Science News

Landslide likely trigger for Himalayan flood

Though scientists on the ground maintain that it is too early to pinpoint the cause, they say it was likely an avalanche.

Vanita Srivastava

doi:10.1038/nindia.2021.23 Published online 8 February 2021

National Disaster Response Force personnel trying to rescue people feared trapped in a tunnel in Chamoli after the floods.

© NDRF

Using satellite images, scientists say the massive Himalayan flood in the northern Indian state of Uttarakhand on 7 February 2021 was likely triggered by a landslide and not by a glacial lake outburst as first thought. At least seven people died and nearly 170 were feared missing after sudden intense floods in the Dhauli Ganga river swept through Chamoli district of the state inundating a power project downstream.

In June 2013, a series of cloudbursts had caused devastating floods and landslides in Uttarakhand, making it one of worst natural disasters in India since the 2004 tsunami. 

Based on the satellite images procured from the San Francisco-based private earth imaging company Planet Labs, Dan Shugar a geoscientist at University of Calgary said that the 7 February disaster was caused by a landslide. “Looks like massive dust deposition over much of the W side of the valley and trigger appears to be the landslide scar that I discovered a few minutes ago, So NOT a typical GLOF (Glacial Lake Outburst Flood),” he tweeted soon after the floods.

In a blog, Dave Petley, Pro-Vice-Chancellor of research and innovation at the University of Sheffield in the UK described the cause of the disaster as a large ‘rockslope detachment’.“…It (the detachment) has moved northwards onto the glacier and turned into a rock and ice avalanche that has moved toward the northwest,” he wrote.

Scientists on the ground are, however, cautious in pinpointing the cause just yet.

Manish Mehta, a senior scientist at the Wadia Institute of Himalayan Geology in Dehradun, part of the team looking for evidence at the site told Nature India that it was too early to say anything. Mehta said that there are 25 glaciers in the area, of which eight are larger than five square kilometers. “Himalayan glaciers have been retreating fast. In the case of flash floods at Reni village of Chamoli, there could be multiple causes. Unless a full study of site is done, it is difficult to arrive at a conclusion.”

Mehta was part of a study group last year trying to quantify the impact of climate change on glaciers in the nearby Rishi Ganga catchment area. The study showed that the glaciers had lost about 10% of surface area between 1980-2017.

Glaciologist D P Dobhal outlines two possible reasons for the Chamoli floods. "A few days back there was heavy snowfall in the region leading to a snow avalanche. The avalanche may have triggered the water body or the glacial lake and caused the flash flood." It is also possible that the avalanche or a sliding rock blocked the river and formed a lake, he says. “When the embankment can’t bear the water load, it could cause floods.”

Saurabh Vijay, an expert in remote sensing of glaciers in high-mountain regions and a professor at Indian Institute of Technology (IIT) Roorkee also maintains that no specific cause may be attributed to the flooding. "Preliminary findings based on daily satellite images provided by Planet Labs clearly indicate the timing and impact of this catastrophic event. The images indicate widespread dust storm in the valley. The region also received snowfall during 2 to 6 February, evident in the satellite images,” he says.

Vijay says that a major landslide may have triggered the event but it is too early to confirm. “It is also important to find evidence about the origin of water that played a major role in this event. Field based glaciologists will soon demystify the potential signs of the events and link information coming from different sources like remote sensing, geology and seismology, although it is a very challenging task,” he told Nature India.

Mohammad Farooq Azam, an assistant professor of glaciology and hydrology at IIT Indore says that the images captured so far show that a hanging glacier near Trishul Peak broke and created an ice-rock avalanche from a height of 5,600 meter.

“This ice-rock avalanche apparently fell vertically down for 2 kms and created the disaster,” he says. The valley bottom, with lots of pre-existing debris, also had some water in it. “The resultant mixture of ice, debris and water that developed could have gone down the valley,” Azam says.

Irfan Rashid, professor in the geoinformatics department at the University of Kashmir maintains that precipitation in the mountains after 2 February 2020 had a high level of moisture. “The satellite images show that there was a hanging glacier and the moisture lubricated the bed of the glacier. This could have led to the formation of an avalanche that moved through a steep slope laden with debris.”

Scientists have been documenting the impact of global warming on glacial retreat in the Hindu-Kush Himalaya (HKH). A recent assessment report of the International Centre for Integrated Mountain Development (ICIMOD) — an intergovernmental body showed that temperatures were rising in the HKH region and called for improved in situ monitoring network for weather, hydrology and glacier change for predicting future hazards.

“Glacier retreat is changing streamflow patterns, in turn, affecting the incidence of glacial-lake outburst floods and exacerbating the risk of flooding and water shortages associated with future climate change,” the report titled “Glacial change and hydrological implications in the Himalaya and Karakoram” says.

Environmentalists blame the large scale construction of hydropower projects and road networks in the fragile Himalayan ecology for the repeated floods in the region.

Update on 18 February 2021:

A team of five scientists from the Wadia Institute of Himalayan Geology, who conducted land-based and helicopter surveys at the site, said that a hanging glacier and a rock mass broke off 5,600 meters above sea level in the Raunthi glacier catchment area cascading a complex slurry of glacial ice, rock fragments, morain debris and snow. This eventually triggered massive flooding downstream.

Institute director Kalachand Sain told Nature India that the break-off glacier and rock mass moved downward to reach the Raunthi stream at an altitude of 3,600 meters and gathered more mass before meeting the Rishiganga river at 2,300 meters. A freshwater lake was formed at this point as the debris tumbled further downstream, devastating small bridges and the Rishiganga hydropower project near Raini village. Carrying even more wood debris, sediments and water along, it triggered a veritable tsunami upstream of the Dhauliganga river, sweeping away villages and the Tapovan hydropower project, Sain said.