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Q&A: Tropical Storm Florence Presents Landslide Threat in Blue Ridge

Bradley Johnson in front of a waterfall
Prof. Bradley Johnson

With Tropical Storm Florence moving out of Charlotte and turning northeast toward the Blue Ridge Mountains, the storm brings a new threat: Torrential rainfall that could trigger landslides.

Dr. Bradley Johnson, associate professor and chair of Environmental Studies at Davidson College, studies landslides and erosion.

"I'm most worried of 12 or more inches of rain falling on the Blue Ridge," he said. "Current forecasts indicate a focus of precipitation on the Blue Ridge escarpment which is the steepest part of the Blue Ridge. If that happens, we could see dozens to hundreds of landslides."

A 2017 Q&A with Johnson about how hurricanes can bring landslides to the Blue Ridge is below.

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 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.

Is there a way to 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 2004 Peeks Creek landslide, the deadliest in North Carolina in 60 years. The group looked at weather data for 2004 and found that 5 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.

Aerial view of landslide
A sequence of major storms in September 2004 triggered at least 400 landslides in Western North Carolina.

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. 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, 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.

Jay Pfeifer