A combination of unrelenting October rainfall and already waterlogged Himalayan slopes caused the deadly landslides in Darjeeling last week, the Geological Survey of India (GSI) has said, stressing that it had issued a high-risk alert hours before the disaster struck.
Deputy Director General Saibal Ghosh, an expert in engineering geology and landslide science, said the GSI issued an “orange-level” warning for Darjeeling on the afternoon of October 4, just before incessant rain lashed the region. “We had provided a landslide forecast in Darjeeling Pulbazar, Jorebunglow Sukiapokhri, Kurseong, Mirik and Rangli Rangliot blocks of Darjeeling district in the afternoon of October 4. It was an operational bulletin issued around 2.15 pm,” Ghosh said.
The GSI’s daily bulletins, available on its ‘Bhusanket’ web portal and ‘Bhooskhalan’ mobile app, are issued for districts considered highly prone to landslides. “We issue this bulletin every day during the monsoon. Apart from these four districts, we also provide bulletins to 17 other districts in eight states, but only to respective state governments, for validation and data collection,” Ghosh said.
He explained that October rainfall is particularly destructive in the Himalayas because slopes remain saturated after months of monsoon rain. “In the Himalayas, the monsoon period is from June to October. During this time, the risk of landslides is always high because of the volume of rainfall. But October rainfall is especially dangerous, because the soil is already fully saturated,” Ghosh observed.
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Darjeeling’s Kurseong area alone recorded 393 mm of rainfall between October 4 and 5. “Basically, rainfall between 130 mm and 150 mm in a single day can trigger a landslide. If such heavy rainfall occurs in October, then it is very dangerous for places like the Himalayas,” he said. “Because after four months of monsoon rains, the soil is already saturated. Even moderate rain can trigger major slides, and here we had an extreme downpour.”
The GSI official warned that had the rain persisted for another day, the scale of devastation would have been far greater. “The rainfall continued from October 4 night till October 5 morning. Had the downpour continued for another day, the devastation would have been much more,” he noted.
Ghosh drew parallels with the catastrophic 1968 landslides, which killed 677 people and buried roads under debris for over a year. “In our records, the dreadful incident of a landslide occurred in 1968 from October 3-5. In three days, there was 1,121 mm of rainfall, 499 mm in a single day,” he recalled. “All the major roads (then NH6 and now NH10) were filled with muck, and it took 1.5 years to remove it.”
Highlighting the fragility of the Himalayan terrain, Ghosh said landslides are “gravitational processes” worsened by human activity. “The Himalayas are a geologically young and unstable mountain chain. The rock masses are already weak, and many slopes are simply waiting for a trigger, often in the form of intense or prolonged rainfall,” he said.
Warning of increasing human interference, he added, “People are building homes across drainage channels, blocking natural water paths. These practices, combined with heavy rain, make landslides inevitable. The state must step in with strict regulations.”
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