Heavy rains inundate reservoirs causing dam failures
Hurricane Matthew dumped copious amounts of rain on the state of North Carolina--nearly 15 inches in some locations. The rains resulted in massive flooding and at least 22 deaths across the eastern portion of the state. In Johnston County alone, multiple swift water rescues had to be performed.
Heavy rainfall also inundated the state in September, saturating the ground and filling reservoirs to capacity. This left little room for the rainwater from Hurricane Matthew. As a result, many of these reservoir dams were overtopped and in some instances, the excessive water and pressure caused those dams to fail.
The rain dumped on the state by Hurricane Matthew had a devastating impact on many of the Cape Fear River Basin dams, the majority of which are privately owned and maintained. As a result of the heavy rainfalls in September, and the deluge of rain provided by Hurricane Matthew, at least 17 dam failures occurred in North Carolina, 13 of which were located in the Cape Fear River Basin.
Most of the failed dams were earthen structures, and in one community near Fayetteville, a dam held back 6.5 million gallons of water that inundated the community once the structure failed. It was also the only road connection from the community to the outside, including fire and police access.
State set around-the-clock watches
The most likely locations for catastrophic dam failures were already well known, and as a result, the Dam Safety Program for the state had inspectors and engineers monitoring and watching over critical dams 24 hours a day during and after Hurricane Matthew to ensure the safety of the residents downstream. No lives were lost as a result of dam failures that occurred after Hurricane Matthew.
Dam classifications and regulations
Of the State's more than 3,200 dams, 1,210 are listed as being high-hazard dams, requiring regular inspection and monitoring. Regular monitoring and inspection of dams is conducted by the state Dam Safety Program that employs 62 inspectors who inspect high-hazard dams at least once every two years, and intermediate-hazard dams once every five years.
The State of North Carolina regulates and inspects all dams that meet the following qualifications:
- 25 feet tall and above
- Impound 50-acre-feet or more in water volume
- All dams classified as high-hazard - regardless of height or water amount held
The classification of a dam as a high or intermediate hazard relates directly to the amount of damage, destruction, and death the failure of the dam is likely to cause downstream should the dam fail for any reason.
A dam that is classified as a high-hazard dam is one that would lead to loss of life, excessive property and road damages should it fail. High-hazard dam failures are those that result in at least $200,000 in losses for flooding of homes, businesses (buildings), public utilities, and railroads. These thresholds are slightly lower for a dam classified as an intermediate hazard.
Unfortunately, if a dam falls outside of these parameters, it might not be inspected regularly--or at all for decades--and it may also remain largely unregulated. This means that needed repairs might be overlooked or not undertaken.
Since a 2014 dam failure that resulted in a coal ash spill into the Dan River, regulations have gotten stronger for owners of dams that are listed as high or intermediate hazard. Such dams must now have emergency action plans and approved downstream inundation areas should a dam failure occur. And copies or access must be provided to local emergency management officials.