What is a slope failure?
Slope failures are significant natural hazards in many areas throughout the world. Generally, a slope failure can be defined as a downward movement of a large amount of slope material. For this reason, slope failures are also referred to as mass movements. Slope failures can occur suddenly in one easily recognized movement, or almost imperceptibly over a period of many years. A slope failure is classified based on how it moves and the type of material being moved:

• Creep: very slow movement of rock or soil downslope, often caused by repeated freezing and thawing of soil moisture.
  
  
• Falls: very rapid fall of rock and earth material from vertical or near vertical slopes. Rockfalls are a common example.
  
  
• Flows: slow to rapid movement of rock, soil, snow, or ice. Types of flows include mudflows, earthflows, debris flows, and snow avalanches. Flows typically have more mixing of material than other slope failures.
  
  
• Slides: slow to rapid movement of soil or rock. This category includes rock slides, earth slides, and slumps. Slides often occur with little mixing of material.
  
  
• Subsidence: slow to rapid collapse of rock or soil into underlying spaces. Sinkholes in karst landscapes are a common example.

Every year in the United States, slope failures cause approximately $2 billion in damage to highways, homes, and other property. Slope failures can be triggered by a variety of natural events including earthquakes, heavy rainfall from thunderstorms, volcanic eruptions, flooding, or freezing and thawing of soil moisture. Several studies have shown that almost any modification of a slope by people increases the risk of slope movement, especially in areas already susceptible to these natural hazards. Avalanches are one of the leading causes of winter fatalities from natural hazards in the western United States. A typical avalanche contains about 100,000 tons of snow and can bury people, cars, and buildings in seconds.

Where do slope failures occur?
Slope failures can be triggered by weather events, geologic events, human modification of the landscape, or most commonly, some interaction of all of the above. Therefore, slope failures occur nearly everywhere slopes exist. Mountainous regions, hilly regions, and coastlines have the greatest risk of slope failures. Also, locations in active tectonic regions are prone to slope failures triggered by earthquakes or volcanic activity. Sinkholes form in karst landscapes, where water has dissolved underlying bedrock, typically limestone or gypsum. The southeastern United States is particularly prone to sinkhole formation; over 50% of the state of Kentucky is subject to these hazards. Along the highways of the Rocky Mountains and coastal mountain ranges of California, rockfalls cause transportation delays, vehicle damage, and road damage every year.

When do slope failures occur?
Slope failures can occur in any season. They are more likely to occur in certain seasons if they are triggered by weather events such as rain, snow, or freezing and thawing of soil water. With the exception of slope failures triggered by geologic processes and avalanches, most slope failures in North America occur between spring and fall. In early spring, snow melt can increase pore pressures in the soil, increasing the risk of slope failures. During summer and fall, intense or prolonged rainfall can trigger slope failures. Freeze-thaw events, which usually happen during spring and fall but also during warm winters, can increase the potential for slope failure by expanding and contracting the water within the soil. In many locations, both geologic and atmospheric processes may play a role in the movement of a slope. In the western United States, wildland fire during the dry season sets the stage for future slope failures by burning vegetation that would normally stabilize the weak mountain soils and intercept rain from intense mountain thunderstorms.

How do we cope with slope failures?
How unstable a slope is depends on many environmental factors, so mitigation of slope failures can be complicated. Generally, vegetation can improve the stability of a slope because plant roots increase cohesion and plant leaves intercept rain drops that could otherwise decrease cohesion. Reducing rainfall runoff at the top of a slope and reinforcing the base of a slope can also improve stability. However, the mitigation of slope failures often needs to be prescribed on a case by case basis because every slope is unique. In areas where slope failures are a hazard, the best solution is not to place any structures on or near a slope that has been determined to be unstable. Avalanche mitigation in the western United States often involves controlled removal of excess snow on mountains through the use of explosives. In the Rocky Mountains and coastal mountain ranges of California, rockfall mitigation using metal nets to catch or deflect small rocks has successfully prevented vehicle and road damage in some areas.