Under [Barometric] Pressure

When winter storms hit, it’s essential that reservoir levels are low enough to absorb the blow. And TVA's River Forecast Center makes sure they are so that floods are avoided. 

Hurricane Joaquin—remember him? The category four hurricane joined forces with another storm system to threaten the Tennessee Valley in late September of 2015. There was the potential for damaging, torrential rain to tear into our region.

“At one point, forecasters were calling for 10 to 15 inches of rain,” says Tom Barnett, senior manager of TVA’s River Forecast Center. “Our computer models were showing that levels on some lakes could rise as much as 30 feet.”

As it turns out, while Joaquin battered South Carolina, the Tennessee Valley received only a glancing blow. North Georgia and western North Carolina got between four and seven inches of rain. But the rest of the region averaged only about two inches.

“Precipitation amounts often change dramatically from the time radar shows a storm forming to when the rain actually starts,” Barnett says. “But, by the time we know a storm is heading our way, there’s generally not enough time to move a significant amount of water out of our tributary storage reservoirs anyway.”

That’s why TVA focuses on keeping reservoirs at or below reservoir-specific flood guide levels year round. These guides specify the amount of flood storage that must be reserved during different times of the year. They’re based on historical rainfall records and decades of operating experience.

Flood Protection Powerhouses

Blue RidgeNottely and Chatuge reservoirs typically do the brunt of the work in reducing potential flooding from hurricanes, just as they did for Joaquin, says Barnett.

“These reservoirs get the majority of hurricane-related rain because of their location on the southern edge of the Tennessee River system. We start drawing them down in August for exactly that reason: because of the increased risk of late-season hurricanes.”

Hurricane season typically starts in May and goes through the end of November. The most active hurricane months are August, September and October.

The flood season in the Tennessee Valley is from December through early May.

“That’s when runoff, the amount of water that ends up in the river system after it rains, is the highest,” says Barnett. “And it’s when we tend to get bigger storms—storms that cover a large area and last for several days.”

When Storms Do Hit

Whether it’s from a hurricane or a winter storm, heavy rain calls for immediate action, Barnett says: “TVA’s River Forecast Center in Knoxville is staffed 24/7, 365 days a year. But it’s all hands on deck when the rain starts. We’ll have people monitoring radar and reservoir levels in real-time, people analyzing hourly data from 250 rain and streamflow gages across the Valley, and people running computer models that show how the water coming in to the system will affect the level of different reservoirs.

Crews also are on call to open and close spillway gates at TVA dams.

At the same time, River Forecast Center staff are in contact with the National Weather Service; local emergency management agencies; cities, towns and businesses along the river; and private property owners, making sure they have the latest information on flows and flood elevations.

“There’s a lot of pressure,” Barnett says. “We know our decisions could affect people’s homes, businesses, crops and even lives. But everyone is clear on the objective: to minimize flood damage.”

That brings Barnett back to a key point: “A lot of people would like us to keep reservoir levels up longer. But we just can’t do that. It takes time to move large volumes of water through the system efficiently, and there’s always the chance of an extreme event—a 100-year or even a 500-year flood. If flood storage space isn’t available when it’s needed, the consequences could be extremely serious. That’s why we draw tributary reservoirs down this time of year.”