Cloudburst & Flash Floods

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    In Context

    • Recently cloudbursts and flash floods were seen in different parts of Himachal Pradesh and Uttarakhand.

    More about the news

    • Isolated areas in these two states have reported heavy rainfall during this time. 
      • The rainfall triggered landslides and flash floods that have disrupted rail and road traffic and resulted in house and wall collapses.
    • Himachal Pradesh was hit by 29 cloudburst incidents in the last three years.

    What are Cloudbursts?

    • About:
      • A cloudburst is a localised but intense rainfall activity. 
      • Short spells of very heavy rainfall over a small geographical area can cause widespread destruction, especially in hilly regions where this phenomenon is the most common.
    • Definition:
      • Not all instances of very heavy rainfall, however, are cloudbursts. 
      • A cloudburst has a very specific definition: Rainfall of 10 cm or more in an hour over a roughly 10 km x 10-km area is classified as a cloudburst event. 
        • By this definition, 5 cm of rainfall in a half hour period over the same area would also be categorized as a cloudburst.
      • During a cloudburst event, a place receives about 10% of this annual rainfall within an hour. 
    • Frequency of Cloudbursts:
      • Cloudbursts are not uncommon events, particularly during the monsoon months. 
      • Most of these happen in the Himalayan states where the local topology, wind systems, and temperature gradients between the lower and upper atmosphere facilitate the occurrence of such events.
      • Are cloudburst incidents increasing?
        • There is no long-term trend that suggests that cloudbursts, as defined by the IMD, are rising. 
        • What is well established, however, is that incidents of extreme rainfall, as also other extreme weather events, are increasing — not just in India but across the world. 
        • While the overall amount of rainfall in India has not changed substantially, an increasing proportion of rainfall is happening in a short span of time. 
        • That means that the wet spells are very wet, and are interspersed with prolonged dry spells even in the rainy season.
    • Consequences:
      • Flash floods are mostly the result of cloudbursts.
      • Even if cloudburst & flash floods is a localised activity, the consequences of these events, are not confined to the small areas. 
      • Because of the nature of terrain, the heavy rainfall events often trigger landslides and flash floods, causing extensive destruction downstream. 

    Flash Floods

    • A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions.
      • Flash-floods are mostly the result of cloudbursts or blockage of river channel due to landslides.
    • The flood problem in the state is mainly during the months of June to August when the southwest monsoon is in progress and snow is melting in the higher reaches.

    Impacts

    • Death or Serious Injury
      • The very nature of flash floods makes them fast and very difficult to predict.
      • Since they can occur without warning, it is no surprise that people can be seriously injured or killed by these natural disasters.
    • Immediate Property Damage
      • In addition to the force of the water, flash floods can carry large debris such as boulders. 
      • This combination can cause heavy structural damage to homes making them uninhabitable and can carry away large pieces of property such as vehicles.
    • Loss of Critical Infrastructure
      • Large debris and flood waters can cause structural damage to bridges and roadways, making travel impossible.
      • Power, telephone, and cable lines can be taken out by flash floods as well.
      • Flood waters can disrupt or contaminate groundwater, making tap water unfit for consumption.
    • Deposited Sediment & Silt
      • Floods can leave behind large amounts of silt and other debris that can make travel difficult and can be costly to remove. 
    • Uninsured or Under-insured Homeowners
      • Being infrequent and unpredictable, many homeowners do not consider flash floods when insuring their homes.
    • Economic Losses
      • Depending on the damage caused, it may prevent local businesses from opening or keep customers from getting to those businesses.

    Initiatives in this direction

    • Flash Flood Guidance Services:
      • It is a robust system designed by the India Meteorological Department (IMD) to provide the necessary products in real-time to support the development of warnings for flash floods about 6-12 hours in advance at the watershed level for the flash flood-prone South Asian countries viz. India, Nepal, Bhutan, Bangladesh, and Sri Lanka.
    • South Asian Flash Flood Guidance System (FFGS):
      • The India Meteorological Department (IMD) launched the South Asian FFGS.
      • It is aimed at helping disaster management teams.
      • Helps governments make timely evacuation plans ahead of the actual event of flooding. 

    Source: IE

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