Floods: causes, flood-prone India and control

Detail

1. What a flood is

• Flood: land goes under water when runoff (rainwater flowing over the ground's surface into streams) is more than a river channel can carry. The extra water spills onto the low, flat land beside the river, called the floodplain.
• Floods are different from most other disasters in three ways:
• Causes are well known. People understand why floods happen.
• Slow and seasonal. Most floods build up over days, mostly in the monsoon (June-September).

• Regional. They hit large river basins, not single points the way an earthquake does.

• World pattern: floods are worst in South, Southeast and East Asia, especially China, India and Bangladesh. These are large monsoon river basins with very many people living on the floodplains.

2. Causes of floods

Type	Cause	How it leads to a flood
Natural	Long, high-intensity rain	Rain falls faster and for longer than the soil can absorb, so the river overflows
Natural	Snowmelt	Summer heat melts Himalayan snow, which raises the water in rivers such as the Satluj and the Brahmaputra
Natural	Storm surge	Cyclone winds push seawater onto the coast. Common on the Odisha, Andhra Pradesh and West Bengal coasts
Natural	Low infiltration	Infiltration means water soaking into the ground. Wet, frozen or clay soil absorbs little, so more water runs off
Natural	Silt	Soil washed off hill slopes settles on the riverbed. A shallower bed holds less water
Human	Deforestation	Without tree roots and leaf litter, rain runs off fast and carries soil with it
Human	Unscientific farming	Ploughing up and down slopes and overgrazing loosen soil, which then silts up rivers
Human	Disturbed drainage	Roads, railway embankments and canals block the natural flow of water
Human	Colonised floodplains and riverbeds	Homes and farms built on the river's "spill zone" are in the path of the water and also narrow the channel

• How silt makes floods worse:
• Hill slopes are cleared → soil erodes.
• The silt settles on the riverbed downstream → the bed rises.
• The river can carry less water → it spills over sooner, even after normal rain.

3. How much of India is flood-prone

• Rashtriya Barh Ayog (RBA, National Flood Commission) was set up in 1976. It was the first national body to study India's flood problem as a whole.
• Its 1980 report estimated 40 million ha (mha) of India as liable to floods [3].
• Newer figure: the Working Group on Flood Management and Region Specific Issues (2011) put the largest area ever affected by floods at 49.815 mha [3]. (NCERT: 40 million ha, RBA 1980.)
• States named as especially vulnerable (Class 7): Uttar Pradesh, Bihar, Kerala, Andhra Pradesh and Assam.

4. Flood-prone geography of India (Figure 6.6)

Zone	States	Type of flood and why it happens
Most flood-prone	Assam, West Bengal, Bihar, plus eastern Uttar Pradesh	Riverine and long-lasting. The Brahmaputra and the Ganga with its northern tributaries carry huge flows and heavy silt from the Himalayas across flat plains
Occasional floods	Northern rivers of Punjab and Uttar Pradesh	Overflow in years of heavy monsoon rain
Flash floods, rising	Rajasthan, Gujarat, Haryana, Punjab	Changing monsoon patterns and blocked drainage channels. In dry regions the ground cannot absorb sudden heavy rain
Winter floods	Tamil Nadu (Coromandel coast)	Come in November-January from the retreating (northeast) monsoon

• Why Tamil Nadu floods in winter:
• From October the monsoon winds retreat and blow from the northeast.
• They pick up moisture as they cross the Bay of Bengal.

• Low-pressure systems and cyclones in the Bay add more rain → coastal Tamil Nadu floods while most of India is dry.

• East vs west India (exercise 2(iv)):

Feature	Eastern India	Western India
Type	Riverine (river overflow)	Flash floods
Duration	Long, sometimes weeks	Sudden and short
Main cause	Large, silt-laden Himalayan rivers, flat plains, very heavy monsoon rain	Short bursts of intense rain on dry ground, blocked channels
Examples	Brahmaputra (Assam), Kosi (Bihar)	Rajasthan, Gujarat

5. Key flood rivers and events

• Brahmaputra (Assam):
• Rises from the Chemayungdung glacier near Lake Mansarovar in Tibet, where it is called the Tsangpo.
• Enters Arunachal Pradesh as the Siang/Dihang, and joins the Dibang and Lohit near Sadiya to become the Brahmaputra.
• Flows west through the Assam valley. In Bangladesh it is called the Jamuna, and it reaches the Bay of Bengal through the Padma-Meghna system.

• It floods every year and also eats away its banks (bank erosion), so Assam loses land each year.

• Majuli: an island in the Brahmaputra (Assam) and the largest riverine island. Floods leave fresh silt on it, which keeps it fertile, but bank erosion is shrinking it.

• Kosi, the "sorrow of Bihar":
• Rises in Tibet and Nepal from seven streams, so it is also called the Sapt Kosi (main streams: Arun, Sun Kosi, Tamur).
• Joins the Ganga near Kursela (Katihar, Bihar).
• It carries very heavy silt and keeps shifting its course.

• Kusaha breach (August 2008): the embankment broke at Kusaha in Nepal. The river swung east into an old course and flooded Supaul, Araria, Madhepura, Saharsa and Purnia districts of north Bihar.

• Kerala floods (August 2018): extreme monsoon rain filled the reservoirs, and dam shutters (including Idukki) had to be opened. It was the state's worst flood in about a century.

6. Consequences of floods

• Losses: crops, roads, rail tracks, bridges and homes are destroyed, and people are displaced.
• Disease: floodwater mixes with sewage, which spreads cholera, gastroenteritis and hepatitis.
• Gain: floods leave fresh fertile silt on farmland, as on Majuli.

7. Flood control

• Structural measures (physical works):
• Embankments: raised earth walls along a river that keep it inside its channel.

• Dams: store floodwater upstream and release it slowly.

• Catchment measures (in the upper area that feeds the river):

• Afforestation: planting trees to slow runoff and hold soil.

• Limits on building in upper catchments.

• Floodplain measures:

• Removing encroachments (illegal buildings) from river channels.

• Depopulating floodplains: moving people to safer ground.

• Coasts: cyclone shelters for areas hit by storm surge.

• Floodplain zoning: the river's floodplain is divided into zones by how often each zone floods, and what may be built in each zone is fixed.
• The Union Government sent a model draft bill on floodplain zoning to all states in 1975 [4].

• Only Manipur, Rajasthan, Uttarakhand and the former state of Jammu & Kashmir have passed such a law [4].

• Flood forecasting (CWC):

• The Central Water Commission (CWC) is the nodal organisation (lead agency) for flood forecasts and early warnings in India [2].
• It issues forecasts for 332 stations: 199 river-level stations and 133 dam/barrage inflow stations. These cover 20 major river basins in 23 states and 2 UTs [2].
• Its basin-wise rainfall-runoff models give 5-day advance flood advisories [2].

• Forecasting is a non-structural measure: it saves lives by giving time to evacuate people and move their belongings, rather than by building walls [2].

• Other options:

• Flood insurance spreads the cost of losses.

• "Room for the river" vs embankments debate: embankments trap silt inside the channel, so the riverbed rises above the land around it, and when the wall breaks the damage is worse (the Kosi in 2008). "Room for the river" (a Dutch approach) instead gives the river space to spread out safely.

• NDMA guidelines: Management of Floods (2008) and Urban Flooding (2010).

8. Flash floods

• Flash flood: a sudden, short flood with little warning. It comes after:
• very intense rain, or
• a cloudburst (a very heavy downpour over a small area in a short time, usually in hills), or

• a dam failure, or a GLOF (Glacial Lake Outburst Flood: a lake held back by glacier ice or loose rock bursts its wall).

• Where: western India (Rajasthan, Gujarat), northern India and the Himalayan hills (Uttarakhand, Himachal Pradesh, Jammu and Kashmir).

• Why so deadly: steep slopes and narrow valleys send water downhill very fast, so there is little time to escape.

9. Urban flooding

• Urban flooding: cities flood even after rain that is not extreme, because:
• drains are too small or blocked, so they overflow;
• concrete and asphalt are impervious surfaces (they do not let water soak in), so almost all rain becomes runoff;

• buildings stand on waterways, lakes and wetlands, so water has nowhere to go.

• Examples:

• Mumbai, 26 July 2005: about 944 mm of rain fell at Santacruz in one day, and the Mithi river overflowed.
• Chennai, December 2015: heavy northeast monsoon rain plus releases from Chembarambakkam lake flooded the Adyar river.

• Bengaluru, 2022: lakes and the drains linking them had been built over, so the water could not flow away.

• New law, Disaster Management (Amendment) Act 2025:

• Got Presidential assent on 29 March 2025 and came into force on 9 April 2025 [7].
• New Section 41A lets states set up an Urban Disaster Management Authority (UDMA) in state capitals and in all cities with a Municipal Corporation (except the NCT of Delhi and Chandigarh) [7].
• The UDMA is chaired by the Municipal Commissioner, with the District Collector as Vice Chairperson [7].
• It prepares an Urban Plan for city risks such as flooding and heatwaves, which the State Authority approves [7].
• Only Karnataka has set one up so far, for Bruhat Bengaluru Mahanagara Palike [7].

10. Case study: Punjab floods 2025 (Class 9)

• Rivers involved:
• Satluj: rises at Rakas Tal near Mansarovar in Tibet, enters India at Shipki La (Himachal Pradesh), and has the Bhakra dam in Bilaspur, Himachal Pradesh.
• Beas: has the Pong dam in Kangra, Himachal Pradesh, and joins the Satluj at Harike in Punjab.
• Ravi: has the Ranjit Sagar dam on the Punjab-Jammu and Kashmir border.

• Ghaggar: a seasonal river from the Shivalik hills that flows through Haryana and Punjab and ends in the Rajasthan desert.

• Natural causes:

• Heavy monsoon rain was made stronger by western disturbances (rain-bearing low-pressure systems that come from the Mediterranean region).
• The rain fell over Punjab, Himachal Pradesh and Jammu and Kashmir.
• The rivers were already high, so they overflowed.

• Very high inflows reached the reservoirs: 3,49,522 cusecs into Pong and 1,90,603 cusecs into Bhakra [6]. A cusec is one cubic foot of water per second.

• Dam releases: releases were decided by a Technical Committee of the partner states (Punjab, Haryana, Rajasthan), the CWC and the BBMB (Bhakra Beas Management Board). It had to balance the rule curve (the set water level a reservoir should hold on each date), dam safety and the limited carrying capacity of the Satluj and Beas downstream [6]. Whether these releases made the flood worse became a major question.

• Human causes:
• Weak, old embankments called dhusi bandh.
• Houses and farms built close to the rivers.
• Silted rivers and dams that hold less water.

• Late or unclear warnings.

• Effects:

• Deaths and people moved into relief camps.
• Damage to the paddy crop and losses of dairy animals and poultry.
• Damaged roads, bridges and border fences.

• Waterborne disease.

• Official toll (as on 27 November 2025, state government data): 40 deaths, 7,161 cattle lost, 14,065 houses damaged, and 1.93 lakh ha of cropped area damaged by hydro-meteorological disasters in 2025 [5].

• Response:
• 35 NDRF teams (National Disaster Response Force) were sent for rescue and relief [5].
• The Prime Minister made an aerial survey of the affected areas on 9 September 2025 [5][8].
• All 23 districts were declared disaster-affected (verify current).

Prelims Hooks

• Rashtriya Barh Ayog: set up 1976, report 1980, found 40 mha flood-prone. The 2011 Working Group put the maximum area ever flooded at 49.815 mha [3].
• Model Floodplain Zoning Bill, 1975: enacted only by Manipur, Rajasthan, Uttarakhand and the former J&K [4]. Trap: Assam and Bihar, the most flood-prone states, have not enacted it.
• CWC is the nodal agency for flood forecasting (not IMD): 332 stations (199 level + 133 inflow) [2].
• Tamil Nadu floods in November-January from the retreating (northeast) monsoon.
• Majuli (Brahmaputra, Assam) is the largest riverine island.
• Kosi = "sorrow of Bihar". The Kusaha breach (2008) happened in Nepal, not in Bihar.
• Dam-river pairs: Bhakra on the Satluj, Pong on the Beas, Ranjit Sagar on the Ravi.
• Urban flood: Mumbai, 26 July 2005, about 944 mm in a day at Santacruz.
• DM (Amendment) Act 2025: Section 41A allows UDMAs, chaired by the Municipal Commissioner [7].
• Eastern India = riverine, prolonged floods. Western India = flash floods.

Mains Points

• Floods are largely human-made in their severity: deforestation, silted beds, encroached floodplains and blocked drainage turn normal monsoon peaks into disasters. The flood-prone area has risen from 40 mha (1980) to a maximum of 49.815 mha flooded (2011) [3].
• Embankment dependence vs floodplain management: embankments raise riverbeds and fail badly (Kosi 2008, the dhusi bandh in Punjab 2025). Only four states have adopted floodplain zoning since 1975 [4]. Non-structural tools are needed: zoning, CWC's 5-day forecasts [2], flood insurance and "room for the river".
• Reservoir operation and inter-state coordination: Punjab 2025 showed the tension between keeping dams safe and protecting areas downstream (Pong and Bhakra inflows) [6]. Rule curves need review, silt must be cleared, and warnings must reach people in time.
• Urban flooding as a governance gap: impervious cities and lost lakes (Mumbai 2005, Chennai 2015, Bengaluru 2022). The UDMAs under the DM (Amendment) Act 2025 are an institutional fix, but only Karnataka has set one up so far [7].