Making the most of rainfall
In three years of drought, lead engineers Nikki Berger and John Hoover and their River Operations colleagues have been working hard to move enough water through the Tennessee River system to provide adequate depth for barges, cooling water for nuclear and fossil plants, water quality, hydro generation and water supplies and recreation for the region.
The drought began in 2006 when the Tennessee Valley had its 27th-lowest total rainfall in 120 years of recordkeeping. In 2007, it hit bottom, with the lowest total rainfall on record, followed by the 15th-lowest annual total in 2008. The lack of hydropower those years cost TVA $1.5 billion for replacement power.
This spring, when it did rain, it poured. But conditions have become dry again, and runoff – the portion of the rainfall that actually makes it into the reservoir system — is only about 78 percent of normal in the eastern part of the Valley.
“We had three years with the water table receding and wells going dry,” says Hoover. “The dry, starved ground is still soaking up lots of that rainfall.”
|“Rainfall is still below normal for the year, and signs are pointing to below-normal rainfall for the next few months,” says Mike Eiffe, program manager in Water Supply.|
In early May, with heavy rains in northeast Tennessee, the River Operations team went to work to meet another basic purpose of the river system: flood control. The rains raised the level of Norris Reservoir to 1,024 feet, about four feet above the target summer level.
To bring the level back down, the flow through the turbines at Norris Dam was increased to the maximum 9,500 cubic feet per second around the clock. But even that could not move water out of the reservoir fast enough to keep up with the water surging in.
On Monday, May 11, Berger gave the order for technicians at Norris Dam to open the three spillway gates – the first time TVA had opened the gates for flood control since 2003.
Those rains helped bring the total rainfall so far this fiscal year to 94 percent of normal for the entire Valley. Hydropower generation is about 82 percent of normal, up from 50 percent in 2008, 70 percent in 2007 and 73 percent in 2006.
Still, says senior Vice President for River Operations Janet Herrin, “We’re not out of the woods.” In fact, in recent weeks the eastern portion of the Valley has received less-than-normal rainfall, and forecasts call for below-normal precipitation for the next few months.
Without sustained inflows into the system, some water in the tributary reservoirs will have to be used keep up minimum flows through the system, and that will affect reservoir levels. It will also affect TVA’s less costly source of power — hydro generation.
So, will the Valley get back to normal rainfall this summer?
“That’s the million-dollar question,” says Hoover. “In the end, we’ll react to whatever Mother Nature provides us, to the best of our ability.”