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Landslide and Flash Floods

  • 16 Jul 2021
  • 5 min read

Why in News

Recently, heavy rains caused flash floods and landslides in many parts of Himachal Pradesh.

Key Points

  • Landslide:
    • About:
      • A landslide is defined as the movement of a mass of rock, debris, or earth down a slope.
      • They are a type of mass wasting, which denotes any downward movement of soil and rock under the direct influence of gravity.
      • The term landslide encompasses five modes of slope movement: falls, topples, slides, spreads, and flows.
    • Causes:
      • Slope movement occurs when forces acting downward (mainly due to gravity) exceed the strength of the earth materials that compose the slope.
      • Landslides are caused due to three major factors: geology, morphology, and human activity.
        • Geology refers to characteristics of the material. The earth or rock might be weak or fractured, or different layers may have different strengths and stiffness.
        • Morphology refers to the structure of the land. For example, slopes that lose their vegetation to fire or drought are more vulnerable to landslides.
          • Vegetation holds soil in place, and without the root systems of trees, bushes, and other plants, the land is more likely to slide away.
        • Human activity which includes agriculture and construction increases the risk of a landslide.
    • Landslide-Prone Areas:
      • The entire Himalayan tract, hills/mountains in sub-Himalayan terrains of North-east India, Western Ghats, the Nilgiris in Tamil Nadu Konkan areas are landslide-prone.
    • Mitigation:
      • Restriction on the construction and other developmental activities such as roads and dams in the areas prone to landslides.
      • Limiting agriculture to valleys and areas with moderate slopes.
      • Control on the development of large settlements in the high vulnerability zones.
      • Promoting large-scale afforestation programmes and construction of bunds to reduce the flow of water.
      • Terrace farming should be encouraged in the northeastern hill states where Jhumming (Slash and Burn/Shifting Cultivation) is still prevalent.
    • Step Taken:
      • The Geological Survey of India (GSI) has done a national landslide susceptibility mapping for 85% of the entire 4,20,000 square km landslide-prone area in the country. The areas have been divided into different zones according to the propensity of the disaster.
        • Improvement in early warning systems, monitoring and susceptibility zoning can reduce the damage caused by landslides.
  • Flash Floods:
    • About:
      • These are sudden surges in water levels generally during or following an intense spell of rain.
      • These are highly localised events of short duration with a very high peak and usually have less than six hours between the occurrence of the rainfall and peak flood.
      • The flood situation worsens in the presence of choked drainage lines or encroachments obstructing the natural flow of water.
    • Causes:
      • It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ice or snow flowing over ice sheets or snowfields.
      • Flash Floods can also occur due to Dam or Levee Breaks, and/or Mudslides (Debris Flow).
      • In areas on or near volcanoes, flash floods have also occurred after eruptions, when glaciers have been melted by the intense heat.
      • The intensity of the rainfall, the location and distribution of the rainfall, the land use and topography, vegetation types and growth/density, soil type, and soil water- content all determine just how quickly the Flash Flooding may occur, and influence where it may occur.
    • Mitigation:
      • Instead of valleys, people should live in areas on slopes with firm ground for safety reasons.
      • In areas where ground fissures have developed, appropriate steps should be taken to check the infiltration of rainwater and surface water.
      • Banning "indiscriminate" and "unscientific" construction works.

Source: TH

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