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Flooding

Water overflowing onto normally dry land, caused by heavy rain, storm surge, dam failure or river overflow. The most common natural disaster in South Asia.

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What is Flooding?

Flooding occurs when water overflows onto land that is normally dry. It is the most common natural disaster in South Asia, affecting hundreds of millions of people every year — more than cyclones, earthquakes, droughts or heatwaves combined.

The economic toll is staggering: India alone loses an average of ₹4 billion (~USD $480 million) per year to flood damage, with peak years exceeding ₹50 billion. The human toll is even greater — homes destroyed, livelihoods lost, health impacts that persist for months.

There are four main types of flooding, each with distinct causes, timescales and response needs:

  1. River (fluvial) flooding — Slow-onset overflow of rivers during monsoon.
  2. Flash flooding — Sudden, rapid floods from intense rainfall or dam break.
  3. Coastal flooding — Storm surge from cyclones combined with high tide.
  4. Urban flooding — Drainage system overwhelmed by intense rain in cities.

River flooding

The Indus, Ganga, Brahmaputra, Meghna, Mahanadi, Krishna, Godavari and Cauvery river systems flood every monsoon to varying degrees. Major causes:

The Brahmaputra valley in Assam floods more than any other Indian region — typically inundating 30–50% of the state every year. Bihar’s Kosi River is called the “Sorrow of Bihar” for its devastating annual floods.

In Bangladesh, the Ganga, Brahmaputra and Meghna combine to drain 1.7 million km² of the subcontinent into a country smaller than Greece. The result: 20–25% of Bangladesh floods every year; in extreme years (1988, 1998, 2007, 2020), 60% of the country goes under water.

Pakistan’s 2022 super-floods affected 33 million people — one third of the population — and caused over $30 billion in damage. The Indus Plain remains highly vulnerable.

Flash flooding

Flash floods develop in hours or minutes — far faster than river floods. Caused by:

Notable events:

See our flash flood glossary entry for details.

Coastal flooding

Storm surge is the wall of seawater pushed inland by tropical cyclone winds and low pressure. When it coincides with high tide, the surge can rise 5–10 metres above normal sea level.

Bay of Bengal storm surges are particularly deadly because:

1970 Bhola Cyclone killed 300,000–500,000 in East Pakistan (Bangladesh) primarily through storm-surge flooding. 1999 Odisha Super Cyclone killed 10,000+. 2008 Cyclone Nargis in Myanmar killed 138,000+.

After Bhola, Bangladesh built the world’s most successful cyclone-warning and evacuation system, reducing cyclone death tolls by 95%+.

Urban flooding

A relatively new but rapidly worsening problem. Indian cities flood several times per monsoon as drainage systems built for 25–50 mm/hour storms get overwhelmed by 100+ mm/hour cloudbursts.

Worst-affected cities:

Causes of urban flooding:

  1. Outdated drains sized for storms that are now too small.
  2. Encroachment on lakes, wetlands, nullahs for housing and roads.
  3. Impervious surfaces (concrete, asphalt) replacing absorptive soil.
  4. Solid waste blocking drains.
  5. Climate change intensifying short-duration rainfall.

Flood early warning

Modern flood warning combines:

The Central Water Commission (CWC) in India issues 3-day flood forecasts for ~325 stations. Bangladesh’s Flood Forecasting and Warning Centre (FFWC) provides similar coverage.

Flood safety

  1. Heed evacuation orders — most flood deaths happen when people stay despite warnings.
  2. Never drive through flowing water — 30 cm can sweep away a car; 60 cm an SUV.
  3. Avoid downed power lines — floodwater becomes electrified.
  4. Boil all drinking water for days after — sewer contamination is universal.
  5. Move valuables and electronics to upper floors before flooding arrives.
  6. Document damage for insurance and government compensation.
  7. Watch for snakes — especially in submerged grasslands.

Frequently asked questions

Which South Asian city floods the most? By annual frequency: Dhaka, Mumbai, Chennai, Bengaluru, Karachi. By population affected: rural Bangladesh during major monsoon years (60% of the country inundated in 1988 and 1998).

Why has urban flooding gotten worse? Three reasons: (1) rainfall intensity is increasing due to climate change, (2) cities have grown faster than drainage infrastructure, (3) natural water bodies (lakes, wetlands, drainage channels) have been built over.

What is “100-year flood”? A flood with a 1% chance of occurring in any given year — not a flood that happens once per century. South Asia has seen multiple “100-year” floods in single decades, suggesting either the statistics or the climate has changed.

Can flooding occur during normal monsoon? Yes — even a normal monsoon causes regular flooding in Assam, Bihar, north Bengal, eastern UP, Bangladesh delta, Sindh and other low-lying areas. These regions are flood-adapted but still suffer crop and infrastructure losses each year.

Where can I check flood risk for my city? Mausam Online shows precipitation forecasts and severe-weather codes on every city page. For acute alerts, consult official sources: IMD Sachet, CWC, BMD, PMD. Live forecasts: Mumbai, Chennai, Dhaka, Karachi, Patna.

Related Terms

flash-flood monsoon storm-surge tropical-cyclone landslide rainfall
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