A flash flood in Rishiganga and Dhauliganga rivers of Uttarakhand killed more than 200 people, washed off property worth crores of rupees and damaged two hydel projects and a bridge. Cause of this debris flow is a topic of debate with several theories floating around. A paper written by Dr Renoj J Thayyen of National Institute of Hydrology and his team has now came up, shedding light into the causes of event. Dr Thayyan is currently heading a team of scientists constituted by the National Disaster management Authority (NDMA) to study the causes of the event. Edited excerpts of the paper, published as preprint recently.
What caused the Uttarakhand flash flood?
Preliminary findings in this study suggest that the debris flow was caused by the detachment of 0.59 sq. km right lobe of a hanging glacier along with a huge rock mass at the bottom, and the resultant ice-rock avalanche. This part of the glacier was located over a mountain slope of 35 degrees at 4700-5555 metres above sea level. The right lobe of the hanging glacier, which got detached, had a length of 1.6 km and an average width of 550 m.
The ice-rock avalanche resulted in a massive debris flow that travelled 14.35 km along Raunthigad to hit the hydropower project barrage and washed off the bridge across River Rishiganga further downstream. About 7.4 km downstream, the debris flow devastated the Tapoban barrage.
(a) Pre-flood NTPC barrage (HEP 2) at Tapovan, b) Post flood photo showing severe damage to NTPC barrage at Tapovan C) Site of washed off bridge across Rishiganga upstream of NTPC barrage D) Site of Rishiganga (HEP-1) project. (Photo courtesy: NTPC and BRO)
This is a very significant event not only in terms of the catastrophic disaster it caused along its flow path, but also in terms of breaking of a large glacier mass which is a very rare phenomenon in the glaciated regions of the world and more so in the Himalayas.
What are the causes of the detachment of the glacier lobe?
The study suggests that the recent changes in weather conditions in the region is primarily responsible for this event through geological, glaciological and permafrost processes. Possible causes include increase in summer monsoon rainfall, reducing snow cover, increase in land surface temperature and permafrost processes.
Monsoon precipitation and mean annual land surface temperature (LST) show an increasing trend since 2012. Snow cover during monsoon months also showed an increasing trend while glacier elevations showed a decreasing trend during the September-November period. Mean land surface temperature rose from -0.3 degrees in 2012 to 0.4 degrees in 2016. These changes in the meteorological factors facilitated an increase in land surface temperature.
(The drainage map of the area showing the Raunthigad catchment and potential source area of the ice-rock avalanche, Rishiganga catchment and Dhauliganga and Alakananda rivers further downstream.)
The central lobe of the glacier had advanced since 2012 and eventually fell off in 2016. This, along with the recent detachment of the right lobe suggests that the LST warming resulted in a reduction of frictional drag, facilitating the advancement and eventual dislodgement of glacier central lobe. The study considers this event as a precursor to the present event.
Permafrost modelling suggests warming of permafrost at around 50 m and conditions favourable for intense frost cracking at 10-15 m. At 40 m depth, the delayed response of 2012-2016 warming produced peak positive temperature conditions and probably facilitated the formation of a thin film of water at the deeper layers acting as a lubricant for glacier sliding along with huge rock mass.
Reason for increase in land surface temperature
The land surface temperature is governed by the changes occurring to the land surface. Here, reducing snow cover exposes large areas surrounding the glacier system to prolonged solar radiation, leading to enhanced warming of LST. Increase in monsoon rainfall also helped this process.
A) Hanging glacier in Google Earth image of 2017. The detached right lobe is marked blue. B) Heli photo of February 9, 2021 of the site showing detachment of the glacier creating an ice-rock avalanche. (Photo courtesy, THDC)
A recent study of glacial lake inventory of the region under the National Mission on Sustaining Himalayan Ecosystem recorded 7 glacier lakes in the area with none vulnerable as per the standard criteria. More studies and better mapping of areas could help predict disasters and save lives.
Any evidence suggesting the role of climate change?
A previous event in 2013 in the same region known as the Kedarnath deluge was caused by a combination of factors, including extreme rainfall, sudden snowmelt under excessive rainfall and breaching of Chorabari lake. This event suggests the role of warming climate. More studies will be needed to link the recent disaster to climate crisis.
Can such flash floods be prevented? Can there be a warning system?
Regular monitoring of glacier change would have helped to identify the advancement and eventual detachment of the central lobe of the glacier in 2016 and could have served as an early warning for the current disaster. Such precursors are common in the Alps and other mountain regions, suggesting an early warning and preparedness is possible in such conditions. It is also appreciated that small glaciers are losing mass at a much faster rate and are highly vulnerable.
Images showing a) Pre-event stream width (marked light blue) and Post721 event scouring (Yellow). The location of the hanging glacier and strike point at Raunthigad (marked red) and the stream reach upstream of strike point (marked blue), showing no signs of activities ruling out any flood originating upstream of strike point.
The current event highlighted the vulnerability of the Himalayan cryosphere system to temperature changes. This also adds a new disaster component in the Himalayas, warranting regular monitoring in areas surrounding infrastructure or developmental projects.
Flood modelling studies suggest a flood volume of 10 million cubic metres (MCM) of debris generating a 24.5 m flow depth and a velocity of 12.7 m/s at the bridge site. The debris travelled 12.4 km before hitting the infrastructure projects. This shows that there is scope of early warning through improved monitoring.