Landslide and Flash Floods

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