Cloudbursts, landslides and floods

The first principle

Water does damage when it arrives faster than the land can absorb or carry it away. A cloudburst delivers a month's rain to a valley in an hour; a landslide happens when water in the soil pushes grains apart and the slope loses the friction that held it; a flood happens when runoff exceeds what a channel or drain can carry. In each case the hazard depends on two things, the rate of supply and the capacity of the ground, and human activity has mostly reduced the capacity, by cutting slopes, sealing soil and filling lakes.

The core

Cloudbursts

The India Meteorological Department (IMD) defines a cloudburst as rainfall of about 100 mm or more in an hour over a small area of roughly 20 to 30 sq km. In the Himalaya, moist monsoon air is forced up steep slopes (orographic lift), cools, and builds a tall cumulonimbus cloud whose strong updraft holds a huge mass of water aloft. When the updraft weakens, or when the cloud is pinned against a valley head, all that water falls in a short burst onto a small catchment. Cloudbursts are worst when a monsoon current meets a Western Disturbance, a low pressure system from the west, because the cold upper air of the disturbance makes the atmosphere more unstable and the convergence feeds more moisture. The Kedarnath disaster of June 2013 followed exactly this meeting, with extreme rain and the breach of the Chorabari glacial lake killing over 5,000 people. Cloudbursts are hard to forecast because radar must catch a single convective cell, and they happen mostly at night in narrow valleys.

Landslides

A slope stays in place while the friction and cohesion holding it exceed the pull of gravity along it. Rain raises the pressure of water in the pores of soil, which pushes grains apart and reduces friction, so most Indian landslides happen in the monsoon. Types include falls of rock from cliffs, slides along a surface, flows of saturated debris and slow creep. The Geological Survey of India (GSI) estimates that about 12.6 per cent of India's land area, excluding snow covered areas, is prone to landslides.

Dimensions

Why landslides differ between the Himalaya and the Western Ghats

• Himalaya. These are young fold mountains still rising as India pushes into Asia, so the rock is folded, faulted and fractured, and earthquakes shake loose material. Valleys are deep and steep, and glacial debris (moraine) lies loose on slopes. Human pressure adds to this: road widening such as the Char Dham project cuts the toe of slopes, tunnels and hydropower works disturb rock, and towns like Joshimath, built on old landslide debris, have sunk as drainage and construction loaded the slope, as seen in January 2023.
• Western Ghats. These are an old, stable escarpment with hard basalt and gneiss and few earthquakes, so the rock itself rarely fails. The danger is a thick layer of weathered soil, often laterite, lying on hard rock. Very heavy monsoon rain soaks this soil until it slides off the impermeable rock below as a debris flow. Deforestation, quarrying, plantation terracing and building on steep slopes remove the roots that bind soil and block natural drainage.
• The Wayanad case. In July 2024 extreme rain on already saturated slopes triggered debris flows at Mundakkai and Chooralmala in Wayanad, Kerala, killing over 200 people, the starkest recent example of the Western Ghats pattern.

Types of floods

India's floods are of four broad kinds. Riverine floods occur on the Brahmaputra and Kosi, rivers carrying huge silt loads from the young Himalaya that raise their beds and shift course; the Kosi breach of 2008 at Kusaha moved the river many kilometres east. Flash floods follow cloudbursts or glacial lake bursts in the hills. Urban floods occur in cities, and coastal floods come with storm surges.

In practice

Why million plus cities, smart ones included, flood

• Lost lakes and nalas. Hyderabad once had thousands of interlinked tanks that stored monsoon water, and many were encroached upon; in October 2020 rain of over 250 mm in a day in parts of the city flooded colonies built on old lake beds. Bengaluru's 2022 floods followed the same pattern, as did Chennai in 2015, where building on the Pallikaranai marsh and the beds of tanks removed natural storage.
• Sealed surfaces. Concrete and asphalt prevent infiltration, so a paved city turns most of its rain into runoff, several times what open land would shed.
• Old, choked drains. Many city drains were designed for 25 to 50 mm of rain an hour and are blocked by silt, plastic and debris.
• Intense short bursts. IMD data show more frequent extreme rain events, as in Mumbai on 26 July 2005, when Santacruz recorded 944 mm in 24 hours, and the high tide stopped drains emptying into the sea.
• Planning failure. The "smart city" label has focused on digital services and roads rather than drainage, so Pune's 2019 flash floods hit new developments along its streams.

Lasting remedies

The National Disaster Management Authority (NDMA) Guidelines on Management of Urban Flooding (2010) ask every city to prepare a stormwater drainage master plan based on its watersheds, to design drains for current rainfall intensity, and to set up local flood warning using automatic rain gauges. Cities should restore lakes, wetlands and floodplains and ban building on them through zoning, and adopt "sponge city" design, as China has since 2015, with permeable pavements, rain gardens and retention ponds. Chennai and Mumbai have built flood warning systems, and Bengaluru's restored lakes show that storage works.

Policy

• National Landslide Risk Management Strategy (NDMA, 2019) calls for landslide hazard mapping, monitoring and early warning, slope stabilisation, building codes for hilly areas, and capacity building.
• Western Ghats reports. The Western Ghats Ecology Expert Panel led by Madhav Gadgil (2011) proposed treating the whole range as ecologically sensitive, with three zones of protection; the High Level Working Group led by K Kasturirangan (2013) narrowed this to about 37 per cent of the area, where mining, quarrying and large construction would be barred. Both have been only partly implemented because states objected.

Issues and way forward

Converting floods into a source of irrigation and navigation

India receives most of its rain in a few weeks of heavy monsoon downpour, so floodwater is a resource that mostly runs to the sea unused. Several approaches can capture it:

• Floodplain and wetland storage. Letting rivers spread onto designated floodplains recharges groundwater and fertilises fields, as the tal wetlands of south Bihar, such as Mokama Tal, do each monsoon.
• Managed aquifer recharge. The Central Ground Water Board's master plan for artificial recharge proposes recharge wells, check dams and flood spreading channels to store surplus monsoon water underground, where it does not evaporate. The traditional ahar pyne system of south Bihar and the tanks of the Deccan stored flood water for winter crops in the same way.
• Multipurpose reservoirs such as those of the Damodar Valley Corporation hold back the flood peak and release water for irrigation and power.
• Inland navigation. The National Waterways Act, 2016 declared 111 national waterways. The Jal Marg Vikas Project on National Waterway 1 (Ganga, Haldia to Varanasi), backed by the World Bank, built terminals and a navigation lock at Farakka, and steady depth from barrages and dredging lets barges run in all seasons.
• River interlinking under the National Perspective Plan proposes moving surplus water from Himalayan rivers to deficit basins; the Ken Betwa link is the first approved, but costs, submergence and ecological harm make the wider plan contested.

There are limits: dredging is costly and silt returns each monsoon, embankments on the Kosi have raised its bed and worsened breaches, and storing floods requires land that is often densely settled. The answer is to combine many small measures rather than rely on a single large project.

Other gaps

• Weak early warning for cloudbursts and landslides. Doppler radar coverage in the hills is thin; IMD has been adding radars, and GSI is piloting regional landslide early warning in Darjeeling and the Nilgiris.
• Unregulated hill construction. Building codes and carrying capacity studies for hill towns, recommended after Joshimath, need enforcement.

Common confusions

• Cloudburst and heavy rain. A cloudburst is a short, intense, localised event over a few square kilometres; heavy rain may last a day over a whole district.
• Landslide and land subsidence. A landslide moves material down a slope; subsidence is the sinking of ground, as at Joshimath, though both may occur together.
• Flash flood and riverine flood. A flash flood rises within hours in small steep catchments; a riverine flood builds over days in large basins.
• Embankment as cure. Embankments protect towns in the short run but trap silt in the channel, so the river bed rises above the land outside.

For the exam

• Keyword bank: cloudburst, orographic lift, Western Disturbance, pore water pressure, debris flow, laterite, ecologically sensitive area, impervious surfaces, stormwater master plan, sponge city, managed aquifer recharge, national waterways, Jal Marg Vikas, interlinking of rivers.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction (data): The Geological Survey of India estimates that about 12.6 per cent of India's land area is prone to landslides, from the young Himalaya to the old Western Ghats.
  • Introduction (report): The Gadgil panel (2011) warned that unregulated quarrying and building in the Western Ghats would make slopes unstable, a warning that Wayanad (2024) bore out.
  • Conclusion: Floods, landslides and cloudbursts are natural, but disasters follow when we reduce the land's capacity to absorb water, so restoring that capacity is the lasting remedy.
  • Conclusion: Cities that give water room through lakes, wetlands and permeable ground, as the NDMA urban flood guidelines urge, will turn floods from a threat into a resource.