A glacial lake outburst flood (GLOF) is being termed as the probable cause for what happened in Chamoli.

What Happened?

• A huge chunk of what is suspected to be a glacier broke and fell off in the Dhauliganga near Raini village in Chamoli District.
• A swollen Dhauliganga river flowed down to Vishnuprayag, where Dhauliganga and Alaknanda meet.
• The force of the river was so much that  the Two power projects — NTPC’s Tapovan-Vishnugad hydel project and the Rishi Ganga Hydel Project — were extensively damaged with scores of labourers trapped in tunnels as the waters came rushing in

Probable Causes:

Glaciers are made of layers of compressed snow that move or “flow” due to gravity and the softness of ice relative to rock. A glacier’s “tongue” can extend hundreds of kilometers from its high-altitude origins, and the end, or “snout,” can advance or retreat based on snow accumulating or melting.Seismic activity and a buildup of water pressure can cause glaciers to burst, but one particular concern is climate change. High temperatures coupled with less snowfall can accelerate melting, which causes water to rise to potentially dangerous levels.

 Glacial lake outburst flood

A glacial lake outburst flood (GLOF) is a release of meltwater from a moraine- or ice-dam glacial lake due to dam failure1,2. GLOFs often result in catastrophic flooding downstream, with major geomorphic and socioeconomic impacts.

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).

When glaciers retreat, they leave a space which becomes a glacial lake being filled with water. When such a lake breaches, it is known as glacial lake outburst flood. It is yet to be investigated what triggered the breach in Chamoli — whether there was an avalanche in the area recently or whether the lake breach was the result of construction, anthropological activities, climate change etc.

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.

But while GLOF is being considered to be the most likely trigger for Sunday’s event, there are questions surrounding this possibility. We don’t know of any big glacial lakes in this region.

What is an avalanche?

An avalanche is falling masses of snow and ice which gathers pace as it comes down the slope. But an avalanche is unlikely to result in the rise of water of that magnitude what Chamoli witnessed.

Cloudburst

What happened in Uttarakhand in 2013 was a multi-day cloudburst. It is a sudden, very heavy rainfall accompanies by a thunderstorm. But it generally happens in monsoon.

In fact, the season in which such a disaster was witnessed has surprised experts as there is no immediate trigger that can be pointed to as the reason why water level rose to that level washing away two hydro projects.

About the Tributaries Involved:

• Dhauli Ganga is joined by Rishi Ganga river at Raini where the disaster at the power project dam took place.
• The river takes a V turn and continues to flow in the opposite direction, toward north, as Dhauli Ganga for another 30-odd kilometres, through Tapovan, until it is joined by Alaknanda river at Vishnuprayag near Joshimath.
• There it loses its identity and Alaknanda flows southwest — through Chamoli, Maithana, Nandaprayag, Karnaprayag until it meets the Mandakini river, coming from the north at Rudraprayag.
• After subsuming Mandakini, the Alaknanda carries on past Srinagar, before joining the Ganges at Devprayag near Kedarnath. Alaknanda then disappears and the mighty Ganges carries on its pan-India journey, first flowing south then west through important pilgrimage centres such as Rishikesh and finally descending into the Indo-Gangetic plains at Haridwar.
• Still on a southern course, the Ganges goes past Bijnor when it loops back on an easterly course toward Kanpur.
• Yamuna, Ramganga and Ghaghara are other Himalayan rivers that join the Ganga.
• Dhauli Ganga is one of the important tributaries of Alaknanda, the other being the Nandakini, Pindar, Mandakini and Bhagirathi.
• The Himalayan rivers pass through environmentally fragile areas. However, like other Himalayan rivers, the Dhauli Ganga too has been dammed. Dhauli Ganga also has a power station of the National Hydropower Corporation Ltd of 280 MW at Pithoragarh in Uttarakhand.