Floods and urban flooding

The first principle

A flood is a river or a drain asked to carry more water than its channel can hold, so the water takes back the land beside it. The flood plain is part of the river, used every few years, and most flood disasters in India happen because towns and roads have been placed on it and then defended with walls. Flood management therefore has two halves, keeping water away from people and keeping people out of the water's way, and India has spent most of its money on the first.

The core

How much of India floods

• The official baseline is old. The Rashtriya Barh Ayog (National Flood Commission), set up in 1976, reported in 1980 that about 40 million hectares, roughly an eighth of India's area, was liable to floods, and it made 207 recommendations.
• The later figure measures something different. The Working Group on Flood Management for the Twelfth Plan found that the largest area flooded in any single year between 1953 and 2010 was about 49.8 million hectares. This is a maximum affected area, not a flood prone area, and India has still not mapped its flood plains.
• Rain is concentrated in time and space. About three quarters of India's rain falls in the four monsoon months, and the silt laden Ganga and Brahmaputra basins suffer most; the monsoon itself is covered in the linked GS1 note.

Four kinds of flood

Floods differ mainly in their cause and in how much warning they give, from days for a river in the plains to minutes for a flash flood below a cloudburst, as the first figure sets out. Flash floods belong to the Himalayan hazards note and coastal floods to the cyclones note, so this note concentrates on riverine and urban floods.

Why cities flood

• Concrete changes the arithmetic. Roofs and roads stop rain from soaking in, and the National Disaster Management Authority (NDMA) notes in its 2010 guidelines that urbanisation can raise flood peaks by 1.8 to 8 times and flood volumes by up to 6 times.
• The sponges are filled in. Lakes, tanks, marshes and stream beds that once stored storm water are built over, so the water has nowhere to wait.
• Drains are too small and choked. Many Indian storm drains were designed for low rainfall intensities decades ago, and silt, plastic and solid waste reduce even that capacity.
• The sea and the river push back. A high tide locks the outfalls of coastal cities, and a river in spate backs up into the drains of river cities.

Dimensions

• Economic and social. Floods destroy crops, houses and roads in the same districts of Bihar and Assam year after year, so households rebuild instead of saving, and the poorest, who live on the lowest land and beside urban drains, are hit first and longest.
• Environmental. Floods also spread silt over flood plains, recharge groundwater and feed wetlands, so a river cut off from its plain loses these services.
• Federal and international. Water is a State subject under Entry 17 of the State List, so States plan flood control while money and forecasting come largely from the Centre. Many floods start in Nepal, Bhutan or China, so data sharing with neighbours matters.
• Climate. A warmer atmosphere holds more moisture, so very heavy rain days are becoming more frequent, as the climate change impacts note explains.

In practice

Cases and the lessons they taught

• Mumbai, 26 July 2005. Santacruz recorded 944 mm of rain in 24 hours while a high tide blocked the outfalls, and the Mithi river, narrowed by encroachment and debris, overflowed into the city. The Fact Finding Committee under Madhav Chitale traced the damage to old drains built for about 25 mm of rain an hour and to the loss of the Mithi's flood plain, so the lesson was drains sized for today's rain and a river given back its room.
• Chennai, December 2015. The Comptroller and Auditor General (CAG) found that the floods killed 289 people and that the release of up to 29,000 cusecs from the Chembarambakkam tank on 1 and 2 December, without an emergency action plan, made the disaster worse. It also found that encroachment of tanks, lakes and river beds had played a major role. The lesson was that urban reservoirs need operating rules and warning plans, and that wetlands are flood defences.
• Kerala, 2018. Rainfall from 1 June to 18 August was 42 per cent above normal, and water had to be released from dozens of dams that were already full. The floods and 341 landslides took 433 lives and moved close to 14 lakh people into relief camps, and the Post Disaster Needs Assessment put recovery needs at about Rs 31,000 crore. The lesson was that dam operation must be planned for extreme years.
• Assam, every monsoon. About two fifths of Assam's area is flood prone, roughly three times the national share, because the Brahmaputra is a braided, silt laden river on a narrow valley floor. Bank erosion, which removes land for good, is as serious as the flood.
• The Kosi, 2008. On 18 August 2008 the eastern embankment of the Kosi breached at Kusaha in Nepal, and the river moved into an old channel in north Bihar, affecting millions of people in Supaul, Araria, Purnia, Madhepura and Saharsa. The lesson, taken up under Issues, is that an embankment does not remove a flood but stores it.

Institutions, law and programmes

• Forecasting by the Central Water Commission (CWC). The CWC issues flood forecasts at 340 stations, 200 of them level forecasts for towns and villages on river banks and 140 inflow forecasts for dams and barrages. Short range forecasts give 6 to 24 hours of warning, and a rainfall based advisory gives an outlook of several days.
• Flood plain zoning. The Union circulated a model bill for flood plain zoning to all States in 1975, which divides a flood plain by flood frequency and limits building in each zone. Only Manipur, Rajasthan, Uttarakhand and the former State of Jammu and Kashmir have enacted such a law.
• NDMA guidelines. The Guidelines on Management of Floods (2008) asked for a balance of structural and non structural measures, and the Guidelines on Management of Urban Flooding (2010) treated urban floods as a separate disaster for the first time, with the urban development ministry as the nodal ministry, city level drainage master plans, local rain gauge networks and protection of water bodies. The Disaster Management (Amendment) Act 2025 adds Urban Disaster Management Authorities for large cities, as explained in the framework note.
• Flood Management and Border Areas Programme (FMBAP). This scheme of the Department of Water Resources has two parts. The Flood Management Programme helps States build flood control, anti erosion and drainage works, with the Centre paying 90 per cent in the special category States and 60 per cent elsewhere, while the River Management Activities and Works related to Border Areas component pays fully for works on rivers shared with neighbouring countries. Its outlay for 2021-26 is Rs 4,100 crore, Rs 2,940 crore for the first part and Rs 1,160 crore for the second.

Issues and way forward

• Embankments are trusted too much. An embankment keeps silt inside the channel, so the bed rises and the land outside cannot drain, and when it breaks, as the Kosi's did in 2008, the water falls on people who had felt safe. Bihar's Water Resources Department counts about 73 per cent of the State as flood prone despite decades of embankment building. The Rashtriya Barh Ayog and the NDMA both asked for embankments to be audited, maintained and combined with drainage and zoning.
• Flood plains are not zoned. With only four States having a zoning law, every new colony on a flood plain adds to future losses, so a national flood plain map made binding through master plans is the basic step.
• Cities plan drains last. Drains are built road by road by different agencies and lakes are handed over for building, so the urban flooding guidelines ask for drains designed for current rain, inventories of water bodies and urban flood warning, which Mumbai and Chennai now run.
• Dams need rules for extreme years. Kerala and Chennai showed that full reservoirs can turn heavy rain into a flood, so rule curves, emergency action plans and downstream warnings matter as much as the dam.
• Natural defences are cheaper. Wetlands and room for rivers store floodwater at little cost, the idea behind the sponge city approach of permeable surfaces and restored lakes.

Common confusions

• Flood prone area and flood affected area. The flood prone area of about 40 million hectares is the Rashtriya Barh Ayog's estimate of land liable to flood, while the 49.8 million hectares of the Twelfth Plan working group is the largest area actually flooded in any one year; they measure different things and should not be called old and new figures of the same quantity.
• Flood control and flood management. Flood control tries to stop water with dams and embankments, while flood management accepts that floods will come and reduces losses through zoning, warnings, insurance and preparedness.
• Urban flooding and riverine flooding. Riverine floods come from a river overtopping its banks and build over days, while urban floods come from intense local rain that drains cannot remove and can happen within hours with no river involved.
• Flash flood and cloudburst. A cloudburst is a rain event, very heavy rain over a small area in a short time, while a flash flood is the rapid rise of water that may follow it, or may follow a dam or glacial lake failure.

For the exam

• Keyword bank: Rashtriya Barh Ayog, flood prone area, flood plain zoning, model bill of 1975, embankment paradox, Kosi breach of 2008, impervious surface, urban flooding guidelines 2010, Central Water Commission forecasting, inflow forecast, rule curve, Flood Management and Border Areas Programme, sponge city, nature based solutions, last mile warning.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction with data: The Rashtriya Barh Ayog estimated in 1980 that about 40 million hectares of India is liable to floods, and in some years the flooded area has reached nearly 50 million hectares, an eighth or more of the country.
  • Introduction with a report: The CAG's audit of the Chennai floods of 2015 found that encroachment of lakes and river beds and an unplanned release from the Chembarambakkam tank turned heavy rain into a disaster that killed 289 people.
  • Conclusion: India's flood policy has spent decades keeping water away from people; the next step is keeping people and buildings out of the water's way through zoning, warnings and room for rivers.
  • Conclusion: Urban floods are made in city planning offices long before the rain arrives, so drainage design, protection of water bodies and urban disaster authorities must work as one system.