Knox News: TVA dams a year after Helene
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TVA's dams held up during Hurricane Helene. A year later, some work is left.
Knoxville News Sentinel
- The Nolichucky Dam needed minor repairs after Helene hit Tennessee and North Carolina.
- None of the other dams the utility owns required any repair work.
A year ago, when TVA's dams were hit by the unprecedented floodwaters from Hurricane Helene, one sustained damage and so did some sites downstream.
But those problems were manageable in comparison to the devastation TVA staffers say the utility helped stave off in East Tennessee.
With deep reflection and research after the deadly storm, utility representatives said their planning ‒ combined with efforts TVA staffers made while Helene raged through the South ‒ kept the riverside cities of Knoxville, Lenoir City, Kingsport and Chattanooga from feeling the brunt.
They estmate their work prevented over $406 million in damage across Tennessee.
Here's what happened.
TVA experts made their typical seasonal planning for water levels last September and reacted quickly when the real danger came into focus.
As the summer ends, TVA usually lowers water levels in its lakes, according to Darrell Guinn, senior manager for the power provider's River Forecast Center, to create room for runoff that comes in the cooler months when there's heavier rainfall. In a normal year, TVA might lower the water level at Douglas Lake by as much as 40 feet, he said.
"It's really important that we have the ability to store excess runoff," Guinn said. "That's what creates floods, is this excess of runoff."
Just ahead of Helene, there was intense rain across the Tennessee Valley that set the stage, Guinn said, for higher water levels. Then the Nolichucky, French Broad and Pigeon rivers - which feed into TVA's Douglas Lake - all saw record rainfall from the hurricane remnants.
There are usually a handful of River Forecast Center staff on duty checking water levels and weather conditions. During Helene, that number rose to about 25.
Much of the water from the mountains ended up in Douglas Lake, which was low enough to catch much of the floods and debris.
But there are limits to the help TVA can offer in a disaster.
"We have the ability to reduce flooding downstream of dams," Guinn said during a meeting with members of the media. "We don't have the ability to regulate flooding on unregulated streams and unregulated creeks that flow into bigger streams and lakes."
One of the dams that control water levels across Tennessee lakes and rivers needed minor repairs after Helene. They're designed to function in extreme weather, and they stood up to the test.
But Helene did stress them.
“Our dams are meant to move water and withstand debris,” TVA spokesperson Scott Brooks said in an email. “They all performed as designed.”
The act of Congress that created the utility in the 1930s tasks it with promoting flood control of the Tennessee River. TVA's system of dams is a key means of addressing the fallout of storms like Helene.
No TVA dam has ever failed, including in the aftermath of Helene. The utility operates 29 power-producing dams and other dams for flood control and recreation.
Nolichucky Dam, south of Greenville, held up against water flows about double the average daily peak of the Niagara Falls as the storm destroyed roads, homes and bridges across East Tennessee. The Nolichucky River at the 112-year-old dam rose by almost 20 feet from 43 feet to 62.57 feet after Helene hit.
A year later, National Oceanic and Atmospheric Administration data showed water levels back at about 42 feet.
-and-go'
As Helene tore through Tennessee, the dams’ status changed, and officials updated statements hour by hour. Just after midnight Sept. 28, the day after the storm, TVA officials issued a precautionary warning for the Nolichucky Dam, saying on social media that a breach could be "imminent" and that, if that happened, flooding could be deadly. But the dam held, and inspections later that day showed the structure was stable.
Helene broke flood records at the dam, triggering water levels 9.5 feet higher than the previous record. The flow of the Nolichucky River, which isn't regulated until it hits Douglas Dam, also intensified. On a normal day, Guinn said, the river's flow is about 3,000 cubic feet per second. During Helene it spiked, rising to 180,000 cubic feet per second.
“Obviously it was a very touch-and-go moment for TVA,” said Lori Spragens, executive director of the Association of State Dam Safety. “But those dams held up great.”
A number of dams – including one in North Carolina owned by Duke Energy – were reported as failing during the storm before evaluators determined they had remained secure.
That wasn’t the case for all dams in Southern Appalachia. Carolina Public Press, a nonprofit newsroom, reported 41 state-regulated dams in North Carolina broke amid the record rainfall.
Spragens said design is the factor that sometimes separates a failing dam from one that survives.
"Anecdotally, most failures happen at embankment dams," she said, rather than at concrete structures.
Between 10 and 15 dams fail every year on average in the U.S., according to research from Stanford University’s National Performance of Dams Program.
"Dam failures in the U.S. are fairly common," said Martin W. McCann Jr., an adjunct lecturer at Stanford University's Department of Civil and Environmental Engineering. "On average, there are 10-20 dam failures each year, with the majority of these being small dams that fail during flood events."
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"On a per-dam basis, the annual probability of a dam failure is generally estimated to be about 0.0001 (or 1 per 10,000) per dam year of operation," McCann told Knox News over email.
A year on, some work continues
After the storm passed, TVA, which owns the Nolichucky Dam, assessed the damage and saw it needed some minor concrete repair work that was finished this summer. The Nolichucky has not generated power for the utility since the 1970s.
The dam's powerhouse, out of commission for decades, was damaged during the storm and removed afterward. So was part of the embankment, which TVA later reinforced.
Some river and stream gauges and observation stations were destroyed.
"Some of those are still in the process of being replaced," Guinn said. "It's a slow process."
But though the dam structure made it through the storm “safe and stable,” TVA is working on the site downstream of the dam, Brooks said. An access area to the river near an Asheville Highway underpass washed out during the storm, he said.
“We are restoring and reinforcing the shoreline on both sides of the river below the dam,” Brooks said. That work could be done by the end of the year, he said, if the weather allows.
Four other dams hit by Helene – Cherokee, Douglas, South Holston and Watauga – required no repairs, Brooks told Knox News.
Mariah Franklin is a growth and development reporter focused on technology and energy. Email mariah....@knoxnews.com.
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