Landslides
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land·slide
the usually rapid downward movement of a mass of rock, earth, or artificial fill on a slope; also: the mass that moves down
Landslides, also known as mudslides or debris flow, can occur in every state. They can be caused by a variety of factors including earthquakes, storms and fires. They
often occur quickly and with little notice. The best way to prepare is to stay informed about changes in and around your home that could signal that a landslide is likely to occur.
The term "landslide" describes a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these. This can happen by the materials falling, toppling, sliding, spreading, or flowing downward. Landslides can be classified based on the type of material involved, mode of movement and, rate of
movement.
Where do Landslides Occur?
Landslides are primarily associated with mountainous regions but they can also occur in areas of generally low relief. In low-relief areas, landslides occur as cut-and-fill failures (roadway and building excavations), river bluff failures, lateral spreading landslides, collapse of mine-waste piles (especially coal), and a wide variety of slope failures associated with quarries and open-pit mines.
Types of Landslides Movements
- Falls - Falls are abrupt movements of masses of geologic materials, such as rocks and boulders, that become detached from steep slopes or cliffs. Separation occurs along discontinuities such as fractures, joints, and bedding planes, and movement occurs by free-fall, bouncing, and rolling. Falls are strongly influenced by gravity, mechanical weathering, and the presence of interstitial water.
- Topples - Toppling failures are distinguished by the forward rotation of a unit or units about some pivotal point, below or low in the unit, under the actions of gravity and forces exerted by adjacent units or by fluids in cracks.
- Flows - There are five basic categories of flows that differ from one another in fundamental ways.
- Debris flow - A debris flow is a form of rapid mass movement in which a combination of loose soil, rock, organic matter, air, and water mobilize as a slurry that flows downslope. Debris flows are usually caused by intense surface-water flow, due to heavy precipitation or rapid snowmelt, that erodes and mobilizes loose soil or rock on steep slopes.
- Debris avalanche - This is a variety of very rapid to extremely rapid debris flow.
- Earthflow - Earthflows have a characteristic "hourglass" shape. The slope material liquefies and runs out, forming a bowl or depression at the head. The flow itself is elongate and usually occurs in fine-grained materials or clay-bearing rocks on moderate slopes and under saturated conditions. However, dry flows of granular material are also possible.
- Mudflow - A mudflow is an earthflow consisting of material that is wet enough to flow rapidly. These are sometimes referred to in the media as "mudslides."
- Creep - This is a slow, steady, downward movement of slope-forming soil or rock. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges.
- Lateral spreads - Lateral spreads are distinctive because they usually occur on very gentle slopes or flat terrain. The dominant mode of movement is lateral extension accompanied by shear or tensile fractures. Failure is usually triggered by rapid ground motion, such as that experienced during an earthquake, but can also be artificially induced.
What Causes a Landslide?
Landslides and Water
Slope saturation by water is a primary cause of landslides. This can occur in the form of intense rainfall, snowmelt, change in ground-water levels and water-level changes along the coastline. Landsliding and flooding are often related due to precipitation, runoff, and the saturation of ground by water. Landslides can actually cause flooding by forming a landslide dam that allow large amounts of ware to back up and if not corrected, can cause overtopping.
Landslides and Seismic Activity
Mountainous areas are vulnerable to landslides if they have also experienced earthquakes. The occurrence of earthquakes in steep landslide-prone areas greatly increases the likelihood that landslides will occur. The 1964 Great Alaska Earthquake caused widespread landsliding and other ground failure and other areas of the United States, such as California and the Puget Sound region in Washington, have experienced slides, lateral spreading, and other types of ground failure due to moderate to large earthquakes. Widespread rockfalls also are caused by loosening of rocks as a result of ground shaking. Worldwide, landslides caused by earthquakes kill people and damage structures at higher rates than in the United States.
Landslides and Volcanic Activity
Landslides that occur due to volcanic activity are some of the most devastating types. Volcanic lava melt snow at a rapid rate thus causing a deluge of rock, soil, ash and water that accelerates rapidly on the steep slope of Volcanoes. It can be devastating to anything in it's path. This type of debris, know as lahars, can reach great distances and can damage structures in flat areas surrounding volcanoes. The 1980 eruption of Mount St. Helens, in Washington triggered a massive landslide on the north flank of the volcano, the largest landslide in recorded times.