Friday, February 27, 2009
Percy Priest Dam is classified high-risk
By Christina E. Sanchez • THE TENNESSEAN • February 27, 2009 Concerns about the limestone that lies under Nashville's J. Percy Priest Dam have led U.S. Army engineers to assign it a spot within the second-highest level of risk. There's no need for those downstream in Nashville, Ashland City or Clarksville to head for higher ground, because the Corps of Engineers says there's no imminent danger of the dam failing. But the results of the agency's recent survey of its 600 U.S. dams will lead to studies in the coming months to determine what work is needed to prevent potential failures, as well as determine the impact of a flood if the dam were to break. "There are no problems that we are aware of, as we were with Center Hill Dam and Wolf Creek," said Michael Zoccola, chief of civil design for the Nashville branch of the Corps of Engineers. "There have been no changes to the dam over its existence, but there are potential for problems to develop." Center Hill and Wolf Creek dams well upstream from Nashville are undergoing multimillion-dollar repairs to plug leaks that threaten the integrity of the structures and caused fears of catastrophic failures that would flood Nashville and other downstream communities. No such leaks have been found at Percy Priest. The problem with all three dams is their limestone foundations, which are subject to erosion. Percy Priest was built in the 1960s on the Stones River in eastern Davidson County, creating a 42-mile-long reservoir that also reaches into Rutherford and Wilson counties. At full summer depth, its 130 billion gallons of water cover 14,200 acres. Design standards of today weren't in place when the dam was built, which creates the potential for the underlying limestone to erode and lead to seepage, Zoccola said. "The Percy Priest dam design was more advanced than Center Hill and Wolf Creek," Zoccola said. "Any dam not being built at this moment is not going to be up to state-of-the-art design." TEMA is prepared While there's no foreseeable danger of catastrophe, the Tennessee Emergency Management Agency says it would be prepared if Percy Priest were to collapse. "Many people envision the huge walls of water they see in the movies would happen here. That's not the case," said Jeremy Heidt, spokesman for the agency. "The rise in water would be sudden, but it could take days to reach the peak flood level." TEMA has no estimates of physical or monetary damage that would result along the Stones and Cumberland rivers in Nashville and beyond if the dam were to fail. They depend on the Corps to provide that data, which now will be gathered in the wake of the new safety ranking. Zoccola said engineers and experts would start creating a plan over the next 90 days on how to further assess the issues identified in the ranking report. "This ranking basically puts us in the queue for funding for more studies," he said. "There will be an in-depth risk assessment process to confirm the ranking." The Corps started ranking its U.S. dams in 2005, issuing Dam Safety Action Classification levels that range from I to V. The lower the number, the higher the risk — and the higher priority for funding. Percy Priest was given a DSAC II. The highest ranking was assigned to the leaky Center Hill Dam, on the Caney Fork River in DeKalb County, Tenn., and Wolf Creek Dam, on the Cumberland River in south-central Kentucky. Five of the 10 dams in the Corps' Nashville district have been ranked. The other two that have received rankings are Dale Hollow Dam on the Obey River near Celina, Tenn., and Barkley Dam on the Cumberland River downstream from Nashville in western Kentucky. They were assigned level three. Assessments are still being finished on the remaining five, including Old Hickory in Nashville and Hendersonville, Cheatham Dam near Ashland City, and Cordell Hull Dam near Carthage, Tenn.
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