Rainfall from Hurricane Ian Could Trigger Landslides in NC Mountain Regions
Forecasts predict the remnants of Hurricane Ian will dump seven inches of rainfall on the North Carolina mountains when the storm makes its way into the region later this week. The estimates are cause for concern and raise the potential for flash flooding and landslides.
Brad Johnson, associate professor and chair of Environmental Studies at Davidson College, studies landslides and erosion. Here he answers some questions about landslides, phenomena that frequently occur in the path of hurricanes.
How do hurricanes 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 overcome the cohesion between grains and start pushing the soil particles 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.
Is there a way to predict when landslides will 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 2004 Peeks Creek landslide, the deadliest in North Carolina in 60 years. The group looked at weather data for 2004 and found that five inches seems like the tipping point for landslides.
While those studies looked at total quantity of rain, the rate of rainfall might be more important. Five or six inches of rain over a week won’t necessarily be a problem because the soil might have enough time to absorb some of that water. However, during more intense rainstorms, soil can become saturated and unable to accommodate additional water. If you start getting really heavy rains on top of saturated soil, that's when things start to get scary: water is unable to absorb, starts to runoff the steep slopes, and can take large amounts of soil and sediment downhill. We call this type of landslide a debris flow. Debris flows might look like a combination of floods and landslides; 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. The steep part is somewhat hard to get away from in the Blue Ridge since that is the defining characteristic of the region. Landslides are a natural phenomenon in areas with sleep slopes that experience large precipitation events like hurricanes or tropical storms.
As for hurricanes, the Blue Ridge ends up in the path of a lot of them. Hurricanes do this large curve as they evolve: they come in from the east but move to the west in the tropical zones. However, as they drift to the north and into the mid-latitudes, they tend to do a U-turn and blow back to the east. Because of that pattern, the Blue Ridge tends to get heavy precipitation from both hurricanes that hit the Atlantic Coast and those that hit the Gulf Coast. For instance, if you get a direct hit on Savannah or Charleston like Hurricane Hugo, it comes across the Blue Ridge. On the other hand, Harvey hit Texas and still managed to bring a lot of moisture to the Blue Ridge.
Many people don’t realize how frequently landslides occur. A 1940 hurricane—a small Category two that brought 20 inches of rain—caused over 2,000 landslides across the Blue Ridge. Most worrisome: There weren’t many people living in those rural counties back then. 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 in 2004 near Franklin, North Carolina, 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. As geologists, we are trained to think about the past as an indicator of what might happen in the future. Homes built on landslide debris are very likely to experience landslides in the future.
- September 28, 2022