Failed dams caused by Hurricane Matthew threaten NC communities
Harriett Shooter can’t see the dam at the north end of J.C. Keith Lake from her house, but looking out during Hurricane Matthew she saw the water rise 10 feet into her back yard in less than an hour and knew the old earthen structure had blown.
“I saw the water coming up, up, up,” she said. “That’s when I knew this wasn’t just rain. It was something more.”
About 6.5 million gallons more. That’s how much water leapt from the little lake in the Rayconda neighborhood in Fayetteville into the big lake when the dam between the two impoundments broke as a result of intense hurricane rain. It severed Ancon Drive – the only safe road in and out of the neighborhood.
The dam was one of at least 17 that burst in North Carolina as Matthew rolled across the state, or in the days afterward, when rain from the massive storm drained off into surface waters downstream. For comparison’s sake, 40 dams failed during Hurricane Floyd.
Thirteen of the 17 dams that are known to have failed during Matthew are in the Cape Fear River Basin, which received extraordinary rainfall during storms at the end of September and again in Matthew.
Inspectors from the state’s Dam Safety Program are still collecting and reporting data on the failures and near-failures. But while the amount of rain that fell on the state from Matthew was a surprise, Bridget Munger, spokeswoman for the program at the Division of Environmental Quality, said the places where the rain was most likely to cause catastrophic failures were well-known.
During Matthew, she said, engineers and inspectors were babysitting critical dams day and night, ready to call for an evacuation if needed.
“Our inspectors are out all the time,” Munger said. “They know the dams in their regions, and they build relationships with the owners.”
Regular inspections and safety code enforcement efforts can’t guarantee that dams won’t break, but they can prevent loss of life and excessive damage to property and infrastructure, said Brad Cole, chief of regional operations for Dam Safety.
None of the 27 deaths state officials attribute to Matthew in North Carolina was caused by a dam failure, Munger said.
Across the state, there are more than 3,200 dams, most of them earthen, most privately owned and all listed in the U.S. Army Corps of Engineers’ National Inventory of Dams. The site lists 1,210 of those as having “high hazard” potential, which has nothing to do with a dam’s physical condition, structural integrity or probability of failure. Hazard potential, for a dam, is the potential for adverse effects downstream if the dam were to fail.
A high-hazard dam is one that, if it failed, would likely cause the loss of human life, damage to roads that carry 250 or more vehicles per day, or more than $200,000 in damage from serious flooding to homes, industrial and commercial buildings, public utilities or major railroads.
A dam is intermediate-hazard if its failure would lead to damage to highways or roads carrying 25 to 249 vehicles per day, interruption of utilities, or between $30,000 and $200,000 in damage from no more than a foot and a half of flooding in isolated homes and commercial and industrial buildings. There are about 735 intermediate-hazard dams in North Carolina.
In North Carolina, the state regulates and inspects dams that are 25 feet tall or higher and impound a water volume of 50 acre-feet or more, or if they are considered high-hazard, regardless of their height or impoundment size. Until 2011 the height limit was 15 feet. The change was brought about to make better use of the Dam Safety Program’s 62 inspectors and to ease regulations on farmers and others.
Dams owned by the Tennessee Valley Authority and by U.S. government agencies are not regulated by the state, but all high-hazard dams under the state’s jurisdiction must be inspected at least once every two years; intermediate dams must be inspected at least once every five years.
All other dams may go decades without ever being inspected.
Coal ash spill brings changes
Most states didn’t regulate early dam construction or maintenance, and failures of some dams built in the 1800s were spectacular and horrific.
In North Carolina, one of the most notable dam failures was the August 1916 collapse of the Lake Toxaway Dam, which held back the state’s largest man-made lake. The remains of a hurricane had dropped heavy rains in the mountains a month before, causing deadly landslides in Transylvania County. Then in August another storm came through, and excessive rain overwhelmed the dam.
The U.S. Geological survey says the flow as the lake emptied was so powerful it carried downstream a boulder weighing 900 tons. Signs of the rushing water are still visible in what is now Gorges State Park. Amazed news reporters of the day said the collapse killed no humans, but one mule.
And yet, North Carolina didn’t put dam safety rules into place until 1967, Cole said, after another series of dam failures brought the issue to national attention. The last major change to the state rules came in 2014, after a massive coal ash spill into the Dan River from a failed impoundment at a Duke Energy Plant near Eden. The General Assembly ordered that owners of high- and intermediate-hazard dams must file emergency action plans with the state that include an approved map of the downstream area that would be inundated in case of failure. Local emergency management officials are to have access to the maps and emergency plans.
A dam’s hazard classification can be changed if the hazard potential changes. For example, when flood maps are updated, they may show a larger area of land subject to flooding or increased development within the area. Increased traffic on a road that travels across a dam or in the flood zone can cause a dam’s hazard potential to rise.
“You’d be surprised if you read some of the inspection reports,” Munger said. Inspectors might point out a gopher burrow that could create an opening through a dam, or note that the lines painted on a road that goes across a dam are no longer straight, indicating the structure has shifted underneath.
If an inspector finds a problem, he or she can issue a “notice of deficiency” to the owners, who are given a time period in which to hire an N.C.-certified engineer to do a more thorough inspection and come up with a fix the state will approve.
Cole recalls only a couple of times in the past two years when the state has had to take the next step, going before a judge to ask that the owners be ordered to make repairs or take the dam down.
Working with owners
The state had worked with the owners of both dams on Keith Lake in Fayetteville since at least 2004 to correct problems, Cole said.
When the state first inspected them, both dams were serving as roadways for the neighborhood. Several years ago, the Rayconda homeowners association chose to close the lower, larger dam, to vehicular traffic, which removed it from the state’s high-hazard list and took it out of the state’s jurisdiction. That left only the upper dam, which carries a section of Ancon Drive, for the state to inspect. It also meant there would be only one way for vehicles to go into and out of the neighborhood of about 220 homes.
Freddy Rivera, a retired U.S. Army sergeant major who serves as the homeowners association president, said that earlier this year, the group hired a contractor to make repairs to the spillway for the upper dam, to better regulate the flow of water from the 2.5-acre lake to the 7-acre lake. But the group hoped the City of Fayetteville would help with the more extensive repairs needed on the dam itself, since the city annexed the neighborhood several years ago and now owns the road across the upper dam. The city balked, Rivera said.
Recently, the city began construction of a new road that will serve as a permanent way into and out of the neighborhood.
But before the work could be completed, Hurricane Matthew took out a section of the upper dam, dumping the contents of the smaller lake into the bigger one. The sudden flow overtopped the second dam as well, causing erosion on the back side of it that chewed off a car-sized chunk.
With the upper dam blown out, Rivera moved aside the giant planters that were blocking the lower dam, and neighborhood residents take turns allowing one car at a time to drive over the damaged structure. Signs warn: “DANGER. Cross at your own risk.”
The city began emergency repairs on the upper dam last week. City officials did not return calls to discuss when the work would be finished, how much it would cost and who would ultimately pay for it.
In the meantime, many residents, including Harriett Shooter, who has lived in the neighborhood for 43 years, are walking across the compromised lower dam, but leaving their cars parked on the far side of it.
“Dams are like anything else,” Cole said. “They need care. It’s not like you can just build them and leave them alone. That’d be like if you built your house in 1950 and just left it alone. You wouldn’t have a very nice house right now.
“Same with the dams. They kind of get overlooked.”
Find a dam
To find out more about a dam near you, including when it was last inspected, go to the North Carolina section of the national dam inventory, http://nando.com/476.
To report a problem with a dam, contact the N.C. Dam Safety Program office serving the region where the dam is located: http://nando.com/477.