With recent flooding in Bedford County, those working for the Tennessee Valley Authority have had to stay on the clock to combat the rising levels of the Duck River.
Normandy Dam, which impounds the Duck, went to a spill rate of 12,200 cubic feet per seconds. This was combined with other lower-level outlets which were releasing another couple thousand to total 15,000 cfs being released earlier this week. Multiply this by 7.48 and you get 112,200 gallons being released per second!
The 17-mile-long reservoir and dam of Normandy was completed in 1976 to aid in the economic development of the upper Duck River region and to operate for flood damage reduction, according to TVA.
It’s the largest of the non-power dams on tributaries of the Tennessee River at 110 feet high and stretching 2,807 feet across the Duck River.
It’s even more incredible to think about when considering that Normandy Dam is only one of 49 dams across the seven states that TVA monitors.
“Part of the reason why we staff around the clock is so we can adjust those gate operations and control the water levels and reduce downstream flooding as much as we can,” said James Everett, senior manager for the River Forecast Center at TVA.
Monday’s cfs rate is not uncommon, Everett assured. For example, 2019 had a similar spill rate, while in 1990, the levels were closer to 20,000 cfs, Everett said.
However, according to Everett, “What is kind of unique is when you look back over the last three years between 2018 and 2020, we have seen valley-wide record-breaking rainfall.”
TVA has rainfall data stretching back to around 130 years for the entire Tennessee Basin, including Normandy. The last three years have shown to be the wettest among those 130 years, with 2020 being the wettest year for the Tennessee Basin.
Everett said we are in a “wet pattern,” and 2021 is an ongoing fourth year of above average rainfall.
Forecasts over the weekend were predicting only an inch or two of rainfall. But that number went up close to 9 inches of rainfall at Normandy Dam.
“When we start to see rainfalls occurring well in excess of what was forecasted then we have to adjust to more a real-time response,” he said. “... Very, very different conditions in reality.”
TVA keeps a river forecast center in Knoxville open 24-7. So, when rainfall rises to above what’s predicted, Everett says they begin to focus on flood control.
When the certain elevation and storage reservoirs can hold is exceeded, water is released through sluices, that is, lower-level gates. There are two of these at Normandy as well as a large 36-inch valve, according to Everette. The water level in Normandy Reservoir varies about 11 feet in a normal year.
“But in these larger flood events, we have to resort to spillway gates, which are large gates that can be opened up to precisely regulate flow from the reservoir downstream,” he said. Which is what they ended up doing in Normandy earlier this week.
For communities like Columbia, Centerville, and Hurricane Mills, who receive heavy rainfalls on top of the Duck River below Normandy Dam, TVA has no ability to regulate the flooding, Everett said.
“Normandy controls a very small portion of that Duck River drainage system. But when you get rainfall up in the upper portions of the drainage system, like what we had over the weekend, it becomes very critical for us to keep a close eye on it and do everything we can.”