While flood risks have fallen with Hurricane Irma’s expected westward track, heavy rains often threaten to detonate potential water bombs across North Carolina.
More than 5,600 small dams in the state are categorized as high-hazard because people would likely die, or property damage would total more than $200,000, if they fail. Mecklenburg County alone has 75 high-hazard dams.
Most dams are privately-owned earthen structures. While they don’t often fail, the dams’ risks are more than theoretical.
Last year, after Hurricane Matthew and tropical storms Julia and Hermine drenched North Carolina, state inspectors found damage to 68 dams. Of those, 22 dams had burst and 46 needed repair.
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One of the damaged dams, in Moore County southwest of Raleigh, could have caused more than $260 million in damage if it had failed, a state analysis found. In June, the Department of Environmental Quality issued an emergency order allowing the dam to be temporarily breached to protect life and property downstream.
In 2015, dozens of dams failed in South Carolina after almost 2 feet of rain fell in parts of the state. State officials advised dam owners Thursday to lower water levels in advance of expected heavy rainfall from Irma.
In North Carolina, state staff are contacting dam owners, conducting pre-storm inspections and preparing field equipment such as laptops and pumps to respond to emergencies, DEQ spokeswoman Bridget Munger said.
Sankar Arumugam, a professor of civil, construction and environmental engineering at North Carolina State University, said small dams typically lack the management applied to utility-scale structures, such as Duke Energy’s dams.
Many small dams are able to store relatively small amounts of water, Arumugam said, making it likelier heavy rain could flood them. Many are also old and often have uncertain maintenance histories.
The largest number of high-hazard dams in the U.S. were built in the 1950s and 1960s, according to a recent paper published in the journal Water Resources Research.
“The age of the dam is something we need to think about, how well maintained they’ve been,” Arumugam said. “If it’s from, say, 1954, we need to be concerned about that.”
In 2011, a dam owned by a homeowners association off Charlotte’s Albemarle Road, began to crumble after heavy rains. The city had planned repairs but had not begun them. More than 30 apartments and three homes had to be evacuated until the Forest Lake dam could be stabilized.
More than 3,000 dams fall under state oversight. The remaining structures are small or, like Duke Energy’s massive hydroelectric dams, are regulated by the federal government.
With Irma on the way, Duke Energy said it had increased monitoring of its 13 hydroelectric dams on the Catawba River. Duke also started moving water downstream Thursday through those dams to free up room to catch Irma’s rainfall.
Duke’s 2014 spill of coal ash from a small basin on the Dan River prompted tighter state oversight of dams, including ash basins.
Legislators required all owners of high- and intermediate-hazard dams to submit emergency action plans. The plans describe potential risks, response actions, emergency notification procedures and maps that show the areas that could be flooded by a dam failure.
High-hazard dams are inspected every two years, while those posing less risk are inspected every five years. Duke says it inspects coal ash basins weekly.
At least three high-hazard dams in Mecklenburg County needed repairs to improve their safety when the Observer examined state records in 2015. About half of the private owners of the Mecklenburg dams had not, at that time, submitted required emergency action plans.
State data last updated in March shows that 56 of Mecklenburg’s 75 high-hazard dams have emergency action plans on file.
Even when owners comply with the law, the Observer found, emergency plans are not accessible to the pubic and leave downstream neighbors in the dark about potential risks.