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TVA River Neighbors

February 8, 2012

In this issue:
Rainfall and Runoff
Current Reservoir Conditions/Operations
The Anatomy of a Spill
More TVA Information

TVA provides these monthly updates on the operation of the reservoir system by email. We sincerely welcome your comments and questions. To provide feedback, sign up for future updates, change your email address, or be removed from this distribution list, please contact riverneighbors@tva.com.


Rainfall and Runoff

At 4.59 inches, rainfall for the month of January was 100% of normal, for the part of the Tennessee Valley above Chattanooga. Rainfall for the entire basin was just above normal, at 108%.

This continues the pattern of precipitation that’s been in place basically since September, with normal or above normal rainfall since then.

With rain falling regularly during January, the ground is more or less saturated. Not much is able to be absorbed, so runoff (the amount that flows over the soil surface and into waterbodies) is increased. January runoff totaled 113% of normal for the area above Chattanooga, and 120% of normal for the Tennessee River watershed.


Current Reservoir Conditions/Operations

TVA tributary reservoirs are all above flood guide levels, and all main-stem reservoirs are outside of their winter operating zones—with the exception of Nickajack and Wilson. Spill operations have been the rule, up and down the river, for the past month. The number of “spill days” during the month of January is a good indicator of how hard TVA’s been working to recover flood storage space. A lot of water’s being routed down through the river system: at Guntersville, Kentucky, and Nickajack alone, spill operations were in effect for at least 25 of the 31 days in January.

Since forecasts call for the prevailing weather pattern (wetter and warmer than normal) to continue, those responsible for operating the reservoir system are conscious of the need to prepare for the likelihood of more rain by continually working to maintain the reservoirs at flood guide levels or within their operating ranges—with the goal of being able to use them for their intended purpose of storing floodwaters. With that in mind, TVA expects spill operations to continue through the middle of February, at various points along the river system.

Tributary Reservoir Elevations

February 1, 2012
Observed Elevations1

February 1
Flood Guide2

South Holston



























Blue Ridge



Tims Ford






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.


The Anatomy of a Spill
A look at why, when, and how we do it

To say that we don’t like to have to do it would be an understatement. As a matter of fact, it’s what you might call the option of last resort. But when the situation warrants it, TVA managers don’t hesitate. Releasing water through the dam’s spillway gates (or “spilling”) is an important tool for managing flows in such a way as to minimize flooding, and the manner in which spill operations are conducted plays a big role in protecting people and property.

A variety of factors are considered when TVA initiates spill operations, including inflows, current reservoir levels (available storage), potential downstream impacts, and predicted rainfall. Computer modeling is an incredibly useful tool for those working in the River Forecast Center, but decisions are made based on real-world conditions up and down the reservoir system.

Since spilling has been occurring very frequently along the main-stem reservoirs in response to recent rainfall events, it seemed like a good time to take a closer look at the process. For that, we turned to Nikki Berger, a Lead Engineer in TVA’s River Forecast Center—ground-zero for those tasked with monitoring precipitation-producing weather systems and making decisions about how the Tennessee River System is operated. The center is staffed round-the-clock, and at any given point in time one individual (the Lead Engineer) is responsible for—among other things—assessing conditions and making judgment calls about initiating spills, determining flow rates/duration, and then terminating spill operations when the need has passed.

Guided by policy

Berger sets the stage for our discussion of spilling by establishing the justification for doing so. “TVA’s Reservoir Operating Policy is the foundation and the framework for everything we do,” she explains. “We are charged with providing flood risk reduction by maintaining the reservoir system in as close to a ‘ready state’ as possible, which means being at or below flood guide on the tributary reservoirs and within operating zones on the main-stem reservoirs. Of course, there are many other public benefits provided by the water in the Tennessee River System—and those are of great importance, as well. That’s why we always try to achieve maximum value from the flows we route through the system, making that water, in effect, ‘work hard’ for the people of the Valley. For example, we are keenly aware of the need to generate clean, cost-effective hydropower. I can promise you that we don’t spill as much as a single teaspoon of water before we’ve determined that it’s simply not possible to run even another drop through the turbines.”

With that said, though, the undeniable fact remains that sometimes spilling is necessary. Let’s take a look at what happens leading up to the point when water begins flowing through the spillway gates.

Inside the River Forecast Center

First of all, constant monitoring helps ensure that those in the River Forecast Center are never “caught off guard” when it comes to developing situations. Volumes of data—both real-time and forecasts—are factored into the decision to spill. “Almost everybody’s interested in the latest weather forecast,” says Berger. “But we take it to a whole new level. To folks who work in TVA’s River Forecast Center, predictions of precipitation are about way more than whether or not we need to take an umbrella when we leave the house: we’re making decisions based on those forecasts that often mean the difference in whether or not someone’s home or property ends up being flooded.”

At any given time, TVA River Forecast Center staff members can take a look at an 8’ x 15’ wall of monitors, displaying up-to-the-minute (and in some cases, up-to-the-second) information from a wide variety of relevant systems. In addition to the always-on Weather Channel, they are able to see a current radar image of the Tennessee Valley, summaries of river temperature and aeration information, status reports for all of TVA’s major generating units, total energy load for the entire TVA power system, and a real-time status map of the river system, with detailed information on each hydroelectric plant’s headwaters, tailwaters, flow rates, output in megawatts being generated, and more. Photo by David Luttrell

During these times, TVA holds as much water in the tributary reservoirs, for as long as possible—after all, that’s what they were designed for: to reduce a flood crest downstream. The flipside, though, is that this flood storage capacity has to be recovered, eventually—in order to maintain the reservoir system in the best possible position to handle future storms. And not every reservoir has enough storage space to hold a large rain event without having to resort to spilling. In the end, everything depends upon precipitation: when it arrives, how much falls, and how long it lasts.

Monitoring the situation

Berger makes it clear that there’s no hard and fast “rule” about precisely when conditions dictate the necessity of initiating spill operations. “At a certain point, though, it simply becomes unavoidable,” she says. “The rain’s probably been falling for a while. You’re watching reservoir elevations all along the river, and headwaters are rising. You’re looking at forecasts that you have a high degree of confidence in, and you’re as certain as you can be that even more rain is coming. As mindful as we always are of our commitments, there just comes a point when all the determining factors align. Even when operating every available generating unit at full capacity—and that’s always the case, before spilling is initiated—we reach the place where we just can’t move enough water through the system fast enough. With more rain on the way, it’s time to spill.”

An array of information—from stream gauges, from the National Weather Service, from the U.S. Army Corps of Engineers—comes into play. State-of-the-art computer modeling means that River Forecast Center staffers can run vast numbers of scenarios to simulate discharges required to achieve specific headwaters. They are constantly evaluating current conditions in light of what’s being forecasted, making necessary adjustments, and refining plans of action.

Preparing and issuing water orders

As rain continues to fall and more is likely on the way, it’s the Lead Engineer’s job to do a “spill calculation,” using a special computer application designed just for that purpose. “For example,” says Berger, “if we need to discharge 20,000 cubic feet of water per second from Chickamauga Dam, we have to determine precisely which gates need to be opened—and to what extent—at a given time, in order to produce the effect we’re after. That’s what we call the ‘gate arrangement,’ and it’s an important aspect of spill operations. Since we’re careful of not causing downstream waters to rise too rapidly and we always want to uniformly distribute flow across the river channel, we often partially open a designated portion of the gates at a given project first, and then open the remaining gates to a certain extent. The next step might be to fully open the first gates, and finally open all the rest to the fullest extent. The same thing happens in reverse, when it’s time to terminate the spill.”

The Lead Engineer takes that information and prepares what’s called a “water order:” a set of specific instructions that guide plant personnel at the dams in their implementation of the gate arrangement. There’s no room for error, according to Berger: “We fax or email the water order to the control room at the plant, and we call to say we’ve sent it. They verify receipt of the water order and then repeat it back to us, to ensure that we’re all on the same page. A water order is a pretty big deal, and we’re all committed to making sure it’s done right.”

Spillway gate types, numbers, and operating methods vary from dam to dam. Although almost all require the use of a gate crew to open or close, a few can be operated remotely, with just the push of a button.

Gate crews are called out

At that point, a gate crew is assembled. They’re the folks who physically move the spillway gates, opening or closing them—either fully or partially, depending on what is called for in the water order. Although gate crew personnel have the skills, training, and years of experience to do the job well, there’s still an unmistakable emphasis on safety, notes Berger: “Implementation doesn’t happen until everybody’s got their safety gear and has been thoroughly briefed about the gate arrangement. Expectations are clear about what’s going to happen and how to do it safely.”

Spilling vs. sluicing

Spilling may be the more commonly known (and certainly the more visually compelling) method, but there’s another way TVA can release water through the dams when flooding concerns arise and the need for discharging water exceeds the rate at which it can be passed through the turbines. Dams on the tributary reservoirs have “sluice gates,” built into tunnels that are located at the bottom of the dams. Just like during spill operations, these gates can be opened to allow for the release of excess water. Since the amount of water discharged through sluicing is much less than that achievable by spilling, sluicing is used mostly in winter, when reservoir levels are below the spillway crest and no other option for recovering flood storage space is available. For some projects without spillway gates, like South Holston and Watauga, sluicing is the only way to discharge excess water. Sluicing is more common at a location like Ocoee #3, where (because automated sluice gates there can be operated remotely) the benefit of achieving an almost immediate response makes sluicing a more viable option.

Whether spilling or sluicing, it takes another water order from the River Forecast Center to terminate these releases. The tricky part is determining just when to do it—particularly when the threat of more rain looms in the not-too-distant future. “It can be a tough call,” admits Berger, “but we always try to keep in mind the need to optimize generating capacity while not negatively impacting our ability to provide flood risk reduction benefits.”

Fast Facts

  • In a typical year, TVA’s River Forecast Center issues around 500 water orders, which serve to document spill operations. During 2011, 987 were issued.

  • Occasionally, spilling operations are initiated for reasons other than flood risk reduction. For example, water from Tims Ford Reservoir may be spilled in order to provide environmental benefits from water temperature regulation—specifically, to enhance the health of threatened and/or endangered mussel species.

  • The number of spillway gates at a given project can vary greatly. There are only three spillway gates at Melton Hill Dam, for example, while Wheeler Dam has 60.

  • The “Discharge of Record” took place at Pickwick Dam on March 17, 1973, with a flow rate of 585,400 cubic feet of water per second. That converts to an astounding 4.38 million gallons per second.



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.

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

Recreation release schedules: View the 2011 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, Upper Bear Creek 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 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.