Construction and controls played role inTaum Sauk reservoir collapse

The power company that operates the failed Taum Sauk reservoir in Missouri has released an expert’s report on what caused the drastic failure of the dam last December. The upper reservoir of the Taum Sauk project on Proffit Mountain in the Ozark Mountains overtopped in the early hours of December 14 and released almost its entire contents — 4.5 billion cubic metres of water — in approximately 30 minutes. According to an article by Charles Norman, CET in the latest issue of the Canadian Dam Association’s magazine Bulletin (Spring 2006), the astonishing result was “roughly the same as the normal flow over Niagara Falls.”
The water rushed down the mountainside to a second reservoir, where it was mostly contained, although it created an estimated 7-metre flood wave in the Black River. However, the torrent had cut a swath of destruction down the mountain, washing away vegetation down to bedrock in some places and destroying a state park. There were no fatalities, but a park superintendent’s house was washed away. The Black River and lower reservoir are now awash in debris and sediment.
The Taum Sauk hydroelectric plant is owned by AmerenUE, which has accepted responsibility. It hired Paul C. Rizzo Associates as dam experts to investigate the causes for the breach and their report was released on April 7. The U.S. Federal Energy Regulatory Commission is also investigating.
Rizzo found that the causes were stability failure and poor design of the dam itself, as well as problems with the plant’s instrument and control systems. The plant was built in 1963 and is a “pumped storage” hydroelectric plant that stored water from the Black River in the Upper Reservoir atop the 1,590-ft. high mountain. The water flowed down a mile-long tunnel inside the mountain, turning turbine generators to produce electricity. When the power demand was low, however, the same turbines ran in reverse to pump water back to the upper reservoir. The plant produced 408 MW of power.
On the fateful day, the upper reservoir ruptured in the northwest corner with a breach 213 metres across.
According to Rizzo Associates’ report, the reservoir’s rock dam wall failed because the material used in the original construction deviated from the original design specifications. Instead of being constructed on bedrock, the foundation rested on up to 18 inches of soft material in some places. Rizzo found that these structural problems could only be determined through borings and visual inspections after the breach had occurred and could not have been found by regular annual inspections.
As well, Rizzo found that the instrumentation system controlling the water levels in the plant were not conservative enough. Another problem was that one single individual was responsible for both operating the plant and for dam safety.
The full report is posted at www.ameren.com