Engineer puts

slopes on the map
By Scott Nicholson
nicholson@wataugademocrat.com

The Watauga County planning board received landslide hazard maps Tuesday night, that suggested some slopes could be at risk but it would remain difficult to predict the triggering weather.

Rick Wooten, engineer with the North Carolina Geological Survey, said Watauga’s map was the second completed in the state.

“We do know there is a significant landslide hazard in Watauga County,” he said. “Certainly not every day, but we do know it can happen in significant storm events.”

The survey is not a regulatory agency, Wooten said, noting that it had provided information to the mining industry during its history. The landslide maps were described as a planning tool and information for developers and the general public, and they didn’t create any regulatory restrictions.

The survey documented 2,253 total landslides, with about 20 percent of the county rated as “high hazard” based on the Stability Index map and 41 percent of the county within potential landslide pathways. Most of those documented landslides occurred in the 1940 flood, which led to 14 deaths.

Wooten said the population had doubled since then and the county was still at risk of landslides, so the county moved to the top of the list for surveying. In Watauga County, 136 structures have been built in tracks of landslides from the 1940 flood. In North Carolina, 45 structures have been destroyed since 1990, with six fatalities.

“Most places, every day, they’re stable,” Wooten said. He noted that major hurricanes were usually the triggering event for landslides. Back-to-back storms had happened during three of the major landslides since 1916, and a major landslide occurs about once every 29 years in the mountain region. “These storms can happen in localized areas practically anytime,” Wooten said.

A “debris flow” is the major danger, with land moving about 30 miles per hour. Usually it occurs when soil flows over bedrock and digs a channel, with the loose material fanning out at the foot of the slope over time. A “blowout” comes from excess pressure that pushes out soil but doesn’t have enough energy to create a soil flow.

Wooten said the slope degree of landslides was usually 20 to 25 degrees, though the number varied, depending on terrain and coverage of soil. Of the documented slopes of the 1940 flood, 90 percent of the landslides were 20 degrees or greater. Wooten said there was more pastureland in 1940, and the increase in forest could work in the county’s favor in future storms. Landslides had been documented all over the county, though most of the historic damage occurred in Deep Gap. Old landslide tracks cross modern roads in 521 different places.

Stability maps showed where the landslides are likely to start, while other maps show where the debris flows are likely to go, usually to the outer edges of earlier flows. Wooten said these could be used to identify property that could face debris flows in the future.

Of Watauga County’s 101 landslides since 1940, 51 percent had occurred on slopes that had been modified by construction or roads. Historically, 7 percent of all landslides have occurred on modified slopes. Wooten also said it was difficult to predict when major storms would occur that could trigger landslides. “It could be 200 years from now or it could be the next hurricane season,” he said.

“If you look at the probability of any particularly spot being below the debris flow...it’s one chance in 5,000 in any given year,” Wooten said. “These maps tell you if something does go wrong, where it’s likely to impact.”

“It’s not the final word at any level,” Wooten said, adding the maps don’t substitute for site-specific investigations by qualified technicians. The geologic survey is headed by the N.C. Department of Environment and Natural Resources and the maps will eventually be posted online for public access. Maps will be available in paper versions in the county planning office.