Landslide

This entry refers to the geological fact. For political usage, see landslide victory. For the Fleetwood Mac song, see Landslide (song).

Rockslide redirects here. For the comic book character see Rockslide (comics)

A landslide is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows. Although gravity acting on an over steepened slope is the primary reason for a landslide, there are other contributing factors affecting the original slope stability:

erosion by rivers, glaciers, or ocean waves create oversteepened slopes
rock and soil slopes are weakened through saturation by snowmelt or heavy rains
earthquakes create stresses that make weak slopes fail (see liquefaction, Hope Slide)
volcanic eruptions produce loose ash deposits, heavy rain, and debris flows.
vibrations from machinery, traffic, blasting and even thunder may trigger failure of weak slopes
excess weight from accumulation of rain or snow, stockpiling of rock or ore, from waste piles, or from man-made structures may stress weak slopes to failure and other structures
groundwater pressure acting to destabilise the slope
in shallow soils, the removal of deep-rooted vegetation that binds the colluvium to bedrock.

In order to clarify the meaning of the word "landslide", the main points of agreement among various authors in the terminological definitions of landslide events can be summarised as follow:

• Landslides represent one category of phenomena included under the general heading of mass movements. The term therefore describes a movement of a mass of rocks or soil from a higher point to a lower one.

• Gravity is the principal force involved. The movement of the masses is due to the action of the force of gravity, but other forces like those due to earthquake or due to water filtration can be involved.

• Movement must be moderately rapid. Creep that affects the regolithe without definition of the sliding surface is therefore a slope movement but is not a landslide.

• Movements may include falling, sliding and flowing. The first is the movement of masses or blocks in free fall; the second is the movement along more or less well-defined surfaces, the third, is movements of masses in a fluid-plastic or viscous state.

• The plane or zone of movement is not identical with a fault. Fault can be a part of a sliding surface.

• Movements should be downwards and outwards with a free face, thus excluding subsidence. Subsidence is a mass movement, in which mass which goes downwards and is induced by gravity and/or specific water conditions.

• The displaced material has well-defined boundaries and usually involves only limited portions of the hillside. This shows that a landslide movement always involves a volume of material whose limits can be recognised or approximated and whose determination is at the basis of the identification of the mechanism movements.

• The displaced material may include parts of the regolithe and/or bedrock. The materials involved can therefore be rocks at various levels of alteration or the product of disgregation phenomena in the past.

• Frozen ground phenomena are usually excluded. Therefore solifluction due to frozen ground phenomena like creep, can be classified as slope movement but is not a landslide.

• On the basis of what is stated above, a complete definition of a landslide event could be the following: "movement of soil or rock controlled by gravity, superficial or deep, with movement from slow to rapid, but not very slow, which involves materials which make up a mass that is a portion of the slope or the slope itself".

Special types of landslides

Debris flow

Slope material that becomes saturated with water may develop into a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses, and cars, thus blocking bridges and tributaries causing flooding along its path.

Muddy-debris flows in the Alpine areas cause severe damage to structures and infrastructure and often claim human lives. Muddy-debris flows can start as a result of slope-related factors, and shallow landslides can dam stream beds, provoking temporary water blockage. As the impoundments fail, a "domino effect" may be created, with a remarkable growth in the volume of the flowing mass, which takes up the debris in the stream channel. The solid-liquid mixture can reach densities of up to 1.8-2 tons/m³ and velocities of up to 13-14 ms-1 (Chiarle and Luino, 1998; Arattano, 2003). These processes normally cause the first severe road interruptions, due not only to deposits accumulated on the road (from several cubic metres to hundreds of cubic metres), but in some cases to the complete removal of bridges or roadways or railways crossing the stream channel. Damage usually derive from a common underestimation of mud-debris flows: in the alpine valleys, for example, bridges are frequently destroyed by the impact force of the flow because their span is usually calculated only for a water discharge. For a small basin in the Italian Alps (area = 1.76 km²) affected by a debris flow, Chiarle and Luino (1998) estimated a peak discharge of 750 m3/s for a section located in the middle stretch of the main channel. At the same cross section, the maximum foreseeable water discharge (by HEC-1), was 19 m³/s, a value about 40 times lower than that calculated for the debris flow that occurred.

Sturzstrom

A sturzstrom is a rare, poorly understood type of landslide. Often very large, these slides are unusually mobile, flowing very far over a low angle, flat, or even slightly uphill terrain. They are suspected of "riding" on a blanket of pressurized air, thus reducing friction with the underlying surface.

Shallow landslides

Landslide in which the sliding surface is located within the soil mantle or weathered bedrock (typically to a depth from few decimetres to some meters). They usually include debris slides, debris flow, and failures of road cut-slopes.

Deep-seated landslide

Landslides in which the sliding surface is mostly deeply located below the maximum rooting depth of trees (typically to depths greater than ten meters). Deep-seated landslides usually involve deep rhegolith, weathered rock, and/or bedrock and include large slope failure associated with translational, rotational, or complex movement.

Related phenomena

An avalanche, similar in mechanism to a landslide, involves a large amount of ice, snow and rock falling quickly down the side of a mountain. Usually the snow builds in cornices or forms over a weaker layer of snow which increases the danger of an avalanche.

A pyroclastic flow is caused by a collapsing cloud of hot ash, gas and rocks from a volcanic explosion that moves rapidly down an erupting volcano.

Historical Landslides

The Storegga Slide - Norway
Cliff landslip of the Undercliff near Lyme Regis, Dorset, England, on 24 December 1839
Frank Slide - Turtle Mountain, Alberta, Canada, on 29 April 1903
The Riñihuazo lanslide in Chile after the Great Chilean Earthquake, on 22 May 1960
The 1966 Aberfan disaster
St-Jean-Vianney, Québec, Canada. Small village near Saguenay river destroyed on May 1971. [1]
Landslides associated with the Mount St. Helens eruption on May 18, 1980.
Thistle, Utah on 14 April 1983
The Mameyes Disaster - Ponce, Puerto Rico on October 7, 1985
Thredbo landslide, Australia on 30 July, 1997
Payatas, Manila garbage slide on 11 July, 2000.
Bluebird Canyon area in Laguna Beach, California, on June 1, 2005 [2]
Southern Leyte landslide in the Philippines on 17 February, 2006
Devil's Slide, an ongoing landslide in San Mateo County, California

External links

OIKOS - Educational European project based on Google Maps Mashups
United States Geological Survey site
British Columbia government landslide information
Slide!, a program on B.C.'s Knowledge Network, with video clips
Geoscience Australia Fact Sheet [3]