Fontenelle Dam - Safety Evaluation

Safety Evaluation

As a result of the catastrophic failure of the closely similar Teton Dam under much the same circumstances in 1976, the Safety Evaluation of Existing Dams (SEED) program was initiated in the late 1970s and early 1980s. The 1983 report on Fontenelle Dam rated the dam "poor," the second-lowest rating. The report noted increasing seepage, similar in nature to the seepage that caused the 1965 failure. Plans were advanced in 1984 to build a 600-foot (180 m) long concrete wall at the east canal outlet works to evaluate its efficacy. However, in May 1985, operators initiated an emergency drawdown of the reservoir after deciding that the dam was "in very serious distress." With the fast drawdown, slumping appeared on the upstream face. The reservoir was entirely drained. In September work began on the trenching of an 840-foot (260 m) test section of 24-inch (61 cm) thick concrete diaphragm wall into the core of the dam, using the "Hydrofraise" system of grout trenching, extending 40 feet (12 m) to 50 feet (15 m) below the base of the dam into bedrock. The reservoir was to be kept empty in 1986, but heavy spring runoff carried logs and tumbleweeds into the reservoir, clogging the outlet works. The reservoir partially filled, preventing flooding in the town of Green River, but creating anxiety about possible failure of the dam, which was retaining fifty feet of water, rising at times five feet in a day. An environmental assessment of the repair program noted that the only alternative was an intentional and permanent breaching of the center section of the dam, leaving local industries without water. The test section of wall was completed, and a full-length wall was considered, but the completed section appeared to have solved the leakage problem.