Expert U.S. panel urges further drawdown at 270-MW Wolf Creek

An independent review panel commissioned by the Corps of Engineers urges the Corps to lower Lake Cumberland another 30 to 40 feet due to �significant potential for failure� of 270-MW Wolf Creek Dam.

Acknowledging the risk of dam failure in January, the Corps announced it would lower the Kentucky reservoir — the largest east of the Mississippi River — to 680 feet above sea level. Normal pool levels are 723 feet in summer and 690 feet in winter.

The agency is conducting a two-year grouting program to strengthen the 5,736-foot-long, 258-foot-tall, rolled earthfill and concrete gravity dam, on the Cumberland River 300 miles upstream of Nashville, Tenn.

However, in a 71-page report released April 24, the Wolf Creek Peer Review Team said it discovered �compelling evidence� a piping failure mode exists at the dam and is advancing.

�At this stage of failure mode development, the panel believes there is significant potential for failure of Wolf Creek Dam under the normal operating conditions,� the panel said.

Recognizing that it is difficult to predict when failure might occur, the group said it is essential for the Corps to take immediate short-term actions to avoid failure and to reduce risks to the public. Expedited investigations, design, and construction of long-term repairs also are needed, it said.

Despite the report, the Corps said it intends to maintain the elevation of the reservoir at 680 feet through the end of 2007 while expediting the grouting program. Power can be generated when the pool elevation is at or above 680 feet.

Keith Ferguson, vice chairman of the peer review panel, told reporters the panel recognized that the Corps might choose a higher elevation than the 640 to 650 feet recommended.

�The panel did recognize the potential to choose a higher elevation and has advised in the report that if that happens it is critical to continue to monitor the structure and grout the dam foundation in an accelerated fashion,� Ferguson said.

The Corps said it expects to complete the first part of the grouting program in September or October, filling voids with grout. The Corps said it would make a decision in the fall about the level for operating the reservoir in 2008.

Panel advances RCC replacement dam option

As disclosed by the Corps last month, the peer review panel also said construction of a new concrete dam section might be a better long-term solution than the Corps’ current seepage repair plan. (HNN 4/5/07) The panel recommends the Corps evaluate building a roller-compacted-concrete dam to replace a 4,000-foot-long earthen embankment versus proposed construction of a concrete cutoff wall through the dam.

The Corps has said it would request full funding to $309 million from Congress to accelerate a program of grouting and construction of the diaphragm wall.

�While there are very significant considerations related to the excavation and preparation of a suitable foundation for an RCC alternative, it would offer a significant increase in reliability and safety,� the group said in its report.

The Corps said it already is taking a second look at the concrete dam proposal it once rejected as too costly. The idea was studied and ruled out several years ago, due to estimated costs of $500 million, compared to the current $309 million plan.

The Corps said the independent peer review validated its high-risk classification of the dam and the interim risk reduction measures it is taking. The Corps said it employs independent project reviews to provide additional insight to assist with its dam safety management and planning decisions.

Screening finds five other dams have high risk of failure

The Corps said it identified high risk of failure at Wolf Creek and five other dams during an initial screening of 130 dams in 2005 and 2006.

Other high-risk structures include:
o 135-MW Center Hill on Caney Fork River in Tennessee. The Corps lowered that reservoir and approved a $240 million rehabilitation plan including grouting.
o Martis Creek Dam, a flood control structure built in 1971 on California’s Truckee River system.
o Isabella Dam, built in 1953 on California’s Kern River. The flood control and water conservation structure controls flows released to Southern California Edison’s 12-MW Borel project (No. 382).
o Clearwater Dam, a flood control structure built in 1948 on Missouri’s Black River. The Corps said in 2005 it expected to spend $90 million to build a concrete cutoff wall the length of the dam to control seepage and eliminate the risk of sinkholes.
o Herbert Hoover dike, a 143-mile embankment entirely encircling Lake Okeechobee in Florida. The Corps has planned action to intercept and control seepage to prevent catastrophic failure due to piping.

The Corps screening found the six dams to be critically near failure or having an extremely high life and economic risk. All dams determined to be of highest risk are to undergo peer review by an independent external panel to ensure the Corps is taking the best approach to reducing risks to the public.

The Corps said it intends to screen for risks the remainder of the 610 dams it operates by the end of September 2010.

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