Types, Causes and Mitigation of Landslides

Types, Causes and Mitigation of Landslides

Question : The district authority in Nawalparasi opened the gates of the Gandak Canal after the landslide at Ramche in Myagdi district blocked the Kali Gandaki river in Nepal. The Himalayan kingdom is also facing threats of flooding following massive landslides. Provide an outline of the types and causes of landslides as well as how to mitigate them.

Landslide is a movement of mass of rocks, earth or debris down a slope

They are a mass wasting denoting down slope movement of soil and rock under the impact of gravity.

A. Types

The different types of landslides include the following:

• Falls: These are abrupt movements of masses of geologic material and boulders that break away from cliffs, slopes. Falls are influences by gravity as well as mechanical weathering

• Topples: Toppling failures are when there is forward rotation of units about some pivotal point wherein gravity and forces act such as fluids in cracks

• Slides: Mass movement wherein the slide material separates from underlying stable material. The different types of slides are rotational and translational.

• Rotational slides are where the slide movement is mostly rotational about an axis parallel to the ground surface and transverse across the slide

• Translational slides have landslide mass movement along a roughly planar surface with lowered rotation or backward tilting

• Spreads: Lateral spreads are landslides which occur on gentle terrain through lateral extension accompanied by tensile fractures

• Flows: 5 basic categories of flow are there including debris flow, debris avalanche, earth flows, mudflows, and creep (seasonal/continuous/progressive)

• These are further subdivided on the basis of type of geologic material into debris, earth or bedrock

• Debris flows and rock falls are the most commonly occurring types of landslides

B. Causes

• Forces acting down-slope exceed the strength of the earth materials composing the slope to cause a landslide

• Landslides can be caused by rainfall, snowmelt, water level changes, ground water depletion, stream erosion, earthquakes, volcanic activity, disturbance by human activities or a combination of these.

• Submarine landslides can lead to tsunamis as well.

• Geological causes of landslides include weak or sensitive materials, weathered materials, sheared, jointed or fissured materials. Other geological factors range from bedding to schistosity, fault, unconformity, contact and contrast in permeability/ stiffness of material

• Morphological causes include tectonic or volcanic uplift, glacial rebound, thawing, vegetation moral, deposition loading slope or its crest, fluvial, wave of glacial erosion of slope or lateral margins or even subterranean erosion through solution, piping . Freeze and thaw weathering as well as shrink and swell weathering can also be other morphological causes of landslides.

• Human causes include water leakage from utilities, excavation of slope or its tow, loading of slope or its crest, drawdown of reservoirs, irrigation, mining, artificial vibration and deforestation.

• Slope saturation by water caused by flooding, rainfall, snowmelt, ground water depletion and water level changes across dams, lakes, reservoirs, canals and rivers causes landslides as well

• Earthquakes such as those in Nepal are also caused by landslides. Another such occurrence is the 1964 Great Alaska quake which caused landslide and loss of ground.
• Landslides due to volcanic activity are the most devastating. One such was the eruption of Mount St. Helens which triggered the largest landslide in recorded history.

C. Mitigation

• Frequency of landslide hazards and type of human activity as well as location determine impact

• Total avoidance of landslide hazard areas or restriction on hazard zone activity is an effective method of management

• Land use policies and regulations should also be in place in areas prone to landslides

• Hazard potentials of sites should be evaluated

• Landslides can be mitigated in following ways:

- The landslide can be covered with an impermeable membrane
- Surface water is directed away from the landslide
- Ground water is drained from the landslide

• Education and awareness about the impact of landslides is also a must

Facts and Stats

• Landslides can move at different speeds

• Debris flows can travel up to 200 miles per hour

• They can also move at average speeds of 30 to 50 miles per hour

• Landslide speed depends on slope angle, debris volume, water content and kind of earths and debris in the flow