Landslides, Cyclones and Disaster-Monitoring Technology

What makes a slope fail?

A landslide starts when a slope cannot hold its soil, rock or debris against gravity. The ground may move slowly or fail suddenly.

Separate the slope’s condition from the event that starts movement. Weak rock, steep terrain and an old slide can make a slope susceptible. Heavy rain or earthquake shaking can then trigger failure.

Water entering the ground can increase pressure between particles and reduce resistance to sliding. Saturation is a common cause, but rainfall is not required for every landslide.

Imagine a road cutting into the foot of a slope. Removing supporting material can reduce stability even before the next storm.

A dry-looking surface does not prove the entire slope is stable. Conditions below the surface also matter.

Sources: USGS landslide processes · NCERT hazards

Do all landslides move in the same way?

No. Landslide is a broad term for several kinds of ground movement.

• Slide: material moves along a surface of weakness within the slope.
• Rockfall: detached rocks fall, bounce or roll from a steep face.
• Debris flow: water and loose material move downhill as a flowing mixture.
• Creep: soil or rock moves slowly downslope over time.

These movements create different dangers. A fast rockfall may strike a road with little warning. A debris flow may travel along a gully and reach settlements below its source.

Slow movement deserves attention too. A slope can damage buildings and roads without one dramatic collapse.

Landslide debris may block a river. Water backing up behind the blockage can flood upstream land. If the blockage fails, released water can flood downstream land.

Sources: USGS movement types · NCERT landslide consequences

Why does warm sea water help a cyclone grow?

A tropical cyclone draws energy from warm, moist air above the sea. Warm water increases the supply of moisture through evaporation.

As moist air rises and cools, water vapour condenses into cloud droplets. Condensation releases heat, helping the storm’s rising air and circulation.

Warm water alone is insufficient. The atmosphere must support organized rising air and a rotating circulation. Earth’s rotation helps turn the moving air.

Vertical wind shear means wind speed or direction changes with height. Strong shear can disrupt the storm’s vertical organization.

A cyclone usually weakens after moving inland because its ocean energy supply is reduced. Heavy rain can still continue inland, so landfall does not end every danger.

Sources: NCERT cyclone formation · IMD cyclone FAQ

Is the eye the most dangerous part?

The eye is the relatively calm, low-pressure centre of a sufficiently developed tropical cyclone. Some cyclones have no clearly visible eye.

The surrounding eyewall contains the strongest surface winds and deep storm clouds. Do not confuse a calm centre with a weak storm.

Someone under the eye may experience a temporary lull while the storm continues around them. Strong winds can return as the other side passes.

The eye is a named part of the storm. A vortex describes rotating flow more generally. A funnel is not the name for a cyclone’s calm centre.

Rainbands extend beyond the centre. Dangerous weather can affect places far from the eye and before the centre reaches land.

Sources: IMD eye and eyewall · NASA cyclone imagery

Why are the Himalayas and Western Ghats vulnerable?

The Himalayas combine steep slopes, relatively young mountain terrain and earthquake activity. Rainfall and human disturbance can add to instability.

The Western Ghats also contain steep slopes and areas of heavy rainfall. Water entering weathered slope material can contribute to landslides.

Both regions face landslides, but their geological settings differ. Avoid explaining every Himalayan and Western Ghats landslide through one identical cause.

Construction can remove support at a slope’s foot or add weight near its top. Mining, vibration, deforestation and poorly managed water can also increase instability.

A hypothetical road-cut example shows the interaction. A steep slope may hold during dry weather, then fail after excavation and sustained rain.

The example explains a mechanism. It does not identify the cause of a particular real disaster.

Sources: NCERT regional vulnerability · USGS human causes

What do the four vulnerability categories mean?

The four broad categories describe regional patterns. They do not give a safety certificate for every building site.

• Very high vulnerability: unstable Himalayan and Andaman–Nicobar mountain terrain, northeast hills, and steep, wet Western Ghats and Nilgiris slopes.
• High vulnerability: Himalayan states and northeast states, with the Assam plains excluded from this broad description.
• Moderate to low vulnerability: lower-rainfall areas such as Ladakh, Spiti and the Aravali, plus rain-shadow and plateau areas.
• Other areas: broadly lower-vulnerability regions, including extensive plains and some southern coastal areas.

Intensity, frequency and combinations of controlling factors distinguish the higher categories. Their regional descriptions therefore overlap.

The lower-rainfall category still includes occasional landslides. Mining and subsidence add hazards in Jharkhand, Odisha, Chhattisgarh and other mining regions.

The “Other Areas” category includes Rajasthan, Haryana, Uttar Pradesh, Bihar, West Bengal and Assam. Darjiling and Karbi Anglong are explicit exceptions.

Regional classifications simplify a complex landscape. Local riverbanks, excavations and mine-waste piles can fail even in generally low-relief terrain.

Broad regional categories and local slope assessments describe different scales. A local assessment remains necessary even in generally low-relief terrain.

Sources: NCERT four categories · USGS low-relief failures

Why does the Bay of Bengal have more cyclones?

The Bay of Bengal has more tropical cyclone formation than the Arabian Sea in the average basin comparison.

The approximate formation ratio is 4:1. This is an average comparison, not a promise about each year’s storms or India’s landfalls.

Storms can develop within the Bay. Disturbances moving from the western Pacific can also contribute to activity there.

The Arabian Sea also produces damaging cyclones. A lower average frequency does not make India’s western coast cyclone-free.

On the east coast, West Bengal, Odisha, Andhra Pradesh and Tamil Nadu provide familiar coastal examples. Actual danger depends on the storm’s track, strength and local coastal conditions.

Formation, landfall and damage are different measurements. A storm forming in a basin need not strike the nearest Indian coast.

Sources: IMD basin frequency and origins · NCERT cyclone distribution

Why can a cyclone damage places beyond its wind path?

A cyclone creates several hazards together. Wind, heavy rain and storm surge can damage different places in different ways.

• Strong winds damage weak buildings, trees, power lines and exposed structures.
• Heavy rain can cause flooding, including after the storm moves inland.
• Storm surge raises coastal sea level above the expected tide during a storm.

Low coastal ground can flood when seawater is pushed inland. The storm’s winds and local coastal shape affect the impact.

A tsunami has a different origin, often an undersea earthquake that displaces water. Storm surge is associated with storm conditions.

Do not wait for the eye to arrive before taking warnings seriously. Rain, waves and strong winds may reach a coast earlier.

Sources: IMD cyclone impacts · NCERT storm surge and tsunami

What is the difference between sensing and GIS?

Remote sensing collects observations from a distance. Satellite instruments can observe clouds, water, land cover and changes on Earth’s surface.

GIS organizes information by location. It lets planners combine observations with roads, settlements, hospitals and hazard maps.

A satellite image and a map layer perform different jobs. The image records an observation; GIS helps analyse where that observation intersects other information.

Imagine a mapped landslide-prone slope above a road and village. Adding those locations helps planners see who and what could be affected.

Adding shelter and route information helps planning further. The safest decision still depends on current conditions and reliable ground information.

A beautifully coloured map can contain old or incomplete data. Combining layers improves analysis only when the layers suit the question being asked.

Sources: NCERT spatial information · ISRO disaster services

How do observations become warnings?

A warning system connects observations, analysis and communication with action. No single instrument completes the whole chain.

• Observe: collect satellite images, radar measurements, rainfall records and local information.
• Analyse: compare changing conditions with maps, models and hazard knowledge.
• Warn: communicate likely impacts to officials and exposed communities.
• Act: use evacuation plans, shelters and local safety instructions.

For cyclones, satellite and radar observations help estimate position, structure and movement. Forecast models help assess future tracks and intensity.

For landslides, rainfall and slope observations can identify dangerous conditions. Mapping previous landslides helps show where investigation and monitoring are needed.

A warning that never reaches a fishing boat or isolated village offers little protection. Communication and preparedness connect the forecast with reduced losses.

The diagram shows why monitoring alone is insufficient. Observations must guide a decision, and the decision must reach people who can act.

Sources: IMD warning services · ISRO disaster support

What can radar reveal about a moving slope?

Synthetic Aperture Radar (SAR) uses radar signals to image Earth’s surface. Interferometric SAR compares radar observations to measure changes in the ground’s position.

Repeated observations can reveal ground movement that is hard to notice in ordinary photographs. Surface changes may help identify slopes needing closer investigation.

Radar measurements require careful interpretation. The observation schedule, ground conditions and usable reflected signals affect what can be measured.

A detected movement is evidence to investigate. It is not a guarantee that the exact time of a future landslide is known.

The Centre for Development of Advanced Computing (C-DAC) and Geological Survey of India (GSI) agreed to collaborate on geoscience applications in 2026.

The planned work includes slope-scale landslide monitoring using SAR and development of early-warning systems. The agreement also covers glacier-lake outburst-flood studies.

An agreement’s scope describes intended collaboration. It does not prove that every Indian slope already has an operational radar warning system.

Sources: PIB collaboration announcement · C-DAC collaboration scope · USGS radar interferometry

Does a hazard automatically become a disaster?

No. A hazard is a potentially damaging event or process. Disaster losses also depend on who is exposed and how vulnerable they are.

• Exposure: people, buildings and other assets located where a hazard can affect them.
• Vulnerability: conditions that make those people or assets more likely to suffer damage.
• Preparedness: plans and abilities that help people respond before and during an emergency.

Two coastal villages may face similar winds. Stronger buildings, accessible shelters and earlier evacuation can reduce losses in one village.

Moving a new settlement away from an unstable slope reduces exposure. Improving drainage may reduce slope instability. These measures address different parts of the problem.

Land-use decisions therefore matter before a disaster occurs. Building first and relying only on rescue later leaves avoidable exposure in place.

Sources: NCERT disaster management · UNDRR exposure · UNDRR vulnerability

Can retaining walls and trees prevent every landslide?

No. Slope stabilization must suit the ground, water conditions and type of movement. A wall is one possible measure, not a universal cure.

• Drainage directs water away and limits harmful increases in groundwater.
• Retaining structures can add support when designed for the particular slope.
• Vegetation and land management can reduce some forms of surface erosion and instability.
• Development controls keep unsuitable construction away from hazardous ground.

A wall beside a road may support part of a slope. It cannot automatically contain a large rockfall from far above.

Water collecting behind a poorly designed structure can create another problem. Drainage and maintenance belong in the same plan as structural support.

Local geological and engineering assessment is necessary. Treating every slope with the same design ignores how landslides differ.

Sources: USGS slope stabilization · NCERT area-specific mitigation

What do building codes and maritime warnings achieve?

Disaster-resistant building codes guide safer structural design. Appropriate construction can reduce damage from earthquake shaking and cyclone winds.

Building codes do not stop the earthquake or cyclone from occurring. Following and enforcing the design requirements is part of reducing vulnerability.

Cyclone shelters and evacuation plans protect people when remaining in exposed homes is unsafe. Local warnings help communities decide when to act.

Maritime warnings provide information for ports, ships and fishing communities. Avoiding dangerous sea conditions reduces exposure.

Warnings must be read for the location and time involved. A forecast for one stretch of coast does not describe every boat’s conditions.

A sound disaster plan combines safer buildings, land-use decisions, monitoring and preparedness. None of these measures replaces the others.

Sources: NCERT preparedness and buildings · IMD coastal and marine warnings

How should you compare the two hazards?

Remember the hazard–damage distinction. A forecast or building code can reduce losses without preventing the natural event.

Sources: NCERT hazards · IMD cyclone FAQ · USGS landslides