Otter Creek Dam Rehabilitation, Piute County, Utah

Otter Creek Dam is located 60 miles south of Richfield, Utah. The dam is an earth and rockfill structure built over 100 years ago for irrigation water storage. The Utah State Engineer established new minimum dam safety standards in 1991. Investigation work on Otter Creek Dam found that it was in good condition, but did not meet the new standards for performance under a major earthquake or flooding event. Dam safety concerns with the embankment were potential for transverse cracking at the interface of the rock abutments and alluvial foundation materials, lack of an intemal filter and drain system, and an over-steepened downstream rock slope. Rehabilitation of the embankment included excavation of the center of the dam, installation of a filter and drain system, and flattening the downstream rock slope. The service spillway was capable of passing 40% of the design flood. Rehabilitation entailed excavation in the spillway bedrock channel to improve flow characteristics and construction of a 120-foot wide emergency fuse-plug spillway. The outlet works did not meet dam safety standards because of the lack of an upstream guard gate. Rehabilitation of the outlet works included construction of a guard gate system and placement of a concrete liner in the tunnel upstream of the operating gates. Challenging issues encountered during the rehabilitation of Otter Creek Dam included construction timing that impacted draining and refilling the reservoir, a lack of original drawings and records, a very soft alluvial foundation ("the bog"), outlet tunnel unknowns, as well as many others. Significant economic savings were realized through the innovative design and construction process. Time and money spent on detailed investigations also enabled construction without major surprises or costly changes. These considerations represent a savings of several million dollars to the State and owner. Significant interaction and cooperation occurred with the owner and the Utah Dam Safety office during design and construction. Financing was provided by a 95% grant, 5% loan to the Otter Creek Reservoir Company from the Utah Board of Water Resources. The total investigation, design, and construction cost of the rehabilitation was $2,200,000.