TVA Reservoir System Update
River Neighbors
Rain and runoff
For the past two months, rain in the eastern part of the Tennessee Valley has been about 60 percent of normal.
Runoff (the amount of water that reaches the river system when it rains instead of being absorbed into the ground) was slightly above normal in February, but only 66 percent of normal in March.
Reservoir elevations
Although both rain and runoff were below normal in March, all main-river reservoirs were within their seasonal operating ranges and tributary reservoirs were at or very close to their targeted seasonal elevations as of midnight, April 4. A combination of factors enabled TVA to keep tributary reservoirs near target elevations despite the dry conditions. These included above-normal rain in the eastern Valley during the last three months of 2009 and TVA efforts to conserve water in the tributary reservoir system by only releasing enough water to meet downstream flow commitments.
Flood guide levels show the amount of storage allocated for flood damage reduction during different times of the year. During the summer, TVA's goal is to meet downstream flow requirements while keeping the reservoir level at the dam as close to the flood guide level as possible to support reservoir recreation. From June 1 through Labor Day, reservoir levels fall below the flood guide only when rain and runoff are insufficient to meet flow requirements. During the rest of the year, the primary objective is to keep the reservoir level at or below the flood guide to ensure there is enough space in the reservoir to store the rain and runoff from flood events.
Reservoir operations
TVA reservoir operations are currently focused on filling tributary storage reservoirs to June 1 target elevations while making sure there is enough water flowing through the river system to meet other needs.
“January rainfall was normal, but February and March have been drier,” said Chuck Bach, TVA General Manager for River Scheduling, “so we are trying to be conservative with the water in the tributary reservoir system.”
Below-normal rainfall in March, April, and May is a concern because of the effects on the spring fill. Not only does it mean lower rainfall totals; it means less runoff. Even when it does rain, not as much of the water reaches the reservoir system when the ground is dry. Plus, more of the water is absorbed by the roots of growing plants during the spring.
Being conservative with the water means only releasing the minimum amount needed to meet downstream flow commitments, said Bach.
“Flow commitments ensure that water is available for a variety of purposes, including navigation, water supply, and aquatic habitat. If there isn't enough rain to provide the water to meet these flow commitments, the water has to come from upstream reservoirs. In that case, we are careful to release only the minimum amount of water needed, and we try to draw the needed water from the upstream reservoirs that are closest to their target levels versus reservoirs that are lower because they’ve had less rain. We hold on to the rest to increase the chances of reaching June 1 elevation targets.”
What happens if the weather is wet?
There isn't much rain in the forecast for the next few weeks, according to Bach. But, if the forecast is wrong and rainfall is above average, TVA could have to increase water releases. “Flood damage reduction has priority. If we get a lot of rain and tributary reservoirs go above their flood guide levels, we're obligated to bring them back down even if that means releasing water during the spring fill. This time of year, we want to keep the reservoirs right at flood guide to give us the best chance of filling. But we've got to keep them from going above flood guide to ensure that we have room to store the water if we do get hit by a major spring storm.”
A lot depends on rain and runoff in April and May, said Bach. “We’ve positioned ourselves to fill the reservoir system on schedule assuming near-normal rainfall and runoff. But we're dependent on rain. We’ll need an average of about an inch of rain a week in the eastern Valley to fill on schedule.”
To see a reservoir's flood guide and track its elevation, go to TVA's Reservoir Information site. Choose a TVA-managed reservoir from the pull-down menu on the right-hand side of the page. Then, from the pull-down menu for “Type of Information,” select “Operating Guide,” then “View Info.”
In answer to your question about reservoir elevations
Why isn't my reservoir filling as fast as some other reservoirs?
Some Nottely Reservoir users have asked TVA this question lately. At the end of March, Nottely was right at its flood guide level, but nearby Chatuge Reservoir was nearly two feet higher than its flood guide level.
In this case, the difference was due to a relatively unique situation. Normally, the water released in the process of generating hydroelectric power at Chatuge Dam is used to meet downstream flow requirements. But the power plant at Chatuge is scheduled to be taken out of service for maintenance in mid-April. TVA is trying to bring the reservoir level up to the spillway crest elevation (1923.5 feet) so the spillway gates can be used to maintain a minimum water flow during the maintenance outage.
Typically, however, differences in reservoir elevations are due to other factors. For example, if your reservoir seems lower than a neighboring reservoir, it could be because there was more rain in the neighboring watershed. It may not have rained at your house, but it might have rained a lot just a short distance away. Reservoir elevations are highly dependent on local rainfall and runoff.
Another possibility is that you are comparing two different types of reservoirs. Large tributary reservoirs fluctuate the most because they do the bulk of the work in controlling floods. They have more storage capacity than other reservoirs and must be drawn down more aggressively—which, of course, means they take longer to fill. Main-river reservoirs don’t fluctuate nearly as much because they have less storage space and because of navigation requirements. Other reservoirs are maintained at near-steady levels year around. These reservoirs are operated primarily to maximize power production or provide local benefits. How your reservoir is operated depends on how it was designed and its purpose within the entire system.
TVA uses a tool called a balancing guide to ensure that tributary reservoirs are treated equitably during the spring fill and summer recreation season. During this time, when we have to release water to meet downstream flow requirements, we are careful to keep the elevation of all reservoirs similar relative to this elevation guide. In other words, if one reservoir is well above its balancing guide level and another reservoir is right at the guide level, we’ll try to draw the needed water from the reservoir with the higher level relative to the guide.
Another thing to keep in mind in comparing the elevation of different reservoirs is their size and shape. Differences in size and shape may cause one reservoir to look full compared to another even though their elevations are in balance. Withdrawing the same amount of water from a deep, bowl-shaped reservoir, for example, will expose more of the bank than withdrawing the same amount of water from a reservoir shaped more like a shallow serving dish. Also, reservoirs have different individual minimum flow requirements to meet water supply and other downstream needs. TVA has to release the water needed to me
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