TVA to raise the height of four dams

KNOXVILLE, TENN. — TVA is raising the elevation of four of its dams to help reduce the risk of flooding in the unlikely event of weather more extreme than any conditions ever recorded in the region. TVA will place temporary, wall-like structures on top of earthen embankments of the Fort Loudoun, Tellico, Cherokee, and Watts Bar dams in East Tennessee, raising the top elevation of each embankment about 4 feet. The extra height would prevent water from overtopping and damaging the earthen embankments.

“TVA evaluates its dam safety program regularly,” said TVA River Scheduling General Manager Chuck Bach. “As technology and standards evolve and more or better data becomes available, we update our flood forecasting calculations to determine whether we need to change our flood control operation or modify our dams to better protect public safety.”

A recent update of TVA’s river modeling program determined that the maximum floodwater elevations could be higher than previously calculated if a highly unlikely, worst-case winter rainfall were to occur in the upper part of the Tennessee Valley watershed.

Bach stressed that this action is not related to recent rainfall that caused flooding in unregulated tributary streams, which was only a fraction of the amount used to forecast a “worst case scenario.”

“The magnitude of rainfall used in the calculations has never actually occurred in the region, but it could produce floodwaters that would be higher than previously thought,” said Bach. “Although the probability of such an event is extremely rare, TVA must design and operate its dams to safely withstand such events.”

He said structures to raise the embankments are scheduled for installation by Jan. 1, 2010, because large regional floods are most likely to occur in winter and early spring. The interim measures are expected to remain in place until long-term, permanent solutions can be identified, evaluated, and implemented.

To calculate the maximum flood levels in its river modeling, TVA assumes an extremely large storm within the watershed area being evaluated, which is critically centered to produce maximum flood levels. The higher predicted flood levels are the result of improved data gathered in part from experience gained during large storms, revised analysis of spillway water flow rates at dams, and higher initial reservoir levels in TVA’s new reservoir operating policy.