Hurricane today. Landslide tomorrow? When hurricanes settle over the Appalachian Mountains, they bring more than downed trees and power lines. Torrential rainfall creates a different hazard: landslides.
The Peeks Creek landslide, the deadliest North Carolina landslide in more than 60 years, happened after hurricanes Frances and Ivan passed over the Blue Ridge within five days of each other.
As Hurricane Irma passes over the mountains of North Carolina and Tennessee, Bradley Johnson, associate professor of environmental studies at Davidson College, says that damage from forest fires in 2016 may increase the risk of landslides in the mountains. Johnson is also chair of the board of supervisors for the Mecklenburg Soil and Water Conservation District. He discusses the risk in this Q&A.
What is it about hurricanes that can trigger landslides?
The easiest way to think about it is to envision a pile of sand at the beach: You add a little water and surface tension creates cohesion between the grains of sand, and the sand gets kind of sticky. However, if you add a lot of water, it turns into a slop pile. The same thing happens to soil on hillslopes.
The question is, do you have enough water to start pushing the granules of soil apart from one another? Once you do, the soil no longer has enough friction to stay in place and begins to flow downhill. In other words, the large rainfall totals that come with hurricanes can cause soil on hillslopes to cross from cohesive to loose.
Can we predict when landslides happen?
We can't predict with any certainty but some scientists did try to establish some thresholds in North Carolina in the wake of the Peeks Creek landslide in 2004. The group looked at weather data for 2004 and found that five inches seems like the tipping point for landslides.
The rate of rainfall might be more important. If you start getting really heavy rains on top of saturated soil, that's where the problem starts. There's not enough time for water to infiltrate everywhere and it starts liquefying the surface soils. We call these landslides debris flows; they are very wet and travel very quickly.
Why does the Blue Ridge experience so many landslides?
It's a combination of two factors: The area has a lot of steep terrain and many hurricanes seem to end up over the Blue Ridge. Hurricanes do this zig-zag where they come in on the Easterlies, get up into the Westerlies and move back out to sea. If you get a direct hit on Savannah or Charleston like Hurricane Hugo, it comes across the Blue Ridge. Harvey hit in a completely different spot and yet it ended up coming this way too.
Many people don't realize how frequently landslides occur. A 1940 hurricane–a small Category 2 that brought 20 inches of rain–caused over 2,000 landslides across the Blue Ridge. There weren't many people living in those rural counties back then, but a lot more call that area home today.
Can homeowners do anything to mitigate the risk of a landslide after their home is built?
Not a lot. That's one of the things that's most troubling. Homeowners' insurance does not apply to landslides, and mountainous counties in North Carolina don't have a lot of zoning to restrict building in areas susceptible to landslides.
After the Peeks Creek landslide, state geologists found that the homes that were hit by the landslide had been built on ground that was just layer upon layer of landslide debris. The homes had been built right on top of previous landslides.
Last fall, there were a lot of forest fires in western North Carolina and Tennessee. Will the damage from those fires increase the likelihood of landslides?
I'm really curious to see how that affects landslides. I think the areas that had fires last year are primed for problems if they get a lot of rain.
A soil scientist from Virginia Tech, with whom I'm collaborating, is finding that soils in the burned areas don't absorb water as readily as those in unburned areas. Two current Davidson environmental studies students are looking at those soils for their senior capstones. They are finding some interesting things. For instance, one of the burned soils from South Mountain State Park was so hydrophobic that it took 53 minutes for a drop of water to sink in to a soil sample. Our preliminary results suggest that those soils aren't absorbing water very well and the result is a lot more water on the surface, which really increases the chance of a landslide or debris flow.