Types of Landslides
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.
The materials may move by falling, toppling, sliding, spreading, or
flowing. The drawing
below is a graphic illustration of a landslide, with the commonly
accepted terminology describing its features.
Although many types of mass movements are included in the general
term "landslide," the more restrictive use of the term refers only to
mass movements, where there is a distinct zone of weakness that
separates the slide material from more stable underlying material.
The two major types of slides are rotational slides and translational
slides. Slide types, images and descriptions at right.
Flows
There are five basic categories of flows that differ from one another
in fundamental ways.
Flow types, images and descriptions at right. Although there are
multiple types of causes of landslides, the three that cause most of the
damaging landslides around the world are these:
Landslides and Water Slope saturation by water is a primary cause of
landslides. This effect can
occur in the form of intense rainfall, snowmelt, changes in
ground-water levels, and waterlevel changes along coastlines, earth
dams, and the banks of lakes, reservoirs, canals, and rivers.
Landsliding and flooding are closely allied because both are related
to precipitation, runoff, and the saturation of ground by water.
In addition, debris flows and mudflows usually occur in small, steep
stream channels and often are mistaken for floods; in fact, these two
events often occur simultaneously in the same area.
Landslides can cause flooding by forming landslide dams that block
valleys and stream channels, allowing large amounts of water to back up.
This causes backwater flooding and, if the dam fails, subsequent
downstream flooding. Also, solid landslide debris can "bulk" or add
volume and density to otherwise normal streamflow or cause channel
blockages and diversions creating flood conditions or localized erosion.
Landslides can also cause overtopping of reservoirs and/or reduced
capacity of reservoirs to store water.
Landslides and Seismic Activity
Many mountainous areas that are vulnerable to landslides have also
experienced at least moderate rates of earthquake occurrence in recorded
times.
The occurrence of earthquakes in steep landslide-prone areas greatly
increases the likelihood that landslides will occur, due to ground
shaking alone or shaking- caused dilation of soil materials, which
allows rapid infiltration of water.
The 1964 Great Alaska Earthquake caused widespread landsliding and
other ground failure, which caused most of the monetary loss due to the
earthquake. 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 due to volcanic activity are some of the most devastating
types. Volcanic lava may melt snow at a rapid rate, causing a deluge of
rock, soil, ash, and water that accelerates rapidly on the steep slopes
of volcanoes, devastating anything in its path.
These volcanic debris flows (also known as lahars) reach great
distances, once they leave the flanks of the volcano and can damage
structures in flat areas surrounding the 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.
Landslide Mitigation
How to Reduce the Effects of Landslides Vulnerability to landslide
hazards is a function of location, type of human activity, use, and
frequency of landslide events. The effects of landslides on people and
structures can be lessened by total avoidance of landslide hazard areas
or by restricting, prohibiting, or imposing conditions on hazard-zone
activity. Local governments can reduce landslide effects through
land-use policies and regulations. Individuals can reduce their exposure
to hazards by educating themselves on the past hazard history of a site
and by making inquiries to planning and engineering departments of local
governments. They can also obtain the professional services of an
engineering geologist, a geotechnical engineer, or a civil engineer, who
can properly evaluate the hazard potential of a site, built or unbuilt.
The hazard from landslides can be reduced by avoiding construction on
steep slopes and existing landslides, or by stabilizing the slopes.
Stability increases when ground water is prevented from rising in the
landslide mass by
(1) covering the landslide with an impermeable membrane, (2)
directing surfacewater away from the landslide,
(3) draining ground water away from the landslide, and
(4) minimizing surface irrigation. Slope stability is also increased
when a retaining structure and/ or the weight of a soil/rock berm are
placed at the toe of the landslide or when mass is removed from the top
of the slope. |