Heavy rainfall from the remnants of Tropical Storm Helene triggered more landslides than state officials had seen in total in the two prior decades, the official in charge of mapping risk told a hurricane recovery committee on Friday.
David Korte, North Carolina's landslide hazard mapping program manager, told the Governor's Advisory Committee on Western North Carolina Recovery that the storm had also caused a shift in how his team is approaching the work of mapping risk in western North Carolina.
"We moved to a focus of resiliency versus hard landslide science. Hard landslide is what we used to do — things like physics and material properties and engineering. We're moving towards more of a resilience focus now," Korte said, adding that he hopes the new maps help emergency managers protect critical infrastructure and shape evacuation routes.
The N.C. Geological Survey responded to 496 landslides that were caused by Helene's heavy rains. That's more than the 358 landslides the state responded to between 1990 and 2023, Korte said.
Helene's slides killed 31 people. About 75% of the landslides impacted a road, structure or river, Korte said.
Most of the landslides — about 88%, Korte said — happened in places that recorded at least 10 inches of rainfall during Helene. The state did not find any in places that saw less than five inches of rainfall during Helene.
Scientists should, Korte said, be able to use precipitation data and soil moisture data to predict where a landslide could happen in the next day or two. That, he added, is effectively the current limit of landslide hazard warning.
"We're not talking half an hour ahead of time. These are fast-moving landslides. They're unpredictable, where they're going to go. We just don't have the science to do that. But we do have the science to say we may have a problem in 24 hours in this area," Korte said.
Shifting priorities
To map landslide hazards, scientists look for places that show evidence of previous slides. So they are looking for evidence of debris flows, rock falls and the soil deposits associated with them, Korte said.
But the current western North Carolina landslide maps show risk in too many places. That makes them useful for someone who is looking for a place to build a home in the mountains but not useful for an emergency manager who needs to make decisions as a storm is bearing down on the region.
Korte said the landslide hazard program is working on a new mapping system that uses a threshold of five inches of projected rain over a 24-hour period, the amount of rain that research indicates triggers a fast-moving slide. By using precipitation forecasts from the Weather Prediction Center, the landslide hazard team will identify "areas of concern." That information and map would be shared with state and local emergency management officials.
Then, the team hopes to turn to a still-being-developed network of precipitation and soil moisture monitoring stations to determine which places are more likely to see slides or debris flows. Places that have higher moisture levels in their soil are at greater risk as heavy rains fall.
"We'll improve our susceptibility maps and remove as many false positives as possible over the next two years and landslide inventory will go on after we are long gone," Korte said.
How landslide mapping is used
Funding for landslide hazard mapping has been a contentious political issue in North Carolina, with Republican budget writers pulling funding from the effort in 2011 amid concerns from western North Carolina developers.
Some funding returned in 2017, with the state budget appropriating $3.6 million in non-recurring funds for landslide mapping.
Senator Ralph Hise, R-Mitchell, addressed some of those concerns during Friday's meeting, asking Korte if he is worried that mapping landslide hazards could stymie development in western North Carolina.
"People are really afraid that when you go out and do this — their entire county — no one's going to be able to build anywhere because the entire county is in a (hazard area)," Hise said.
Korte addressed that history head-on Friday, noting the program had been shut down before and making it clear that his goal is to show where risk is. Decisions about where development should take place and what measures are necessary would, he continued, remain with local government officials.
"I'm leaving it up to others what to decide what to do with the property value assessments and those kinds of things. There are going to be areas that the map will point out that the site might need some work, or should need some work, before you build a house on it," Korte said.
Korte also noted that many western North Carolina real estate agencies require their agents to use the state's map to disclose landslide risk when they sell a house.
In a disaster recovery bill last year, the General Assembly appropriated $3 million to map landslide hazards in areas impacted by Helene.
As that bill was working its way through the legislature, the Governor's Advisory Committee on Western North Carolina Recovery recommended an appropriation of $6.1 million for landslide mapping. The mapping funds would be used, the committee said, to create landslide hazard maps in 15 counties that haven't yet been covered and to re-do maps in the nine counties where they already exist.
The committee also called for the General Assembly to appropriate an additional $600,000 to create the early warning system for landslides
Friday, Korte told the recovery committee that the landslide mapping program currently has three staff members who work on the hazard maps. He would like to double the size of the landslide hazard team.
That would include three people to install and maintain monitoring stations that are part of an early warning network for landslide risk. The state has nine of those stations, but bears, insects and vandals often render them ineffective.
"Maintenance on those has been neglected because landslide mapping has been more important to our customers recently, and we don't have the person power to do both good landslide mapping and properly maintain monitoring stations," Korte said.
