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TVA River Neighbors

Taking care of TVA dams

Ensuring that TVA dams remain structurally sound and in good working order is no small task. Just ask Rusty Tompkins, manager of dam safety at TVA.

photo of dam inspection

An engineer on TVA’s Rope Access Team inspects one of the four spillway gates at Fontana Dam. Team members must complete a minimum of 40 hours of training, pass a test administered by outside instructors, and undergo a two-day recertification process every year to keep their skills current. 

“TVA’s dam safety group is responsible for 49 dams across the Tennessee Valley—most of which were built in the 1930s and 1940s. These dams have held up remarkably well over the years, which says something about their original design and construction and about TVA’s ongoing commitment to dam safety.”

TVA’s dam safety program has several components, including modifications to meet modern-day design criteria for earthquakes and floods.

TVA’s dam safety group began reevaluating TVA dams for extreme earthquakes in the 1970s. Reevaluations were undertaken due to advances in the understanding and modeling of potential earthquakes in the eastern United States. Similarly, in the 1980s, meteorological research showed that larger floods were possible than those considered in the original design of TVA dams. The dam safety group reevaluated all TVA dams for these higher floods, which resulted in 19 dams being modified either by increasing the height of the dam, increasing spillway capacity, or both. (A spillway is a passage for excess water to flow over or around a dam.)

Inspections are another important component of TVA’s dam safety program. TVA uses a layered approach to inspections, according to Tompkins. ”Our dam safety engineers perform a comprehensive inspection and evaluation every five years—including the ‘up-close’ view afforded by rope access.” Specially trained crews rappel down the face of dams and spillway bays looking for significant cracking of the concrete blocks that make up the dam structure or other deficiencies that might turn into safety concerns. Divers do the same inspections below the water level, looking at trash racks, spillway gate aprons, and intake gate guides. 

Teams of mechanical, civil, and electrical engineers also go over each facility every two and a half years, and civil and instrumentation engineers perform an annual inspection on each dam in the TVA system, says Tompkins. “We check previous inspection reports, instrumentation readings, and drawings to tell us what we can expect when looking at the structure and to help us determine if anything is different from the last inspection.” 

All dams have instruments which measure settlement, seepage, and stresses which are regularly reviewed. In addition, plant personnel perform monthly “walk-downs” at each location.

“This layered approach helps to ensure that potential safety concerns are identified and corrected before they become major problems,” says Tompkins. 

Finally, TVA’s dam safety group is responsible for emergency action planning. Each dam has a plan for handling dam emergencies that is coordinated with local and state emergency management agencies. Emergency drills are held periodically, and a dam safety emergency center is maintained in Chattanooga with a second facility in Knoxville.

“Dams provide tremendous benefits to society—from river navigation, flood damage reduction, and hydroelectric power to water supply, wildlife habitat, and recreation,” says Tompkins. “But they also represent a public safety issue. TVA does everything it can to protect the lives and property of people who live and work around its dams and to ensure that these dams will be around—and providing multiple benefits for the Tennessee Valley region—for many years to come.”

 

Concrete can grow?  Yes, indeed.

TVA has been dealing with the concrete-growth phenomenon at Chickamauga, Fontana, and Hiwassee dams since soon after they were built.

Concrete growth is a reaction that occurs between alkali in the cement and the limestone used to make the concrete. This reaction causes the concrete to swell, resulting in development of high internal stresses and structural deformation of unconfined features such as spillway piers and navigation lock walls.

For a massive dam such as Fontana, the effects of the growth can be managed. However, at Chickamauga, the unrestrained growth of the concrete in the dam has resulted in significant forces being applied to the walls of the lock.

Slot-cutting to relieve stresses and post-tensioning to add structural strength are only interim measures. Since the growth cannot be managed in the long-term, a new lock at Chickamauga is being constructed, and the existing lock will be closed and stabilized.

 

 

 

           
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