Wednesday, August 8, 2018

Fact about landslide

The biggest reason for a landslide is that there is a slope and material goes down the slope because of gravity. But other things also contribute to landslides: erosion by rivers, glaciers, or ocean waves makes slopes too steep. rock and soil slopes made weak through saturation by snowmelt or heavy rains.

This article is about the geological phenomenon. For other uses, see Landslide (disambiguation).

A landslide near Cusco, Peru in 2018.
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A NASA model has been developed to look at how potential landslide activity is changing around the world.
The term landslide or, less frequently, landslip,[1] refers to several forms of mass wasting that include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows and debris flows. Landslides can occur underwater, in which case they are called submarine landslides, and in coastal and onshore environments. Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting slope stability. Typically, pre-conditional factors build up specific surface or sub-surface conditions that make a slope prone to failure, whereas the actual landslide often requires a trigger before being released.

The Mameyes Landslide, in the Mameyes neighborhood of barrio Portugu├ęs Urbano in Ponce, Puerto Rico, which buried more than 100 homes, was caused by extensive accumulation of rains and, according to some sources, lightning.
Landslides occur when the slope changes from a stable to an unstable condition. A change in the stability of a slope can be caused by a number of factors, acting together or alone. Natural causes of landslides include:

groundwater (pore water) pressure acting to destabilize the slope
loss or absence of vertical vegetative structure, soil nutrients, and soil structure  (e.g. after a wildfire – a fire in forests lasting for 3–4 days)
erosion of the toe of a slope by rivers or ocean waves
weakening of a slope through saturation by snow melting, glaciers melting, or heavy rain
earthquakes adding loads to barely stable slope
earthquake-caused liquefaction  destabilizing slopes
volcanic eruptions.
Landslides are aggravated by human activities, such as

deforestation, cultivation and construction, which destabilize the already fragile slopes.
vibrations from machinery or traffic
blasting
earthwork which alters the shape of a slope, or which imposes new loads on an existing slope
in shallow soils, the removal of deep-rooted vegetation that binds colluvium to bedrock
construction, agricultural or forestry activities (logging) which change the amount of water infiltrating the soil.

The landslide at Surte in Sweden, 1950. It was a quick clay slide killing one person.

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.

Debris flow is often mistaken for flash flood, but they are entirely different processes.

Muddy-debris flows in 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, resulting in 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 2,000 kg/m3 (120 lb/cu ft) and velocities of up to 14 m/s (46 ft/s) (;). 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 derives 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 (0.68 sq mi)) affected by a debris flow, estimated a peak discharge of 750 m3/s (26,000 cu ft/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 (670 cu ft/s), a value about 40 times lower than that calculated for the debris flow that occurred.

Earthflow

The Costa della Gaveta earthflow in Potenza, Italy. Even though it moves just some mm/a and is hardly visible, this landslide causes progressive damage to the national road, the national highway, a flyover and several houses that were built on it.

A rock slide in Guerrero, Mexico
An Earthflow is the downslope movement of mostly fine-grained material. Earthflows can move at speeds within a very wide range, from as low as 1 mm/yr (0.039 in/yr) to 20 km/h (12.4 mph). Though these are a lot like mudflows, overall they are more slow moving and are covered with solid material carried along by flow from within. They are different from fluid flows which are more rapid. Clay, fine sand and silt, and fine-grained, pyroclastic material are all susceptible to earthflows. The velocity of the earthflow is all dependent on how much water content is in the flow itself: the higher the water content in the flow, the higher the velocity will be.

These flows usually begin when the pore pressures in a fine-grained mass increase until enough of the weight of the material is supported by pore water to significantly decrease the internal shearing strength of the material. This thereby creates a bulging lobe which advances with a slow, rolling motion. As these lobes spread out, drainage of the mass increases and the margins dry out, thereby lowering the overall velocity of the flow. This process causes the flow to thicken. The bulbous variety of earthflows are not that spectacular, but they are much more common than their rapid counterparts. They develop a sag at their heads and are usually derived from the slumping at the source.

Earthflows occur much more during periods of high precipitation, which saturates the ground and adds water to the slope content. Fissures develop during the movement of clay-like material which creates the intrusion of water into the earthflows. Water then increases the pore-water pressure and reduces the shearing strength of the material.

Debris slide

Goodell Creek Debris Avalanche, Washington, USA

A debris slide is a type of slide characterized by the chaotic movement of rocks, soil, and

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