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TVA River Neighbors

Reservoir operations update

It’s rained and rained. Most reservoirs are at near-normal levels for this time of year. The drought is over, right?

photo

Cherokee Dam

Unfortunately not, says TVA River Scheduling General Manager Chuck Bach.

“Conditions have definitely improved as a result of the wet weather in December and January. But abnormally dry to moderate drought conditions still persist in the eastern Tennessee Valley according to the U.S. Drought Monitor."

The U.S. Drought Monitor is a weekly report compiled by a group of government and academic scientists who track droughts across the United States. Click here for the latest U.S. Drought Monitor map.

The drought won’t be over until there’s enough rainfall to replenish groundwater supplies and deep soil moisture—a slow process, according to Bach.

“The eastern Valley got just over 11 inches of rain from Dec. 1 to Jan. 31. But we need a lot more to make up for rainfall deficits of almost 20 inches in 2007 and almost 10 inches in 2008.

“A lot of the recent rain soaked into the dry ground instead of running off into the river and reservoir system. We need an extended period of wet weather before runoff will return to normal.

What is a flood guide?

Flood guides show the amount of storage allocated in a reservoir for flood damage reduction during different times of the year.

Typically, flood guide levels are lower in winter because that’s when large, flood-producing storms are most likely to occur. In the summer, less room is reserved for flood storage, so flood guide levels are usually higher.

From June 1 through Labor Day, TVA’s goal is to keep the reservoir level at the dam as close to the flood guide level as possible to support recreation. During this time, elevations fall below this line only when rainfall and runoff are insufficient to meet downstream flow requirements. During the rest of the year, the primary objective is to keep reservoir levels from rising above the flood guide to be ready for flood events. 

“The good news is that tributary reservoirs, on average, were about four and a half feet higher on February 1 than they were on the same date last year, and most were close to their flood guides, which is where we want them to be this time of year.”

The higher pool levels are due in part to the recent rainfall, but also to the fact that TVA has operated the reservoir system in a water conservation mode since the drought began more than two years ago.

“TVA has conserved water in tributary reservoirs by releasing only enough water to meet downstream flow requirements,” says Bach. “That’s limited how much hydroelectric power we can generate, but it has helped us reduce the impact of the drought on tributary reservoir elevations.”

Looking ahead to summer, Bach is cautiously optimistic. “If rainfall is near normal over the next several months, most tributary reservoirs should fill to summer pool elevations by June 1.”

The only exception is Watauga Reservoir, which is still below flood-guide level due to lack of local rainfall. It will take above-normal rainfall in that area between now and June for Watauga to fill.

So why isn’t TVA holding on to the rain we’ve been getting the past couple of months? Wouldn’t that help in reaching summer pool elevations—especially if we have a dry spring?

These are questions Bach hears a lot.

“In fact, we’re holding every drop of water that we can in Watauga since it is below flood guide. But storing more water in reservoirs already at flood guide just isn’t an option at this time of year.

“December to April is when major floods are most likely to occur in the Tennessee Valley because that's when large storms are most likely to occur. So we’ve got to be sure there’s room to store the rain from those large storms in the tributary reservoir system. That’s why it was built. If the reservoirs that make up that system already are at flood guide levels, we have to release water when it rains to recover storage space to be ready for the next significant rain.”

The timing of rainfall is crucial when it comes to the spring fill, says Bach. “From mid-March on, the risk of major flood-producing storms decreases, so that’s when we like to see wet weather. Based on historical rainfall data, we aren’t likely to need as much flood storage space, so we can start holding the water and bringing tributary reservoir levels up.”

Meanwhile, Bach is pleased that the drought’s grip on the Tennessee Valley appears to be loosening. “A return to more normal rainfall is inevitable, and it can’t come soon enough.”

 

Tributary reservoir elevations

Reservoir

Observed Elevation*

Flood Guide Elevation*

Feb. 1, 2008

Feb. 1, 2009

Feb. 1

June 1

South Holston

1694.1

1710.1

1708.0

1729.0

Watauga

1937.9

1945.0

1952.0

1959.0

Cherokee

1045.4

1045.9

1045.0

1070.9

Douglas

955.2

955.4

954.0

994.0

Fontana

1656.4

1655.1

1653.0

1703.0

Norris

989.3

1002.2

1000.0

1020.0

Chatuge

1915.5

1918.6

1918.0

1926.0

Nottely

1759.0

1762.6

1762.0

1777.0

Hiwassee

1486.2

1486.5

1485.0

1521.0

Blue Ridge

1666.5

1668.7

1669.4

1687.0

*In feet


 

 

 

 

 

 

 

 

 

 

 

 

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Correction

In our last issue, one of the columns in the table showing August 20 reservoir elevations was mislabeled. The numbers shown in the column labeled “normal level” are actually flood guide elevations. Flood guide elevations show the amount of storage allocated in each reservoir for flood damage reduction and vary seasonally. Although TVA tries to fill tributary reservoirs to flood guide elevations during the summer, it takes above-normal rainfall to reach these elevations.

           
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