Landslides

A landslide is the movement of any mass of rock, soil, or debris down a slope.  This process, known as mass movement, is driven by gravity and may occur instantaneously or quite imperceptibly.  Landslides are usually triggered by heavy rainfall, rapid snow melt, stream incision, or earthquakes, while certain man-made changes to the land, such as slope modification or drainage alteration, can greatly increase the likelihood of landslides.  Landslides are capable of wrecking buildings, rupturing gas, water, and sewer mains, and knocking out power and telephone lines while blocking transportation routes.  Soil creep and slumping cause property damage gradually, whereas rock slides and debris flows can sweep away people and property instantaneously.  In the United States, landslides annually cause up to $2 billion in damages and take between twenty-five and fifty lives. 

Landslides occur in many manifestations and are usually classified according to the type of material involved and the mode of downslope movement.  The material can range from loose earth to blocks of solid rock to tree trunks.  Modes of transport include freefall, bouncing, sliding down a slip surface, flowing in a slurry, or moving particle-by-particle.  The following are some of the more important types of mass movement:

• Rockfalls entail large blocks of bedrock breaking off a cliff face and tumbling downslope.
• Rockslides occur when a detached section of bedrock moves down an inclined surface, frequently along a bedding plane.
• Earthslides involve masses of soil moving down a slip face, usually the top of the bedrock. 
• Creep is the slow, continuous, imperceptible downslope movement of soil and rock particles.
• Slumps result from the rotation of a cohesive unit of soil or rock down a slip surface, leaving a curved scarp.
• Debris flows develop on steep slopes as a result of heavy rainfall or rapid snow melt that saturates the soil, which under the extra weight and lubrication breaks loose and becomes a slurry that takes everything with it, including large trees and houses. Channeled debris flows can reach speeds approaching a hundred miles an hour and strike without warning.
• Avalanche is a term reserved for landslides composed of snow and ice.

Landslides are Virginia's most widespread geologic hazard.  The most disastrous landslide events are associated with heavy rainfall along the steep slopes of the Blue Ridge Mountains and the Appalachians, but slumping, sliding, and creep can occur even on fairly gentle slopes if local conditions exceed the natural stability of the site.  Areas that are prone to mass movement include areas of previous landslides; the bases of steep slopes, particularly slopes burned by forest and brush fires; the margins of drainages; and developed hillsides, particularly where septic systems are used.  Research in North Carolina has revealed that about fifty-six percent of recent landslides happened on slopes that had been altered in some way by development.  Click here for indicators of an unstable slope.

Aerial view of debris flow chutes and flood deposits from the June 27, 1995 flood at Kinsey Run, near Graves Mill in Madison County, Virginia. Photo from The debris flows of Madison County, Virginia:  34th Annual Virginia Geological Field Conference Guidebook.

Significant rain events occur on a regular basis in Virginia.  On June 27, 1995, a storm over Greene and Madison counties triggered more than a thousand debris flows.  Hurricane Isabel (September of 2003) and Tropical Depression Gaston (August 2004) wreaked havoc in the Richmond area, leaving hundreds, if not thousands of landslides in their wake, and causing millions of dollars in property damage.  Virginia’s deadliest natural disaster occurred on the night of August 19, 1969, when remnants of Hurricane Camille dumped over twenty-five inches of rain during a five-hour period along the Blue Ridge in central Virginia, triggering hundreds of debris flows that killed 153 people.  Countless buildings were destroyed and more than a hundred bridges were swept away in parts of Nelson, Amherst, and Rockbridge counties.

The potential for landslides in most of the counties in western Virginia is high.  As shown in the map below, adapted from USGS Landslide Overview Map of the Conterminous United States, at least half of the Commonwealth falls into zones of high potential.

Counties in Virginia that are susceptible to landslides.  Red = high potential; orange = moderate potential; yellow = moderate to low potential; green = low potential. 

The Virginia Department of Emergency Management (VDEM) provides basic information about landslides in the Commonwealth, and sponsors regional mitigation plans outlined in the Federal Disaster Mitigation Act of 2000.  Unfortunately, these plans are incomplete.  The problem is that there is no simple, universal methodology for determining vulnerability to landslides, and the available data provides only the broadest indications of areas that could expect a landslide.  What are lacking are detailed landslide hazard maps for VDEM and other state agencies to use in planning and mitigation. 

Homeowners insurance does not cover landslide damage.  Anyone who intends to buy or build on sloping land should be wary of potential problems.  Effective, on-site geological analysis is necessary to identify former landslide deposits and existing planes of weakness in bedrock, such as fractures, foliation, and bedding surfaces.  Although the cause of most landslides cannot be mitigated, detailed geologic investigations and effective land-use practices can greatly reduce the risk. Click here to learn more about what to do before, during, and after a landslide.

References:

Carter, M. W., and Berquist, C. R., Jr., 2005, A preliminary investigation of the Chimborazo Hill landslide of August 30, 2004:  Virginia Minerals, v. 48, n. 4, p. 25-36.

Cruden, D. M., 1991, A simple definition of a landslide:  Bulletin of the International Association of Engineering Geology, n. 43, p. 27-29.

Eaton, L. S., Bailey, C. M., and Gilmer, A. K., 2004, The debris flows of Madison County, Virginia:  34th Annual Virginia Geological Field Conference Guidebook, 27 p.

Hack, J. T., 1956, Erosion by catastrophic floods in the Ridge and Valley province, Virginia:  Virginia Journal of Science, v. 7, n. 4.

North Carolina Geological Survey, 2005, When the Earth Moves (Bechtel, R., ed.): North Carolina Geological Survey Information Circular 32, 24 p.

Radbruch-Hall, D. H., R. B. Colton, W. E. Davies, Ivo Lucchitta, B. A. Skipp, and D. J. Varnes, 1982, Landslide Overview Map of the Conterminous United States, Geological Survey Professional Paper 1183, U.S. Geological Survey, Washington.

Ress, D., 2004, City damage may top $15 million:  Richmond Times-Dispatch, September 1st, Section B, p. 1. 

Ritter, D. F., 1986, Process Geomorphology:  Wm. C. Brown Publishers, Dubuque, Iowa, 579 p.

Varnes, D. J., 1978, Slope movement types and processes, in Schuster, R. L. and Krizak, R. J., eds., Landslide Analysis and Control: Transportation Research Board Special Report No. 176, National Academy of Sciences, Washington, D.C., p. 11-33.

Virginia Division of Mineral Resources, 1969, Natural features caused by a catastrophic storm in Nelson and Amherst counties, Virginia:  Virginia Division of Mineral Resources Virginia Minerals, Special Issue, 20 p.

Web sites:

USGS Landslide Handbook - A Guide to Understanding Landslides
http://pubs.usgs.gov/circ/1325/

USGS landslide hazards program
http://landslides.usgs.gov/

USGS landslide hazard map
http://landslides.usgs.gov/learning/nationalmap/

USGS site of debris flow hazards in the Appalachians
http://pubs.usgs.gov/fs/2008/3070/fs2008-3070.pdf

USGS site on debris flow hazards in Virginia’s Blue Ridge
http://landslides.usgs.gov/docs/faq/fs159-96.pdf  

Details of Hurricane Camille
http://sciencepolicy.colorado.edu/about_us/meet_us/roger_pielke/camille/report.html 

Online report on the Madison landslides in 2004
http://web.wm.edu/geology/virginia/madison/9-27-1995.html

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