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Saturday, April 10, 2021

Movement still occurring in Chamoli: study

On February 7, a portion of the Nanda Devi glacier broke away and caused an avalanche in the Alaknanda river system (Dhauli Ganga, Rishi Ganga and Alaknanda rivers).

Written by Esha Roy | New Delhi |
March 4, 2021 5:54:53 am
The flood washed away hydroelectric stations and left a large number of labourers trapped, many of them feared dead.

A consortium of international glacier and permafrost scientists have found through high resolution satellite imagery indications that mass movement activities are still occurring in the zone covering Chamoli in Uttarakhand, which recently witnessed flash floods.

The study by Gaphaz or the Glacier and Permafrost Hazards in Mountains — a scientific standing group of the International Association of Cryospheric Sciences and the International Permafrost Association — has also found that another slope failure and avalanche can be critical for people and infrastructure downstream, close to the riverbed.

“Large volumes of material have been eroded and deposited along the river channel. In combination with water from rivers, snow melt, heavy (monsoon) rainfall or overflow of temporary lakes, debris flows can be triggered from these depositions. Erosion by the flood probably has undercut some slopes, and this instability could affect roads, villages and other infrastructure located far above the river bed,’’ says the preliminary study exploring the causes behind the Chamoli incident, adding that further investigation is required.

On February 7, a portion of the Nanda Devi glacier broke away and caused an avalanche in the Alaknanda river system (Dhauli Ganga, Rishi Ganga and Alaknanda rivers). These floods washed away hydroelectric stations and left a large number of labourers trapped, many of them feared dead.

While scientists are still investigating the cause of the floods, early findings reveal that a major rock/ice avalanche detached itself at an elevation of about 5,600 m above sea level from a north facing slope northeast of Trisul Peak in the Nanda Devi mountain. An analysis of satellite imagery from the area shows that this event occurred due to failure deep within the bedrock of the mountain, and the glacier ice was most probably entrained with the collapsing block of bedrock.

The study has ruled out the possibility of a glacier lake outburst flood as well and also said temporary lakes formed due to damming are unlikely to have had a direct affect on the flooding.

The Gaphaz report finds that the depth of the failure plane is more than 100 m below surface, where no seasonal temperature variations are expected. The zone is in permafrost conditions, meaning ground temperatures are perennially below zero. There is speculation that heat fluxes from the warmer south face of the mountain to the colder north face, where the avalanche detached, could have warmed the frozen bedrock, leading to the avalanche.

Furthermore, liquid water from snow and ice melting could have infiltrated the bedrock in cleft systems and destabilised the rock. “However, the initiation of the failure as well as the eventual trigger of the avalanche remain unclear. It is also important to note that unstable geological configuration and steep topography can, on its own, be a sufficient driver of large slope failures,’’ says the study.

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