TVA River Neighbors Message
River Neighbors
Rainfall and runoff
The eastern Tennessee Valley received 4.5 inches of rain in the month of January, which is about normal. Runoff was 3.8 inches, which is slightly above normal.
Runoff is important to reservoir elevations because it is a measure of the water that reaches the river system when it rains instead of being absorbed into the ground. Runoff is affected by soil conditions and by the intensity, amount, and duration of rainfall.
For the past four months, rainfall was 3.6 inches above normal, and runoff was 5.7 inches above normal.
Reservoir elevations
Most reservoirs in the TVA system are higher than normal for this time of year due to heavy runoff from rain and snowmelt and TVA efforts to temporarily store the extra water to reduce downstream flood damage. Main-river reservoirs are near summer elevations, except for Nickajack and Wilson which are within their seasonal operating zones.
Flood guide levels show the amount of water storage available in the reservoir to help with flood damage reduction during different times of the year. During the winter, TVA’s goal is to keep the reservoir level at the dam from rising above the flood guide to be ready for ready for large rains that could cause flooding. During the summer, the primary objective is to keep reservoir levels as close to the flood guide level as possible to support recreation. In summer, reservoir levels fall below the flood guide only when rainfall and runoff are insufficient to meet downstream flow requirements.
Reservoir operations
Higher-than-normal rain in 2009, along with recent rain and snowfall, has TVA reservoirs above seasonal levels.
“This year is beginning like last year ended, with a lot of precipitation,” said Chuck Bach, TVA General Manager for River Scheduling. “During the last three months of 2009, rainfall and runoff totals were significantly above normal, and now in 2010 we are getting a significant rain or snow every week. We’ve run the hydroelectric turbines at TVA dams at maximum capacity to move all that water through the system so we’d have room to store additional rain. But, with all of the recent rain and snow, inflows have been so high, we haven’t been able to release the water through the turbines fast enough to recover all the storage space needed. We’ve also had to spill water at all main-river and some tributary power dams in the TVA system to bring reservoir levels down so we’ll be ready if we get more rain in the weeks ahead.”
TVA tries to avoid spilling, according to Bach. “Hydro is our most efficient and economical power source, so we try to release as much water as we can through the turbines at our dams. But our number-one priority is providing the flood-reduction benefits that the TVA system of dams and reservoirs was built to provide. That means spilling water if necessary to ensure that we have adequate flood-storage space.”
All nine hydroelectric dams on the Tennessee River and most of the 20 power-producing tributary dams have been running at maximum power generating capacity. Those dams have generated 7,958 gigawatts of power in the past four months, compared to a normal generation of 5,853 gigawatts.
Bach says TVA already is looking ahead to the spring fill. “In the weeks ahead, we’d like to keep reservoir elevations from dropping below flood-guide levels so that, when the spring fill starts in mid-March, we’ll be in a good position to reach summer target levels on schedule. If the rain continues, that shouldn’t be difficult.”
In answer to your question:
Wouldn’t it be a good idea to hold on to some of the rain we get this winter to help fill reservoirs to full pool this summer?
Although holding on to extra rain would help the spring fill, TVA River Scheduling Manager Chuck Bach says TVA must be very careful about going above winter flood-guide elevations.
“Summer storms typically affect only a portion of the Valley, but winter storms can cover the entire Valley for several days, with one storm followed by another even larger storm three to five days later. History provides clear evidence of this pattern. During the past 140 years, the largest flood events occurred in March 1867, February-March 1875, April 1886, March 1897, March 1917, January-February 1957, March 1963, March 1973, April 1977, May 1984, February-March 1994, and May 2003. Plus, runoff is higher in the winter and early spring because vegetation is dormant and the ground is wetter.
“As we get closer to March, we’ll start looking at the possibility of holding more water in the tributary system to help reach June 1 levels. But holding extra water in reservoirs already at flood-guide levels isn’t an option.
“If we were to go above flood guide levels and then got hit by a major winter storm and didn’t have the flood storage space we need, we’d be getting calls from people with water in their homes and businesses and from members of Congress who hold us responsible for operating the reservoir system for the purposes it was built—to improve navigation and reduce flood-damage.”
Dam safety modifications
TVA is raising the elevation of some of its dams to help reduce the chance of flooding in the unlikely event of a worst-case winter rain event.
Temporary, wall-like structures have been installed on top of four dams—Cherokee, Fort Loudoun, Tellico, and Watts Bar—increasing their effective height to prevent water from overtopping and damaging the structures should such an event occur.
TVA River Scheduling Manager Chuck Bach says a recent update of TVA’s river modeling program determined that a worst-case storm could produce more rain than the dams could handle, even with the floodgates wide open. Such an event is highly unlikely. But, because it is possible, federal guidelines and nuclear operating regulations require TVA to be prepared for it.
TVA evaluates its river modeling program regularly as modeling technology evolves and more or better data becomes available. Factors affecting the most recent evaluation included improved modeling technologies and data, alterations to TVA dams, and changes in TVA’s reservoir operating policy since the last evaluation.
Additional analyses are being conducted to determine if other dams need to be modified. Permanent solutions will be developed with public input under the guidelines of the National Environmental Policy Act.
Member Comments…
»
Alabama Lakes
»
Pickwick Lake
Login
|
Home
