Bartlett Dam Modification: Erosion Study of the Service Spillway

Bartlett Dam is a multiple arch concrete dam located northeast from Phoenix Arizona. The dam is 287 feet high with a hydraulic height of 188 ft. The dam has a crest length of approximately 800 feet and contains approximately 182,000 yds of reinforced concrete. The dam was instructed between 1936 and 1939. The service spillway was designed to pass a flood with a peak Inflow of 175,000 ft³/s through three 50- by 50-foot crawler-type gates which have a design capacity of 175,000 ft³/s at reservoir water surface elevation 1798 feet. Dam Safety evacuations of Bartlett Dam revealed that the dam was unable to withstand the maximum credible earthquake (MCE) and could not safety pass floods exceeding 43 percent of the probable maximum flood (PMF). Modifications to safely pass the PMF included raising the service spillway discharges from 175,000 ft³/s to 284,700 ft³/s (an increase of 62 percent). The integrity of the service spillway under the PMF flows was investigated as part of the dam safety evaluations. The service spillway channel experienced headcutting during floods in 1978 and in 1979. Of special concern was the potential for headcutting during the PMF and the failure of the sewice spillway. The issues involved the increased discharge through the service spillway during a PMF and the failure mechanisms necessary to compromise the service spillway crest structure and permit the uncontrolled release of the reservoir contents. This was considered a significant threat due to the demonstrated erodibility of the service spillway foundation. A plunge pool has formed since the dam was constructed due to erosion immediately downstream from the spillway flip bucket. 6 pp., 1 reference.