Current forecasts indicate that Hurricane Helen could reach the region later this week, bringing 10 to 15 inches of rain to the mountains of North Carolina. This is a concerning prediction, increasing the potential for flash flooding and landslides.
Brad Johnson, a professor of environmental studies at Davidson College, studies landslides and erosion. Here he answers our questions about the potential impacts of the coming storm.
To contact Johnson, email japfeifer@davidson.edu.
How does heavy rain cause landslides?
The easiest way to think of it is to think of a pile of sand on the beach. When you add a little water, surface tension causes the sand particles to stick together and the sand becomes sticky. But if you add a lot of water, it turns into a pile of sand. The same thing happens with soil on a hillside.
The question is whether there is enough water to overcome the cohesive forces between the particles and push the soil particles apart. If there is enough water, the soil no longer has enough friction to stay in place and starts to flow downhill. In other words, the large amounts of rainfall that accompany a hurricane or tropical storm can cause the soil on a hillside to transition from a cohesive to a loose state.
Is there a way to predict when a landslide will occur?
While predictions can’t be made with any certainty — at least not for a specific location for a specific storm — scientists have established some thresholds for North Carolina following the 2004 Peaks Creek landslide, the state’s deadliest in the past 60 years. A group of scientists looked at weather data from 2004 and concluded that 5 inches appears to be the tipping point for a landslide.
These studies looked at total rainfall, but the amount of rainfall may be more important. Five or six inches of rain in a week isn’t necessarily a problem because the soil has plenty of time to absorb that water. However, during more intense rainstorms, the soil becomes saturated and can’t absorb any more water. When really heavy rain starts falling on saturated soil, things get scary. The water can’t be absorbed and starts to run downhill, carrying tons of soil and sediment downstream. These types of landslides are called debris flows. Debris flows may look like a combination of a flood and a landslide. They’re very wet and they move very fast.
While we can’t predict specific locations, it’s easy to look ahead when we think about whole regions. For example, if a storm dumps 10 inches of rain in the Blue Ridge Mountains, there’s a very good chance that a landslide will occur somewhere. It’s just hard to predict exactly where it will occur. But we do know what types of terrain are more susceptible to landslides; areas at the bottom of steep slopes and at the headwaters of rivers are very dangerous. More specifically, areas that have seen landslides in the past are also more likely to experience them again, which is one of the reasons why Appalachian states are making a great effort to map past landslides.
Why do so many landslides occur in the Blue Ridge Mountains?
It’s a combination of two factors. This region has a lot of steep terrain, and many hurricanes and tropical storms seem to come over the Blue Ridge Mountains. The steep terrain of the Blue Ridge Mountains is a feature of this region, so it’s hard to avoid them. Landslides are a natural occurrence in areas with steep slopes where heavy rains like hurricanes and tropical storms occur.
