Landslides, Mudslides, and Rockslides

Factors involved: Geography, geological forces, gravitational forces, human activity, ice, plants, rain, snow, temperature, weather conditions

Regions affected: All

Definition

“Landslide” is a general term referring to any perceptible mass movement of earth materials downslope in response to gravity. The deadly forms of landslides, such as debris avalanches and mudflows, can move at speeds in excess of 249 miles (400 kilometers) per hour and can bury entire cities. The death toll from a single event can be greater than 100,000. Landslides cause more deaths and cost more money each year than all other natural disasters combined.

Science

Mass movement is the proper term for any form of detachment and transport of soil and rock materials downslope. Some forms of mass movement have extremely slow velocities, less than 0.4 inch (1 centimeter) a year. Landslides include all forms of mass movement having speeds of greater than 0.04 inch (1 millimeter) a day.

Landslides can be divided into as many as fifteen different classes. The basis for the classification is the type of material that moves (for example, mud) and the general nature of the movement (for example, flow). The names of most of the individual classes are merely a combination of the two terms used in making the classification. For example, when very small particles called mud are saturated with water and flow down a slope like a liquid, the landslide is classified as a “mudflow.”

The types of materials that are involved in a mass-movement event are called debris, mud, rock, sand, and soil. These terms refer to the size of the particles that are moving. The word “soil” is used by earth scientists for particles that are less than 0.08 inch (2 millimeters) across. The word “mud” refers to the smaller pieces of soil, whereas “sand” indicates the larger-sized soil fragments. The term “rock” is used for particles that are greater than 0.08 inch (2 millimeters) across. The term “debris” is used when there is a mixture of soil and rock; however the rock sizes usually predominate in most debris.

Civil engineers, who build highways, bridges, dams, and other construction projects, have slightly modified the classification of materials. They consider “soil” any unconsolidated material, which they divide further into two classes, called “earth” when the particle size is small and “debris” when the particle size is large. The term “rock” is reserved for material that started as distinct, rigid, rock layers within the earth. Rock will usually break up into gravel-size particles during a mass movement.

The nature of the movement can be a “slide,” “flow,” “fall,” or one of a number of special terms in which a mixture of different movements occurs. There are several key characteristic movements associated with a slide, which physically resembles a child’s slide in a playground. The material usually moves as a single mass. The moving material is coherent; it does not break apart, nor do the individual fragments take differing contoured paths down the slope. Also, the base of the sliding material is usually a single, well-defined surface. A “translational slide” occurs when the surface at the base of the moving material is a flat plane having a uniform slope, which roughly corresponds to the slope on the land surface prior to the mass movement. A “rotational slide” occurs on a curved basal surface, where the upper part of the surface is steeper and the lower part is gentler, giving the surface a spoon shape.

The mass movement called a “flow” has a motion similar to that of a shallow mountain stream: the entire mass behaves as a fluid. The individual particles of moving material take contoured paths that diverge, converge, and collide with one another as they proceed down the slope. The basal surface beneath the flowing material is more undulating, having higher and lower elevations in different areas of the flow. In most cases flows have higher water content than slides; however, the fluid nature of a flow can also be generated by internally trapped air.

A “fall” occurs when material either free-falls down a cliff face or bounces down a very steep slope. A special movement called a “topple” happens when the material rotates around a fixed pivot axis near the base of the column before the fall occurs. The rotation may proceed slowly over a period of years, but this fall is the fastest of all types of movement.

Two special categories of motion are often associated with natural disasters. An “avalanche” is a special category of flow, in which a highly disaggregated material is fluidized by entrapped air and moves at very fast speeds. A “spread” is a vertical combination of a coherent upper layer that slides downslope on a lower, more fluid layer that flows. Spreads commonly occur during an earthquake, when the dry coherent material above the groundwater table laterally spreads out and sinks into the water-saturated flowing material below the water table.

Often, people are not present at the location of a landslide, and the nature of movement must be deduced from the deposits formed. The standard technique to distinguish a slide from a flow is to make a ratio of the depth (thickness) of the moving material divided by the length (or distance) the material moves down the slope. This ratio is called the depth-length ratio. Flows move greater distances down the slope even though they generally involve less thickness of flowing material. Flows, thus, have small values for the depth-length ratio, compared to slides, which are thick and move a short distance downslope.

Of the fifteen classes of landslides, which are defined by the type of material and the nature of movement, all can be disasters in terms of property loss, but less than half are life-threatening. The most common disaster is when debris moves by a rotational slide; this class is called a “slump.” A slump generally moves slowly, taking hours, days, months, or even years to complete its travel down the slope. The main block of material in a slump often breaks into a series of smaller blocks that appear as backward-tilted steps. A small, slow-moving earthflow typically develops at the toe of the slump. Few lives have been lost because of slumps, but when a slump develops in a city or town, every home in a section of several square blocks will have broken foundations and loss of vertical orientation of their walls and will probably need to be razed.

Mudflows and debris flows are the landslides that have generated the greatest death tolls. These events involve thick masses of mud or debris saturated with water and flowing with the consistency of wet cement. They can move at speeds of 31 miles (50 kilometers) per hour and faster. Normally, they develop after a long period of rainfall, which saturates slope materials and causes them to move. These flows also occur after sudden melting of frozen soils, often brought on by spring snowmelt. They are particularly numerous in years with heavy snowfalls and deep snowpack. As the snow melts, the water seeps into the subsurface of the slope, saturating the soil or rock mass and beginning the landslide. Mudflows are usually unexpected, and the slurry of mud and debris rushing down the slope can destroy homes, wash out roads and bridges, fell trees, sweep away cars, and obstruct roads and streams with a thick deposit of mud.

A special class of mudflow or debris flow called a “lahar” is produced when material from a volcanic eruption is ejected onto snowfields, glaciers, or crater lakes at the summit of a stratovolcano. An eruption of Nevado del Ruiz, an ice-capped Andean volcano in Colombia, in 1985 killed no one. However, the lahar produced by the melting glacier rushed 37 miles (60 kilometers) down the valley and killed 25,000 people in the city of Armero. Lahars can travel at speeds of 93 miles (150 kilometers) per hour, and when these thick deposits of mud come to rest they become as firm as concrete in a matter of a few hours.

Mudslides can be distinguished from mudflows by the coherence of the moving mass. One eyewitness in a mudslide reported that the ground became soft and he sank to his ankles, making walking difficult while he moved several hundred yards downslope on top of a mudslide. People unfortunate enough to be atop a mudflow would immediately sink into it and become part of the churning fluid.

The landslide categories of rockslides, rockfalls, and rock avalanches are also usually lethal. A vivid example is the Vaiont Dam Disaster, where a slab 1.2 miles (2 kilometers) wide by 1 mile (1.6 kilometers) long and 820 feet (250 meters) thick slid into the Vaiont Reservoir in Italy in 1963. The drop into the reservoir took less than one minute. The rockslide splashed a wave over the dam, producing a downstream flood that killed almost 3,000 people in a town 1.5 miles (2.5 kilometers) from the dam. The dam itself survived. A rock avalanche in 1962 in Peru moved 3.9 million cubic yards (3 million cubic meters) of mountain 12.4 miles (20 kilometers) down a valley in seven minutes. Observers said the landslide bounced from one side of the valley to the other at least five times before it spread out over a populated valley at the base of the mountain, killing 60 people. The same valley experienced another rock avalanche in 1970 as part of the Great Peruvian Earthquake. Also known as the Ancash earthquake and Yungay avalanche, it exacted a death toll of about 70,000.

The material of a rockslide differs from that of a rock avalanche in the amount of fracturing found in the rock. Rockslides involve crack development at a specific horizon where there is expansion within the rock mass. Cracks form within the rock over a relatively narrow zone; the fracturing does not penetrate the whole rock mass. Rock avalanches develop when the fracturing is continuous all the way down to the sliding surface. An avalanche involves independent movement of fragments in the entire mass above the sliding surface, as opposed to the rockslide, which involves a single direction of movement for the material above the layer of continuous cracks.

Rockfalls in mountainous regions are often controlled by an increase in temperature, causing a thaw. Rockfalls can be so continuous in mountains that spring climbing on some European peaks must be completed by 10 a.m. Several people are killed each year in the Rocky Mountain region of the United States because of rockfalls, usually motorists struck by bouncing rocks clearing the retaining wall.

All landslides are a form of slope failure. They happen when the shear stress within a slope exceeds the strength of the slope material. Then the slope fails, and millions of cubic feet of rock and soil materials can shear away from the slope and move hundreds or thousands of feet down the hill. There are a half dozen or more factors that affect slope stability. These forces can cause shear stresses to exceed the forces that hold the slope in place. The most significant factor promoting landslides is an increase in the angle of the slope: the steeper the slope, the more prone it is to landslides. The angle of the slope always increases directly above any region where construction has cut a relatively flat region into the hillside, such as a road, the leveling for a house foundation, or a quarry site. Fills for roads and waste from mines and quarries are often placed on slopes, making them steeper than the normal angle of rest. Slides will begin until the angle of rest (usually about 35 degrees for coarse material) is attained. The naturally steep walls of river gorges and glaciated valleys are therefore common sites for landslides.

Another common factor contributing to landslides is the addition of water to the area. Water lifts or pushes the grains apart in the soil or rock, reducing the internal friction of the soil and counteracting the gravitational forces that hold the slope in place. Much in the same way air pressure in a car’s tires lifts the car, high water pressure in the pores of rock or soil will lower the stability of the slope. This added water can come from heavy rains, melting snow, or even ponds and reservoirs. The area of Southern California is like a desert most of the year; however, it can receive heavy rains in later winter and early spring, which corresponds to the landslide season. Human influence has added water to the ground by construction of septic tanks, ponds, reservoirs, or irrigation canals. In one case in Los Angeles, a man went on vacation leaving his lawn sprinklers running, which caused an earthflow.

Landslides can also be caused by earth tremors. Earthquakes, volcanic eruptions, and even heavy machinery or trains passing on nearby roads or railroads have been known to induce tremors that start landslides. Most of the victims of the 1998 earthquake in Afghanistan were killed not by the earthquake itself but by landslides caused by the quake. In January 1994, the Northridge, California earthquake, triggered more than 11,000 landslides over an area of approximately 3,861 square miles (10,000 square kilometers). The largest measured rockslide had a volume in excess of 130,790 cubic yards (100,000 cubic meters). Dozens of homes were destroyed or damaged, roads were blocked, and an oil-field infrastructure sustained damage from the slide.

Another factor that promotes landslides is the removal of lateral or basal support from a slope. In nature, this occurs because of erosion by either meandering rivers or wave action on ocean cliffs. Every year numerous million-dollar homes are lost to earthfalls from wave erosion along the Pacific coastline.

Vegetation changes contribute to landslides in a variety of ways. In high mountain valleys the bedrock is wedged apart by roots of trees. In regions of rockslides the depressions created where small-scale movements have occurred are often the very sites where trees will take root and grow. On gentler slopes vegetation helps to anchor loose soil materials and prevent landslides. Wildfires have been responsible for promoting landslides by destroying tree cover; areas freshly clear-cut by the logging industry or cleared for housing developments have also been reported as sites of increased landslide activity.

The repeated freezing and thawing of water in cracks can be responsible for rockfalls. The process is called frost wedging, in which the expansion during freezing widens the crack and allows the water to penetrate deeper into the rock when the thaw occurs. Individual blocks can be wedged out of the cliff face, falling independently or causing such a loss of cohesion that larger portions of the cliff face can collapse.

Geography

Mass movement occurs in varying degrees almost everywhere. Huge landslides have been identified on the Moon, on Mars, and beneath the Atlantic Ocean on continental margins. A landslide discovered on Mars in 1978, was about 37 miles (60 kilometers) long and 31 miles (50 kilometers) wide.

The number of landslides increases in regions that have steep slopes, high precipitation, sizable fluctuations in seasonal temperatures, much clay in the soils, and frequent earthquakes and volcanic eruptions. Some countries that are among the hardest hit by landslides are Switzerland, Italy, Japan, China, Peru, and Colombia.

Landslides occur in every state of the United States. California, West Virginia, Utah, Kentucky, Tennessee, Ohio, and Washington have the most severe landslides. Many of the disastrous landslides in the United States have occurred in the West; these states are among the most arid, and the occurrence of landslides is strongly correlated with unusually heavy rainfalls or the melting of winter snowpack.

Once a landslide has occurred in a given area the chances of a repeat occurrence are very high. Governments spend a considerable amount of time and money attempting to identify geographic regions where landslides have occurred. Satellite images are used to identify large landslides by noting changes in soil and vegetation cover. Photographs taken from planes are used to record the extent of sliding land.

Prevention and Preparations

The standard method used to evaluate the potential of a landslide is the determination of the “factor of safety.” A numerical value is determined for every factor related to the occurrence of a landslide. The factor of safety is a ratio in which all the values that resist landsliding are divided by the sum of all the values that favor a landslide. The slope is considered stable when the factor of safety has a value that is greater than 1. Landslides are considered imminent when the value is less than 1.

Myriad techniques and equipment are used to assess the instability of a slope. Conventional surveying methods measure and record the development of cracks, subsidence, and uplift on slopes. Tiltmeters are used to record changes in the slope inclination near cracks and areas of weakness. Inclinometers and rock noise instruments are installed to record movements near cracks and ground deformations. Dating cracks and subsidence and upheavals of slope areas can help scientists assess the past changes in climate and denudation, along with rainfall and earthquake and volcanic activities, which can act as triggers for future slope failures.

Recording air-temperature thresholds forecasts the onset of landslides brought on by snowmelt. Research is demonstrating that 85 percent of landslide events occurs within two weeks after the first yearly occurrence of a six-day average temperature of 58 degrees Fahrenheit. This sort of forecasting can allow ample time to prepare persons in the area to evacuate. One of the safety problems of landslides is that they normally happen within seconds, pouring tons of material on homes and buildings in their path, not allowing the populace enough time to evacuate the area.

Trends from past measurement coupled with current monitoring of slopes increase the ability to predict future landslides. Monitoring rainfall and pore water pressure are other ways to try to predict potential landslides. However, predicting landslides is a very inexact science because some cracks can form on slopes and cause landslides within minutes of their formation. Other slopes have been known to sustain cracks, subsidence, or buckling for years and then fail suddenly, with little or no warning.

Local officials are turning more to the development of landslide hazard mapping. Each area is rated as to the potential for movement and assigned to one of six designations. Areas of similar designation are grouped together as regions on a map. Local legislation places restrictions on and develops greater monitoring of the areas having the highest hazard rankings. In San Mateo County in California the hazard maps are used to restrict the number of homes that may be built there. The normal density allowed is one home per 5 acres, whereas high-hazard areas are restricted to one home per 40 acres.

People living in landslide areas need to note common warning signs of potential slope failure. Some signs of landslides are doors or windows sticking or jamming for the first time on a home. New cracks appearing in plaster, tile, brick, or the foundation of houses can be a precursor of earth movement. Widening cracks on paved streets or driveways also indicate movements in landslide areas. Sometimes underground utility lines will begin to break as result of earth movement. Water will sometimes break through the ground in new locations, and fences, retaining walls, utility poles, and trees will tilt more. A faint rumbling sound, increasing in volume, can be heard as the landslide nears. If any of these warning signs are experienced, evacuation plans should be made. It is recommended that there be at least two planned evacuation routes, because roads may become inaccessible from deposit of slide materials.

Japan spends approximately $4 billion annually to try to control mud- and debris flows. The Japanese government has built sabo dams along the river systems in urban areas to trap mud and rock that slide down the mountains. In the United States an American version of these dams is found in Los Angeles County, where there is a system of temporary fortifications to protect areas such as Pasadena and Glendale from debris flows that originate in the San Gabriel Mountains and canyons after hard rains.

The best form of landslide prevention is to not build on areas where landslides have occurred, at the base of slopes, at the base of minor drainage hollows, at the base or top of old fill slopes, or on hillside developments where leach-field septic systems are used. Unfortunately, landslide, rockslide, and mudslide areas are very scenic and are known to entice people to build houses. The West Coast, one of the most slide-prone areas in the world, is a prime example of an area that attracts building in spite of the dangers of landslides.

Rescue and Relief Efforts

A variety of agencies are usually dispatched to the scenes of disastrous landslides. Search and rescue teams are trained in recovery techniques that are appropriate for landslides, such as rescue dogs and proper digging methods. The dangers that are associated with disease, hunger, and lack of water and shelter are handled by entities such as state governments, the Federal Emergency Management Agency (FEMA), and the American Red Cross. The National Landslide Hazards Program, within the United States Geological Survey, responds to emergencies and disasters to provide information on the continuing potential for movement while rescue efforts are taking place.

The National Flood Insurance Program was amended in 1969 to include payment for damage incurred by mudslides caused by flooding. Most homeowners’ insurance policies do not cover damage caused by landslides. Federal assistance is available for areas declared a national emergency.

Impact

The United States Geological Survey reported that more people died from landslides in the last three months of 1985 than were killed during the previous twenty years by all other geological hazards (such as earthquakes and volcanic eruptions). In terms of property damage, landslides have cost Americans three times the combined costs from all other natural disasters, including hurricanes, tornadoes, and floods. The average annual statistics for the United States report 25–50 people killed and $1.5 billion in damage.

Landslides are a major worldwide hazard. Thousands of people are killed each year across the world in landslides. A region of southern Italy experienced a series of landslides in 1973, causing 100 villages to be abandoned and 200,000 people to be displaced. A single mudflow event in the Gansu Province of China in 1920 is thought to have been the deadliest landslide, with an estimated 200,000 people killed. Property damage from landslides worldwide is estimated to be in the tens of billions of dollars.

Historical Overview

Historically, the deadliest landslides have occurred in the mountainous regions of Asia, Europe, and the Americas. While landslides are also frequently experienced in Africa and Australia, the quantity of slides and the resultant loss of life in those regions do not compare to those in other parts of the world. Most landslides occur in hilly or mountainous regions where sloping conditions make such activity more likely, but they can happen almost anywhere.

In the United States, landslides and rockslides have occurred most frequently in the Rocky Mountain region and along the Pacific coast. Utah, Colorado, California, and Washington have been the most susceptible to landslide disasters. West Virginia holds that distinction on the U.S. East Coast, primarily as a result of slope instability caused by mining and the debris and waste that it creates. The world's largest historic landslide occurred in Washington state in 1980 when the Mount St. Helens erupted, causing a landslide measuring 2.8 cubic kilometers in volume. Alberta, British Columbia, and Quebec are considered the most landslide-prone provinces of Canada.

The largest and most devastating landslides have been caused by earthquakes. Most landslides occur with little or no warning, often in tandem with seismic activity. In one of the worst slides in recorded history, a 1920 earthquake in Gansu Province, China, sheared off unstable cliffs, destroying 10 cities and killing 200,000.

Human activity has also been a major contributor to the death toll caused by landslides. Ground that is normally stable may slide after human activity alters its natural state. Many deadly landslides have occurred when development altered slope and groundwater conditions. In Virginia, a state not considered a prime site for landslide activity, 8 people were killed in 1942 when a coal waste heap slid into a river valley near the city of Oakwood. The worst landslide in the history of Wales occurred in 1966, when a human-made slag heap outside of Aberfan shifted, sending 2 million tons of rock, coal, and mud downhill into the city and killing 147 people, most of them children. Deforestation was a major factor in the 2006 Leyte mudslide that buried villages in the Philippines; more than 2,000 people were missing or confirmed dead.

Scientists were long unaware of the potential for destruction from underwater landslides. A scientific team that visited the site of a 1998 tsunami in Papua New Guinea later concluded that the deadly waves were probably caused by an underwater landslide set in motion by a small earthquake. This theory forced many scientists to seriously consider the possibility of a connection between landslides and tsunamis.

Unlike many other natural disasters, landslides often have a long-lasting effect on the physical environment. Landslides have collapsed mountains, sent rivers on new and destructive courses, and created huge lakes that inundated populated fertile valleys. A 1925 landslide sent some 50,000 cubic yards of debris into the Gros Ventre River of Wyoming, creating a natural dam 350 feet high. A 3-mile-long lake formed behind the dam. It is not unusual for a landslide to permanently displace animals and humans.

Property damage from landslides is a common occurrence throughout the world, resulting annually in billions of dollars in property damage. A variety of methods are now employed throughout the world to prevent landslides. One way of avoiding catastrophe is diversion and drainage of water before it reaches potential problem areas. Building contractors consider the potential for landslide damage to buildings and other structures prior to excavation and construction. The disposal of construction, logging, and mining waste is closely monitored by many governments in efforts to avoid potential slide disasters.

Bibliography

Bloom, Arthur L. Geomorphology: A Systematic Analysis of Late Cenozoic Landforms. 3d ed. Upper Saddle River, N.J.: Prentice Hall, 1998. Chapter 9, entitled “Mass Wasting and Hillslopes,” provides a low-level technical discussion of factors contributing to landslides.

Bryant, Edward A. Natural Hazards. 2d ed. Cambridge, England: Cambridge University Press, 2005. A nontechnical book that cites nearly twenty additional readable references on land instability.

Cooke, R. U., and J. C. Doornkamp. Geomorphology in Environmental Management. Oxford, England: Clarendon Press, 1990. This book provides details on hazard assessment and risk calculations. It gives detailed examples from many countries, including the United States.

Easterbrook, Don J. Surface Processes and Landforms. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 1999. This college textbook is quite good for a general audience. It provides excellent descriptions, pictures, and accounts of over ten classes of landslides.

Erickson, Jon. Quakes, Eruptions, and Other Geologic Cataclysms: Revealing the Earth’s Hazards. Rev. ed. New York: Facts On File, 2001. One of the books in the series entitled The Living Earth. Contains a chapter on earth movements that provides a descriptive treatment of landslides.

“Landslide Hazards.” USGS, US Department of the Interior, www.usgs.gov/natural-hazards/landslide-hazards. Accessed 5 Feb. 2019. The United States Geological Survey page on Landslide Hazards includes information about the National Landslide Hazards Program.

Plummer, Charles C., David McGeary, and Diane H. Carlson. Physical Geology. 11th ed. Boston: McGraw-Hill Higher Education, 2007. A superb introductory textbook. A chapter is devoted to mass wasting and landslides, including descriptions of common forms of landslides and a section on prevention.

Ritter, Dale F., R. Craig Kochel, and Jerry R. Miller. Process Geomorphology. 4th ed. Dubuque, Iowa: Wm. C. Brown, 2002. This book provides the technical details of how to evaluate all factors involved in the calculation of the factor of safety. Requires a good background in mathematics, including trigonometry and vectors.