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TVA River Neighbors
November 2006

 

Reservoir operations update

In answer to your questions about winter reservoir operations

image of flooding

In an average year, lowering tributary storage reservoirs prior to the start of the winter flood season helps avert about $224 million in flood damage in the Tennessee Valley and along the Ohio and Mississippi rivers.

When reservoir levels begin to drop, the number of phone calls and e-mails to TVA with questions about winter reservoir operations goes up. For answers, we turned to Chuck Bach, manager of TVA’s River Scheduling.

Is this year’s drawdown on schedule?

Most tributary reservoirs are actually at slightly higher levels than they would be under normal rainfall conditions. Because the first eight months of the year were relatively dry, we were careful to conserve as much water as possible so we’d have the flows we need for water quality and hydropower generation going into the fall months. Then September and October turned out to be fairly wet. That’s left us with a little more water in storage than usual. Water levels will continue to drop through December, and we expect to reach winter operating levels on schedule by January 1.

What is the winter flood guide level on my reservoir?

Reservoirs are drawn to their lowest level by January 1 to get ready for flood-producing storms. These storms typically occur in winter and early spring when vegetation is dormant and runoff is the highest.

To see the expected winter elevation range for your reservoir, go to TVA’s Reservoir Information page. Choose your reservoir from the pull-down menu and then select Operating Guide. You’ll see a graph with a shaded band labeled Expected Elevation. Based on computer simulations using more than 100 years of historical rainfall and runoff data, you can expect your reservoir to be in the shaded area an average of eight out of 10 years on any given date.

How was the winter flood guide level on my reservoir determined?

How far a reservoir is drawn down to get ready for wintertime storms varies depending primarily on the reservoir’s original flood storage allocation. The engineers who designed TVA’s water control system determined the flood storage allocation for individual reservoirs based on their drainage area, size and shape, historical rainfall data, and other variables that influence a reservoir’s ability to store and release water at given times of the year.

Some of these flood storage allocations have been modified over the years based on subsequent analyses of rainfall and runoff characteristics of the drainage basin and physical limitations of the reservoir system. The most recent analysis, completed in 2004, resulted in a decision to allow higher winter water levels on the 11 tributary reservoirs that supply the bulk of TVA’s flood storage capacity. On average, winter pool levels on these reservoirs are 10 feet higher than they would have been under TVA’s previous operating policy.

Why are some reservoirs so much higher than others this time of year? Do some reservoirs get special treatment?

If your reservoir seems lower than a neighboring reservoir, it could be because there was more rain in that 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 storing floodwaters. They have more storage capacity than other reservoirs and must be drawn down more aggressively. Main-river reservoirs don’t fluctuate nearly as much because they have less storage space and because of navigation requirements. Other reservoirs, which are operated primarily to maximize power production or provide local benefits, are maintained at near-steady levels year around. How your reservoir is operated depends on how it was designed and its purpose within the entire system.

In 2004, we began using a new tool, called a balancing guide, to ensure that tributary reservoirs are treated equitably during the summer recreation season. When we have to release water during the summer 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. Our Reservoir Information page lets you track this for yourself.

When will my reservoir start going back up?

The spring fill typically begins in mid-March. Most reservoirs fill to their highest level by June 1. Again, you can track your reservoir’s elevation on TVA’s Reservoir Information page and see how it compares with the expected operating range and actual pool levels for the same date last year.

 

 

 

 

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