Why do landslides occur, and what triggered the tragedy in Wayanad?

Landslides are not considered to be as devastating as droughts, storms, or floods. They are also less studied because they are localised events. But their frequency is increasing as the effects of climate change intensify, and they are causing significant losses of lives and livelihoods.

How prone is India to landslides?

India accounts for about 8% of global fatalities due to landslides, and during the period 2001-21, landslides caused 847 deaths and displaced thousands, according to a team from IIT-Madras that has worked on the phenomenon.

Despite the significant number of fatalities however, landslides were not given enough importance in India until the 2013 Kedarnath landslide and floods, the team has noted.

The IIT-M team has developed a high-resolution India Landslide Susceptibility Map (ILSM) using machine learning models.

According to a paper published in IIT-M’s Shaastra magazine, the ILSM shows that 13.17% of the country is susceptible to landslides, which is more than what was believed previously. And 4.75% area is considered “very highly susceptible”.

“Sikkim has the largest land area (57.6%) that is landslide-prone, while outside of the Himalayas, Kerala is the most vulnerable state with over 14% 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 paucity of data on landslides,” the paper says.

In 2023, the Indian Space Research Organisation (ISRO) released a “Landslide Atlas of India”, identifying the landslide hotspots around the country. Among the highly vulnerable areas in the atlas were the Himalayas and the Western Ghats.

What factors cause landslides?

Landslides are natural phenomena that usually occur in mountainous regions with steep slopes. During a landslide, large amounts of rock, boulders, loose mud, soil, and debris roll down slopes and hillsides, gathering great momentum and often taking vegetation or buildings along.

Landslides are seen as being caused by (i) conditioning factors and (ii) triggering factors.

CONDITIONING FACTORS are related to the soil topography, rocks, geomorphology, and slope angles, among other factors. These factors make some parts of the country more vulnerable to landslides than other parts.

TRIGGERING FACTORS are intense rainfall, and anthropological activities such as thoughtless changes of land use, road and bridge building, haphazard and unscientific construction, and large-scale destruction of forests.

How did these broad factors apply in the case of Kerala?

S Abhilash, director of the Advanced Centre for Atmospheric Radar Research (ACARR) at Cochin University of Science and Technology, Kochi, told The Indian Express that the cutting of trees to create plantations is contributing to landslides in Kerala.

“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 hold the top soil to the bedrock, are cut down. These trees are being replaced by big plantation crops, such as tea and coffee, which have shallow roots,” he said.

Research carried out by Dr Abhilash and his team in 2019 in Puthumala, which is close to the place where the landslides of this week took place, found that certain atmospheric conditions made the area vulnerable to very heavy rainfall in a small window, which increased the likelihood of 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 over two to three hours… So, that type of rainfall is becoming more common along Kerala,” Dr Abhilash said.

Indeed, two weeks of continuous rainfall, 50-70% above normal for this time, set the conditions for the disaster in Wayanad. This rain saturated the topsoil, and when a day of extremely heavy rain akin to a mini cloudburst followed, the landslide event was triggered.