Context: Parts of Uttarakhand on Sunday witnessed massive flooding after a chunk of the Nanda Devi glacier broke off at Joshimath in Uttarakhand’s Chamoli district. Bringing back haunting memories of the 2013 Kedarnath calamity, the glacier break caused large-scale devastation in the upper reaches of the ecologically fragile Himalaya.

• Watch the video: https://youtu.be/-ktcBrQt__s

Reason behind this burst

• As of now, the incident does not seem to have any direct linkage with construction-related activities, or the presence of big dams, but climate change as a factor is not something that can be ignored, particularly in the formation of proglacial lakes. 
• A majority of the glaciers in the Himalayas are known to be receding, all leading to the formation of several proglacial lakes.
• Retreating glaciers, like several in the Himalayas, usually result in the formation of lakes at their tips, called proglacial lakes, often bound only by sediments and boulders. 
• If the boundaries of these lakes are breached, it can lead to large amounts of water rushing down to nearby streams and rivers, gathering momentum on the way by picking up sediments, rocks and other material, and resulting in flooding downstream.

Key highlights

• A flood has occurred in Uttarakhand's Chamoli district after a glacial burst caused an avalanche. 
• Casualties are feared, and people in living in villages near the Dhauli Ganga, and Alaknanda rivers have been evacuated. 
• Rescue teams have been rushed to the spot, and several districts of the state have been placed on high alert.
• As per the available information, over 150 labourers working at a power project in Tapovan-Reni are missing while rescue teams have recovered 10 bodies so far.
• The power project was completely washed off and homes along the way were also swept away as the waters rushed down the mountainsides in a raging torrent.

What glacial lake outburst flood (GLOF)?

• A glacial lake outburst is a type of outburst flood that takes place occurs when water dammed by a glacier or moraine is released. To simplify it, GLOF is an outburst that happens when a dam containing a glacial lake fails.

GLOFs have three main features

• They involve sudden (and sometimes cyclic) releases of water.
• They tend to be rapid events, lasting hours to days.
• They result in large downstream river discharges (which often increase by an order of magnitude).

Why does a glacier break?

• A glacier break or burst can occur for a number of reasons like erosion, a build-up of water pressure, an avalanche of snow or rocks, and also an earthquake under the ice.
• A glacier burst can also be triggered by massive displacement of water in a glacial lake when a large portion of an adjacent glacier collapses into it.
• Although glacier lakes can vary in volume, they are known to hold millions to hundreds of millions of cubic metres of water and a failure to contain ice or glacial sediment can result in water being released even for days.

As per the Experts

• “There is high confidence that current global glacier shrinkage caused new lakes to form and existing lakes to grow in most regions, for instance in South America, High mountain Asia and Europe".
• Glacier retreat and permafrost thaw are projected to decrease the stability of mountain slopes and increase the number and area of glacier lakes (high confidence). 
• Resulting landslides and floods, and cascading events, will also emerge where there is no record of previous events (high confidence),” says the Special Report on Oceans and Cryptosphere (SROCC) by Inter governmental Panel for Climate Change (IPCC).

What happened in Uttarakhand in 2013?

• In June 2013, a multi-day cloudburst centred on Uttarakhand caused devastating floods and landslides.
• According to the state government, more than 5,700 people were presumed dead in the disaster. A number of bridges and roads were destroyed and more than 3 lakh people were trapped in the valleys leading to the Char Dham pilgrimage sites.

Additional Information

About Tapovan Hydro-Electric Power Dam

• Also known as Rishi Ganga Project was completely washed off following a glacier burst in the Chamoli district.
• The dam is called Tapovan Hydro-Electric Power Dam. 
• It is on the confluence of Dhauli Ganga and Rishi Ganga.