As Kerala grapples with Wayanad tragedy, a look at why landslides occur

Shaastra, a science and technology magazine from the institute, adds that ILSM shows that 13.17 per cent of the country is susceptible to landslides – more than previously believed – and that 4.75 per cent is ‘very highly susceptible’.

“Sikkim has the largest land area (57.6 per cent) that is landslide-prone, while outside of the Himalayas, Kerala is the most vulnerable state with over 14 per cent of its land mass in the ‘very high susceptibility’ category. Some areas in the Eastern Ghats, around Odisha, too, are susceptible – which previous studies had missed. Arunachal Pradesh has the largest susceptible area (31,845 sq km), which other models had missed because of the paucity of data on landslides,” it adds.

At present, the country is grappling with the early morning landslides in Kerala’s Wayanad that killed more than 250 people who were asleep when tragedy struck.

Factors that cause landslides

Landslides are a natural phenomenon usually seen in steep mountainous regions where large quantities of material, composed of rock, boulders, loose mud, soil and debris, are driven down the slope with force. In 2023, the Indian Space Research Organisation (ISRO) released the ‘Landslide Atlas of India’, identifying the landslide hotspots in the country. Among the highly vulnerable areas are the Himalayas and the Western Ghats.

Two factors mostly cause landslides – a conditioning factor and a triggering factor. Conditioning factors are related to the soil topography, rocks, geomorphology and slope angle, among others. The main triggering factors for landslides in India are intense rainfall, and anthropological activities such as land use changes, road network building, unscientific construction and large-scale destruction of forests.

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“Most of the landslides that are taking place in Kerala are around plantation areas indicating another major triggering factor – agricultural activities, such as monocropping, in which large, native trees, which serve to hold the topsoil to the bedrock, are cut down. These trees are being replaced by big plantation crops, such as tea and coffee, which have shallow roots,” S Abhilash, director of the Advanced Centre for Atmospheric Radar Research at Cochin University of Science and Technology, told The Indian Express.

Abhilash and his team have conducted research and found that after 2019 in Puthumala – 17 people died in a landslide in Puthumala, close to the recent landslide site – the region is highly susceptible to landslides. “What we observed in 2019 was that it was due to a kind of mesoscale organization of big convective cloud clusters, which produced more than 5-10 centimetres of rain consecutively for two to three hours. So, that type of rainfall is becoming more common along Kerala,” said Abhilash.

Two weeks of continuous rainfall, 50 to 70 per cent above normal for that period, set the conditions in Wayanad. The topsoil, mixed with rocks and so on, got saturated in the rainfall. In addition to this antecedent two weeks’ rainfall, there was one day of extremely heavy category, or mini cloudburst type of extremely heavy rainfall, which triggered the event.