Geologist tells Polk 40-percent slope increases landslide chance

Published 5:00 pm Tuesday, July 17, 2012

The Polk County Planning Board heard facts about landslides during a July 12 meeting, as well as a geologist’s opinion that a 40-percent slope is the point at which the chances of a landslide increase.
Rick Wooten, senior geologist for geohazards and engineering geology with the N.C. Department of Environment and Natural Resources (DENR), Division of Land Resources, presented data from past landslides around the state. Wooten said landslide damage and losses can be avoided. He said most insurance policies do not cover damage to homes from landslides.
Since 1916, there have been at least 46 deaths in North Carolina related to landslides, according to Wooten’s data. Between 1990 and 2011, 57 landslides occurred in the state, resulting in six fatalities, with 40 structures destroyed or condemned and 24 structures damaged.
Studies also show that between 1990 and 2011 there was one fatality and 24 structures were destroyed or condemned as a result of landslides on slopes altered by human activity, or on modified slopes.
Approximately 52 locations in Polk County have had landslide activity, Wooten said. He mentioned the Saluda grade railroad that gave way in 2004 and the instability of Chocolate Drop Mountain.
He also said recurring weather patterns trigger landslides in western North Carolina. For example, he said, landslides can be expected when an area receives 5 inches of rain in a 24-hour period.
The biggest storm known in Watauga County occurred in 1940 and triggered 1,200 landslides and 14 deaths. Wooten said tracking past landslide data is important to see where they occurred. He said since 1940 in Watauga County 136 structures have been built in the tracks of the landslide.
Other landslide areas were reviewed, including Maggie Valley, where Wooten said there were some warning signs prior to a slide. He said a 2010 landslide took a two-story house and “turned it into sticks.”
There were about 400 landslides in western North Carolina during the heavy rains of Hurricanes Frances and Ivan in 2004, Wooten said.
“The good news is it’s not all that hard to avoid damage from landslides,” Wooten told the planning board. “It’s kind of pay now or pay later.”
The planning board asked Wooten at what point of slope planners need to start being concerned.
Wooten said at 40 percent (or 22 degrees) is when there begins to be an increase for slides.
“That’s not to say all 40 percent slopes are unstable,” Wooten said. “That’s the point the stability can be affected. A lot of times it’s a combination of things.”
The planning board has been struggling to determine an exact slope at which restrictions should be placed on building. The planning board is working to draft a unified development ordinance (UDO) that combines all the county’s ordinances, including its mountainside and ridgeline protection ordinance (MRPO), which formerly placed restrictions on any commercial building above 1,650 feet in elevation.
A UDO committee approved a draft UDO ordinance earlier this year that eliminated the elevation restrictions in the MRPO and asked the planning board to replace elevation with slope restrictions. The slope that has been considered previously is 30 percent. Restrictions may include requiring geological and soil studies prior to building.
Planning board member Wayne Horne, who chaired last week’s meeting, said the board is trying to determine at what slope the county will require engineering.
“What we’re trying to do is determine the slope factor before engineering kicks in,” Horne said. “Why we’re doing this is if you have a mountain property, we’re trying to get you to (build) on the 30-percent side instead of the 60-percent side.”
The planning board will adopt a recommended UDO ordinance and send it to the Polk County Board of Commissioners for final adoption. Commissioners will be required to hold a public hearing prior to adopting the final UDO.
The planning board’s next workshop will be next Wednesday, July 25 at 5 p.m.

Sign up for our daily email newsletter

Get the latest news sent to your inbox