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. The investigation work on Otter Creek Dam found that the dam was in good condition, but did not meet the new standards for performance under a major earthquake or flooding event. Design and construction occurred to upgrade the dam to meet Utah State standards. Dam safety concern with the embankment were potential for transverse cracking at the interface of the rock abutments and alluvial foundation materials, lack of an internal filter and drain system, and an over-steepened downstream rock slope. Rehabilitation of the embankment included excavation of the center section of the dam, installation of a filter and drain system, and flattening of the downstream rock slope of the dam. The service spillway was capable of passing approximately 40% of the design flood. Rehabilitation entailed excavation of rock in the bedrock channel to improve flow characteristics and construction of a 120-foot wide emergency fuse-plug spillway. The dam safety concern with the outlet works was the lack of an upstream guard gate. Divers were used to inspect the upstream portion of the outlet tunnel to allow final design of a guard gate system. A 7-foot by 4-foot guard gate and system was constructed at the entrance to the tunnel and a 6-inch concrete liner was constructed within the existing upstream tunnel to allow closure of the guard gate under a full reservoir condition. There were many issues encountered and lessons learned during the rehabilitation of Otter Creek Dam. A critical concern was the construction timing that impacted draining and refilling the reservoir. There were many design issues dealt with including a lack of original drawings and records, a foundation consisting of soft alluvial materials (the "bog"), bedrock unknowns impacting the emergency spillway configuration, and outlet tunnel unknowns. The remote location, high elevation, and winter construction contributed to various construction issues. Coordination with many government agencies involved was also critical. Significant interaction occurred with the Dam Safety division in the State Engineer's office during design and construction. Financing was provided through a 95% grant, with the remaining 5% coming in the form of a loan to the irrigation company from the Utah Board of Water Resources. The total investigation, design and construction cost of the rehabilitation was $2,200,000. 10 pp.