The image is overlaid with topographical lines and yellow dots that mark the starting points of five separate rivers of mud, rock and water that, in 1977, merged into one powerful, devastating track — a debris flow, Wooten explains. “In the mountains, landslides are likely to happen where they’ve happened in the past; the geologic conditions haven’t changed,” he says.
Although Buncombe County is “pretty stable ground,” the ’77 slide wasn’t the first in this uninhabited area, and it won’t be the last: “Sometimes the modern debris flows have so much material in them that they spill over the banks of the channels cut through the older deposits. That is what we found” here during a statewide landslide survey, Wooten says.
Debris flows are the most common type of landslide, particularly in mountainous Western North Carolina, he explains. When such slides happen near homes and highways, the results can be devastating, such as a 2004 slide in Macon County’s Peeks Creek area that destroyed 15 homes and killed five people after Hurricane Ivan brought torrential rains to the region.
When there’s a slide, large or small, Wooten’s the guy everyone calls. He helps figure out what happened and how.
Wooten is the senior geologist for North Carolina’s Geohazards and Engineering Geology division. He has studied slope movements for nearly 30 years (even longer, if you count collecting rocks as a kid). Based in Asheville, he was a featured speaker at the June 5 Rockslide Conference hosted by the Land-of-Sky Regional Council, which aimed to give local governments, businesses, engineers and others the chance to learn about events such as the December 2012 rockslide that closed part of Interstate 40 near the Tennessee-North Carolina line for about a month (a 2009 one closed the corridor for much longer).
Slides happen, often in the same locations, time and again. In 2010, a Maggie Valley slide cut a 3,000-feet-long swath down the mountain and was 175 feet wide in spots. A statewide survey of landslides, spearheaded by Wooten and launched not long after the deadly Peeks Creek event, would have finished mapping Haywood County, but budget cuts halted the project in 2011 with just four counties completed — Buncombe, Henderson, Macon and Watauga.
A few weeks before the conference, Wooten met with Xpress in the field to talk about his ongoing work.
“The term landslide gets used a lot,” he says as we get ready to tour the Bent Creek site. “Typically a landslide is pretty much what it says — where the earth slides down the hill. It’s an intact mass,” says Wooten.
Geologists define slope movements by the materials being moved and the way that they travel down the slope, he continues. “Earth” or “mud” flows mean that more than 20 percent of the material caught up in the slide is tiny — silt- or clay-sized. In WNC, “debris” flows are “dominated by sand with gravel- to boulder-size rock fragments,” Wooten explains. Such slides “move like a thick liquid [and] can reach speeds in excess of 30 miles per hour.”
Debris slides are the second-most common — a soil mass or debris that moves intact down the slope but can become a flow if hits a water way or there have been heavy rains. In short, Wooten notes, flows and slides “are usually a ‘muddy mess.’”
And however you define slides, something has to set them off, he says.
“The way [geologists] look at it, there are causes, then there are triggers … usually some sort of blast event. Here [in WNC], rainfall is usually the trigger, but there are other factors — what kind of rock, what kind of soil, [the] shape of the land, what’s been modified, cuts, excavations, fills,” he says.
For a slope movement to occur on a section of unmodified “natural” mountainside, there must be 5 to 10 inches of rainfall within a 24-hour period. In areas where there have been changes to the slope — such as cutting into the bank to build a road or house — it can take as little as 3 inches to induce a landslide.
In 2009, about 4 inches of rain occurred while Nikki Donin’s elderly parents slept in their Maggie Valley home. The rain — and a poorly reconstructed slope for a home above — triggered a slide that swept them downslope. The Donins survived with nothing worse than a broken nose, although the home was flattened and scattered like a discarded game of Pick-up-sticks (see “Finding Stable Ground in Landslide Country,” Feb. 4, 2009, Xpress).
So far this year, WNC has had above-average rainfall, resulting in more recorded slope movements than what Wooten sees in a typical season. He estimates that there were more than 100 slides in January alone. One mudslide took out Long Branch Road in Maggie Valley and briefly trapped a family and their dog in their home.
Slope steepness is another contributing factor for slides. Many WNC homes are built on steep land, and Wooten notes that most slides occur where the grade measures 20 degrees or greater.
As he walks down the trail, Wooten stops to point out how the rocks themselves tell a story: “A tell-tale sign that a deposit … is from a debris flow, modern or ancient, is when the rock fragments are stacked up on top of each other shingle-style. The rock fragments in younger debris flows are usually hard; those in older deposits are more weathered and crumbly. The soil mixed in with the rock pieces of younger debris flow deposits is usually dark brown; yellow to orange in older deposits (tens of thousands of years); and red to dark red brown in really old deposits (hundreds of thousands of years).”
It can be important to know where slides have happened, and could happen again, especially in populated areas. In the 1940s, the remnants of a hurricane caused heavy flooding, landslides and more than a dozen deaths in Watauga County, possibly the most landslide-prone area in WNC.
“The maps are like a landslide inventory: Where they’ve happened, the paths they’ve taken. The bulk of those deposits may be prehistoric, tens or hundreds of thousands of years old,” says Wooten. Most happen in remote areas, he says. Many will happen in the same area again, sooner or later.
Leaving the trail, he beelines hundreds of yards through the forest, straight to mounds of rock that seem to have been precisely placed one atop the other. Such piggybacked boulders indicate a fast-flowing slope movement, he says. Other signs, easily visible to the untrained eye, are streams that run on top of the ground rather than in channels carved by centuries of water flow.
What’s another way to determine an area’s landslide history?
Take to the skies. According to Wooten, aerial photography has been instrumental in helping geologists discover slope movements that otherwise might have gone unnoticed. Comparing old and new photographs can help, and laser technology makes the process even easier. Many scientists use maps created with Light Direction and Ranging (LiDAR) — mounted to an aircraft flying over a designated area, the system creates a precise topographical, he says. “You have an accurate reading taken every 20 feet,” says Wooten.
Returning to the car, Wooten unfastens his utility belt, places it in the trunk and then opens his laptop. He points out other features on the LiDAR-generated map and talks further about “run-out zones” and other indications of the ’77 slide’s path.
At one point, he comments, “Not everyone is fortunate enough to do what they love.”
And with more slopes moving, more sites to analyze and more maps to make, there’s no sign that Wooten will be put away his hammer and compass anytime soon.
— Mat Payne is a former Asheville resident and now a reporter in Clayton, Ga.
News Editor Margaret Williams contributed to this report. She can be reached at 251-1333, ext. 152, or email@example.com.