With February's rainfall total nearly 7 inches above normal in Chattanooga, the rain-swollen Tennessee River has remained at least 4 feet above its normal winter levels for most of the month in Chattanooga and has flooded low-lying areas around Savannah, Tennessee for weeks.
But without the network of 49 dams built and operated by the Tennessee Valley Authority to help harness America's fifth biggest river, much of Chattanooga's downtown and low-lying areas would have been underwater. TVA estimates without its dams, the river would have risen 16 feet higher than it did in Chattanooga this month and caused an estimated $772 million of flood damage in Chattanooga.
"Without our dams, we estimate the river would have risen to about 15 feet above its flood stage so there would have been significant flooding throughout much of Chattanooga," said James Everett, senior manager of TVA's River Forecast Center in Knoxville.
Chattanooga is the drainage point in the Tennessee River basin for more than 20,000 square miles of East Tennessee, western Virginia, western North Carolina and North Georgia. Prior to the creation of TVA in 1933, Chattanooga was nearly completely underwater during major floods in 1917 and in the worst flood ever in 1867.
"There's an enormous area that flows into the Tennessee River and goes through Chattanooga," Everett said.
The Nickajack Gorge just west of downtown Chattanooga on the Tennessee River also would back up during heavy rains prior to TVA building its dams on the Tennessee River and its tributaries.
TVA estimates more than 90% of the projected flood damage that would have occurred in the Tennessee Valley without its network of dams are in Chattanooga.
TVA uses its upstream reservoirs to hold back rainfall runoff to limit downstream flooding during periods of heavy rain. TVA is able to release water and drawn down its reservoirs over time to limit flooding.
With rainfall in the Tennessee Valley more than double the normal level so far this year, TVA has kept its spillways on its major dams open throughout February, which has kept a strong current in the Tennessee River and shut off barge navigation in Chattanooga for most of the past month.
"We've had a little bit of a break in our rainfall and we're trying to recover as much as we can of our storage capacity in our reservoirs by continuing to spill water through most of our dams," Everett said.
TVA did scale back the flow of the river to below 100,000 cubic feet per second through the Nickajack Gorge on Monday to allow barge traffic to resume, at least for now, Everett said.
"But navigation could be closed again if we get much additional rainfall next week," he said.
Last year was the wettest January and February on record with nearly 17 inches of rain across the Valley in those two months. But this year's rainfall total in the same two months topped 16 inches and is expected to end up being the second wettest start for the first two months of any year in TVA history, TVA spokesman Travis Brickey said.
Across the Tennessee Valley, TVA estimates its flood control efforts have helped avoid nearly $1 billion this month of structural damage to buildings and other facilities in low-lying areas.
The savings from flood damages by TVA this month were only about half of the nearly $1.4 billion TVA estimates its dams helped save in flood damages for all of 2019 in Chattanooga, which is the most vulnerable city for flood damages in TVA's 7-state region.
During an average year, TVA's reservoir operations avert approximately $300 million in flood damage. TVA estimates it has averted more than $8.6 billion in flood damage since it completed its first dam, Norris Dam, in 1936.
Contact Dave Flessner at firstname.lastname@example.org or at 423-757-6340.