Natural Hazards and Disasters

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (India: Physical Environment), Chapter 6.

When does a shaking ground become a disaster?

Two identical earthquakes strike two places. One is a forest in a mountain valley. The other is a crowded port city with tall buildings on reclaimed land. Only the second makes international news. The ground shook equally, yet only one event is called a disaster. This chapter is about that gap between nature's event and human loss, and about the six hazards that matter most in India: earthquakes, tsunamis, tropical cyclones, floods, droughts and landslides.

Hazard versus disaster

A natural hazard is a feature or circumstance of the natural environment that has the potential to harm people or property or both. It can be swift, like a tsunami, or a permanent part of the setting, like ocean currents, steep slopes and unstable rock structures in the Himalayas, or the extreme climate of deserts and glaciated areas.

A natural disaster is the harm actually done. It is relatively sudden, it kills and destroys on a large scale, and it disrupts society in ways people cannot control. An event becomes a disaster when the magnitude of destruction and damage is very high.

The official definition adds three tests: it is an undesirable occurrence from forces largely outside human control, it strikes quickly with little or no warning, and it needs more effort than normal emergency services can provide.

Why it matters. Not every hazard becomes a disaster. The missing ingredient is vulnerability: people, buildings and activities sitting in the path of the hazard.

Who causes disasters? Not nature alone

For a long time, geography treated disasters as nature's work and people as helpless victims. That view is now incomplete, because human actions cause some disasters directly and make others worse indirectly.

Two views. For the exam: a disaster is a mix of natural hazard and human vulnerability, and human made disasters are rising in number and size. Another reading: some scholars argue that the word "natural" in "natural disaster" hides the human choices behind the losses, such as where cities are built and who is pushed onto risky land. Disaster scholars call this the social production of vulnerability.

The change in thinking has a clear logic. Earlier, communities in hazard prone areas avoided disturbing the delicate balance of their ecosystem, so disasters did less damage. Technology has since given people the power to intensify activity in risky places, so vulnerability has grown. Colonising the flood plains of rivers, and building large cities and port towns such as Mumbai and Chennai right on the coast because land there is costly, expose millions to cyclones, hurricanes and tsunamis.

India's vastness, environmental diversity, large population, a long colonial past and continuing social discrimination have all increased its vulnerability. About two thirds of its area, and an equal share of its population, is vulnerable to disasters.

Four families of natural disasters

Classifying disasters helps governments prepare the right response. Learn the grid below, because "which category?" is a favourite question.

Pause and predict: a tropical cyclone starts in the air but its worst damage is a rush of sea water over the coast. Which of the two categories does the cyclone itself belong to, and which does the storm surge belong to?

Check your answer: the cyclone is Atmospheric. The storm surge is Aquatic. The grid sorts events by what the event is, not by where the damage falls.

The world starts to organise: Yokohama and after

The Yokohama conference noted that disasters hit the poor and disadvantaged hardest, especially in developing countries that are ill equipped to cope. It stressed that every country has the sovereign responsibility to protect its citizens, gave priority to developing, least developed, landlocked and small island countries, urged stronger national capacity and law with the help of local communities and non governmental organisations, and promoted regional and international cooperation in capacity building, sharing of technology and mobilising resources.

Earthquakes: the least predictable disaster

Earthquakes are the most unpredictable and highly destructive of all natural disasters. The most devastating are tectonic earthquakes, caused by a sudden release of energy during movements in the earth's crust, and they affect a very large area. Earthquakes linked to volcanic eruptions, rockfalls, landslides, mining subsidence or reservoirs filling behind dams affect only a limited area and cause less damage.

Why the Himalayas shake. The Indian plate moves about one centimetre per year towards the north and north east, and the Eurasian plate keeps blocking it. The two plates are effectively locked together, so energy builds up as stress. When the lock breaks, the sudden release is an earthquake along the Himalayan arch.

The most vulnerable parts of India are Jammu and Kashmir, Ladakh, Himachal Pradesh, Uttarakhand, Sikkim, the Darjeeling subdivision of West Bengal and all seven north eastern states. The central western parts of India have also had severe earthquakes: Gujarat in 1819, 1956 and 2001, and Maharashtra in 1967 and 1993. Scientists find it hard to explain quakes in the old, stable Peninsular block, and one recent idea points to a fault line along the Bhima (Krishna) river near Latur and Osmanabad in Maharashtra.

India has been divided into five zones, based on a study of more than 1,200 past earthquakes by the National Geophysical Laboratory, the Geological Survey of India, the Department of Meteorology and the National Institute of Disaster Management.

Effects of earthquakes. The shaking harms three things at once, and chain effects are possible in each.

Earthquakes also tear open fissures through which water and other volatile material gush out, and they trigger landslides that can block rivers and form reservoirs. Sometimes rivers change course and cause floods. Because transport and communication links are wrecked, relief reaches victims slowly.

What can be done. An earthquake cannot be prevented, so the answer is preparedness and mitigation, not cure: seismological monitoring centres (GPS helps track the movement of plates), vulnerability maps and public education, changes in house types and building designs with fewer high rise buildings, large industries and big urban centres in vulnerable areas, and mandatory earthquake resistant design with light materials for major construction.

Tsunami: the wave that hides at sea

A tsunami (harbour wave, or seismic sea wave) is a high vertical wave created when an earthquake or volcanic eruption suddenly moves the sea floor and displaces ocean water. After the first big disturbance a series of afterwaves follows, swinging between high crest and low trough as the water settles.

Over deep water the wave has a very long wavelength and a small height, so a ship at sea is lifted only a metre or two, slowly, and the rise and fall take several minutes. That is why a tsunami is so hard to detect in the open ocean. As it enters shallow water near the coast its wavelength shortens while its period stays the same, and its height rises, sometimes to 15 metres or more.

Beyond the textbook. For the exam: the impact of a tsunami is small over the open ocean and large near the coast, because the wave changes with water depth. If a question uses the textbook's wording about speed, the book says the wave is faster in shallow water. In reality: the speed of a tsunami grows with water depth, so it travels fastest, at close to the speed of a jet aircraft, over deep ocean and slows sharply as it nears the shore. The slowing is exactly what squeezes the wave's energy into greater height. Why it matters: the depth link is what you need for the exam, and the physics shows why the same wave is harmless offshore and deadly on the beach.

Tsunamis are frequent around the Pacific ring of fire, notably Alaska, Japan, the Philippines, and the island and coastal countries of South east Asia, plus Sri Lanka and India. Coasts are densely populated centres of human activity, so tsunami losses are far larger than for other coastal hazards. The tsunami of 26 December 2004 killed more than 300,000 people. Losses are too large for any one government to handle, so mitigation needs international cooperation, and India joined the International Tsunami Warning System after 2004.

Tropical cyclones: heat engines of the sea

A tropical cyclone is an intense low pressure area, confined between 30°N and 30°S, around which high velocity winds blow. It extends 500 to 1,000 km horizontally and 12 to 14 km vertically. Think of it as a heat engine: warm, moist air from the ocean condenses, releases latent heat, and that heat powers the storm.

Four starting conditions are needed. Learn them as a set.

Why not near the equator? The Coriolis force is absent near the equator, so no cyclone forms between 0° and 5° latitude.

Where cyclones form for India. Because of its peninsular shape, India's cyclones arise over the Bay of Bengal on the east and the Arabian Sea on the west. Most originate between 10° and 15° N in the monsoon season. In the Bay of Bengal, cyclones mostly develop in October and November, west of 92°E. By July their birthplace shifts to around 18°N and west of 90°E, near the Sundarban delta.

Pause and predict: a cyclone crosses the coast and keeps moving inland for hundreds of kilometres. What happens to its strength, and why?

Check your answer: it weakens. Its energy comes from latent heat released by warm moist air over the sea. Move away from the sea and the supply fades, so the force falls as distance from the Bay of Bengal or the Arabian Sea increases.

Storm surge. Coastal areas are often hit by severe cyclonic storms with an average velocity of about 180 km/h. The very high horizontal pressure gradient and very strong surface winds push sea water across the coast along with heavy rain. This abnormal rise in sea level is called a storm surge. It inundates settlements and fields, damages crops and destroys structures.

Cyclones are more predictable in time and place than earthquakes, tsunamis and volcanic eruptions, and monitoring techniques now track their intensity, direction and magnitude. So management is possible: cyclone shelters, embankments, dykes, reservoirs and afforestation to slow the winds. Even so, losses keep rising in countries like India, Bangladesh and Myanmar, largely because so many people live in vulnerable coastal areas.

Floods: slow, predictable, human assisted

A flood is the inundation of land and settlements when water rises in a channel and spills over. Unlike most disasters, the causes of floods are well established, they are relatively slow to build, and they occur in well identified regions at expected times of the year.

Floods occur when surface run off exceeds the carrying capacity of rivers and streams and spreads over neighbouring low lying flood plains. Other causes are a storm surge on the coast, very heavy rain for a long period, melting ice and snow, reduced infiltration and a load of eroded material in the water from rapid soil erosion.

Floods in China, India and Bangladesh are especially frequent and severe. Humans help create and spread floods through indiscriminate deforestation, unscientific farming, disturbing natural drainage channels and settling in flood plains and river beds.

Floods bring harm and one benefit. They destroy crops, damage roads, rails and bridges, leave millions homeless and spread cholera, gastro enteritis and hepatitis. But they also deposit fertile silt on farmland. Majuli in Assam, the largest riverine island in the world, grows good paddy after the annual floods of the Brahmaputra. The text adds that this benefit is insignificant beside the grave losses.

Control measures are flood protection embankments, dams, afforestation, discouraging major construction in the upper reaches of flood creating rivers, removing encroachments from river channels and depopulating flood plains. Cyclone centres give relief in coastal areas hit by a storm surge.

Droughts: when water runs short

Drought is an extended period of water shortage caused by inadequate precipitation, excessive evaporation and over use of water from reservoirs and groundwater. It is complex because it combines meteorology (precipitation, evaporation, groundwater, soil moisture, run off) with farming practices such as the crops grown, plus socio economic and ecological conditions.

A rule that surprises students: if an area has more than 30 per cent of its gross cropped area under irrigation, it is left out of the drought prone category.

About 19 per cent of India's area and 12 per cent of its population suffer from drought every year. About 30 per cent of the country is drought prone, affecting around 50 million people. The cause is the monsoon: it varies hugely and unpredictably, so one region can flood in one season and parch in another.

Consequences. Crop failure brings scarcity of grain (akal), of fodder (trinkal) and of water (jalkal). When all three occur together it is trikal, the most devastating form. Cattle die, people and livestock migrate, and contaminated water spreads gastro enteritis, cholera and hepatitis.

Management. Short term steps are safe drinking water, medicines, fodder and water for cattle, and shifting people and livestock to safer places. Long term steps are identifying groundwater aquifers, transferring river water from surplus to deficit areas, inter linking of rivers, building reservoirs and dams, using remote sensing and satellite imagery to find suitable basins, spreading drought resistant crops with training, and rainwater harvesting.

Landslides: small, local, costly

A landslide is the rapid sliding of a large mass of bedrock. It is less dramatic than an earthquake or cyclone but its impact on the environment and the economy is just as serious. What sets it apart: most disasters are driven by large regional factors, while landslides are controlled by highly localised ones, namely geology, geomorphic agents, slope, land use, vegetation cover and human activity. That makes monitoring difficult and expensive.

Landslides block roads and rail lines, choke channels with rockfalls and can divert river courses, which leads to floods. They make movement between places risky and costly and hold back development.

Mitigation is area specific: restrict road and dam construction and large settlements in high vulnerability zones, limit farming to valleys and moderate slopes, plant forests on a large scale, build bunds to slow water, and encourage terrace farming in the north eastern hill states where jhumming (slash and burn, or shifting cultivation) is still practised.

Disaster management: before, during, after

All hazards need not become disasters, and natural disasters cannot be eliminated. The best option is mitigation and preparedness, in three stages.

The Disaster Management Bill, 2005 defines disaster as a catastrophe, mishap, calamity or grave occurrence from natural or man made causes, or from accident or negligence, causing substantial loss of life or human suffering or damage to the environment, of a magnitude beyond the coping capacity of the affected community.

Beyond the textbook. For the exam: the Disaster Management Bill, 2005 and the National Institute of Disaster Management are the examples of steps taken by the Government of India. In reality: the Bill was passed in 2005 and is now the Disaster Management Act, 2005, under which the National Disaster Management Authority was set up. Why it matters: a current affairs style question may use "Act" and "NDMA", so know that they grew out of the same framework.

Exam traps

60-second revision

Where this chapter connects

• Earlier in the course: Interior of the Earth explains earthquake waves and why the crust moves, and Distribution of Oceans and Continents explains the plate movement that locks the Indian plate against Eurasia.
• Process link: Geomorphic Processes covers landslides, mass movement, weathering and soil erosion at the process level.
• Weather link: Atmospheric Circulation and Weather Systems covers tropical cyclones and the Coriolis force in depth.
• Water link: Drainage System places the flood prone rivers, such as the Brahmaputra, on the map.