An Introduction to Landslides in Western North Carolina
Landslides pose significant risks to public health and safety, local economies, and home and business owners.
Extreme weather patterns—such as periods of above-normal rainfall or drought—are associated with a changing climate. They are already impacting people in western North Carolina.
These extreme weather patterns are also linked to landslide events.
Landslides and debris flows usually occur in areas where they've happened before.
Mapping where landslides have occurred in the past helps identify areas where they could happen again.
Landslide mapping and other resilience planning allow emergency responders, planners, and city and county staff be better prepared to help protect people and assets from losses associated with landslide events.
Introduction
A landslide is the movement of a mass of rock, debris, or earth down a slope. Landslides are one type of what geologists call "mass wasting"—any movement of soil and rock down a slope that moves as a mostly unified mass under the influence of gravity.
From 1990 to 2016, the North Carolina Geological Survey (NCGS) responded to more than 175 requests for assistance on landslide events from government agencies, the public, and consultants.
NCGS geologists have since investigated over 200 landslides in the Blue Ridge Mountains of western North Carolina. At the time of publication, these landslides have resulted in five deaths, destroyed more than 25 homes and damaged at least 40 others, and impaired nearly 80 roads.
Read on to learn more about the types of landslides, what causes them, and explore maps developed by NCGS to learn where landslides have occurred in the past and the potential for future risk.
To explore historical landslides in the region, please visit Historical Landslides in Western North Carolina .
What is a Landslide?
A landslide is the movement of a mass of rock, debris, or earth down a slope. Landslides are one type of what geologists call "mass wasting"—any movement of soil and rock down a slope that moves as a mostly unified mass under the influence of gravity.
The word “landslide" encompasses five types of slope movement: falls, topples, slides, spreads, and flows.
Slide (A, B, C)—In a slide, a soil or rock mass moves downslope either on a surface of rupture or on a thin zone of intense shear strain. Slides do not occur simultaneously across the entire surface of a rupture; rather, they grow from an area of local failure. Slides can be rotational (where the surface of the rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface and transverse across the slide) or translational (where the landslide mass moves along a roughly planar surface with little rotation or backward tilting). Both types of slides can be caused by intense rainfall or by rises in groundwater due to flooding, irrigation, and rises in stream levels.
Fall (D)—During a fall, rock or soil detaches from a slope and descends rapidly by falling, bouncing, or rolling. Falls occur on steep slopes and are triggered by the undercutting of slopes by streams or rivers, weathering, human activity, or earthquakes.
Topple (E)—A mass of soil or rock that rotates forward out of a slope, which may be driven by the weight of upslope material. Topples can be triggered by water seeping into, or ice forming in, cracks and by vibration, undercutting, differential weathering, excavation, and stream erosion.
Flow (F,G,H,I)—A flow is a continuous movement of material down a slope that resembles the movement of a fluid. Flows can be further divided into debris flows, volcanic debris flows, debris avalanches, earthflows, and creeps.
Spread (J)—A spread occurs when a mass of soil or rock expands and the fractured mass sinks into soft underlying material. Spreads are divided into block spreads, liquefaction spreads, and lateral spreads, which typically occur in flat or gently sloping landscapes.
These slope movements are further subdivided by the type of geologic material involved: bedrock, debris, or earth (a combination of rock and soil).
The type of movement and the material involved determines the potential speed of the landslide and the distance it will travel, as well as the volume of displaced material and possible impacts.
Every landslide, or slope movement, is unique and often unpredictable. Landslide events are best evaluated on a case-by-case basis.
Examples of debris flows, debris slides, and rock slides.
Anatomy of a Landslide
Multiple debris flows can happen in the same location. These create features called “composites.” Debris flows can become inactive, and older debris flows can reactivate.
A scarp is the edge of a steep slope of a debris flow, and an initiation zone or process point is where a debris flow initiates.
The red area in the picture at right shows ground rupture lines at the upper edge of the vertical offset along the scarp.
The diagrams at right show the anatomy of a debris flow and a cross section of a rock slide.
Common Landslide Types
Common landslide types (or slope movements) in western North Carolina include debris flows, debris and earth slides, and rock slides.
Debris source and deposit areas.
DEBRIS FLOWS are rapidly flowing mixtures of soil, rock particles, and water that are often referred to as "mudslides.” This is the most common type of the 3,290 landslides that occurred before June 30, 2011, in western North Carolina. Debris flows typically begin with a mass of rocky silt-sand that has a low clay content and is therefore not cohesive. Because of the low cohesion, this type of soil mass can “liquefy” in heavy rains, allowing it to move rapidly down a slope at speeds approaching 30 miles per hour.
Debris flows often begin in depressions, hollows, the headwaters of mountain streams, and in areas where bedrock is overlaid with a thin layer of soil that is, at most, six feet deep.
This slide damaged North Carolina Highway 194 north of Newland, in Avery County, in July 2013.
DEBRIS (soil-rock mixture) or EARTH (clay-silt soil) SLIDES move at a slower rate than debris flows because the water content is too low for the mass to liquefy. High clay content can cause material to be more cohesive and therefore move at a slower rate. Unlike quick-moving debris flows, debris and earth slides typically move at a rate of several inches or feet per day. After the initial movement of a debris or earth slide opens tension cracks and scarps in the landscape, water is able to infiltrate deeper into the mass of material involved in the slide. In wet weather conditions, the movement of a debris or earth slide can become self-perpetuating, as additional water further destabilizes the material and further movement widens existing cracks and scarps—which opens pathways where more water can infiltrate.
This rockslide occurred on February 3, 2012, on U.S. Interstate Highway 40 West at Mile Marker 7 in North Carolina, near the Tennessee border.
ROCKSLIDES and ROCKFALLS usually occur along roadways, but can occur on any modified or natural rock slope. They occur on steep, exposed faces made of bedrock or boulder-rich rock deposits. In western North Carolina, rockslides are common along major transportation routes and have resulted in significant direct and indirect costs in the region.
Where Do Landslides Occur?
Damage from a landslide that covered U.S. Highway 176 and neighboring properties in Polk County on the night of May 18, 2018. Click on the image for a larger view.
Landslides can take place almost anywhere in the world in a variety of landscapes—including cultivated lands, slopes, and forests—and do not necessarily happen on steep slopes. However, certain types of landslides are associated with hilly and mountainous terrain.
Natural patterns like climate, wildfire, and the courses of streams and rivers also influence where landslides occur, as do human activities such as clearing trees and vegetation and construction on slopes.
Some types of landslide only occur in specific terrains or in the presence of particular geologic materials. Debris torrents, for example, take place in channels and ravines, and, as discussed above, rockfalls occur on steep, exposed faces made of bedrock or boulder-rich rock deposits.
What Causes Landslides?
Landslides are triggered primarily by natural occurrences—including rain, melting snow, changes in water level, stream erosion, changes in groundwater, earthquakes, and volcanoes—and by human activity. Factors such as soil type, landform, and geology also influence landslide events.
Heavy rainfall and the subsequent increase in groundwater is a common trigger for landslides in western North Carolina.
Widespread development of landslides coincides with major rainfall events, particularly rainfall events that occur as the remnants of tropical storms pass into the mountains of western North Carolina.
Widespread occurrence of landslides typically occurs when 10+ inches of rain falls within 24 hours.
Localized landslides typically occur when at least five inches of rain falls within a 24-hour period.
The chart combines data on the dates and amount of rainfall (blue lines in the lower graph) and the dates and number of landslides (black lines in the upper graph) in western North Carolina between 1876 and 2010.
Human activities that contribute to landslides include:
- altering drainage patterns,
- clearing vegetation, and
- destabilizing slopes by undercutting a slope bottom and/or loading the top of a slope beyond its bearing strength.
Activities such as irrigation, draining or creating reservoirs, and improper slope excavation can destabilize once-stable slopes.
Field investigations have determined that many debris flows and landslides have occurred where slope modification by humans was a contributing factor. The majority were fill failures that mobilized into damaging debris flows and debris slides.
For areas that have already been mapped for landslide hazards, there is a strong correlation between areas identified on the maps as having increased potential for debris flows and debris slides and landslides that have actually occurred in those areas.
Landslides and debris flows usually occur in areas where they've happened before.
In western North Carolina, correlations between rainfall and debris flow events indicate that on slopes modified by human activity, debris flows can be triggered by rainfall events with lower rates and durations than rainfall events that trigger debris flows on unmodified slopes.
Landslides have been triggered by three inches of rainfall or less in a 24-hour period on slopes modified by human activity that had pre-existing signs of instability.
Activities that contribute to slope instability include:
- lack of compaction during construction,
- inadequate ground surface preparation,
- large woody debris, and
- the incorporation of untreated acid-producing rock into embankments.
Rainfall intensity and duration is an important factor in triggering debris flows. From January to July in 2013, multiple above-normal rainfall events caused significant landslides in the Asheville area.
Six storm events triggered slope failures in western North Carolina between January and July 2013 during a long period of above-normal rainfall.
Landslide Hazard Maps
The landslide hazard mapping program is intended to provide the public, local government, and state emergency agencies with a planning tool indicating areas where landslides have occurred in the past and those areas vulnerable to future slope movements.
Data Description
The data produced by the landslide hazard mapping program are described in the maps below. Browse the data to learn landslide terminology and how they contribute to understanding landslide events in western North Carolina.
Note: The maps below are static. You can interactively explore the data in the WNC Landslide Hazard Data Viewer , which allows you to click on each element to learn additional information.
Landslide Points
Landslide points identify the initiation (source) of slope movements. Attributes of this layer include descriptions of slope movement type, location, dimensions, movement dates, and geomorphic, hydrologic, and other site data for individual slope movements, where known.
Landslide Outlines
Landslide outlines show the approximate areal extents of relatively recent, individual slope movements where their initiation (source) areas are known. Outlines were determined from field investigations, features visible in various vintages (1940–current) of aerial photography and orthophotography, and a variety of digital maps derived from 6-meter pixel resolution LiDAR digital elevation models.
Landslide Deposits
Landslide deposits represent the areal extents of significant volumes of earth, debris, and rock fragments that have accumulated primarily as a result of past debris flows and debris slides and, to a lesser extent, rock falls and rock slides. These deposits indicate areas that can be affected by future landslides, and can become unstable in some circumstances. Debris flow deposits mainly occur in valleys and can transition upslope into debris slide, rock fall, and rock slide deposits nearer steep source areas.
Potential Debris Flow Pathways
The Potential Debris Flow Pathways layer shows where landslides have gone in the past—and where they might go in the future—by delineating areas likely to be in the path of potential slope movements. Changes in the landscape as a result from human activity, future debris flows, and other types of landslides can alter the potential pathways of subsequent debris flows; therefore, this data represents general areas that could potentially be affected by future landslide impacts.
While areas outside of the green potential debris flow pathway areas are unlikely to be damaged by landslide activity, modification or alteration of slopes could result in slope movements outside the marked areas.
Probability of Sliding Map
The Probability of Sliding map is a susceptibility map indicating where landslides may initiate. The predicted relative hazard rankings (High and Moderate) indicate susceptibility of the initiation of naturally occurring, shallow, translational slope movements (for example, debris/earth flows and debris/earth slides) in response to a rain event of approximately 5–6 inches or more within a 24-hour period.
The data on this map does not predict that shallow translational slope movements will occur; rather, it forecasts, if they do occur, where they are more likely to initiate given the assumptions and input parameters used in the production of this data.
Learn More
Explore historical landslide events and more in StoryMaps, access digital copies of printed educational materials, and access tools and resources on the project website .
To explore the regional landslide data, visit the interactive WNC Landslide Hazard Data Viewer , which displays data for all counties in western North Carolina that have been mapped to date.
Acknowledgments
North Carolina Geological Survey
- Rick Wooten
- Corey Scheip
- Jesse Hill
- Tommy Douglas
- David Korte
UNC Asheville's NEMAC
- Karin Rogers
- Greg Dobson
- Jim Fox
- Nina Hall
- Ian Johnson
- Dave Michelson
- Kim Rhodes
- Mary Spivey
- Matthew Geiger
- Katie Caruso and Kelsey Hall (undergraduate interns)
