Heavy rains from Florence caused a slope to collapse at a coal ash landfill at a closed power station near the North Carolina coast, Duke Energy says.

Duke spokeswoman Paige Sheehan said late Saturday about 2,000 cubic yards (1,530 cubic meters) of ash were displaced at the L. V. Sutton Power Station outside Wilmington and that contaminated runoff likely flowed into the plant's cooling pond.

The company has not yet determined whether the weir that drains the lake was open or if contamination may have flowed into the Cape Fear River. That's roughly enough ash to fill 180 dump trucks.

Florence slammed into the North Carolina coast as a large hurricane Friday, dumping nearly three feet (1 meter) of rain and swelling the region's rivers. The resulting flooding forced swift-water rescues and left several people dead.

Sheehan said the company had reported the incident to state and federal regulators "out of an abundance of caution."

The coal-fired Sutton plant was retired in 2013 and the company has been excavating millions of tons of ash from old waste pits and removing it to safer lined landfills constructed on the property. The gray ash left behind when coal is burned contains toxic heavy metals, including arsenic, lead and mercury.

Duke has been under intense scrutiny for the handling of its coal ash since a drainage pipe collapsed under a waste pit at an old plant in Eden in 2014, triggering a massive spill that coated 70 miles (110 kilometers) of the Dan River in gray sludge.

In a subsequent settlement with federal regulators, Duke agreed to plead guilty to nine Clean Water Act violations and pay $102 million in fines and restitution for illegally discharging pollution from coal-ash dumps at five North Carolina power plants. The company is in the process of closing all of its coal ash dumps by 2029.

Spokeswoman Megan S. Thorpe at the state's Department of Environmental Quality said state regulators will conduct a thorough inspection of the site as soon as safely possible.

"DEQ has been closely monitoring all coal ash impoundments that could be vulnerable in this record breaking rain event," Thorpe said. She added that the department, after assessing the damage, will "hold the utility accountable for implementing the solution that ensures the protection of public health and the environment."

There are at least two other coal-fired Duke plants in North Carolina that are likely to affected by the storm.

The H.F. Lee Power Station near Goldsboro has three inactive ash basins that flooded during Hurricane Matthew in 2016, exposing a small amount of coal ash that may have flowed into the nearby Neuse River. The old waste pits are capped with soil and vegetation intended to help prevent erosion of the toxic ash beneath.

The Neuse is expected to crest at more than nine feet (3 meters) above flood stage Monday and Sheehan said the company expects the same ash basins are likely to be inundated again.

At the W. H. Weatherspoon Power Station near Lumberton, Sheehan said it had already rained more than 30 inches (75 centimeters) by Saturday evening, causing a nearby swamp to overflow into the plants cooling pond. The Lumber River is expected to crest at more than 11 feet (3.3 meters) above flood stage Sunday, which would put the floodwaters near the top of the earthen dike containing the plant's coal ash dump.

Environmentalists have been warning for decades that Duke's coal ash ponds were vulnerable to severe storms and pose a threat to drinking water supplies and public safety.

"Disposing of coal ash close to waterways is hazardous, and Duke Energy compounds the problem by leaving most of its ash in primitive unlined pits filled with water," said Frank Holleman, a senior attorney at the Southern Environmental Law Center who has battled the company in court.

"In this instance, it appears that Duke Energy has not done enough to ensure that its new Wilmington landfill safely stores coal ash. After this storm, we hope that Duke Energy will commit itself to removing its ash from all its unlined waterfront pits and, if it refuses, that the state of North Carolina will require it to remove the ash from these unlined pits."