Landslides

Introduction

A landslide or debris flow is a downhill movement of a significant amount of rock and soil. Mudslides are a type of landslide that occurs when the loose material is mixed with a large amount of water. A mudslide can move quickly and travel miles along a valley or other channel, causing many fatalities: A single mudslide in Venezuela in 1999 killed approximately 20,000 people. In the United States, landslides kill 5-50 people every year and do more than $2 billion of damage. As of 2008, the greatest loss of life in a single landslide in U.S. history was in Puerto Rico in 1985, when a landslide killed 129 people. A single 1983 landslide in Thistle, Utah, was the most expensive single landslide in U.S. history, causing over $400 million in direct and indirect costs.

Historical Background and Scientific Foundations

Landslides occur on Earth and on many other bodies in the solar system, including Mars, Jupiter’s moon Io, and various asteroids. They occur whenever a steeply sloping mass of loose material—sand, rock, soil, or any combination of these, often (on Earth) mixed with water—is set into motion. This often occurs when intense rainfall increases the density and mobility of the mass and it begins to flow. Motion may also be triggered by an earthquake, volcanic eruption, or artificial explosion. A single heavy storm or strong earthquake can trigger thousands of landslides over a large area. Landscapes are an important contributor to the evolution and erosion of mountainous landscapes.

Landslides also occur underwater along the edges of continental shelves and along the steep slopes of some islands. A large submarine landslide, as an underwater landslide is called, may trigger a landslide tsunami. Although not covering as large an area of coast as a typical large earthquake tsunami, a landslide tsunami can be deadly: In July, 1998, a 50-ft (15-m) high, 6-mi (20-km) long landslide tsunami struck the coast of Papua New Guinea, a large island in the western Pacific, killing over 2,000 people.

Once set in motion, a landslide or debris flow can move as slowly as a few inches/centimeters a year or accelerate to speeds of up to 35 mph (56 km/h) and travel great distances along valleys or over level ground. The areas at greatest danger from landslides are the bottoms of canyons, stream channels, and areas where such features empty out. Most often, a landslide begins at the top of a small gully (small, eroded cut in the land); once a small amount of material begins to move, it entrains other material, and the landslide grows until all the material that is free to move is doing so. Slopes that have been altered for roads or building construction or that have been deforested or burned over by wildfires are more likely to give way than undisturbed slopes or slopes that are stabilized by vegetation. In the United States, many landslides in the western part of the country occur in the fall and winter, when rains come after the summer fire season.

According to the U.S. Geological Survey, all 50 U.S. states experience landslides, with 36 states having moderate to highly severe landslide hazards. Most landslide damage occurs in the Pacific Coast region (including Alaska and Hawaii), the Rocky Mountains, the Appalachian Mountains, and Puerto Rico. Seismically active areas are most at risk, because earthquakes are effective triggers for landslides. For example, several thousand landslides were triggered in central California by the 1980 Mammoth Lakes earthquake sequence.

WORDS TO KNOW

DEFORESTATION: A reduction in the area of a forest resulting from human activity.

EROSION: The wearing away of the soil or rock over time.

Impacts and Issues

Globally, landslides are one of the most common natural disasters, causing thousands of deaths and billions of dollars worth of damage every year. Direct impacts include property destruction (no structure can withstand a large landslide) and death; landslides cause death more often than injury, because if a person is caught up in or buried under a debris flow at all, their chances of survival are usually small.

Landslides also cause indirect impacts by destroying facilities that people depend on for well-being, what the World Health Organization calls “lifeline systems.” These include roads, communication lines, pipelines for fuel and water, electrical transmission lines, and healthcare facilities. By destroying livestock and crops, landslides in Asia and South America have also increased the vulnerability of rural sustenance farmers in developing countries to food insecurity, also making them more vul-

nerable to other disasters. Indirect costs include rescue, cleanup, tourism losses, and lost productivity.

Life and property losses from landslides are increasing around the world, including the United States, for two reasons. First, more people are in harm’s way, as increasing populations settle locations vulnerable to landslide. Second, increasingly frequent intense-rainfall events in many parts of the world, possibly a result of global climate change, are increasing the frequency of landslides, especially in deforested parts of Asia, Latin America, Australia, and New Zealand.

Predicting landslides would save many lives and greatly reduce property damage. Despite strong motivation for the development of predictive techniques, the science of landslide prediction remains crude. Site-specific monitoring systems similar to those used to detect the early-warning signs of volcanic eruptions can be installed to measure water pressure, subsurface slippage, and subtle movement of the surfaces. However, such systems are expensive. Regional-scale landslide prediction relies on satellite data to monitor soil moisture in landslide-prone areas. Based on such information, landslide warnings may be issued. A landslide warning system was under development by the U.S. National Aeronautics and Space Administration (NASA) in the early 2000s, but had not yet been activated as of mid-2008.

See Also Earthquakes; Floods

BIBLIOGRAPHY

Books

Cornforth, Derek. Landslides in Practice: Investigation, Analysis, and Remedial/Preventative Options in Soils. New York: Wiley, 2005.

Periodicals

Keefer, David K., and Matthew C. Larsen. “Assessing Landslide Hazards.” Science 316 (2007): 1136–1138.

Stone, Richard. “Too Late, Earth Scans Reveal the Power of a Killer Landslide.” Science 311 (2006): 1844–1845.

Web Sites

Centers for Disease Control and Prevention (CDC). “Landslides and Mudslides.” http://www.bt.cdc.gov/disasters/landslides.asp (accessed May 10, 2008)

Federal Emergency Management Administration (FEMA). “Landslide and Debris Flow.” http://www.fema.gov/hazard/landslide/index.shtm (accessed May 10, 2008).

National Geographic. “Satellites Can Warn of Floods, Landslides Worldwide, Scientists Say.” May 25, 2006. http://news.nationalgeographic.com/news/2006/05/060525-flood-warn.html (accessed May 10, 2008).

U.S. Geological Survey. “Landslides Hazard Program.” http://landslides.usgs.gov/ (accessed May 10, 2008).