Drivers, causes and impacts of the 2023 Sikkim flood in India
Summary author: Walter Beckwith
Concrete dam of the Teesta III hydropower plant completely destroyed by the flood disaster
In a comprehensive analysis, researchers present the divers, causes, and impacts of the catastrophic 2023 Sikkim glacial lake outburst flood (GLOF). The findings 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 Himalayan region. South Lhonak Lake – perched at 5200 meters above sea level in the Upper Teesta basin of Sikkim, India – is among the region's largest and most rapidly expanding glacial lakes, posing severe hazards due to its potential for GLOFs. These hazards were realized on October 3, 2023, when the glacial 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. Combining high-resolution satellite imagery, seismic and meteorological data, field observations, and numerical modeling, Ashim Sattar and colleagues present a comprehensive and multidisciplinary analysis of the event. According to the findings, the outburst was triggered when a landslide containing 14.7 million cubic meters (m3) of frozen sediment collapsed into the lake, generating a ~20-meter tsunami-like wave that breached and eroded the frontal moraine containing the waterbody, releasing roughly half of the lake’s volume (~50 million m3 of water) and ~270 million m3 of sediment into the Teesta River valley. Moreover, Sikkim et al. show that climate warming intensified the event, as heavy rainfall primed the landscape for landslides that compounded sediment transport and downstream destruction in the Teesta Valley, which impacted Sikkim, West Bengal, and Bangladesh and damaged infrastructure as far as 385 kilometers away from the flood’s origin. 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.
Journal
Science
Article Title
The Sikkim flood of October 2023: drivers, causes and impacts of a multi-hazard cascade
Article Publication Date
30-Jan-2025
Earth scientists study Sikkim flood in India to help others prepare for similar disasters
UCalgary scientist says it’s important to determine what happened and what can be learned
University of Calgary
image:
Dan Shugar, an associate professor with the Department of Earth, Energy and Environment in the Faculty of Science at the University of Calgary, says it’s important to analyse what happened in the Sikkim flood and what can be learned from it as rapid climate warning affects mountain regions around the world.
view moreCredit: Courtesy: Dan Shugar/University of Calgary
Experts from the global Earth science community – including a scientist from the University of Calgary – have pieced together what happened during the massive Sikkim flood to try to help others prepare for similar disasters.
On Oct. 3, 2023, a multi-hazard cascade in the Sikkim Himalaya, India, was triggered by a permanently frozen (permafrost) lateral moraine – debris from erosion along a glacier – collapsing into South Lhonak Lake.
“A landslide went into a lake and that triggered a wave that eroded a dam at the end of the lake, which resulted in a slurry-like flood for hundreds of kilometres,” explains Dr. Dan Shugar, PhD, an associate professor with the Department of Earth, Energy and Environment in the Faculty of Science.
Known as a glacial lake outburst flood, it killed at least 55 people, left dozens more missing, damaged agricultural land and destroyed a hydropower dam.
The Sikkim flood was declared one of the worst climate-related disasters to have occurred on the continent that year by the World Meteorological Organization’s State of the Climate in Asia 2023 report.
A new paper in the prestigious journal, Science, presents a collaborative effort by scientists, academics, government departments, non-governmental organizations and others to investigate the event.
Shugar, a geomorphologist who’s a co-author on the paper, says it’s important to forensically analyse what happened and what can be learned from it as rapid climate warning affects mountain regions around the world.
The paper looks at the drivers and causes and assesses the downstream impacts of the hazard cascade using high-resolution satellite imagery, seismic data, meteorological data and field observations. It also explores the triggers of the flood and reconstructs its hydraulic dynamics, evaluates downstream implications and considers the long-term impacts of the event.
“The assessment indicates that the high hazard level arises not only from the flood itself but also from the subsequent processes it triggers,” says the paper.
Dr. Ashim Sattar, PhD, the study’s lead author who’s an assistant professor in the School of Earth, Ocean and Climate sciences at the Indian Institute of Technology, adds that the Sikkim flood had devastating effects for downstream regions.
“This is high time to be building resilience in downstream regions that are exposed to such potentially catastrophic events in the Himalaya,” he says in a statement.
“The threat of these catastrophic events is growing, urging us to act with urgency in protecting both our environment and communities in the Himalaya and similar regions around the world.”
Sattar says glacial lakes are growing in number and size, so they need to be critically and urgently evaluated for downstream hazards and damage potential.
“Climate change is leading to changes in permafrost temperatures, increasing the risk of slope failures that can lead to avalanches or trigger glacial lake outburst floods in the high mountains.”
In Canada, for example, a glacial lake outburst flood in British Columbia’s southern Coast Mountains destroyed forest and salmon spawning habitat in November 2020.
Shugar, who has studied both events, says they can have serious implications for people and infrastructure.
“This study (on the Sikkim flood) is a good example of a mountain disaster galvanizing the global Earth science research community to work together on a common goal,” he says.
“Advances in Earth observation technologies over the last decade have dramatically improved our ability to understand these sorts of events, and ultimately, leading to disaster risk reduction.”
Journal
Science
Article Title
The Sikkim flood of October 2023: Drivers, causes and impacts of a multihazard cascade
Article Publication Date
30-Jan-2025
Climate change increases risk of successive natural hazards in the Himalayas
University of Zurich
image:
Flooding and destruction of Rangpo, over 130 km downstream of South Lhonak Lake
view moreCredit: Praful Rao
An international study has investigated the causes and impacts of the devastating flood disaster in the Himalayas in October 2023, which destroyed large areas along and surrounding the Teesta River in Sikkim, India. A research team from nine countries, including researchers from the University of Zurich (UZH), analyzed the complex drivers, causes and consequences of this flood cascade and reconstructed the exact time of its onset.
Massive damage caused by tsunami wave
On 3 October 2023, approximately 14.7 million cubic meters of frozen moraine material collapsed into South Lhonak Lake, triggering a tsunami-like impact wave up to 20 meters high. The subsequent glacial lake outburst flood breached the moraine and released approximately 50 million cubic meters of water – enough to fill 20,000 Olympic-sized swimming pools. The flood caused massive damage along the 385-kilometer-long valley, washing away some 270 million cubic meters of sediment and inundating infrastructure such as hydroelectric power plants on the Teesta River. At least 55 people were killed, and 70 others were reported missing.
“This event is a stark reminder of the vulnerability of high mountain regions to the effects of climate change,” says Christian Huggel, co-author of the study and head of the Environment and Climate research group at UZH. “The thawing of permafrost and the instability of rock, ice and moraine structures pose major risks.”
High-resolution remote sensing data crucial
Using state-of-the-art scientific methods, the researchers analyzed the dynamics and effects of the flood disaster in detail. High-resolution satellite images, digital elevation models and numerical simulations provided a detailed reconstruction of the event. Seismic data helped the researchers to determine the exact time of the moraine collapse, while geomorphological analyses quantified the volume of water and sediment released. The combination of satellite technology and physical models provided a comprehensive picture of the disaster and its far-reaching consequences.
“The use of high-resolution remote sensing data was crucial to understanding the complex processes and cascading effects of the flood in detail,” explains first author Ashim Sattar, a former postdoctoral researcher at UZH and now an assistant professor at the Indian Institute of Technology in Bhubaneswar. “Collaboration among researchers from different disciplines was key in gaging the full extent of this event.”
Urgent need for early warning systems
The flood not only destroyed infrastructure, including five hydroelectric power plants, but also caused massive erosion and sedimentation, with serious consequences for farmers and local businesses. “Our findings highlight the urgent need for early warning systems and international cooperation to address such challenges,” emphasizes Sattar. The study also shows that the instability of the moraines had been evident years before the event, with shifts of up to 15 meters per year. This underlines the need for coordinated monitoring of critical high mountain areas and further preventive measures that could have mitigated the damage.
Better risk assessment
The researchers emphasize that similar disasters are likely to become more common in the future as rising temperatures increase the risk of glacial lake outbursts. “The case of South Lhonak Lake is a reminder to take climate risks in mountain regions worldwide more seriously,” says Christian Huggel. Ashim Sattar adds: “We need better risk modeling and assessment, as well as robust adaptation strategies, to minimize future disasters.”
The team also calls for stronger regulation of hydropower development in high-risk areas, better monitoring of glacial lakes, and the integration of early warning systems. The study provides important insights that can help to better prepare local communities for the growing challenges of climate change.
Literature
A. Sattar et al. The Sikkim flood of October 2023: Drivers, causes and impacts of a multihazard cascade. Science. 30 January 2025. DOI: 10.1126/science.ads2659
Journal
Science
Method of Research
Computational simulation/modeling
Subject of Research
Not applicable
Article Title
The Sikkim flood of October 2023: Drivers, causes and impacts of a multihazard cascade
Article Publication Date
30-Jan-2025
Northern Lateral moraine Colla [VIDEO] |
High-resolution satellite images of South Lonak Lake before and after the flood disaster
Credit
Pléides
Numerical modelling of the glacial lake outburst flood at South Lonak Lake, Sikkim, India, showing the progressive flooding of the Teesta River valley (top) and the observed and modelled erosion during the lake outburst flood (bottom).
Numerical modelling of the glacial lake outburst flood at South Lonak Lake, Sikkim, India, showing the progressive flooding of the Teesta River valley (top) and the observed and modelled erosion during the lake outburst flood (bottom).
Credit
Ashim Sattar et. al.
Ashim Sattar et. al.
