Landslide

The word landslide is mixture of two American English word land + slide in 1841. Before that was landslip (1670s), which is preferred in Britain.

The usually downward movement of a mass of rock, debris, earth or artificial fill on slop downs the slope.

The term “landslide” is used as when soil, rock and vegetation movement occurred as downward and outward by gravitational influence.

Mass movement of rock, soil or debris elements create a slope, it can be occurred by natural or human induces, towards the lower and external part of the slope.

In a simple word, Landslide is a collapse of mass of the earth rock from mountain or cliff.

 

Types of Landslide

The various types of landslides can be differentiated by the kinds of material involved and the mode of movement. The most common types of landslides are given bellow:

  1. Slides
  2. Falls
  3. Topples
  4. Flows
  5. Lateral Spreads

1. Slides

The term ‘slide’ refers only to mass movements, where there is a specific zone of weakness that divided the slide material from more stable underlying material. The two major types of slides are rotational slides and translational slides.

  • Rotational slide

The movement of the earth material is rotational in nature. It is also known as Slump. The surface rupture is curved upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface.

Rotational Slide (Slump)
Rotational Slide (Slump)
  • Translational slide

The earth mass is mainly in place after it slides downhill on a plane preface.  A block slide is a translational slide in which the moving mass consists of a single unit or a few closely related units that move down slope as a relatively coherent mass.

Translational Slide
Translational Slide

2. Falls

These are usually affected by gravity after large rocks or boulders are separated from their parent rock. They usually fall along steep slopes. It’s called free falls of material from mountain or cliff.

Falls
Falls

3. Topples

Topple refers to the earth mass rotates forward about a pivit. The result is usually a slope without collapse. It is usually cause by cracks or fracture in the bedrocks.  The end-over-end motion of rock down a slope is topple.

Toppels
Toppels

4. Flows

Viscous to fluid-like motion of debris is called flows. There are five basic categories of flows that differ from one another in fundamental ways.

Flows
Flows
  • Debris flow

Debris flow involves the rapid downhill movement of loose earth materials like soil, rock, and organic matter usually with water.

  • Debris avalanche

Debris Avalanche is similar to Debris Flow but has a more rapid flow. This is a variety of very rapid to extremely rapid debris flow.

  • Earth flow

Earth flow refers to the earth material is finer and is washed away leaving a depression bowl at the head. It has a characteristic “hourglass” shape.

  • Mud flow

Mud flow is made up of fine silt, sand and clay material saturated with water and flowing very rapidly. Mud flows and debris flows are commonly referred to as “mudslides.”

  • Creep

Creep is the unnoticeable slow, stable, downward movement of slope-forming soil or rock. This movement is caused by shear stress enough to produce permanent deformation, but too small to produce shear failure.

5. Lateral Spreads

Lateral spreads are distinctive because they usually occur on very gentle slopes or flat terrain.

Lateral Spread
Lateral Spread

 

Causes of Landslide

Landslides are occurred mainly by three reasons. These reasons can be described briefly. Main causes of landslides are given below in three categories.

1. Geological causes

  • Weak or sensitive materials
  • Weathered materials
  • Tectonic or volcanic uplift
  • Sheared, jointed, or fissured materials
  • Adversely oriented discontinuity (bedding, fault, unconformity, contact, and so forth)
  • Contrast in permeability and/or stiffness of materials

2. Morphological causes

  • Glacial rebound
  • Fluvial, wave, or glacial erosion of slope toe or lateral margins
  • Subterranean erosion (solution, piping)
  • Deposition loading slope or its crest
  • Vegetation removal (by fie, drought)
  • Thawing
  • Freeze-and-thaw weathering
  • Shrink-and-swell weathering

3. Human causes

  • Excavation of slope or its toe
  • Loading of slope or its crest
  • Drawdown (of reservoirs)
  • Deforestation
  • Irrigation
  • Mining
  • Artificial vibration
  • Water leakage from utilities

Landslide Prone Area

Place and location that may be prone to landslides include the following:

  • Location: The bottom of slopes and canyons are examples of locations that may be prone to landslides. Which areas received high amounts of rainfall and have slopes these area also great conditions for landslides. More seismic activity or earth movements regions are also locations that may be prone to landslides.
  • Human activity: Human activities such as mining, building, and road constructions have often exposed the land and slopes to landslides. Landslide can be occurred that location where wildfires and deforestation found.
  • Frequency of landslide events: This area is also a location for landslide where it occurred before. It will be occurred again because the condition which is considering landslide it may be present there.

Landslide Mitigation

Mitigation is the use of action plan to reduce risks pre to during and post- disaster. It is related to short-term, mid-term and long-term measures. Some significant steps for landslide disaster mitigation are as follows:

Short Term (Next 1 Year)

  • Develop a Team: Develop consist of about 10 member’s team. This team will observe the area where landslides can be occurred. They will take steps to protect these people who are in vulnerable area. This team will also be part of main rescue team.
  • To protect the Vulnerable Zone: Prepare or constriction of a retaining wall, drain and strong wall near the hill to protect vulnerable zones.
  • Stop Hill Cutting and Other Activities: To protect landslide it is forbidden to all types of hill cutting, sand collection and other activities which all causes landslide.
  • Reshaping Hill: Increase part of vulnerable hills should be leveled by the concern authority.

Mid Term (Next 2 Year)

  • Awareness Program: To arrange different kinds of training, workshop and seminar to develop awareness. Non Governmental Organization and Governmental Organization will be involved to increase awareness these people who live in this vulnerable area.
  • Information Center: Information center will be provided all types of data, photographs, related documents and information about landslides.
  • Cover the Hill: Cover the all necked hills by herbs, shrubs, trees etc. It will protect the top soil and eventually it control erosion from hills.
  • Shift the Vulnerable People: Vulnerable people will be shifted sequentially from vulnerable area to a safe place when any chance of land slide occurred at any time.

Long Term (Next 3 Year)

  • Develop Policy: Hill management policy will be developed where cover the hill cutting, land use, hill use etc. Policy is the starting point for risk reduction activities.
  • Develop Fund: Develop a fund for emergency time and develop employee’s socio-economic condition that lives in vulnerable area. Because, poverty forced people to live in dangerous and unsafe shelter.
  • Residence Project: Take steps to create a residence project for employees (staff and officer).

Effects of Landslide

Landslides are a distractive hazard. In Bangladesh human are mostly suffered by landslides. No one can run out when landslide approached. Main effects are given below-

  1. Loss of life

At a greater risk of death by landslide which communities living at the foot of hills and mountains. A landslide carries huge amount of rocks, heavy debris and soil. That can kills lots of people on impact. According to WHO-World Health Organization, confirmed that 160 deaths and 187 injured by landslide in Bangladesh in 2017. Every year it occurred in the hill zone of Bangladesh and killed lots of people.

  1. Lead to economic decline

Property distraction is one of the major impacts of landslide. After landslide, the affected area normally undergoes rehabilitation. Economic condition of the area has broken down where landslide affected.

  1. Damage of infrastructure:

Landslide can damage to property resolution from the force flow or mud. Infrastructure land can be destroyed by landslide occurred. Such as-

  • Destruction a building and placement.
  • Land massive collapse can cause the destruction of a city.
  • Damage to roads.
  • Affect communication system of roads and communication systems.
  1. Affects beauty of landscape:

The erosion left behind by landslides leaves behind rugged landscapes that are unsightly. The pile of soil, rock and debris downhill can cover land utilized by the community for agricultural or social purposes. Debris flows usually uproot trees and wipe out vegetation and wildlife in its path.

  1. Impacts river ecosystems:

Landslide depositional materials can find way into rivers and block their natural flow. Many river habitats like fish can die due to interference of natural flow of water. Communities depending on the river water for household activities and irrigation will suffer if flow of water is blocked.

 

References

  • Highland L, 2004, Landslide Types and Processes, USGS- Science for a changing world, Fact Sheet 2004-3072, July 2004
  • Sultana T. 2013, landslide disaster in Bangladesh: a case study of Chittagong university campus, impact journals, issn(e): 2321-8851; issn(p): 2347-4580 vol. 1, issue 6, nov 2013, 35-42

 

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