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Landslides
Hazards, Federal Emergency Management Agency (FEMA),
U.S. Department of Homeland Security
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Landslides and Debris Flows (Mudslide)
Natural Hazards, Are You Ready?
Federal Emergency Management Agency (FEMA),
U.S. Department of Homeland Security
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Mass-Wasting and Mass-Wasting Processes
Prof. Stephen A. Nelson,
lecture notes,
Natural Disasters,
Earth & Environmental Sciences 204 (EENS 204),
Department of Earth & Environmental Sciences,
Tulane University
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Modeling the Interaction of Landslides, Debris Flows,
and the Channel Network
Stephen T. Lancaster, Ph.D., & Gordon E. Grant,
Department of Geosciences, Oregon State University
(OSU) &
USDA Forest Service,
U.S. Department of Agriculture (USDA)
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THE SCIENCE OF SLIDES: A Primer on
How Debris Flows Work
Jen Shaffer,
Logging and Landslides: A Clear-Cut Controversy,
Wildfire News
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Some Definitions and Features of Landslides
Mountain Risks and Hazards,
International Centre for Integrated Mountain Development (ICIMOD)
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USGS Landslide Hazards
U.S. Geological Survey (USGS), U.S. Department of the Interior
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Definitions: A landslide is “the
perceptible downward sliding or falling of a relatively dry mass of earth,
rock, or mixture of the two.” [Dr. C.F. Stewart Sharpe,
1938 or 1939]
A landslide is “a rapid displacement of a mass of rock,
residual soil, or sediments adjoining a slope, in which the center of
gravity of the moving mass advances in a downward and outward
direction.” [Karl von Terzaghi, “Mechanism of
Landslides”, 1950]
There appears to be a great deal of inconsistency in application of the term
landslide, even among technical sources. The generic
usage of the term by the public is more accurately defined as mass
movement; however, that term encompasses far more types of ground
displacement than those attributed to landslides alone. The collection of
all types of soil and rock displacement from higher to lower elevation due
to the effect of gravity is referred to as mass wasting.
(Professor Stephen
A. Nelson at Tulane University has provided the best descriptions of
mass wasting processes we have found to date in
his EENS 204 course notes.)
A landslide begins when the cohesive (frictional) forces binding the material
in a portion of a slope are overcome by the force of gravity; the
affected portion of the slope then begins to slip, separating from the
parent portion and sliding downhill. Landslides are most often initiated
by infiltration of water into a layer of soil or between layers of rock
of differing composition (bedding planes). The water acts as a
lubricant, reducing cohesion sufficiently to allow gravity to prevail.
The process may be gradual, on the order of hours, days or years
(creep), or it may be catastrophic, occurring within a matter of
seconds. Vibration is another initiator of landslides; earthquakes
frequently result in catastrophic landslide events.
Landslides can best be segregated into slope failures and
sediment flows. A slope failure may further be characterized as
a fall, slide or slump.
Falls, the most rapid type of landslide, originate on cliffs or
steep slopes and drop vertically or at a sharp angle. Falls can range
in size from a small stream of dirt or pebbles to the sudden shearing of a
massive section of rock face. The impact of a large rockfall can generate
a shock wave capable of felling trees and structures.
A slide is a mass movement of soil or rock that occurs as a coherent
unit by slipping along one or more failure surfaces. A slide exhibits
an orderly pattern of movement without a rotational component. Slides are
sometimes referred to as translational slides to distinguish them
from rotational slides, or slumps.
A slump is a slide having a downward rotational component along a
concave shear surface such that the horizontal movement at the base of
the slide zone is greater than that at the top. Slumps have a tendency
to come to rest with both the upper surface and the greater mass of
dislocated material left relatively undisturbed in what is known as a
slump block.
A sediment flow is a mass movement whose internal structure has
become disaggregate, chaotic and turbulent. The rock and/or soil
involved in a sediment flow is mixed with water or air which imparts a
lubricating effect to the flow. Sediment flows are further categorized as
granular flows if they contain less than about 20 percent water
and slurry flows if they contain between 20 and 40 percent water.
Flow containing more than 40 percent water grades into stream flow. Such flow
in an area where no water is usually present, or a sudden stream flow
significantly higher than normal flood stage, even if carrying sediment,
might best be characterized as a flash flood. For practical
reasons, these flows may still be described as slurry flows of specific
type (e.g., mudflow, lahar), especially if they deposit considerable
quantities of sediment. (See
Lahar, Mud & Quicksand
Hazards for more information on slurry flows.)
Both granular flows and slurry flows can entrain debris as they progress
down a slope or channel; if sufficient foreign material is accumulated,
the flow may be termed a debris flow. Debris can be
defined both as “material differing substantially in nature from the
main components of flow” and as “material that is not a part
of the original components of flow” [author’s definitions].
Runout is the horizontal travel distance achieved by a landslide.
Equations have been derived to predict landslide runout based upon
parameters such as slope angle, depth to slip surface and height at which
the slide is initiated. Both empirical and mathematical models have been
used to predict landslide runout.
Unexpected behavior has been observed in certain large sediment flows,
which possessed runouts significantly greater than modeling had
predicted. Some of these flows were shown to have run uphill for
considerable distances. Furthermore, speeds of some sediment flows appear
to be greater than expected, reaching in excess of 170 mph. New
theories of landslide motion and interaction based on landslide computer
modeling have been proposed to account for these phenomena. Some
landslide computer models suggest that large sediment flows, while
being internally chaotic, may actually generate and ride upon a cushion
of air and pulverized material that exhibits laminar flow
characteristics. This essentially allows the flow to move
faster and farther than conventional landslide models predict by
reducing the frictional component of the flow.
Landslides and debris flows pose a threefold hazard; they have the
ability to crush, bury and dislocate. The force exerted by earth,
rock and debris can crush structures, people and animals. Both granular
flows and slurry flows can flow around and over objects to entrap them or
bury them entirely. For humans and animals, this may result in death due
to trauma, asphyxiation or hypothermia. Finally, landslides dislocate
objects they come in contact with. Dislocation can include the uprooting
of trees, severing of utility lines such as electric, telephone, gas, water
and sewer (resulting in additional hazards), tossing of vehicles off of
roadways, and destruction of roads, railways and bridges.
Authored by Kenneth L. Anderson.
Original article published 23 May 2004.
Follow links to the right to learn more about landslides and debris flows, which are
processes involving mass movement and mass wasting.
At the left margin, Related Links address topics of interest
pertaining to geologic hazards and other security issues. View the
Security & Consumer Protection SiteMap
for a complete list of security and consumer protection topics.
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