Mass Wasting - Landslides & Debris Flows
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Mass Wasting - Landslides & Debris Flows

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 2040 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, updated 6 December 2012.

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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