Landslides and land subsidence

Detail

What a landslide is

• Landslide: a large mass of rock, soil and debris moves quickly down a slope because of gravity. It moves along a failure plane, also called a shear plane. This is a surface of weakness where the upper mass breaks away from the ground below.
• A landslide looks less dramatic than an earthquake. But it does serious harm to the environment and the economy.
• Example: the road to Srinagar (Jammu and Kashmir) is often blocked by landslides.

• Example: rock falls repeatedly hit the Konkan Railway along the Western Ghats coast.

• Local factors control landslides. These are the rock type (geology), geomorphic agents (running water, ice and wind that shape the land), the steepness of the slope, land use, vegetation cover and human activity.

• These factors change from one hillside to the next.
• So a landslide is hard to predict, and monitoring is slow and costly.

Causes: step by step

• Heavy, continuous rain
• Water soaks into the soil and rock, so the slope becomes heavier.
• The water also cuts the friction that holds grains and rock blocks together.

• The slope fails and the mass slides down.

• Earthquakes and volcanic shaking: shaking loosens rock on a slope. Areas hit by earthquakes stay unstable for years afterwards.

• Steep slopes and loose rock: on a steep slope, gravity pulls harder along the surface. Loose rock that is not packed together (unconsolidated rock) gives way easily.
• Human causes: cutting forests (deforestation), mining, building roads, unplanned building on hillsides and poor drainage.
• Cutting into a slope for a road removes the support at its foot (the toe).
• Water from blocked drains seeps into the slope.
• The slope becomes weak and fails.

Why the Himalayas and the Western Ghats slide

Feature	Himalaya	Western Ghats and Nilgiris
Age and stability	Young fold mountains that are still rising and tectonically active (plate movement still goes on beneath them)	Old, stable block of the Peninsular plateau
Rock	Sedimentary and unconsolidated rocks that break easily	Hard crystalline rocks
Main triggers	Steep slopes, earthquakes, monsoon rain, road and dam building	Steep cliffs on the western face, very intense monsoon rain
Weathering	Freezing and thawing at height, plus erosion	Mechanical weathering caused by temperature change: rock heats by day and cools by night, so it cracks and breaks apart
Result	Landslides are very frequent	Landslides happen even though the rock is hard

Landslide vulnerability zones of India (NCERT, Figure 6.9)

• Landslide vulnerability zones: areas grouped by how likely the land is to slide. The grouping uses geology, slope, rainfall, earthquake activity and human activity.

Zone	Areas
Very high	Young, unstable Himalaya; Andaman and Nicobar Islands; high-rainfall, steep-sloped Western Ghats and Nilgiris; the north-eastern region; areas often shaken by earthquakes; areas with heavy road and dam building
High	Conditions are like the very high zone, but the factors are less severe. Covers all Himalayan states and north-eastern states, except the Assam plains
Moderate to low	Trans-Himalayan Ladakh and Spiti (Himachal Pradesh), which get little rain; the Aravalis; rain-shadow parts of the Western and Eastern Ghats and the Deccan plateau (a rain shadow is the dry side of a mountain, away from the wind). Landslides caused by mining and subsidence occur in Jharkhand, Odisha, Chhattisgarh, Madhya Pradesh, Maharashtra, Andhra Pradesh, Karnataka, Tamil Nadu, Goa and Kerala
Other areas (largely safe)	Rajasthan, Haryana, Uttar Pradesh, Bihar, West Bengal (except Darjiling district), Assam (except Karbi Anglong), and the southern coastal plains

• Trap: Darjiling (West Bengal) and Karbi Anglong (Assam) are hilly pockets inside states that are otherwise "safe".
• National extent: about 0.42 million sq km, or 12.6% of India's land area (not counting snow-covered areas), is prone to landslides (GSI) [2].

Consequences

• Roads and railways get blocked for long periods.
• Rock falls damage houses, crops and power and communication lines.
• River damming and diversion
• Debris falls into a river channel and blocks it.
• A temporary lake forms behind the blockage.

• The natural dam bursts or the river changes course, and floods follow downstream.

• Travel and trade between places (spatial interaction) become costly. This slows development in hill areas.

Mitigation

• Landslide mitigation: steps taken to reduce how often landslides happen or how much harm they do. These steps must suit each area.
• Restrict roads, large dams and big settlements in high-vulnerability zones.
• Keep farming to valleys and areas with gentle (moderate) slopes.
• Grow forests on a large scale (afforestation).
• Build bunds (low banks of earth or stone) to slow the flow of water down the slope.
• In the north-east, use terrace farming in place of jhum.
• Jhum is shifting cultivation: farmers clear forest by slash-and-burn, farm the land for a few years, then move on.
• It leaves slopes bare and unstable.
• Terraces are flat steps cut into the hillside. They slow runoff and hold the soil in place.

National mapping and early warning

• National Landslide Susceptibility Mapping (NLSM) by GSI covers all landslide-prone hill areas at 1:50,000 scale. Landslide susceptibility means how likely an area is to slide.
• Covers the Himalaya, the Tertiary belt of north-east India (young, soft rocks) and the Western Ghats.
• Spread over parts of 19 states/UTs, about 4.3 lakh sq km.

• 33,904 landslides were checked on the ground (field-validated). New post-disaster data is added every year [7].

• National Landslide Forecasting Centre (NLFC): set up by GSI and inaugurated on 19 July 2024.

• It warns of landslides caused by rain.

• It uses rainfall thresholds (the amount of rain beyond which slopes usually fail), Numerical Weather Prediction (NWP) models (computer weather forecasts) and observed rainfall [7].

• Landslide Atlas of India: released by NRSC (ISRO) on 18 March 2023 [3].

• It is an inventory of about 80,000 landslides mapped between 1998 and 2022 [3].
• It covers 17 states and 2 UTs in the Himalaya and Western Ghats and ranks 147 districts by how exposed they are to landslides [3].
• It used images from IRS-1D, the Resourcesat and Cartosat series, Sentinel-1 and 2, Pleiades and WorldView satellites [3].
• The atlas has three kinds of inventory: seasonal (rainy seasons of 2014 and 2017), event-based (for example Kedarnath and Kerala) and route-wise (tourist and pilgrim routes) [3].
• It also ranks the districts of Himachal Pradesh by landslide risk exposure [2].
• Rudraprayag (Uttarakhand) ranks as the most exposed district, followed by Tehri Garhwal. Mizoram and Uttarakhand are among the most affected states.

Case study 1: Malpa landslide (1998)

• Date and place: 18 August 1998, at Malpa in the Kali river valley, Pithoragarh district, Uttarakhand. Here the Kali river forms the India–Nepal border.
• Toll: about 220 people died, including pilgrims on the Kailash Mansarovar Yatra (route to Tibet through the Lipulekh pass).
• Cause: steep valley walls, weak Himalayan rock and heavy monsoon rain. Rockfall and debris buried the village (NCERT exercise 1(ii)).

Case study 2: Wayanad landslides (2024)

• Date and place: 30 July 2024, in the villages of Mundakkai, Chooralmala and Punchirimattom, Wayanad district, Kerala. Wayanad sits on the Western Ghats.
• Type: debris flows: mud, boulders and uprooted trees mixed with water and rushed down the stream valleys like a river of debris.
• Trigger: extreme rain falling on soil already soaked by weeks of monsoon rain.
• Toll: over 200 people died.
• Warnings: the Union Home Minister told Parliament that the Centre sent early warnings to Kerala on 23, 24 and 25 July 2024. On 26 July it warned of more than 20 cm of rain and possible landslides that could kill people [6].
• Policy link: the disaster brought back the debate on two reports about Ecologically Sensitive Areas (ESAs). ESAs are zones where mining, quarrying and big building projects are restricted.
• Gadgil Committee report (2011), from the Western Ghats Ecology Expert Panel.
• Kasturirangan Committee report (2013), from the High-Level Working Group.

Land subsidence

• Land subsidence: the ground surface sinks, slowly or suddenly. NCERT's Table 6.1 lists it as a terrestrial (land-based) natural hazard.

Cause	Process	Example
Mining	Coal is dug out underground → empty spaces are left → the roof rock caves in	Jharia coalfield (Dhanbad, Jharkhand) and Raniganj coalfield (West Bengal), both in the Damodar valley
Underground coal fires	Coal seams burn underground → the burnt-out space collapses	Jharia, where fires have burned for decades
Groundwater pumping	Too much water is pumped out → pore spaces in clay and silt close up → the land sinks	Fast-growing cities on alluvial plains
Natural collapse	Limestone dissolves underground, or loose old debris settles	Karst areas (limestone regions with caves and sinkholes); towns built on old landslide debris

Case study 3: Joshimath land subsidence (2023)

• Place: Joshimath, Chamoli district, Uttarakhand. It sits above the Alaknanda valley and is a gateway town for Badrinath and Auli.
• What happened: in January 2023, wide cracks opened in houses, roads and fields.
• A strip of land about 350 metres wide was affected [4].
• At a high-level review on 8 January 2023, it was reported that one NDRF team and four SDRF teams had reached the town [4].
• The district administration moved affected families to safer places [4].

• Hundreds of families were relocated in total.

• Why it happened

• The town is built on old landslide debris: loose material that settles easily.
• Poor drainage and no proper sewer system let water seep into the ground.
• Unplanned, heavy building put extra load on the slope.

• Nearby Tapovan-Vishnugad hydropower tunnelling and road work disturbed the slope.

• Response: the State Government stopped all building work in the Joshimath area. This included the Tapovan-Vishnugad power project and the Helang-Marwari bypass road [5].

• Satellite monitoring: ISRO's satellite data showed the ground sinking fast in the weeks around January 2023.

Prelims Hooks

• 12.6% of India's land area (about 0.42 million sq km, not counting snow-covered areas) is landslide-prone (GSI) [2].
• Landslide Atlas of India: by NRSC/ISRO, released in March 2023. It maps about 80,000 landslides (1998-2022) and ranks 147 districts [3]. Rudraprayag is the most exposed district.
• NLSM is mapped at 1:50,000 scale. GSI's National Landslide Forecasting Centre was inaugurated in July 2024 [7].
• Malpa landslide (1998): Kali valley, Pithoragarh, Uttarakhand. It hit Kailash Mansarovar pilgrims.
• Wayanad (Kerala), 30 July 2024: Mundakkai, Chooralmala, Punchirimattom. These were debris flows.
• Joshimath is in Chamoli district (not Pithoragarh or Rudraprayag). Project linked: Tapovan-Vishnugad [5].
• Trap: Ladakh and Spiti are in the moderate to low zone, not very high, because they get little rain. Darjiling and Karbi Anglong are vulnerable pockets inside otherwise "safe" states.
• Trap: the Western Ghats are made of hard, stable rock yet fall in the very high zone, because of steep cliffs, intense rain and weathering caused by temperature change.
• Jharia (Jharkhand) and Raniganj (West Bengal) are known for subsidence caused by coal mining and coal fires. NCERT lists subsidence as a terrestrial hazard.

Mains Points

• Distribution (GS-I): landslides cluster in places where weak geology meets heavy rain and human pressure.
• The young Himalaya and the north-east slide because of active tectonics and soft rock.
• The Western Ghats slide because of steep scarps (cliff faces) and intense orographic rain (rain caused when moist wind is forced up a mountain).

• Mining belts on the plateau face subsidence instead.

• Development versus fragility (GS-III): Joshimath (2023) and Wayanad (2024) show how roads, tunnels, quarries and unplanned towns weaken unstable slopes.

• Answer frame: carrying-capacity studies for hill towns; Gadgil and Kasturirangan ESA zoning; stricter environmental clearances for hydropower and highway projects.

• Moving from relief to risk reduction (GS-III, Sendai Framework)

• Map the risk: GSI's NLSM at 1:50,000 and NRSC's Landslide Atlas [7][3].
• Forecast rain-triggered landslides through the NLFC [7].

• Plan land use in high zones, follow the NDMA landslide guidelines (2009) and turn warnings into evacuations at the local level.

• Mitigation package: afforestation, bunds, terracing in place of jhum, proper slope drainage and retaining structures. In mining areas, fill old mine voids with sand (sand stowing), put out coal fires and resettle people living over unstable ground.