State PCS - West Bengal (WBCS)

• 25 Sep 2025

Kolkata Floods 2025

Why in News?

Kolkata and its suburbs recorded the third-highest September rainfall since 1978, receiving 251.4 mm in 24 hours. The peak hourly rainfall of 98 mm, just below the 100 mm cloudburst threshold, led to urban flooding.

What is Urban Flooding?

• About: Urban flooding is the waterlogging of densely populated areas caused by heavy rain, overflowing rivers, or poor drainage, disrupting transport, damaging infrastructure, and posing health risks.
• Causes of Urban Flooding: 
  • Natural: 
    • Heavy Monsoon Rainfall: Intense rains in regions like the Western Ghats and northeast India often overwhelm urban drainage (2015 Chennai floods).
    • Topography: Cities in floodplains or low-lying areas (Mumbai, Kolkata) or with poor natural drainage (Bengaluru) face higher flood risk.
    • Climate Change: Increasing rainfall intensity and frequency causes flash floods (2023 Delhi floods).
  • Anthropogenic:
    • Rapid Urbanization: Encroachment on wetlands and loss of natural drainage (e.g., Bengaluru’s 80% lakes lost) increase runoff.
    • Inadequate Drainage: Outdated systems (e.g., Mumbai’s British-era drains) fail during heavy rainfall.
    • Solid Waste Mismanagement: Blocked drains worsen flooding (Himachal Pradesh 2023).
• Causes of Kolkata Floods 2025: The heavy rainfall over Kolkata was triggered by a low-pressure area moving toward coastal Gangetic West Bengal, causing strong moisture convergence and clouds reaching 5–7 km. 
  • Disfigured drainage, choked canals, high tide, and the decline of waterbodies due to unchecked urban expansion worsened flooding. 
  • The United Nations Intergovernmental Panel on Climate Change’s Sixth Assessment Report (AR6) (2021) had predicted sharply increasing short rainfall episodes in the city.

Cloudburst

• About: A cloudburst is a sudden, intense rainstorm delivering over 100 mm of rain in under an hour across a small area (around 10 km²), often accompanied by hail and thunder. 
  • Common in mountainous regions, especially the Himalayas, cloudbursts are difficult to predict but can trigger flash floods and landslides due to their localized, extreme rainfall.
• Causes: A cloudburst occurs when moist air rises over mountains, cools, and condenses into heavy rainfall. Strong upward currents enlarge raindrops, which fall suddenly when the currents weaken. 
  • In India, it often happens when monsoon clouds move from the Bay of Bengal or Arabian Sea to the Himalayas, releasing intense rain.