An independent panel of experts urges immediate action to avert catastrophic failure of the Corps of Engineers’ 135-MW Center Hill Dam on Tennessee’s Caney Fork River.
The panel report, issued Sept. 7, said there is significant potential for catastrophic failure of the 250-foot-tall concrete and earthen embankment structure under normal operating conditions. The report confirmed findings of an earlier Corps in-house report on uncontrolled seepage that has existed since at least the late 1960s.
ï¿½The continued progression of this piping will eventually cause loss of the reservoir through large cavities in the abutments, under the concrete section of the dam, or it will cause collapse of the embankment section as sinkholes develop in the upstream slope resulting in rapid erosion, breach of the dam, and loss of the reservoir,ï¿½ the report said.
Because it is impossible to estimate when the dam might fail, the panel declared immediate action is essential to reduce risks to the public. It said investigations, grouting, and construction of other remedial measures must be expedited.
ï¿½Emergency response entities should be notified to be on an enhanced state of preparedness to follow the emergency action plan,ï¿½ the report said. ï¿½Surveillance of the dam and the distress indicators should be maintained at a high level.ï¿½
Short term: Lower reservoir, expedite grouting
In the short term, the panel recommends immediate lowering of the reservoir level as far as possible while still operating and providing most project benefits. It recommends keeping the pool level at or below elevation 620 to 630 all year. The Corps previously modified operations to target pool levels below 630 in the winter and below 648 in the summer.
The panel also recommends completing a planned grouting program for the main embankment dam, saddle dam, left abutment, and right abutment rim as soon as possible. It said the remedial action is necessary to reduce the risk of dam failure and to establish an appropriate restricted reservoir level.
The panel said grouting should employ the newest methods to make sure a grout curtain seepage barrier is effective, and that important information on foundation conditions is collected for design of a proposed cutoff wall. The Corps took bids Sept. 6 for installation of the grout curtain. (HNN 8/10/07)
Long term: Consider RCC dam at new location
In the longer term, the panel recommends reconsideration of previous dam repair proposals, including possible dam replacement at a site upstream or downstream from the current location. The panel noted the Corps previously ruled out building a new dam, concluding the economic consequences of removing the existing dam, treating the foundation, and building a new dam would devastate surrounding communities.
Options could include a new roller-compacted-concrete dam or partial or full removal and replacement of the existing embankment dam. The panel said a cutoff wall installed from the foundation level would have a much longer life than one installed from the embankment crest and that a concrete dam would be much more resistant to erosion than present embankments.
A new alternatives analysis should evaluate and compare short- and long-term risk reduction measures, the panel said. By taking that approach, the panel said, a ï¿½risk informedï¿½ decision could be made considering the best value over the long term versus one based on least cost and least negative consequences. The panel said the Corps should proceed with the design and construction of an alternative based on best value.
Center Hill Dam, near Lancaster, Tenn., consists of a 1,382-foot-long concrete section and 778-foot-long earth embankment. It is among six dams, including 270-MW Wolf Creek Dam in Kentucky, that the Corps previously identified as being critically near failure or possessing extremely high life or economic risk.
The Corps uses independent peer review panels to validate that it is taking the best approach to reduce risks to the public. The agency noted it employs independent project reviews to provide additional insight to assist with its dam safety management and programming decisions.