Kerala’s 2024 Wayanad Landslide: Understanding the Triggers, Contributors
An extreme rainfall event — about 572 mm rainfall in just over 48 hours — was identified as the immediate trigger for the disaster. But the occurrence of a landslide is not determined by rainfall alone. It is the result of a complex interaction between this climatic trigger and pre-existing terrain vulnerabilities
On the night of July 29, 2024, people of the Vellarimala hill ranges in Kerala’s Wayanad District didn’t realise what was in store as the ground under their feet was slowly beginning to shift. A few hours later, on July 30 early morning, the mountainside finally gave way in its entirety. The result: Kerala experienced the most catastrophic landslide disaster in its history.
When it rains, it pours. The first landslide occurred at around 1 am and was soon followed by a catastrophic one at 4.30 am. The gushing muddy water and massive boulders following the second landslide washed away the Mundakkai and Chooralmala settlements of Meppadi Panchayat and killed over 400 people.
The debris flow impacted Punchirimattam, Mundakkai and Chooralmala settlements along the riverbank. The landslide crown was in the Vellolipara Mala.
The trigger
Satellite data indicate the debris flow had a runout distance of approximately 8 km, a scale that devastated the three villages in its path. An extreme rainfall event — about 572 mm rainfall in just over 48 hours — was identified as the immediate trigger for the disaster.
Landslides have become a common phenomenon in Kerala during the monsoon season, and some of them are massive and consequently disastrous, as occurred on 30 July in the Mundakkai and Chooralmala area. Changing climatic patterns, particularly the intensification of short-duration, high-intensity rainfall driven by increased atmospheric moisture, have become a dominant trigger for major landslides in Kerala. The frequency of these events has shown a marked increasing trend in recent years.
Complex interplay
However, the occurrence of a landslide is not determined by rainfall alone. It is the result of a complex interaction between this climatic trigger and pre-existing terrain vulnerabilities. Key geological and hydrological factors include the geologic structure of the slope, as well as the hydraulic and mechanical properties of the topsoil. Among these, soil composition is critical. A slope’s stability is heavily influenced by the soil’s shear strength (its resistance to sliding) and its drainage capacity. When intense rainfall saturates soil with poor drainage, the resulting pore pressure can drastically reduce shear strength, leading to slope failure.
The primary trigger for major landslides in Kerala is the short-duration, high-intensity rainfall events. However, the impact of the climatic forcing is determined by the inherent vulnerability of the landscape. Key natural factors that influence a slope’s stability include: 1) Slope angle, 2) Surface ruggedness, 3) Soil/regolith thickness over the underlying bedrock, 4) Integrity of the soil structure, and 5) Density of stream channels in a given area. The external forcing is generally determined by the scale of human activities, such as changes to the landscape, soil, and drainage brought about by land utilisation and construction, as well as the extent of agricultural activities and built-up areas.
The landslide hazard in Wayanad has been significantly exacerbated by a long history of human activities that have altered the landscape’s natural stability. Deforestation, initiated during the British colonial era and continued for cash crops, like tea, rubber, coffee, and cardamom, has changed soil conditions over time. Unscientific land-use practices have compounded these vulnerabilities. The factors — historical deforestation, specific agricultural methods, and disruptive construction — collectively represent a multi-generational alteration of the landscape that has dramatically increased its susceptibility to failure during intense rainfall events.
Rainfall intensity is one major factor that triggered the Mundakkai landslides. It seeps through the soil, resulting in matric suction dissipation, increasing the pore-water pressure, and reducing the soil’s overall strength. The matric suction dissipation refers to the loss of apparent cohesion of unsaturated soil due to an increase in its water content, which significantly reduces the soil’s shear strength. The study under World Weather Attribution (WWA) shows the high rainfall event in August 2024 was the third heaviest spell in the region, with the heaviest spells in 2019 and 1924, when the downpour was 10% heavier, triggering landslides.
Mirroring the 2019 landslide event
The landslide in Mundakkai and Chooralmala in Wayanad occurred about 2-3 km from Puthumala, where a similar version of mass wasting, although smaller in scale, occurred on 29 August 2019, and killed 17 people. The 2024 Mundakkai landslide event mirrors the sequence of the 2019 Puthumala disaster. In both cases, high-intensity rainfall served as the major trigger, pushing the slopes past a tipping point. Notably, the Mundakkai event also began as a minor failure before escalating.
Effect of land-use activities
In order to understand the landslide vulnerability of a region, it is important to collect such historical and geological data on the previous landslides, if any, in the region. A historical (archival) database on landslides should also be made available in the District and Panchayat offices for the residents’ reference and development of land-use strategies. With all the historical information available on landslide incidents in Mundakkai and neighbouring areas, one question that needs to be raised is why the authorities allowed the built-up area to be expanded on the floodplain in Mundakkai, including a school. The information on an earlier landslide in 1984 in Mundakkai and the mechanics of the landslide that occurred in 2019 in Puthumala, about 3 km from Mundakkai, should have been appropriately used by the authorities to model the vulnerability of Mundakkai for the possible scale of an incident in future and inform the public.
External anthropogenic forcings — including unplanned land utilisation, disruptive construction practices, and intensive agricultural activities — significantly perturb natural systems. These activities alter the landscape, degrade soil structure, and impair natural drainage, leading to decreased slope stability and fundamental changes in hydrological properties such as surface runoff and infiltration rates. Given these compounded risks, experts strongly recommend the implementation of strict, scientifically informed land-use policies and engineering standards to prevent such destabilising practices and mitigate landslide hazards.
Much needed measures
Bioengineering works have emerged as a promising and innovative approach to landslide stabilisation, offering a sustainable and environmentally friendly solution to a pressing issue. Tap/fibrous roots form a dense network and increase soil shear strength. Restricts erosion and promotes biodiversity. Bamboo is flexible and fast-growing. Unstable soils could be stabilised through vegetative cover, where soil erosion and mass movements are predominant.
Currently, the available landslide hazard susceptibility map (e.g. Wayanad District) presents only the broad region of the occurrence of landslides; information on the specific locations where landslide initiation can occur or its probable flow path is presently not available. High-resolution maps showing areas of landslide vulnerability should be brought to the attention of residents.
Ignored warning
It is also important to involve community groups in risk reduction, capacity building and disaster resilience. An exemplary example of community participation in the forecast efforts originated near Mundakkai, the site of the current landslide disaster. The Hume Centre for Ecology and Wildlife Biology investigators had warned the district authorities regarding the landslide about 16 hours before the event. They used the measure of rainfall from their rain gauge, which they established in Puthumala, not far from Mundakkai.
However, this warning was not acted upon with sufficient seriousness by the district authorities. An effective disaster mitigation strategy must move beyond top-down directives and actively foster community participation and empowerment. Trained local volunteers can be instrumental in this approach, conducting micro-level vulnerability surveys and acting as crucial communication links to disseminate warnings and information.
The available landslide susceptibility maps are static. The continuous monitoring of vulnerable zones may be an option for generating a dynamic landslide susceptibility map in such areas. Global developments show that with the help of InSAR images, dynamic landslide susceptibility maps may become practical. A significant advancement in landslide hazard assessment involves integrating Multi-Temporal InSAR (MT-InSAR) data with machine learning techniques to create highly accurate susceptibility maps.
