Why in news?

• The Chandigarh-Manali highway was blocked on June 26 following flash floods and landslides.
• Flash floods were witnessed in Khotinallah near Aut (in HP) on the Pandoh–Kullu stretch due to a heavy downpour.

What’s in today’s article?

• Flash floods

Flash floods

• About
  • Flash floods are sudden and rapid floods that occur within a short period, typically within hours of heavy rainfall or other intense water accumulation events.
    • According to the US’s meteorological agency, the National Weather Service, flash floods are caused when rainfall creates flooding in less than 6 hours.
  • They are characterized by a swift rise in water levels in rivers, streams, or urban areas, often with little to no warning.
• Factors responsible
  • Apart from heavy rain, flash floods can also be caused by dam or levee failures, ice or debris jams, or sudden release of water from natural reservoirs such as glacial lakes.
  • In India, flash floods are often associated with cloudbursts – sudden, intense rainfall in a short period of time.
  • Factors such as the intensity and duration of rainfall, the steepness of terrain, the nature of the soil, and the presence of man-made structures that impede water flow are also responsible for flash floods.
• Features
  • Flash floods are known for their extreme force and velocity, carrying a tremendous amount of water, debris, and sediment.
  • They can overwhelm drainage systems, cause rivers to overflow their banks, and inundate low-lying areas.
  • Flash flooding commonly happens more where rivers are narrow and steep, so they flow more quickly.
  • They can occur in urban areas located near small rivers, since hard surfaces such as roads and concrete do not allow the water to absorb into the ground.

How flash floods are different from floods in general?

• Speed of Onset:
  • Flash floods have a rapid onset, occurring within a short span of time, often within a few hours or even minutes.
  • On the other hand, floods typically develop gradually over a more extended period, often days or weeks, as a result of sustained rainfall or melting snow.
• Duration:
  • Flash floods are short-lived events that subside quickly once the intense rainfall or water accumulation event ends.
  • Floods, in general, can last for days, weeks, or even months, as they are the result of prolonged precipitation or continuous water inflow.
• Intensity:
  • Flash floods are characterized by their high intensity. They involve a sudden surge of water, often leading to significant destruction.
  • Floods, although they can also be powerful and destructive, tend to have a lower peak intensity compared to flash floods due to their longer duration and slower rise in water levels.
• Warning Time:
  • Flash floods provide little to no warning time since they occur rapidly.
  • In contrast, floods typically offer more advance warning as they develop gradually, allowing for evacuation plans to be implemented and emergency measures to be taken.
• Geographic Scope:
  • Flash floods are usually localized events, occurring in specific areas where intense rainfall or other factors lead to rapid water accumulation.
  • General floods, on the other hand, can cover larger areas, including river basins, coastal regions, or expansive low-lying areas.

How common are flash floods and floods?

• Floods
  • According to government data from a project by the Assam State Disaster Management Authority, India is the worst flood-affected country in the world, after Bangladesh.
  • It accounts for one-fifth of the global death count due to floods.
  • Nearly 75 per cent of the total Indian rainfall is concentrated over a short monsoon season of four months (June to September).
    • As a result, the rivers witness a heavy discharge during these months.
  • About 40 million hectares of land in the country are liable to floods according to the National Flood Commission, and an average of 18.6 million hectares of land are affected annually.
• Flash floods
  • Flash floods have been commonly witnessed in cities like Chennai and Mumbai.
  • Depression and cyclonic storms in the coastal areas of Orissa, West Bengal, Andhra Pradesh, and others also cause flash floods.
  • As per the experts, flash floods may in the future, begin to take place after wildfires that have been taking place more frequently.
  • This is because wildfires destroy forests and other vegetation, which in turn weakens the soil and makes it less permeable for water to seep through.

Ways to deal with flash floods

• Early warning systems, including weather monitoring, river gauges, and emergency alerts, play a crucial role in mitigating the impact of flash floods.
• It is essential for individuals and communities in flood-prone areas to be aware of the risks, have emergency plans in place, and follow the guidance provided by local authorities during such events.

Steps taken in India

• IMD has been using Doppler radars, a flash floods forecasting and warning systems to predict the occurrence of flash floods.
• The NDMA also advices the inhabitation of low-lying areas along the rivers, nullah and drains to be regulated by the state governments/State Disaster Management Authorities (SDMAs)/ District Disaster Management Authorities (DDMAs) as a preventive measure.
• Central Water Commission (CWC)/ National Remote Sensing Agency (NRSA)/ state governments/ SDMAs also check for landslides and blockages in rivers with the help of satellite imageries.