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Land Slides

A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, 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 the original slope stability. Typically, pre-conditional factors build up specific sub-surface conditions that make the area/slope prone to failure, whereas the actual landslide often requires a trigger before being released.

 

CAUSE

Landslides occur when the stability of a 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:

Landslides are aggravated by human activities, Human causes include:

 

TYPES

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 tons/m³ and velocities of up to 14 m/s (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 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) affected by a debris flow, Chiarle and Luino (1998)[citation needed] 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.

 

Earth Flow

Earthflows are downslope, viscous flows of saturated, fine-grained materials, which move at any speed from slow to fast. Typically, they can move at speeds from 0.17 to 20 km/h. Though these are a lot like mudflows, overall they are slower moving and are covered with solid material carried along by flow from within. They are different from fluid flows in that they 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: if there is more 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 creates the intrusion of water into the earthflows. Water then increases the pore-water pressure and reduces the shearing strength of the material.

 

Debris avalanche

A debris avalanche is a type of slide characterized by the chaotic movement of rocks soil and debris mixed with water or ice (or both). They are usually triggered by the saturation of thickly vegetated slopes which results in an incoherent mixture of broken timber, smaller vegetation and other debris.[2] Debris avalanches differ from debris slides because their movement is much more rapid. This is usually a result of lower cohesion or higher water content and commonly steeper slopes.

Movement

Debris slides generally start with big rocks that start at the top of the slide and begin to break apart as they slide towards the bottom. This is much slower than a debris avalanche. Debris avalanches are very fast and the entire mass seems to liquefy as it slides down the slope. This is caused by a combination of saturated material, and steep slopes. As the debris moves down the slope it generally follows stream channels leving a v-shaped scar as it moves down the hill. This differs from the more U-shaped scar of a slump. Debris avalanches can also travel well past the foot of the slope due to their tremendous speed.[3]

 

Sturzstrom

A sturzstrom is a rare, poorly understood type of landslide, typically with a long run-out. Often very large, these slides are unusually mobile, flowing very far over a low angle, flat, or even slightly uphill terrain.

 

Shallow landslide

Landslide in which the sliding surface is located within the soil mantle or weathered bedrock (typically to a depth from few decimetres to some metres). They usually include debris slides, debris flow, and failures of road cut-slopes. Landslides occurring as single large blocks of rock moving slowly down slope are sometimes called block glides.

Shallow landslides can often happen in areas that have slopes with high permeable soils on top of low permeable bottom soils. The low permeable, bottom soils trap the water in the shallower, high permeable soils creating high water pressure in the top soils. As the top soils are filled with water and become heavy, slopes can become very unstable and slide over the low permeable bottom soils. Say there is a slope with silt and sand as its top soil and bedrock as its bottom soil. During an intense rainstorm, the bedrock will keep the rain trapped in the top soils of silt and sand. As the topsoil becomes saturated and heavy, it can start to slide over the bedrock and become a shallow landslide. R. H. Campbell did a study on shallow landslides on Santa Cruz Island California. He notes that if permeability decreases with depth, a perched water table may develop in soils at intense precipitation. When pore water pressures are sufficient to reduce effective normal stress to a critical level, failure occurs.

 

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 regolith, weathered rock, and/or bedrock and include large slope failure associated with translational, rotational, or complex movement. These typically move slowly, only several meters per year, but occasionally move faster. They tend to be larger than shallow landslides and form along a plane of weakness such as a fault or bedding plane. They can be visually identified by concave scarps at the top and steep areas at the toe.

 

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.
  • 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

 

LANDSLIDE READY RECKONER

Areas that are generally prone to landslides

  • Old and/or recent existing landslides
  • Base or top of slopes
  • Base of minor drainage hollows
  • Base or top of an old fill slope
  • Base or top of a steep cut slope

 

Areas generally safe from landslides

  • Hard, non-jointed bedrock that has not moved in the past
  • Flat-lying areas away from slopes and steep river banks
  • The nose of ridges, set back from the tops of slopes

 

Landslide warning signs

  • Sticking or jamming of doors or windows
  • Appearance of cracks in plaster, tile, brick, or foundations
  • Pulling away from the building of outside walls or stairs.
  • Slow development of widening cracks on the ground or on paved areas such as streets.
  • Breakage of underground utility lines
  • Appearance of bulging ground at the base of a slope
  • Emergence of flowing ground water in new sites
  • Sudden decrease in creek water levels though rain is still falling or just recently stopped
  • Tilting or moving of fences, retaining walls, utility poles, or trees.
  • Faint rumbling sound that increases in volume as the landslide nears.The ground slopes downward in one specific direction and may begin shifting in that direction under your feet.

 

Immediate steps for imminent Landslide

  • Contact your local Fire, Police or Public Works Department
  • Inform affected neighbors
  • Leave the area quickly

 

Actions to be taken before Intense Rainfall

  • Become familiar with the land around you. Slopes, where landslides or debris flows have occurred in the past, are likely to experience them in the future
  • Buildings should be located away from known landslides, debris flows, steep slopes, streams and rivers, intermittent-stream channels, and the mouths of mountain channels
  • Observe the patterns of storm-water drainage on slopes near your home, and watch especially the places were runoff water converges, increasing flow over soil-covered slopes. Observe the hillsides around your home for any signs of land movement, such as small landslides or debris flows or progressively tilting trees
  • Contact your local authorities to learn about the disaster management response, and develop your own emergency plans for your family and business.

 

During Intense Rainfall

  • Be observant. Many landslide and debris flow casualties occur when people are sleeping. Listen to radio for warnings of intense rainfall. Intense short bursts of rain may be particularly dangerous, especially after longer periods of heavy rainfall and damp weather.
  • Unusual sounds might indicate moving debris, such as trees cracking or boulders knocking together. A trickle of flowing or falling mud or debris may precede larger landslides. Be alert for any sudden increase or decrease in water flow in streams or channels. Such changes may indicate landslide activity upstream, so be prepared to move quickly
  • If you live in areas susceptible to landslides and debris flows, consider leaving if it is safe to do so. If you remain at home, move to a part of the house farthest away from the source of the landslide or debris flows, such as an upper floor, but keep an escape route open should it become necessary to leave the house.
  • Be alert when on the roads. Embankments along roadsides are particularly susceptible to landslides. Watch the road for collapsed pavement, mud, fallen rocks, and other indications of possible landslides or debris flows.

 

After Intense Rainfall

Be alert for signs indicating land movement. Landslides can occur weeks or months after intense storms

 

Things to Remember

  • Mudflows tend to flow in channels, but will often spread out over a floodplain. They generally occur in places where they have occurred before
  • Landslides and mudflows usually strike without much appreciable warning. The force of rocks, soil, or other debris moving down a slope can devastate anything in its path. Take the following steps to be ready.
  • Plant ground cover on slopes and build retaining walls.
  • In mudflow areas, build channels or deflection walls to direct the flow around buildings
  • Remember: If you build walls to divert debris flow and the flow lands on a neighbor's property, you may be liable for damages

 

Precautions to be taken during landslides

If inside a building:

  • Stay inside
  • Take cover under a desk, table, or other piece of sturdy furniture

If outdoors:

  • Try and get out of the path of the landslide or mudflow
  • Run to the nearest high ground in a direction away from the path.
  • If rocks and other debris are approaching, run for the nearest shelter such as a group of trees or a building
  • If escape is not possible, curl into a tight ball and protect your head.

 

After Landslide

  • Stay away from the slide area. There may be danger of additional slides.
  • Check for injured and trapped persons near the slide area. Give first aid if trained.
  • Remember to help your neighbors who may require special assistance--infants, elderly people, and people with disabilities.
  • Listen to a radio or television for the latest emergency information.
  • Stay away from the slide area. There may be danger of additional slides.
  • Check for damaged utility lines. Report any damage to the utility company
  • Check the building foundation, chimney, and surrounding land for damage
  • Replant damaged ground as soon as possible since erosion caused by loss of ground cover can lead to flash flooding

 

Images of Landslides:

 
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