Western North Carolina Landslide Fact Sheet

Following the devastating landslide in Macon County that claimed five lives last September, residents across Western North Carolina came to the unnerving conclusion that the hillside above their homes could be waiting time bombs.

While predicting a landslide is impossible, geologists say they can determine landslide prone areas by analyzing a range of factors, including the soil type, slope, hydrology and level of human disturbance. The state has sanctioned a landslide risk analysis for 25 counties in WNC, a project that could take five to 10 years. The result will be a map defining the features and characteristics that are indicators of landslide probability.

The landslide in Macon County — technically called a debris flow — was triggered by heavy rains that liquefied the soil and sent it sliding off the mountain.

“It will initiate in a little cove or hollow not too far from the ridge line and follow an existing stream or a channel,” said Rick Wooten, a geologist with the North Carolina Geological Survey. A snowball effect fuels the debris flow, getting larger and faster as it continues down the mountain.

The Peeks Creek debris flow flattened everything in its path, carving out a 200-foot wide swath of destruction. There was no warning and little time to escape. Geologists have estimated the mass of water, rock, mud and uprooted trees was traveling at 33 miles per hour.

“There are people in Peeks Creek who were standing there and looked out and saw it and heard it, and knew enough not to run to the basement but ran uphill. They are doing fine now. That’s cutting it a little closer than I would like to cut it,” Wooten said. “From all reports it does sound like a tornado or freight train.”

Many people in Peeks Creek thought it was a tornado that hit them, between the swath of destruction and the freight train noise that accompanied it, a sound typical of tornados but also large debris flows.

The first task in identifying slide-prone areas is coming up with a set of variables thought to contribute to landslides, Wooten said. Next, they will do a pilot study in Macon County. If the selected variables indicate Peeks Creek is at high risk for landslides, then the variables are accurate at least to some degree, Wooten said. But if Peeks Creek emerges as low risk, then clearly they will need to chose a different set of variables to predict landslide prone areas.

Old landslides will provide a further litmus test for whether the selected variables are making accurate predictions. The trained eye can detect signs of previous debris flows decades after they occurred, providing several opportunities to test the equation, Wooten said.

After the Macon County pilot and when they are comfortable with their selected variables, they will move on to the other counties.

What are the variables?

While slope is a necessary factor for a landslide to occur, the sheer steepness of the mountain alone is not a good indicator. The key factor is the depth of the soil.

Landslides occur where there is a shallow layer of soil covering the bedrock. Heavy rainfall causes water to pool up against the bedrock under the soil.

The worst subsurface condition occurs in an area where the bedrock is slightly concave, allowing the pooled water to collect in a pond.

“When that uplift force exceeds the strength of the soil, that’s when things start to move and head down the hillside at a pretty rapid clip,” Wooten said.

Human disturbance on a slope is also a factor. Tree roots hold the soil in place and suck up excess moisture in the roots. Clearing the trees can lead to a loss of stability.

Slope modifications, such as cut and fill areas, alter the natural drainage pattern and can be a landslide trigger. Freezing and thawing of soil can also cause upheavals, making an area more prone to landslides.

Landslides can be costly. A 1997 slide that collapsed a section of Interstate 40 in Haywood County cost $10 million to repair and stabilize, Wooten said. That’s cold costs, not counting lost tourism revenue or other indirect economic losses.

Homeowners insurance does not cover landslides, and there really is no such thing as a separate landslide policy.

“The residents and communities are left holding the bag,” Wooten said.