TVA River Neighbors
Tennessee Valley Authority (TVA)
Rainfall amounts for the part of the Valley above Chattanooga were 100% of normal for the month of October. And that’s a good thing, considering the fact that October is typically the driest month of the year in the Tennessee River watershed.
It was a different story in the western part of the Valley, where rainfall was only 48% of normal for the month. But for the calendar year to date, we’re in good shape: in the eastern part of the Valley, rainfall has been 97% of normal; below Chattanooga, the total so far is 100% of normal.
As far as runoff goes, October totals were at 86% for the eastern part of the Valley and only 29% of normal below Chattanooga. The picture looks better in terms of the calendar year. To date, runoff stands at 97% above Chattanooga and at 106% of normal for the western part of the Valley.
Flows through the system are at or slightly above what would typically be expected in an average year. That’s the consensus among those responsible for operating the TVA reservoir system.
For TVA tributary reservoirs as a whole, water in storage is slightly above median levels. Each individual tributary reservoir is either at or slightly above the median level.
The main-river reservoirs are all within their normal operating ranges for this time of year. Chickamauga, Guntersville, Wheeler, Pickwick and Kentucky are all at or approaching winter flood guide levels. The winter drawdown for Fort Loudoun and Watts Bar began on November 1; those reservoirs will be within their winter operating zones by the first of December
¹Elevations above mean sea level, as of 12:01 a.m. on this date
²Flood 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.
There’s no outward sign that it has occurred. You’d never be able to tell just by looking. Yet a profound natural change happens within every TVA reservoir at about this time of year: without any advance notice, during the course of a single day, the lake “turns over.”
And TVA’s River Forecast Center Manager David Bowling is relieved when it does. He and his colleagues schedule releases from the reservoirs, deciding how much water to route downstream, when to release it, and for how long. “We have quite a bit more flexibility after the reservoirs turn over,” he says. “It makes that one aspect of our work significantly easier.”
Reservoir turnover happens when autumn’s cooler air temperatures trigger a process by which the lake’s various water temperature layers—which have stratified over the course of the past several months—disappear, leaving the reservoir essentially the same temperature from top to bottom.
Before that point in time, Bowling and his co-workers have to be aware of (and, in some cases, must “ration”) the amount of cold water available from the deepest part of the reservoirs. “All during the year, but especially during the summer season, we are constantly keeping tabs on thermal concerns—doing what we can to time releases of cold water to help regulate river temperatures downstream of TVA’s fossil and nuclear plants,” he says. “Our operating policy includes a commitment to provide minimum flows, which means we release water at certain times and at rates designed to benefit aquatic life and improve downstream water quality. The cold, oxygen-rich water we release is of particular importance to trout fisheries in the tailwaters.”
As a matter of fact, anglers are among the few folks—with the exception of “limnologists,” who study the thermal dynamics of reservoirs—who are aware of the layers of different water temperatures, referred to as the “thermocline.”
Autumn foliage is reflected in the waters of Norris Reservoir.
Because cold water is heavier than warm water, it’s always found on the bottom of the reservoir. During the summer months, the warm air temperatures and sunlight heat up the top layer. Anglers use their knowledge of this thermal stratification to know where they’re more likely to have success. As the top layer of the lake gets warmer, the fish naturally gravitate toward the deeper, cooler water. Savvy anglers fish deeper during that time of year.
But something interesting occurs when the first cold weather of autumn arrives. “After a number of days and nights in the 30- to 40-degree range,” explains Bowling, “these brisk air temperatures start to cool off the top layer of the reservoir. The result is that, during this relatively brief period of time, there are actually three water temperature layers: a cold top layer, a warm middle layer, and a cold bottom layer.”
In terms of simple physics, that arrangement is not sustainable indefinitely. Eventually, the heavier top-most layer of cold water sinks down through the warmer water. After this mixing has taken place, there are no more warm or cold layers—just a reservoir with uniformly cold water from top to bottom. At this point, the lake is said to have “turned over.”
“TVA collects temperature profiles as part of our ongoing monitoring efforts,” says Bowling. “So we know exactly when each reservoir turns over. It really doesn’t change how we operate the river system, to any great extent. But it does provide us with a much greater volume of cold water, which can then be routed very effectively throughout the system.”
It’s a particularly useful time of year to have plenty of cold water to work with, since water is already being withdrawn from the tributary reservoirs in order to provide flood storage space. “We have the opportunity to be judicious with regard to the timing of our releases,” says Bowling. “This flexibility allows us to run less water on days when temperatures are mild and power demand is not as great, and also to run more water during times when there’s increased demand for power. Our goal is to meet all our other obligations—like providing minimum flows—while maximizing the value of the water used to generate electricity.”
The distinction is an important one. TVA is not cutting slots into Fontana Dam because of any problems that have arisen. TVA is cutting slots into Fontana Dam in order to prevent potential problems from occurring.
“The idea is to address these issues well in advance of impacts to either dam safety or spillway operations,” says civil engineer John Barrett. He works in TVA’s River Operations Engineering and Support Services group and serves as technical lead for the project, which began on October 10th and is scheduled for completion by December 1st.
It’s not the first time that slot-cutting has happened at Fontana, and it won’t be the last. That’s because of the type of concrete mix that was used for the dam’s construction back in the early 1940s. The presence of a certain type of rock (or “aggregate”) mixed with cement caused a chemical reaction to take place. The result is commonly referred to as “concrete growth.” Basically, it means that the concrete gradually expands over time, or—in situations where the concrete is constrained and is thus unable to expand—the building of stress can cause cracks to form.
“It’s something TVA has been aware of for quite some time now,” says Barrett. “Unfortunately, concrete growth is also an issue at several of our other dams which were built about the same time—Hiwassee and Chickamauga, most notably.” During the “Golden Age” of dam construction, no one knew of the potential for this type of chemical reaction to take place. Since that time, TVA has helped pioneer engineering solutions to mitigate problems associated with concrete growth, and standards have been implemented to make sure that the reaction doesn’t take place in new construction.
The path of the cut on the downstream face of Fontana Dam
“As a known condition,” says Barrett, “it’s really a maintenance issue for us. These ‘concrete growth dams’ are performing as designed, and we stay on top of things—taking appropriate action when necessary—to make sure that continues to be the case.”
If the situation were allowed to progress unchecked, it would eventually mean that spillway gates would start to bind. If they were to actually get stuck and could not be raised, that would naturally impact public safety—from several different perspectives.
Barrett explains: “When scheduling reservoir releases, TVA always prefers to pass water through the turbines, to make wise use of the resource by generating electricity. But there are times when heavy rains over a period of time makes that impossible. If we didn’t have the option of spilling water during flood control operations, rising waters in the reservoir could result in shoreline properties being flooded. And in a worst-case scenario, the water would rise above, or ‘over-top’ the dam.”
All of which provides ample reasons for TVA to keep a close eye on the progression of concrete growth at each of the dams where it is an issue. “An extensive monitoring program is our first line of defense,” says Barrett. “We make it our business to be constantly aware of conditions, so that any change related to concrete growth is immediately obvious. There are no surprises.”
Streams of data from hundreds of highly sensitive instruments placed at different locations throughout the dam’s structure are painstakingly analyzed on a monthly basis. Based on those results, projections are made as to the rate of growth and when action will be needed to prevent future problems.
Once a year, TVA performs what is called a “full travel test,” sending a pair of engineers out to inspect and physically hoist the spillway gates—making sure they operate smoothly when maneuvered from a fully closed to a fully open position. These inspectors report their findings, which virtually dove-tail with the data obtained from monitoring efforts.
When all signs point to the need for measures to be undertaken to address the situation, then a plan is put into place for providing some extra “room” for the concrete to continue its persistent growth. The most effective method for doing this is to cut two slots into the dam’s surface, one on either side of the spillway bays.
It’s a complex process, in terms of both timing and execution. TVA closed vehicle traffic across the top of the dam on October 10th, in order to help ensure worker safety. Work began on the first 93-foot slot on October 17th. With labor continuing around-the-clock, it was successfully completed just seven days later. No small feat, given the enormity of the task. “It takes a specialized saw, which is kind of like a massive version of a bandsaw, to make the 5/8-inch-wide cuts,” explains Barrett. “A continuous wire moves in a loop and is pushed downward through the concrete with hydraulically powered pulleys. The wire actually passes through a series of pulleys—some of which take up the slack while others make necessary adjustments—including a mobile pulley, which is attached to the saw itself. To give you an idea of the scale we’re talking about here, the main pulley is three feet in diameter.”
The primary seal features several layers of protection, helping to ensure that the slot is water-tight.
Cutting of the first slot was accomplished thanks to the use of a “cofferdam,” which is attached to the upstream side of the dam and allows water to be pumped out of the resulting space—thus making it possible for the work to take place “in the dry.” Work on the second slot was scheduled in order to take advantage of lower elevations resulting from the winter drawdown of the reservoir, and will begin on November 14th, weather permitting. If all goes according to plan, the road across the dam will be re-opened some time in early December.
Regardless of the complexity of the procedure or the justification for it, there’s still a nagging question that begs an answer: just how can you cut two vertical slots in a dam without water leaking through the openings? After the cut is made, neoprene pads are glued to the concrete surface of the dam, upstream of the slot. On top of that is placed a single continuous piece of extremely heavy-duty material, similar to what is used for conveyer belts. Steel plates positioned on either side of the slot are then bolted to the face of the dam. Together, these layers of protection span the cut, from top to bottom, and form an absolutely water-tight seal.
In addition to being impenetrable by water, the seal must also be flexible and elastic, says Barrett: “Despite their massive size and appearance of solidity, all dams ‘move’ a little—due to thermal variation. They’re designed that way, in order to allow for the expansion and shrinkage of the concrete during summer’s heat and winter’s cold. The primary seal has to be formed in such a way as to shift along with the dam itself as conditions change.”
Even the most ingenious engineering, however, can’t halt the slow but steady growth of the concrete. Years from now, the slot-cutting procedure will no doubt need to be repeated at Fontana. But when it happens, it won’t be undertaken as a response to a problem. It’ll be done in order to keep problems from happening in the first place. “It’s always wiser and more cost-effective to do maintenance on something than to wait for an issue to begin impacting dam safety or operations,” says Barrett. “The idea is to be proactive, instead of reactive.”
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