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October 4, 2010

In this issue:

Rain and runoff
Reservoir elevations
Reservoir operations
Holding back the floodwaters
TVA installs warning system at Ocoee No. 1 Dam
Blue Ridge Dam rehabilitation project update
Reminder: Watts Bar Lock to close Oct. 13
More TVA information

TVA provides monthly updates on the operation of TVA-managed reservoirs by e-mail. To sign up for future updates, provide feedback, change your e-mail address, or have your address removed from this distribution list, please send an e-mail request to reservoirupdate@tva.com.

 

Rain and runoff

Most of September was dry in the eastern Tennessee Valley, but the rainfall total for the month ended up above normal thanks to two days of steady rain at month’s end. Eastern Valley rainfall for the year totaled 31.7 inches at the end of September, which is 7.4 inches below normal.

Eastern Valley Rainfall

Month

Observed rainfall

Normal rainfall

Percent of normal

January

4.51

4.6

98

February

2.68

4.23

63

March

2.79

4.82

58

April

2.69

4.17

65

May

4.50

4.23

106

June

2.91

4.28

70

July

3.96

4.97

80

August

4.01

4.28

94

September

3.68

3.38

109

Runoff (the amount of water that reaches the river system when it rains instead of being absorbed into the ground) is currently 87 percent of normal in the eastern Valley.

 

Reservoir elevations

On average, the elevation of tributary storage reservoirs decreased by about five feet during the month of September as TVA continued the annual drawdown to winter flood-damage-reduction levels. Tributary reservoirs were slightly higher than normal on Oct. 1, but comfortably below their flood-guide elevations. (Flood-guide elevations reflect the amount of storage allocated in tributary reservoirs for flood-damage reduction during different times of the year. The operating objective is to keep the reservoir level at the dam at or below this elevation to be ready for flood events. A reservoir may rise above its flood-guide elevation as a result of large inflows, but the water level is lowered to the flood-guide elevation as soon as it can be done without increasing downstream flood damage.)

Blue Ridge Reservoir is a special case. TVA began a deep drawdown on Blue Ridge in mid-July. The reservoir is being lowered to an elevation between 1620 and 1630 feet above sea level—compared to its normal winter flood-damage-reduction level of 1668—as part of a project to rehabilitate the 79-year-old dam. Get an update on this project below.

Reservoirs along the main Tennessee River—Fort Loudoun, Watts Bar, Chickamauga, Nickajack, Guntersville, Wheeler, Wilson, Pickwick, and Kentucky—were all within their normal operating ranges on Oct. 1.

Tributary Reservoir Elevations¹

Reservoir

Oct. 1, 2010
Observed Elevation

Jan. 1
Flood Guide Elevation2

South Holston

1716.4

1708

Watauga

1949.4

1952

Cherokee

1054.7

1045

Douglas

975.6

954

Fontana

1682.4

1653

Norris

1008.1

1000

Chatuge

1921.1

1918

Nottely

1768.2

1762

Hiwassee

1503.5

1485

Blue Ridge

1633.7

1668

Tims Ford

886.6

873.1

Normandy

869.9

864

1 Water elevation at the dam in feet above mean sea level
2 Flood guide elevations show the amount of storage allocated for flood damage reduction during different times of the year. The amount of storage varies with the potential flood threat. Flood-guide elevations are lowest from Jan. 1 through mid March because winter storms are generally larger, occur more frequently, and produce more runoff. Flood-guide elevations increase between mid-March and June 1 as the risk of flooding decreases. They are highest from June 1 through Labor Day to support summer reservoir recreation. After Labor Day, TVA begins the unrestricted drawdown to Jan. 1 flood-guide elevations.

 

Reservoir operations

Since Labor Day, TVA river operations have focused primarily on two objectives, according to David Bowling, manager of TVA’s River Forecast Center in Knoxville. “We are working to lower reservoir elevations to winter flood-damage-reduction levels and to schedule water releases in the most economical way.”

He says that represents a major shift in focus.

“During the summer, we ration the water to keep tributary reservoirs at summer elevations, Bowling says. “Under normal conditions, we release only enough water to meet minimum flow requirements, which are set to ensure an adequate flow of water through the river system. We generate hydroelectric power with the water we release to meet those requirements, but we don’t release any extra water for power purposes.

“That changes in September. After Labor Day weekend, flow restrictions are lifted. We start releasing water from tributary storage reservoirs at a faster rate with the goal of lowering water levels to winter flood-damage-reduction levels as efficiently as possible.”

Lowering water levels efficiently is mostly a matter of timing, according to Bowling. “As we release water to create the flood-storage space we need in winter, we try to get the most the value we can from it. That means releasing it through the hydroelectric turbines at TVA dams—preferably on days and at times when power demand is highest.”

So why not keep reservoir levels up longer and save the water for colder weather when power demand goes up?
Bowling says delaying the unrestricted drawdown would likely waste water. “We’ve got to move a lot of water through the system to get ready for the winter flood season, and it takes time to move that much water efficiently.

“Beginning the unrestricted drawdown after Labor Day minimizes the chance that we will have to spill water to reach winter flood-damage-reduction elevations on schedule. That’s important to all of us because hydroelectric power is TVA’s least expensive source of generation, and spilled water doesn’t generate any hydroelectric power.

 

Holding back the floodwaters

This article continues our series on the many ways the Tennessee River system touches our daily lives. In our last issue, we talked about the benefits of river transportation. In upcoming issues, we’ll highlight power generation, water supply, water quality, and recreation benefits.

“How high’s the water, Mama?
Two feet high and risin’
How high's the water, Papa?
She said, it’s two feet high and risin' ...”

Those who lived in the Tennessee Valley before TVA were able to relate to the lyrics of this popular Johnny Cash tune. Many saw the big floods and the scenes of devastation firsthand—the mud-red river pouring over its banks, cutting across fields, submerging streets and entering homes.

Such scenes were part of the reason TVA was created: to tame the river. The TVA Act specifically instructs TVA to operate the dams and reservoirs in its control to regulate the flow of water in the Tennessee River and its tributaries for the purposes of promoting navigation and controlling floods and, consistent with those purposes, to generate electric power.

The TVA reservoir system can’t prevent all flooding. Many streams across the Valley aren’t regulated—that is, they aren’t controlled by an upstream dam—so there is no way to stop local flooding. Also, unusually large storms can produce more rain and runoff than TVA-managed reservoirs have room to store. Consequently, TVA focuses on reducing flood damage.

TVA is able to reduce flood damage by temporarily storing the water in upstream reservoirs during large rainfall events. When the rain stops and the danger of flooding is over, TVA gradually lets the water out at a controlled rate to get ready for the next storm. This lowers the downstream flood crest and the resulting property damage.
Reducing flood damage is contingent, however, on having enough space to store the floodwaters. Most federal dam owners reserve the same amount of flood storage in their reservoirs year-round. But, to optimize the value of the water, TVA allows for a seasonal variation in the use of storage space. Historical rainfall records show that the largest storms in the Tennessee Valley usually occur in winter and early spring, so that’s when TVA reserves the most storage space. In the summer, when storms tend to be smaller and less frequent, reservoirs are allowed to fill to higher elevations, leaving less room for flood storage.

In managing tributary reservoir elevations, TVA follows reservoir-specific flood guides. These guides specify the amount of storage that must be reserved for flood-damage-reduction during different times of the year based on historical rainfall records and many decades of operating experience. To see flood guides for tributary reservoirs, go to TVA’s Reservoir Information Web page. Click on the down arrow in the “Select a reservoir” box, choose a reservoir, and then click “View info.” You’ll see a page with current reservoir operating information. Use the “Operating guide” link on the right-hand side of the page to see a graph with a blue line labeled “flood guide.”
Seasonal use of flood storage allows higher water elevations in summer, which benefit recreation, navigation, and other interests. But it also requires vigilance to ensure that reservoirs do not exceed their flood-guide elevations except to regulate a flood and calls for an immediate response to heavy rainfall.

To meet this challenge, TVA’s River Forecast Center is staffed around the clock, including weekends and holidays. Employees check satellite data from rain and stream gauges, run computer models to simulate runoff, and constantly monitor rainfall events. Then they develop an operating plan based on this information, which specifies how much water will be stored in and released from each reservoir in the system. This plan is updated throughout the day based on observed inflow and changes in predicted rainfall.

The reservoir system in the eastern portion of the Tennessee Valley was planned primarily to protect Chattanooga from flooding. Seven reservoirs provide most of the storage: Norris Reservoir on the Clinch River; Fontana Reservoir on the Little Tennessee River; Douglas Reservoir on the French Broad River; Cherokee Reservoir on the Holston River; and Chatuge, Nottely and Hiwassee Reservoirs in the Hiwassee River basin.

Kentucky Reservoir, near the mouth of the Tennessee River, provides more than 40 percent of the available flood storage in the entire TVA reservoir system. This space can be used to reduce flood crests on the Mississippi at Cairo, Ill., by as much as two to three feet.

Flood-storage space in other reservoirs along the main Tennessee River is limited by topography and the requirement in the TVA Act to provide a nine-foot waterway for commercial navigation. However, these reservoirs also provide important flood-damage-reduction benefits. Fort Loudoun/Tellico, Watts Bar, and Chickamauga are important in protecting Chattanooga, and storage space in Pickwick, Wheeler and Guntersville is used to supplement storage in Kentucky Reservoir and to reduce flooding immediately below these dams.

Learn more about how TVA reduces flood damage.

Fast facts

  • In an average year, the TVA system prevents about $240 million in flood damage in the TVA region and along the lower Ohio and Mississippi Rivers.
  • To date, the operation of this system has prevented more than $5.4 billion in flood losses across the TVA region, including about $4.9 billion in damages averted at Chattanooga.
  • The largest flood in recent years occurred in May 2003, when floodwaters at the Walnut Street Bridge in Chattanooga crested at just over six feet above flood stage. During this event, the city suffered about $17 million in flood damages. However, without the TVA system of dams, the water levels at Chattanooga would have been 8.2 feet higher, causing over $425 million in flood damages.
  • The largest flood since construction of the TVA reservoir system occurred in March 1973 when floodwaters in Chattanooga rose to a record level of almost seven feet above flood stage.
  • From Jan. 1 through March 15, the TVA reservoir system has the capacity to store about 10 million acre-feet of water (a volume equal to one foot of water covering 10 million acres of land).
  • About 5 million acre-feet of flood-storage capacity, including 3.3 million acre-feet in Kentucky Reservoir, is maintained during June, July, and August to protect against summer storms, which typically affect smaller areas and tend to be shorter and less frequent.

 

TVA installs warning system at Ocoee No. 1 Dam

TVA has installed a new safety warning system at the Ocoee No. 1 hydroelectric plant in Polk County, Tenn., to improve public safety during water releases from the dam.

The Ocoee No. 1 warning system, which began operation in mid-September, includes signs, warning horns and strobe lights to alert the public to water discharges through the dam that will cause a rapid rise in water levels.

“The horns and strobe lights will be automatically activated prior to water-level changes as a safety warning to the public,” says TVA Senior Vice President of River Operations John McCormick. “When the horns and strobe lights are activated, anglers and the public should move to a safe area immediately.”

More information about avoiding the dangers surrounding dams, locks and powerhouses is available at www.tva.com/river/hazwater.

 

Blue Ridge Dam rehabilitation project update

On Friday, Sept. 24, construction personnel working on the Blue Ridge Dam rehabilitation project found a depression measuring 1 foot deep by 15 feet wide in the construction area at the bottom of the dam. Construction was temporarily suspended so that a detailed investigation could be performed. That same day, TVA’s dam safety engineers and a third-party engineering firm inspected the construction area and the entire dam and reviewed instrumentation readings. They concluded that the depression resulted from soil settlement in the affected area probably due to the construction activity and was not a threat to the safety of the dam.

Inspections and instrumentation evaluations conducted over the weekend by additional independent engineering consultants confirmed that this was a localized event and not a dam safety issue. Work at the site resumed on Monday, Sept. 27.

The investigation into what caused the soil to settle is continuing, however, soil removal associated with the drilling of holes for foundation supports was the likely cause. TVA and an engineering consultant will continue to monitor conditions at the dam to ensure its stability.

Repairs to the penstock at Blue Ridge Dam, the large underwater pipe which carries water from the reservoir to the turbines in the powerhouse, are scheduled to begin later this month when the elevation of the reservoir reaches 1630 feet above sea level. (The elevation was 1633.7 on Oct. 1.)

Visitors to the area are reminded that the road across the dam is closed to both vehicles and pedestrians and will remain closed for the duration of the project. The Fannin County offices on the Blue Ridge side of the dam are still open.

Read more about the project and sign up for e-mail updates.

 

Reminder: Watts Bar Lock to close Oct. 13

The U.S. Army Corps of Engineers will close Watts Bar Lock for three weeks beginning at 6 a.m. on Wednesday, Oct. 13. There is no auxiliary lock available at Watts Bar, so this will affect river traffic between Chattanooga and Knoxville—including fans planning to travel by boat from Chattanooga and points south to the Tennessee-Alabama football game in Knoxville on Saturday, Oct. 23.

The closure is for inspection and major repairs to the underwater components of the lock. The lock is scheduled to reopen at 6 p.m. on Wednesday, Nov. 3.

Closure schedules are subject to change. Check the Nashville District's Navigation Notices Web page for updates.

 


Get more information on TVA.com

The links below will take you to reservoir-related information on TVA’s website.

TVA’s reservoir operating policy:  Learn how TVA manages the flow of water through the Tennessee River system to provide navigation, flood damage reduction, power supply, water quality, water supply, recreation, and other benefits.

Reservoir information:  Get detailed information about individual reservoirs, including observed and predicted elevations and releases at TVA dams, reservoir operating guides, water quality improvements, fish population survey results, and more.  To check reservoir information from your cell phone or other mobile device, go to http://m.tva.com.

Rainfall and stream flows:  Get the latest information on daily rainfall and stream flows across the Valley.

Recreation release schedules:  View the 2010 schedule for water releases for rafting, kayaking, and canoeing below these TVA dams:  Apalachia, Ocoee No. 1, Ocoee No. 2, Ocoee No. 3, Norris, Watauga/Wilbur, and Tims Ford.

Map of TVA reservoirs and power plants:  Our interactive map is your guide to the entire TVA power system, including fossil and nuclear plants, dams and reservoirs, and visitor centers.  You’ll find interesting facts about each facility and learn how they work together for the purposes of power supply, river management, and economic development.

Water supply FAQs:  Get answers to frequently asked questions about obtaining a water intake permit, improving water quality around intakes, inter-basin transfers, and more.

Dangerous areas around TVA dams:  If you like fishing or enjoy swimming and boating on TVA-managed reservoirs, you need to be aware of the possible hazards around dams, locks, and powerhouses.

How to lock through:  Find out what you need to do to safely approach a navigation lock, secure your boat in the lock chamber, and exit the lock.

Reservoir health ratings:  See the latest monitoring results for TVA-managed reservoirs.

Campgrounds and day-use areas:  Get information here about campground fees and amenities as well as picnic pavilion reservations.

TVAkids.com:  TVA’s got a Web site just for kids!  Learn about how TVA makes electricity, reduces flood damage, protects wildlife, and more.  There’s a section for teachers, too.

TVA Heritage:  Read about the people who founded TVA, shaped its purpose, and built its power plants.  TVA Heritage offers fascinating glimpses of the agency’s 76-year history.

Get more information by phone

For the latest information on reservoir elevations and stream flows, call TVA’s Reservoir Information Line from a touch-tone phone:

  • From Knoxville, TN:  865-632-2264
  • From Chattanooga, TN:  423-751-2264
  • From Muscle Shoals, AL:  256-386-2264
  • From all other locations:  800-238-2264 (toll-free)

For answers to questions on how your reservoir is operated, call TVA River Operations at 865-632-6065.

For answers to questions about recreation, permitting procedures, reservoir land management plans, and other environmental issues, call TVA’s Environmental Information Center at 1-800-882-5263.

 
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