Glen <br />Canyon Dam's spillways since they were placed in operation in 1964. <br />During these operations, cavitation caused severe damage to the <br />concrete lining of the spillways, leading to the Bureau of <br />Reclamation's (Reclamation) assessment of potential modifications to <br />correct the problem. The spillways at Hoover Dam were also damaged <br />from flood releases during 1983. As a part of dam safety <br />investigations, Reclamation computed probable maximum floods for <br />each structure in the following manner. <br /> <br />Upper Limit Design Rainstorms (ULDRS) were developed for three <br />locations in the Colorado River drainage above Hoover Dam. For each <br />location, the rainfall magnitude, spatial and temporal <br />distributions, and seasonal variations were determined from <br />transposed and moisture maximized historical events. Storms ranging <br />in size from 5,000 to 100,000 square miles were tested in the <br />hydrologic computer model to arrive at the 40,000-square-mile <br />critical storm area. The magnitude of ULDRS was estimated as <br />averaging from 6.93 to 7.29 inches in depth for 72-hour storms for <br />the three locations. Historical storm data indicated the <br />possibility of two large rain events occurring within a few days of <br />each other; therefore, design storm sequences were developed that <br />incorporated two ULDRS or near-ULDRS events. <br /> <br />The 100-year snowmelt event was selected as the antecedent flood <br />affecting the basin prior to the onset of the ULDRS. A statistical <br />analysis of floodflows was the basis for developing the daily <br />undepleted snowmelt base flood, which had an annual volume of <br />26.7 million acre-feet at Hoover Dam. Approximately 3.6 million <br />acre-feet of depletions were subtracted from the snowmelt flood to <br />account for present streamflow regulation and use in the upper <br />basin. <br /> <br />A hydrologic model was developed to convert excess precipitation to <br />runoff and to generate the flood hydrographs. Input to the model <br />consisted of the system configuration, rainfall, lag times, loss <br />rates, dimensionless unit hydrographs, and starting reservoir <br />elevations. The 167,000-square-mile drainage basin above Hoover Dam <br />was divided into 99 subbasins, of which 65 were above Glen Canyon <br />Dam. Starting reservoir elevations for the major dams upstream from <br />Glen Canyon were determined by simulating forecasted inflow <br />conditions and routing the 100-year snowmelt flood through the <br />facilities. Release decisions were based on current flood control <br />operating criteria for the Colorado River reservoir system. <br /> <br />The most critical flood condition for Glen Canyon Dam occurs in <br />August. A San Juan Mountains storm, followed a day later by a <br />Boulder Mountains storm, produces the largest flood at Lake Powell. <br />This probable maximum flood for Glen Canyon Dam has a peak inflow of <br />697,000 ft3/s and 60-day volume of 5.8 million acre-feet. Routing <br />this flood through the reservoir results in a maximum water surface <br />elevation of 3,709.8 feet, which is more than 5 feet below the dam <br />crest elevation. <br />