On February 7, 2021 massive floods washed away the Rishiganga hydroelectric power project and the Tapovan project on the Dhauli Ganga river (a tributary of the Alaknanda) near Chamoli district of Uttarakhand. The floods were most likely caused as a result of breaking of a part of the Nanda Devi glacier. The research to ascertain the exact cause of the disaster is under way. As per a top scientist from the Defence Research and Development Organisation (DRDO), the glacier burst led to an avalanche and a deluge that passed through the Alaknanda river system in the upper reaches of the Himalayas.
The reason behind the glacier burst is attributed to seismic activity and a build-up of water pressure. Climate change is also considered to be a major concern in this regard.
The Himalayas are home to many glaciers and glacial lakes. This region is among the areas identified for potentially deadly glacial bursts and floods. A glacial lake is a water body that originates from glacier activity. Proglacial lakes are formed after glaciers retreat. Even as glacial lake outburst flood (GLOF) is being regarded as the reason behind the floods, there are doubts around this possibility. GLOF event is the result of breaching of boundaries of glacial lakes. When this happens, a large amount of water rushes down to nearby streams and rivers, causing flooding downstream.
Glaciers, Glacier Lakes, and Glacier Lake Bursts :
A glacier is a mass of ice moving under its own weight. It is made of layers of compressed snow. The movement of glaciers is attributed to gravity and to the softness of ice compared to rock. Depending on the accumulation or melting of snow, the end of a glacier can either move ahead or retreat. When the snout of a glacier retreats, it leaves behind lakes that are bound by sediment and boulders. These lakes are called proglacial lakes. The boundary of these lakes can be easily breached because of their structural weakness, additional water, or pressure. Such a breach would result in the downstream flooding of the rivers that originate from the glacier. Glacial lake bursts can also be caused by seismic activity. Another likely reason for glacial bursts is climate change. The rising temperature due to global warming and reduced precipitation can cause snow to melt faster, as a result of which the water in rivers rises to dangerous levels. This could result in devastating floods in the adjoining areas.
However, glacier burst or breaking off of glaciers is not a common phenomenon in winters. Glacial walls are bound tightly during winters, which prevents glacial bursts. Also, information gathered using satellites and google earth images do not give evidence of any proglacial lakes close to the region where the disaster occurred. This event could have been triggered by water pockets or lakes inside glaciers.
As per glaciologist, D.P. Dobhal, the most probable cause of this incident appeared to be a snow avalanche. In such an event, a sudden change in temperature after snowfall causes melting and slippage of the fresh show on the surface due to the higher ambient temperature.
A few days before the incident, there had been heavy snowfall in the area for two days. Then, suddenly, the weather became somewhat warmer, resulting in some melting of snow. This might have caused an avalanche resulting in a series of events leading to the flash floods.
Most of the area having glaciers contain huge quantities of debris. The slippage of snow carries with it some of this debris. As such, events become quite intense and erode everything that comes in the way.
Geologists studying these glaciers have also pointed towards the impact of climate change. The glaciers in the Himalayan region have witnessed reduced precipitation since the 1980s and have been retreating fast. The equilibrium line altitude is that zone on a glacier where the mass lost by it is balanced by the mass gained over the period of a year. Fluctuations in the equilibrium line altitude caused by reduced precipitation coupled with rising temperature could have caused such an incident. The exact cause is difficult to ascertain due to a lack of monitoring and the remote location of the glaciers.
According to the latest reports of the UN Intergovernmental Panel on Climate Change, the retreat of glaciers and the thawing of permafrost result in a rise in the number of glacier lakes or cause the existing ones to swell. Such developments near steep river valleys and unstable mountain walls can easily trigger glacier lake bursts.
Concerns were raised after the earthquake in the Tehri Dam region in 1991 and the floods that wreaked havoc in Kedarnath in 2013. Despite the threat of seismicity, landslides, and unstable glacial lakes, India has been increasingly investing on the growth of hydropower and dam-related activities in the Himalayan region. Evidence suggests that siltation reduces the life of dams considerably, increasing the possibility of future disasters. So, the impact of this policy on the Himalayas should be considered. It has been suggested that low impact power projects should be preferred over projects that require large dams and reservoirs. Large power projects should be allowed in those areas, where the impact will be least.