Why heavy rain repeatedly triggers landslides in Kerala

Neetha K Gopal, head of the IMD’s Meteorological Centre in Thiruvananthapuram, told The Indian Express that the intense spell was being driven by an offshore trough extending from the south Gujarat coast to Kerala, strong southwesterly winds across the Arabian Sea, and a strong north-south pressure gradient along the west coast. Together, these conditions had strengthened the monsoon flow and produced widespread rainfall across Kerala.

“This was not rainfall confined to one or two places. It was a widespread heavy rainfall event,” Gopal said. Of the 109 IMD stations in Kerala that recorded rain on Saturday, 53 received more than 7 cm of rainfall, adding that some stations recorded rainfall in the “extremely heavy” category, which IMD defines as more than 20.4 cm in 24 hours. She added that a large quantity of rain had fallen within a short period, leaving little time for the water to drain away.

“It was the cumulative effect of the preceding rainfall, combined with today’s intense and widespread rainfall activity,” Gopal said, underscoring that the landslides were not triggered by a single burst of rain alone.

Why do landslides occur?

Landslides are natural disasters that occur mainly in mountainous terrains with conducive soil, rock, geology and slope conditions. A sudden movement of rock, boulders, earth or debris down a slope is called a landslide.

While they are fairly common, landslides are extremely hazardous and pose a threat to human and animal lives, damaging property, roads and bridges, disrupting communication lines and snapping power lines.

Landslides generally result from a combination of two sets of factors:

• Conditioning factors, such as soil topography, rocks, geomorphology, and slope angles, among others, make some parts of the country more vulnerable to landslides than other parts.
• Triggering factors include intense rainfall and anthropogenic activities such as haphazard land use, unscientific construction, and large-scale destruction of forests.

Most landslides in India are rain-induced events. Other natural causes include earthquakes, snow-melting and undercutting of slopes due to flooding. They may also be caused by anthropogenic activities such as excavation, cutting of hills and trees, excessive infrastructure development, and overgrazing by cattle.

Why Kerala is especially vulnerable

Kerala’s landslide risk is largely concentrated along the Western Ghats, especially in central and northern districts, such as Idukki, Pathanamthitta, Kottayam, Wayanad, Kozhikode and Malappuram. The region’s steep slopes, geology and changing land use make it inherently susceptible to landslides.

District Administration deploys boats to rescue people from the flood-affected area at Ranni in Pathanamthitta on Saturday. Flash floods from the hilly regions caused overflow of the Pamba River. (ANI Video Grab)

During spells of intense or prolonged rainfall, water infiltrates the loose soil and reaches the interface between the soil and the underlying rock, weakening the bond between the two and increasing the likelihood of slope failure, KS Sajinkumar, Professor of Geology at the University of Kerala, told The Indian Express after the 2024 Wayanad landslides. “The terrain here has two distinctive layers, a layer of soil sitting atop hard rocks… water reaches the rocks and flows between the soil and the rock layers. This weakens the force that binds the soil to the rocks and triggers the movement,” he said.

This combination is significant because preceding rainfall can progressively saturate slopes, while a subsequent burst of heavy rain can add large quantities of water over a short period, increasing runoff and the likelihood of slope failure.

Geography

In 2023, the Indian Space Research Organisation’s National Remote Sensing Centre, Hyderabad, released a landslide atlas based on landslide events recorded between 1998 and 2022, primarily along the Himalayas and the Western Ghats.

The database showed that while the Western Ghats recorded fewer landslide events than the Himalayan region, Kerala accounted for a disproportionately high share of landslide fatalities.

Human intervention

Land-use changes and human interventions have also contributed to the fragility of the region, with quarrying, road laying, and the expansion of plantations altering the stability of slopes and natural drainage.

“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 in 2024.

Concerns about the cumulative impact of development in the Western Ghats have long been expressed. The 2011 Western Ghats Ecology Expert Panel, chaired by ecologist Madhav Gadgil, recommended that mining, quarrying, polluting industries and large construction projects in ecologically sensitive areas along the mountain range be strictly regulated. However, several states objected to the contents of this report, and the Centre constituted a High-Level Working Group on the Western Ghats headed by space scientist K Kasturirangan.

The Kasturirangan Committee subsequently proposed a smaller Ecologically Sensitive Area and a less stringent regulatory framework than the Gadgil panel. Since then, successive landslide disasters in Kerala have repeatedly revived the debate over balancing development in the Western Ghats with ecological protection.