Sonam Rinzin, a third year PhD student in the School of Geography, Politics and Sociology, was part of an international team of scientists that undertook a comprehensive analysis of the devastating glacial lake outburst flood (GLOF).

As glaciers retreat, they leave behind moraine or ice dams that trap meltwater. These lakes can suddenly burst and create a GLOF that can be highly destructive, damaging property, infrastructure, and agricultural land. They can also lead to significant loss of life.

Sonam was involved in simulating the chain of events that took place in the 2023 Sikkim flood. This included analysing the effect of mass moraine failure entering the lake, its interaction with lake water and subsequent impulse wave generation, and moraine dam failure. These simulations were based on pre- and post-event data and numerical modelling.

The team’s findings, published in Science, stress the urgent need to enhance GLOF hazard assessments and improve prediction and early warning systems as melting glaciers steadily raise the risk of GLOFs in the Himalaya region.

Sonam said: “The 2023 catastrophic GLOF event of Sikkim left enormous destruction and damage downstream and is an example of how hazardous glacial lakes can be for communities in the Himalaya.

“This novel scientific effort, which investigated the intricate causes, dynamics, and consequences of this catastrophic flood cascade is enormously important, not only for understanding the cause and consequences of the event itself, but also for highlighting the urgent need for collaborative efforts to reduce and mitigate GLOF risk in the Himalaya.

“As an early-career scientist, I feel privileged to have contributed to this significant research and to have collaborated with over 30 scientists from 10 countries. This experience provided me with a valuable opportunity to interact with some of the world's leading scientists while also enhancing my academic and technical skills.”

Widespread devastation

South Lhonak Lake – the source of the 2023 flood – is perched at 5,200 metres above sea level in the Upper Teesta basin of Sikkim, in north east India. It is among the region's largest and most rapidly expanding glacial lakes, posing severe hazards due to its potential for GLOFs.

These hazards were realised on 3 and 4 October 2023, when the lake experienced a catastrophic outburst, unleashing a flood cascade that claimed 55 lives, left 74 missing, and caused widespread downstream devastation, including the destruction of the Teesta-III hydropower dam.

According to the findings, the outburst was triggered when a landslide containing 14.7 million m³ of frozen sediment collapsed into South Lhonak lake. This generated a 20-metre tsunami-like wave that breached and eroded the frontal moraine containing the lake’s water and released around 50 million m³, a volume equivalent to 20,000 Olympic-sized swimming pools. This enormous volume of water then rushed down the valley causing severe damage to downstream infrastructure.

The research team – which comprised scientists from 10 countries, including the universities of Hull, Leeds, and Cambridge in the UK, and non-governmental organisations and other stakeholders in India and Bangladesh - show that climate warming intensified the event, as heavy rainfall primed the landscape for landslides and compounded sediment transport and downstream destruction in the Teesta Valley. This not only impacted Sikkim and West Bengal in India but also affected Bangladesh and damaged infrastructure as far as 385 kilometres away from the flood’s origin.

Need for improved early warning systems

According to the authors, the findings underscore the inadequacy of current GLOF models, which often fail to account for erosion, sediment transport, and cascading processes, and highlight the need for enhanced early warning systems, policy reforms, and adaptive risk management strategies, particularly in remote, high-altitude, vulnerable glacial regions like the Himalayas.

Professor Dave Petley, Vice-Chancellor at the University of Hull and an expert in the study and management of landslides, was the UK lead for the international collaboration. He said: “This research highlights the need for early warning systems; for mechanisms of risk management and mitigation; for research; and for evaluation of the risks to the growing number of hydropower schemes in this area. There is an urgency to this work, but of course many developing countries are facing reductions in aid. Understanding precisely what happened is a key to understanding future risks – and to ensuring that communities are better protected.”

The study was led by Dr Ashim Sattar, Assistant Professor at the Indian Institute of Technology, Bhubaneswar, who said: “As we work to reduce the risks of GLOFs in the Himalaya, it is clear that we need a multi-faceted approach, including early warning systems, strengthened regulatory frameworks, a paradigm shift in GLOF modeling approaches, and robust preparation programs and community education.” 

In February 2023, a Newcastle University-led team produced the first global assessment of areas at greatest risk of GLOFs and identified priority areas for mitigation. The research highlighted that 15 million people live within 50 km of a glacial lake and that High Mountain Asia - which encompasses the Tibetan Plateau, from Kyrgyzstan to China, and parts of India and Pakistan - has the highest GLOF danger, with 9.3 million people potentially at risk.

Reference

Sattar et al., ‘The Sikkim flood of October 2023: Drivers, causes and impacts of a multihazard cascade’, Science (2025) DOI: 10.1126/science.ads2659

Press release adapted with thanks to the University of Hull