Tibble Fork Dam Rehabilitation

The Tibble Fork Dam, owned by the North Utah County Water Conservancy District, is in the Uintah-Wasatch- Cache National Forest approximately 7.5 miles northeast of Cedar Hills, Utah at approximately 6400 ft. above sea level. The dam was constructed in 1966, and was designed to retain a normal pool of 166 acre-ft. and 250 acre-ft. at the auxiliary spillway crest elevation. In 2011, a Dam Safety Study identified several deficiencies, including undersized spillway, inadequate seepage collection, and seismic stability concerns related to liquefaction. An Environmental Assessment (EA) was completed by the US Department of Agriculture, Natural Resources Conservation Service in January 2015, which evaluated options to rehabilitate or breach the dam. The EA preferred alternative included raising the dam 15 feet to restore storage capacity lost to sedimentation. The Owner retained a design team in October 2015 to complete final investigation and design. The design was completed in May 2016, and construction began in June 2016. The design included excavation and replacement of potentially liquefiable soils beneath the downstream embankment, construction of a downstream stability berm, raising the normal reservoir pool level 18 feet, raising the dam crest 15 feet, installation of a cement bentonite cutoff wall with a maximum depth of 75 feet, installation of a grout curtain in bedrock and overburden materials, and construction of a new spillway. The spillway was designed with a box inlet, concrete chute, and 28-foot high SAF stilling basin. The construction work was substantially complete in the Spring of 2017. 17 pp.