Landslide vulnerability zones

Topic: Natural Hazards and Disaster Management in India · NCERT: Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"

Meaning

Landslide vulnerability zones are parts of India grouped by how likely the land there is to slide. The grouping uses geology (rock type), slope, rainfall, earthquake activity and human activity, and NCERT shows it on a map (Figure 6.9). There are four groups: very high, high, moderate to low, and largely safe.

The zones tell planners where to limit roads, dams and towns, and where warnings matter most. 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].

Explanation

How a landslide happens

• 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 weak surface where the upper mass breaks away from the ground below.

• Heavy, continuous rain is the most common trigger

• Water soaks into the soil and rock, so the slope gets heavier.
• The water also reduces the friction that holds grains and rock blocks together.

• The slope fails and the mass slides down.

• Earthquakes shake rock loose. Areas hit by earthquakes stay unstable for years afterwards.

• Steep slopes and unconsolidated rock (loose rock that is not packed together) give way easily.
• Human activity weakens slopes
• Cutting into a slope for a road removes the support at its foot (the toe).
• Blocked drains let water seep into the slope.
• Cutting forests (deforestation), mining and unplanned building add to the damage.

What decides the zone

• Each zone depends on how these local factors combine:
• Geology: young, soft rock or old, hard rock.
• Slope: steep or gentle.
• Rainfall: heavy or little.
• Earthquake activity.

• Human pressure: roads, dams and settlements.

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

The four zones (NCERT, Figure 6.9)

Zone	Areas	Why
Very high	Young, unstable Himalaya; Andaman and Nicobar Islands; Western Ghats and Nilgiris; the north-eastern region; areas often shaken by earthquakes; areas with heavy road and dam building	Weak rock, steep slopes, heavy rain and earthquakes all come together
High	All Himalayan states and north-eastern states, except the Assam plains	Same conditions as the very high zone, but less severe
Moderate to low	Ladakh and Spiti (Himachal Pradesh) in the trans-Himalaya; the Aravalis; rain-shadow parts of the Western and Eastern Ghats and the Deccan plateau	Little rain, or old and stable land. A rain shadow is the dry side of a mountain, away from the moist wind
Other areas (largely safe)	Rajasthan, Haryana, Uttar Pradesh, Bihar, West Bengal (except Darjiling district), Assam (except Karbi Anglong), and the southern coastal plains	Flat land with gentle slopes

• Mining and subsidence landslides happen inside the moderate to low zone. They occur in Jharkhand, Odisha, Chhattisgarh, Madhya Pradesh, Maharashtra, Andhra Pradesh, Karnataka, Tamil Nadu, Goa and Kerala.
• Subsidence means the ground sinks, slowly or suddenly.

Why two very different mountain ranges are both "very high"

Feature	Himalaya	Western Ghats and Nilgiris
Age	Young fold mountains that are still rising. They are tectonically active: the plates beneath them are still moving	Old, stable block of the Peninsular plateau
Rock	Sedimentary and unconsolidated rocks that break easily	Hard crystalline rocks
Triggers	Steep slopes, earthquakes, monsoon rain, building of roads and dams	Steep cliffs on the western face, very heavy monsoon rain
Weathering	Freezing and thawing at height, plus erosion	Mechanical weathering: rock heats up by day and cools at night, so it cracks and breaks apart

Where it occurs

• Himalaya (very high)
• Malpa landslide (18 August 1998): Kali river valley, Pithoragarh district, Uttarakhand. The Kali forms the India–Nepal border here. About 220 people died, including pilgrims on the Kailash Mansarovar Yatra.
• Rudraprayag (Uttarakhand) is the most exposed district in India, followed by Tehri Garhwal.

• The road to Srinagar (Jammu and Kashmir) is often blocked by landslides.

• Western Ghats (very high)

• Wayanad, Kerala (30 July 2024): debris flows in Mundakkai, Chooralmala and Punchirimattom. A debris flow is mud, boulders and trees mixed with water, rushing down a stream valley. Over 200 people died.

• Rock falls repeatedly hit the Konkan Railway along the Western Ghats coast.

• North-east (very high and high): Mizoram is among the most affected states. Karbi Anglong is the vulnerable hilly pocket of Assam.

• Subsidence in hill towns: Joshimath, Chamoli district, Uttarakhand (January 2023). The town is built on old landslide debris.
• Subsidence in mining belts: the Jharia (Jharkhand) and Raniganj (West Bengal) coalfields in the Damodar valley.
• National mapping and warning
• National Landslide Susceptibility Mapping (NLSM) by GSI maps all landslide-prone hill areas at 1:50,000 scale. It covers about 4.3 lakh sq km in parts of 19 states/UTs, and 33,904 landslides were checked on the ground [4].
• The Landslide Atlas of India, released by NRSC (ISRO) on 18 March 2023, maps about 80,000 landslides (1998–2022). It covers 17 states and 2 UTs and ranks 147 districts by how exposed they are [3].
• The National Landslide Forecasting Centre of GSI was inaugurated on 19 July 2024. It warns of landslides caused by rain using rainfall thresholds (the amount of rain beyond which slopes usually fail) and computer weather forecasts [4].

Don't confuse with

• Land subsidence: the ground sinks downward, as at Jharia or Joshimath. In a landslide, a mass moves down a slope along a shear plane.
• Landslide susceptibility (NLSM): GSI's detailed map of how likely each hill area is to slide, at 1:50,000 scale [4]. The vulnerability zones are NCERT's broad four-group national map.
• Seismic zones: these group areas only by earthquake risk. Landslide zones combine earthquakes with rain, slope, rock and human activity. For example, Ladakh is a hilly earthquake area but is only moderate to low for landslides because it gets little rain.
• Western Ghats vs rain-shadow Ghats: the wet, steep western face is very high. The rain-shadow parts of the Western and Eastern Ghats are moderate to low.

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].
• Trap: Ladakh and Spiti are in the moderate to low zone, not very high, because they get little rain.
• Trap: Darjiling (West Bengal) and Karbi Anglong (Assam) are vulnerable hilly pockets inside states that are otherwise "safe". The Assam plains are left out of the high zone.
• Trap: the Western Ghats have hard, stable rock but are still in the very high zone, because of steep cliffs, very heavy rain and mechanical weathering.
• The Andaman and Nicobar Islands are in the very high zone. The Aravalis are in the moderate to low zone.
• Landslide Atlas of India: NRSC/ISRO, March 2023, about 80,000 landslides, 147 districts ranked [3]. Rudraprayag is the most exposed district.

Mains Points

• Distribution (GS-I): landslides cluster 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 heavy orographic rain. This is rain that falls when moist wind is pushed up a mountain, cools and condenses on the side facing the wind.

• Dry trans-Himalaya and old plateau areas are safer. Mining belts there face subsidence instead.

• Using the zones to plan (GS-III)

• Restrict roads, large dams and big settlements in high-vulnerability zones.
• Keep farming to valleys and moderate slopes. Plant forests on a large scale (afforestation) and build bunds (low banks of earth or stone that slow the water running downhill).
• In the north-east, replace jhum (shifting cultivation: forest is cleared by slash-and-burn, farmed for a few years and then left) with terrace farming (flat steps cut into the hillside that hold the soil).

• Joshimath (2023) and Wayanad (2024) show what happens when building ignores slope fragility. This links to the Gadgil (2011) and Kasturirangan (2013) reports on Ecologically Sensitive Areas.

• From relief to risk reduction (Sendai Framework)

• Map the risk with GSI's NLSM at 1:50,000 [4] and NRSC's Landslide Atlas [3].
• Forecast landslides caused by rain through the National Landslide Forecasting Centre [4].
• Make sure local authorities act on warnings and evacuate people in time.

Related concepts

• Landslide
• Landslides in the Himalayas and Western Ghats
• Landslide mitigation
• Malpa landslide
• Landslide Atlas of India
• Land subsidence
• Joshimath land subsidence (2023)
• Wayanad landslides (2024)

Read more

• Landslides and land subsidence