Rapid adjustments to catchment sediment yield following a catastrophic rock-ice avalanche and debris flow, Uttarakhand, India

Lead Research Organisation: Northumbria University
Department Name: Fac of Engineering and Environment


On 7th February 2021 a massive rock-ice avalanche originating from a mountain ridge in Chamoli District, Uttarakhand, Indian Himalaya, transformed into a fast-moving and catastrophic debris flow which travelled along the Rishiganga, Dhauliganga, and Alaknanda rivers. The flow killed hundreds of people, destroyed or damaged mature and under-construction hydropower projects, and caused severe modification to the channel and wider valley floor landscape, including the destabilising of steep valley sides. Once the flood subsided, rapid post-event analysis revealed that sediments deposited by the debris flow were more than 20 m thick in places, and that the flow was capable of transporting boulders exceeding 20 m in diameter.

The next 12 months are a crucial period for this river system because this is when we predict that newly deposited sediments will be eroded and transported in vast quantities, and we believe that most of this activity will occur within a distance of around 50 km from the avalanche source, which includes four hydropower facilities and riverside settlements and infrastructure. This 're-activation' of sediments left behind by the flood has implications for local hydropower operators, who need to anticipate these elevated sediment loads and plan accordingly to reduce the risk of blockage to dam outlets and tunnels, avoid reduced discharge capacity, and damage to mechanical equipment. In addition, there is a high risk of further valley flank instability as this new drape of sediment is removed and banks that were undercut by the initial flow become more unstable, or undercutting is initiated in new areas. We also anticipate that sediment deposition could also present a hazard where these deposits intersect with valley floor energy and transport infrastructure.

To urgently predict rates and patterns of post-flood channel modification we will use a computer model that is capable of simulating river flow and the erosion, transport, and deposition of sediment. We will run this model for an initial period of one year (including the summer monsoon, which brings an order-of-magnitude increase in river discharge) and we will generate critical summary datasets that can be rapidly communicated to in-country end users. We already have access to most of the data that we require to set up and run the model, and project partners are well-placed to provide missing data that we need to perform initial runs and perform regular checks on model performance.

The work will be carried out by an international team comprised of experts in extreme floods and numerical flood modelling, the hydrology of high mountain landscapes, and community adaptation to (rapid) environmental change. The team includes researchers from the UK, India, Canada and the USA with a collective track record of delivering high quality science to inform real-world decision-making.

Follow-on work will broaden the scope of the work to look at sediment transport and deposition over a much larger area: analysis of satellite imagery shows that the initial sediment plume generated by the flood travelled >150 km in ~24 h and we anticipate that annual re-activation of flood sediment will have significant impacts on the hazard posed by this extreme event.


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Description This research combined satellite- and field-based observations with computer modelling to reconstruct landscape adjustment in river channels affected by the 7 February 2021 'Chamoli event', a catastrophic avalanche comprised of rock and glacier ice which turned into a fast-moving and highly destructive debris flow. Now that the main programme of work has been completed, we can report that our primary findings are that:

- Of the initial debris deposit volume of 10.4 ± 1.6 million m3, 67% of this material (7.0 ± 1.5 million m3) was removed in the first 12 months post-event by fluvial action;

- The median erosion rate in our 30 km-long study domain was -2.3 ± 1.1 metres per year, which drove rapid channel re-establishment and sediment export;

- Most material was removed by river flows in the pre-monsoon (March-June) and monsoon (June-September) periods;

- The results of computer modelling (for simulating water and sediment transport through the landscape) suggested that high flows activated a series of sediment 'waves' comprised of coarse material, and that these waves travelled at between 0.1-0.3 km/day.

- Increases in suspended sediment concentrations up to three orders of magnitude above pre-disturbance norms were detected up to 85 km from the event source, caused by the system effectively 'flushing' fine, easily erodible sediment from the avalanche/debris flow deposits;

- In combination, elevated suspended and bedload sediment transport following the Chamoli event may have impacts on riparian assets (including hydropower), water quality, and ecosystem services for many years - quantifying these impacts should be a priority for future work;

- In sum, our findings show that the Ronti Gad-Rishiganga-Dhauliganga-Alakanda rivers represent a high-mountain fluvial cascade that has a short 'relaxation time' (time taken to recover toward a pre-disturbance state) and a high degree of 'resilience' (ability to recover toward a pre-disturbance state) to a cataclysmic geomorphological perturbation.

Additional discoveries:

- In the course of this research we discovered evidence of previously undocumented and recurring large ice avalanche activity from Ronti Peak, Chamoli District, Uttarakhand. This avalanche (or series of avalanches) totalled ~8 million cubic metres and occurred toward the end of August 2021. This has been an unanticipated outcome of the award.
Exploitation Route Arguably the most significant finding of their work is the potential presence of large-scale and fast-moving bedload sediment waves, which originated from the avalanche deposits. The PI is actively pursuing a programme of work to quantify the downstream impact of these waves, and has applied for funding from the British Society for Geomorphology and the Mount Everest Foundation (decisions pending), and will submit a NERC 'seedcorn' proposal in March 2023, to obtain funding with which to conduct this work.

Related - The PI and Co-I Dunning are involved in an allied project led by French academics to investigate the recent and ongoing destabilization of the town of Joshimath. Modelling outputs from this Urgency work may feed into this effort in order to shed light on whether channel adjustment following the 2021 Chamoli event has exacerbated slope movement on adjacent hillsides. Those findings will be communicated to in-country decision makers via project partners in due course.

The new ice avalanche(s) that we discovered remain poorly understood, but we have made our wider network aware of this finding, including in-country contacts. They may choose to take our findings and build on these to better unravel the triggers and dynamics of the avalanches in question, which could shed new knowledge on the frequency and magnitude of extreme and potentially hazardous mass movements in this region of the Himalaya.
Sectors Communities and Social Services/Policy,Education,Energy,Environment

Description Invited keynote lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact I was invited by an Indian project partner to give a keynote lecture ('Post-flood assessment of the Chamoli Disaster') at the 42nd Annual Convention and Seminar of Association of Exploration Geophysicists, which was hosted by the Wadia Institute of Himalayan Geology (WIHG) in Dehradun, Uttarakhand, India. I delivered this lecture via Teams on 3rd December to an audience of ~60 or so, including a large proportion of students and early career researchers. In a follow-up e-mail, the conference organisers stated: 'Thank you for accepting our invitation and deliver[ing] a highly educational talk and sharing knowledge to the geo-science fraternity and young researchers joining the conference ... Your talk related to the Chamoli Disaster was interesting to all of us.' Shortly after this talk, I was invited by colleagues from WIHG to co-convene a session at the forthcoming Asia Oceania Geosciences Society conference (AOGS22).
Year(s) Of Engagement Activity 2021
URL https://www.wihg.res.in/?page_id=1477