1976 жылғы Тетон бөгетінің күлдірауы

Dam in Idaho, US that collapsed in 1976

The Teton Dam was an earthen dam in the western United States, on the Teton River in eastern Idaho. It was built by the U. S. Bureau of Reclamation, one of eight federal agencies authorized to construct dams. Located between Fremont and Madison counties, it suffered a catastrophic failure on June 5, 1976, as it was filling for the first time. The collapse of Teton Dam killed 11 people and 16,000 livestock. The dam cost about $100 million to build and the federal government paid over $300 million in claims related to its failure. Total damage estimates have ranged up to $2 billion, and the dam was not rebuilt.

Interest in building a dam in the eastern Snake River Plain had arisen for many years to control spring runoff and provide a more constant water supply in the summer. The area had suffered a severe drought in 1961, followed by severe flooding in 1962. The Bureau of Reclamation proposed the Teton Dam in 1963 and Congress passed, without opposition, an authorizing bill the following year. The planned dam was to be an earthen structure 310 feet (94 m) high and 0.6 mile (1 km) long, creating a reservoir 17 mile (27 km) in length. The impounded water would be used to generate hydroelectric power. An environmental impact statement was issued for the dam in 1971, but it did not raise the possibility of a collapse. The tuff, a late Cenozoic volcanic rock, is 1.9 million years old. The dam site is composed of basalt and rhyolite, both of which are considered unsuitable for dam construction because of their high permeability. This was confirmed by long term pump in tests at rates of per minute. Test cores, drilled by engineers and geologists employed by the Bureau of Reclamation, showed that the canyon rock at the dam site is highly fissured and unstable, particularly on the right side (as one faces the direction of flow). The widest fissures were determined to be wide. The bureau planned to seal these fissures by injecting grout into the rock under high pressure to create a grout curtain in the rock. In addition, an investigation of the area by geologists of the U. S. Geologic Survey indicated that it was seismically active; five earthquakes had occurred within 30 mile (48 km) of the dam site in the previous five years, two of which had been of significant magnitude. This information was provided to the Bureau of Reclamation in a memorandum, but the geologists' concerns were considerably watered down in the 6 month redrafting process before the USGS sent the final version of the memo to the USBR in July 1973. This necessitated using twice as much grouting as had been originally anticipated; the total injected grout included 496,515 ft3 of Portland cement, 82,364 ft3 of sand, 132,000 pounds of bentonite, and 418,000 pounds of calcium chloride, injected into 118,179 linear ft of drilled holes.

The dam was completed in November 1975 and filling the reservoir began at the standard rate of 1 feet (0 m) a day. However, snows were heavy that winter, and five months later, the project's construction engineer requested permission to double the filling rate to deal with the additional spring run off, while continuing to inspect for leaks and monitor the groundwater. A month later, though monitoring showed that groundwater was flowing a thousand times faster than had been originally anticipated, the filling rate was doubled again, to 4 feet (1 m) a day. At 11:55 am MDT (UTC−17:55), the crest of the dam sagged and collapsed into the reservoir; two minutes later, the remainder of the right bank third of the main dam wall disintegrated. Over (many times the average flow rate of Niagara Falls) of sediment filled water emptied through the breach into the remaining 6 mile (10 km) of the Teton River canyon, after which the flood spread out and shallowed on the Snake River Plain. By 8:00 pm, the reservoir had completely emptied, although over two thirds of the dam wall remained standing. placed blame for the collapse on the permeable loess soil used in the core and on fissured (cracked) rhyolite in the abutments of the dam that allowed water to seep around and through the earth fill dam. The permeable loess was found to be cracked. The combination of these flaws is thought to have allowed water to seep through the dam and to lead to internal erosion, called piping, that eventually caused the dam's collapse. An investigating panel had quickly identified piping as the most probable cause of the failure, then focused its efforts on determining how the piping started. Two mechanisms were possible. The first was the flow of water under highly erodible and unprotected fill, through joints in unsealed rock beneath the grout cap and development of an erosion tunnel. The second was "cracking caused by differential strains or hydraulic fracturing of the core material." The panel was unable to determine whether one or the other mechanism occurred, or a combination:

The fundamental cause of failure may be regarded as a combination of geological factors and design decisions that, taken together, permitted the failure to develop. A wide ranging controversy ensued from the dam's collapse. According to the Bureau of Reclamation, bureau engineers assess all reclamation dams under strict criteria established by the Safety of Dams program. Each structure is periodically reviewed for resistance to seismic activity, for internal faults, and for physical deterioration. and again in May 1985; and the Jackson Lake Dam, which would have failed during an earthquake on the nearby Teton Fault.

Teton Canyon ends about 6 mile (10 km) below the dam site, where the river flows onto the Snake River Plain. When the dam failed, the flood struck several communities immediately downstream, particularly Wilford at the terminus of the canyon, Sugar City, Salem, Hibbard, and Rexburg. Thousands of homes and businesses were destroyed. The small agricultural communities of Wilford and Sugar City were wiped from the river bank. Five of the 11 deaths attributed to the flood occurred in Wilford. The similar community of Teton, on the south bank of the river, is on a modest bench and was largely spared. One Teton resident was fishing on the river at the time of the dam failure and was drowned. An elderly woman living in the city of Teton died as a result of the evacuation. An estimated 80% of existing structures were damaged in the Hibbard and Rexburg area, whose population was about 10,000. The Teton River flows through the industrial, commercial, and residential districts of north Rexburg. Much of the damage in the area was done by thousands of logs dislodged from a lumber yard. Dozens of them hit a bulk gasoline storage tank a few hundred yards away. The gasoline ignited and sent flaming slicks adrift on the racing water. The force of the logs and cut lumber and the subsequent fires practically destroyed the town. The flood waters traveled west along the route of the Henrys Fork of the Snake River, around both sides of the Menan Buttes, damaging the community of Roberts. The city of Idaho Falls, even further down on the flood plain, had time to prepare. At the older American Falls Dam downstream, engineers increased discharge by less than 5% before the flood arrived. That dam held and the flood was effectively over, but tens of thousands of acres of land near the river were stripped of fertile topsoil. The Bureau of Reclamation set up claims offices in Rexburg, Idaho Falls, and Blackfoot. By January 4, 1977, seven months after the disaster, victims had filed over 4,800 claims totaling $194 million and the federal government had paid 3,813 of those claims worth $93.5 million. The claims program was originally scheduled to end in July 1978, but continued until January 1987, when the federal government had paid 7,563 claims for a total amount of $322 million. No plans have been made for rebuilding the Teton Dam, but its reconstruction has been discussed over the years. Footage and interviews are featured in Catastrophe No Safe Place (1980), hosted by Charles Bronson and Jill Ireland.