The Experience in Design, Implementation and Operation of the Coolidge Dam Early Warning System

Coolidge Dam is located on the Gila River in Southeast Arizona. It provides irrigation, recreation, and power to the San Carlos Irrigation Project. The dam is a multiple dome reinforced concrete structure. The hydraulic height is 220 feet and the crest length is 920 feet. Overflow control is provided by a chute spillway located on each abutment. Reservoir capacity at the spillway crest (El. 2511 ft) is 912,400 acre-feet. The dam was completed in 1928.

In 1989, the Bureau of Reclamation issued a report that presented recommended structural modifications to Coolidge Dam to correct the safety deficiencies that existed. The safety deficiencies noted in that report were potential abutment and buttress instability and the abutment erodibility during an overtopping flood event. The recommended plan of action is designed to prevent failure during normal operating conditions as well as during the maximum credible earthquake or probable maximum flood.

The actions include modification of Coolidge Dam by:
- Armoring sections of the downstream rock surfaces with a concrete mat to provide erosion protection during overtopping
- Building a support wall under a potentially unstable rock wedge beneath the left dome buttress
- Removing and replacing damaged concrete in spillway chute walls and floors
- Scaling loose material and installing rockfall netting on slopes above left and right abutment spillways
- Monitoring rock mass movement above the right spillway.

Immediate interim actions recommended included a reservoir elevation restriction, instrumentation to enhance dam monitoring, and an emergency downstream warning system. 7 pp., 2 references.