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Geologist speaks on problems with steep-slope development in WNC
By Justin Goble
The problems that arise from development on steep slopes was the main topic of an Aug. 8 keynote address by geologist Rick Wooten at Western Carolina University.
Speaking as part of a WCU seminar on development in Western North Carolina, Wooten, the senior geologist with the N.C. Division of Land Resources’ Geological Survey, addressed the hazards of building in steep areas.
According to Wooten, landslides have been occurring on slopes throughout this area for thousands of years. Heavy rains from thunderstorms and hurricanes, such as 2004’s Ivan and Frances, have triggered slides in particularly steep areas.
“It’s a recurring phenomenon in WNC,” Wooten said. “The intensity and duration of the rain puts down so much water that it overcomes the strength of the soil. That, and there’s a situation we see in the mountains where thunderstorm cells park over an area and don’t move. It’s a weather situation we have to watch out for.”
Geologists can predict where future slides will happen by determining where they have occurred in the past, Wooten said. Since previous slides have cut paths into the mountains’ rock, discharge from future landslides will usually follow them. However, many of those areas have been used for development.
Macon County’s Peeks Creek slide, caused by heavy rains from Hurricane Ivan, is one of the clearest examples, Wooten said. Once the soil was weakened by the rainfall, the mud and rock followed the path of a previous slide, doing a tremendous amount of damage to several homes and killing five.
“At Peeks Creek, the slide material was travelling about 35 miles per hour and discharging 45,000 cubic feet of material per second,” Wooten said. “That’s equivalent to 16 or 17 18-wheelers going by at once doing 35 miles per hour.”
Closer to home, Wooten pointed to Jackson County’s greatest natural disaster – the flood of 1940 that devastated Cullowhee’s Canada and Caney Fork communities and killed four. The entire county was affected, however, because the high waters rendered every bridge that crossed the Tuckaseigee River unusable. He showed slides of landslides in Canada and Caney Fork, which destroyed homes and caused the fatalities. Those landslides also followed paths cut by previous ones, the geologist said.
There was significantly less development then,” Wooten said. “If we have a repeat of that event, we can expect much more damage in the area.”
More recently, Wooten said the county’s airport, which was built in the mid 1970s, has caused damage to homes below it. Landslides shortly after the airport’s construction shortened the runway, damaged several homes and led to lawsuits against the county. An isolated thunderstorm in August 2005 triggered more earth movement and has led to another pending lawsuit. (See related story on page 1A.)
“There was a scarp (steep rockfill) that became a slow-moving slide,” Wooten said. “(The airport) was a well-intentioned design, but everything had to go right for it to be fine. It turned into a liability for Jackson County. Now the runway can’t hold the weights it was engineered to hold, and something that was built in the 1970s is creating problems today.”
The geological survey is currently mapping areas in WNC to determine where slides have happened in the past. Wooten said his office should have the Jackson County maps completed sometime next year.
Another issue Wooten raised is building on “debris fans,” where the material collected from previous slides comes to rest at the bottom of a slope. Wooten said many developers choose such sites for homes and apartment complexes without considering the damage a slide could cause to the property.
“They’re popular places to develop,” Wooten said. “They’re out of the floodplain, and they don’t require a lot of heavy excavating. But there’s a source area above it. If that’s not disturbed, it could be stable. But if the area above is disturbed or developed, there could be a slope failure that lands on homes in the area below. People talk about what’s ‘up slope,’ but you have to look at what’s ‘down slope’ as well.”
Acid-producing rock is another problem developers need to consider, Wooten said. Many rocks and minerals aren’t dangerous on their own but can cause harm once exposed to other elements.
“Pyrite, in it’s natural setting, is fine,” Wooten said. “When dug up and exposed to rain, it forms a mild sulfuric acid. That can wreak havoc on houses.”
To accommodate for that, Wooten suggested developers get a soils report before starting construction.
For more information, visit http://www.geology.enr.state.nc.us.
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