Guwahati: When torrential rain battered Assam’s hill district of Dima Hasao in May 2022, the slopes did not fail one by one—they collapsed in thousands.
In just seven days, between May 11 and 18, more than 5,178 landslides were triggered across the district, burying villages, displacing nearly 57,000 people and submerging Haflong railway station under thick debris.
A new scientific study has uncovered why the devastating May 2022 landslides in Dima Hasao were so explosive and widespread—triggering more than 5,178 landslides in just seven days and submerging Haflong railway station under a river of mud.
The findings, published in Landslides, show that a dangerous mix of geology, changing land use and intense monsoon patterns is pushing Assam’s only hill district towards an escalating landslide emergency.
The study, conducted by Rupam Sonowal, Pallab Jyoti Hazarika, Mukunda Saikia, Vimha Ritse, Nabajit Hazarika and Amiya Baruah, provides the most comprehensive explanation yet for the May 2022 landslide disaster in Dima Hasao, where over 5,178 landslides occurred within just seven days.
Between May 11 and 18, 2022, Dima Hasao received 540 mm of rainfall, nearly 140% above the monthly average and significantly higher than even the region’s July peak monsoon levels.
This extreme rainfall initiated thousands of slope failures, with the worst impacts seen in Haflong and Mahur. At least 200 houses were destroyed, 57,000 residents displaced, and the Haflong railway station was engulfed by debris.
The authors highlight that Dima Hasao’s hills are composed of highly weathered sedimentary and meta-sedimentary rock units, including sandstone, shale and siltstone.
These rocks contain extensive vein-like weak layers, the result of long-term tectonic deformation and weathering.
These weak layers introduce mechanical anisotropy—meaning the rocks have different strengths in different directions—which reduces slope stability. The weakened layers allow rapid water infiltration during intense rainfall, further lowering shear strength and making the slopes more prone to failure.
A key finding of the study is that grain-size distribution directly influenced the type and behaviour of landslides.
Laboratory experiments carried out by the authors show that slopes dominated by finer grains (around ~2 mm) tended to fail in brittle, discrete fault-like scarps. The experiments also showed that slopes enriched with coarse sand and gravel transformed into low-viscosity slurry during rainfall, leading to rapid, flow-like landslides.
The rheological tests demonstrated that increasing the proportion of coarse sand from 5% to 30% caused the yield stress of the material to drop drastically—from approximately 1,750 Pa to 600 Pa.
“This reduction in strength allowed the debris to move quickly downslope, consistent with simulations showing debris-flow velocities of ~42 m/s near Haflong railway station,” the authors note.
The study examined LANDSAT data from 1993 to 2023, documenting extensive shifts in land use across Dima Hasao. The authors found major conversions of forest land to agriculture, forest land to built-up areas, and forest land to barren land. The data show thousands of hectares affected across these categories.
“Such changes reduce vegetation cover, expose soil, and alter hydrological patterns—factors known to increase slope susceptibility during heavy rainfall.”
The study concludes that the landslide crisis was not caused by rainfall alone.
Instead, the disaster emerged from the combined influence of extreme monsoon precipitation, weak, weathered and anisotropic rock formations, slope materials with grain-size distributions conducive to rapid flow failures, and three decades of deforestation and expanding built-up areas. These factors acted simultaneously, creating conditions for clustered landslides of exceptional magnitude.
The authors suggest that Dima Hasao urgently needs microzonation-based hazard planning, slope stability assessments that integrate grain size, rheology and rock deformation patterns, sustainable urban and infrastructure development strategies, and better monitoring of rainfall thresholds and pore-water pressure.
These steps could help reduce landslide vulnerability in one of India’s most sensitive and geologically active hill districts.
Also Read: Water-rich but thirsty: Structural flaws undermine Meghalaya’s supply systems
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