Heavy October rainfall on saturated Himalayan slopes caused Darjeeling landslides, says GSI

Kolkata: Claiming that it had issued a high-risk landslide forecast for Darjeeling on October 4, hours before the region was hit by intense rainfall that triggered devastating landslides, the GSI on Wednesday said the calamity was a result of heavy downpour that significantly weakened the natural stability of the Himalayan terrain.

GSI officials said they had issued an orange-level (high-risk) warning for Darjeeling around 2.15 pm on October 4, hours before incessant rainfall hit 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," GSI Deputy Director General Dr Saibal Ghosh told PTI.

Ghosh, an expert in engineering geology and landslide science, said the GSI has been issuing daily bulletins for four districts in the country — Darjeeling, Kalimpong, Nilgiris (Tamil Nadu), and Rudraprayag (Uttarakhand) — in operational mode, which means the public also has access to the data.

He said these alerts were also available on the GSI's 'Bhusanket' web portal and 'Bhooskhalan' mobile app.

"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," he said.

The forecast issued just hours before the landslides was based on significant rainfall forecasts, prevailing risk conditions and the understanding that the region’s slopes were already saturated and vulnerable, he said.

Ghosh, who has over three decades of experience working in the Himalayas, said such warnings are based on real-time rainfall forecasts provided by the IMD, along with geological assessments carried out by the GSI.

On the reason behind the devastating landslide in Darjeeling, Ghosh explained that heavy rainfall in October, after an entire monsoon season, poses an acute threat to Himalayan slopes.

"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," he said.

Referring to the 393 mm of rainfall recorded in Kurseong on October 4-5, he said, "The timing of this year's event made it worse." "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. 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," he explained.

If the heavy downpour continued beyond October 5, the devastation could have been more, the scientist said.

"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 added.

Historically, October has seen some of the worst landslide events in Darjeeling and Sikkim Himalayas.

Ghosh cited the 1968 disaster, which claimed around 677 lives and left major roads buried under muck 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. That led to floods, landslides, and 677 reported deaths. All the major roads (then NH6 and now NH10) were filled with muck, and it took 1.5 years to remove it," he said.

He added that several other major landslide events have also occurred in subsequent years, including in 1978, 1979, 1981, 1993, 1998, 2001, 2006, 2007, 2015, and 2023.

"Technically, landslides are gravitational processes. Hillsides are naturally prone to collapse, but whether a slope will fail depends on both intrinsic geological conditions and external triggers like rainfall, earthquakes, or human activity," he said.

"In the case of Darjeeling, the region's complex terrain, steep slopes, and fractured rock formations make it highly susceptible. 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 added.

Ghosh also raised concerns about growing human interference on unstable slopes.

"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," he said.