The past 12 months have been the wettest year in the Tennessee Valley in the 131 years of rainfall records, according to the Tennessee Valley Authority.
From October 2019 through September 2020, the Tennessee River basin was drenched with 75.74 inches of rainfall, or 50% more than normal. That's the wettest 12-month period ever by about 2.8 inches and, for the first time, rainfall has been above average every month for 12 consecutive months.
If rainfall continues at its current pace, calendar year 2020 also will be the wettest calendar year on record, surpassing the previous record high year in 2018 and also topping the second highest year on record, last year.
"There is definitely a cyclical nature on hydrology, and we have tended to see prolonged periods of heavy rains and droughts in the past," said James Everett, manager of the Tennessee Valley Authority's River Forecast Center. "But the past three years have been the wettest years on record and have really been extraordinary."
Although the next few days should be sunny and dry, the latest tropical storms moving up from the Gulf Coast and the Atlantic Ocean - Gamma and Delta - could bring another couple of inches of rain or more to the Tennessee Valley by this weekend, according to the National Weather Service.
Through its network of 49 dams on the Tennessee River and its tributaries, TVA has managed the record rains to limit flooding. TVA estimates it has averted nearly $1 billion in damage so far in calendar 2020, including $780 million of flood damage in Chattanooga that the agency estimates would have resulted if TVA didn't have its dams and river management system in place upstream of the city.
"It's been a challenge to manage all of the rainfall runoff in the region, but this is what the TVA dams and reservoirs were built to do, to help control floods as well as to help ensure river navigation, power generation and recreation and water quality," Everett said.
BY THE NUMBERS
* $780 million - Value of flood damage averted in Chattanooga by TVA in 2020* $9 billion - Total amount of flood damage TVA has averted through its 87-year history* 75.74 inches - Rainfall average in the Tennessee Valley in fiscal year 2020 (October 2019 through September 2020), or nearly 25 inches above normal* 58.35 inches - Rainfall so far this year in the Tennessee Valley, just 8.7 inches below the total rainfall for all of 2018, which is the highest year on record for precipitation in the Tennessee Valley.Source: Tennessee Valley Authority
In February when rainfall totaled nearly 7 inches above normal in the watershed upstream of Chattanooga, the rain-swollen Tennessee River rose nearly 4 feet above its normal winter levels for most of the month. But without TVA's dams, Everett said, the river would have risen 16 feet higher than it did in Chattanooga and left much of the city underwater, causing an estimated $720 million in flood damage.
Because Chattanooga straddles the Tennessee River and is the drainage route for rainfall runoff from more than 20,000 square miles in parts of Tennessee, Georgia, Virginia and North Carolina, the Scenic City is the most vulnerable community for flooding in the region from America's fifth biggest river.
Before the creation of TVA in 1933, Chattanooga was nearly completely underwater during major floods in 1917 and in the worst flood ever in 1867.
Heavy rains have limited barge traffic on the river during parts of February, March and April in Chattanooga. Heavy flows in the Nickajack Gorge require the Army Corps of Engineers to restrict some boat traffic following heavy rains.
But the above-average precipitation this year has helped TVA generate more of its cheapest power source - the electricity produced by the 29 power-generating dams on the Tennessee River and its tributaries. TVA gets about 10% of its total power from hydro generation and the rain provides the "fuel" for such power, generated by falling water through TVA's dams.
TVA has about 3,000 megawatts of generating capacity at its 109 hydro units, and the abundant rains have helped keep the agency's fuel costs this year 12% below average, TVA spokesman Scott Fiedler said.
Everett said TVA now is lowering the levels of its tributary lakes down to winter pool levels by the end of the year to create more water storage capacity during the colder months when vegetation is absent and there is more runoff that can cause flooding, especially during the spring.
Contact Dave Flessner at [email protected] or 423-757-6340.