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The Sikkim flood of October 2023: Drivers, causes, and impacts of a multihazard cascade.

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Title: The Sikkim flood of October 2023: Drivers, causes, and impacts of a multihazard cascade.
Authors: Sattar, Ashim, Cook, Kristen L., Rai, Shashi Kant, Berthier, Etienne, Allen, Simon, Rinzin, Sonam, de Vries, Maximillian Van Wyk, Haeberli, Wilfried, Kushwaha, Pradeep, Shugar, Dan H., Emmer, Adam, Haritashya, Umesh K., Frey, Holger, Rao, Praful, Gurudin, Kori Sanjay Kumar, Rai, Prabhakar, Rajak, Rajeev, Hossain, Faruk, Huggel, Christian, Mergili, Martin
Source: Science. 3/21/2025, Vol. 387 Issue 6740, p1-20. 20p.
Subjects: Glacial lakes, Floods, Economic impact, Climate change, Land use
Abstract: On 3 October 2023, a multihazard cascade in the Sikkim Himalaya, India, was triggered by 14.7 million cubic meters of frozen lateral moraine collapsing into South Lhonak Lake. The impact generated an ~20-meter tsunami-like impact wave, which breached the moraine and drained ~50 million cubic meters of the lake's water. The ensuing glacial lake outburst flood (GLOF) eroded ~270 million cubic meters of sediment, which overwhelmed infrastructure, including hydropower installations along the Teesta River. The physical scale and human and economic impacts of this event prompt urgent reflection on the role of climate change and human activities in exacerbating such disasters. Insights into multihazard evolution are pivotal for informing policy development, enhancing early warning systems (EWS), and spurring paradigm shifts in GLOF risk management strategies in the Himalaya and other mountain environments. Editor's summary: In October 2023, South Lhonak Lake in Sikkim, India, unleashed a devastating glacial lake outburst flood and ensuing sediment displacement that caused substantial loss of life and infrastructure damage. Sattar et al. report a multifaceted examination of the causes and consequences of this disaster, drawing from satellite imagery, seismic and meteorological data, field observations, and numerical modeling. Their results point to the pressing need to enhance prediction capabilities and early warning systems as melting glaciers raise the risk of such floods in the Himalayan region. —Jake S. Yeston INTRODUCTION: On 3 October 2023, a multihazard cascade began with a permafrost landslide into South Lhonak Lake at 5200 m above sea level in Sikkim, India. This triggered a glacial lake outburst flood (GLOF) that traveled 385 km along the Teesta River to Bangladesh. The disaster caused 55 deaths, and 74 persons were reported missing in Sikkim. This event caused extensive damage to infrastructure, making it one of the most disastrous multihazard events recorded in the region. The severity of these impacts stem from the large volume of water that was released, the massive amount of eroded material, and the recent construction of infrastructure and settlements along the Teesta Valley. RATIONALE: High mountain regions are experiencing rapid environmental and societal changes. The combined effects of the changing cryosphere and population growth, along with construction, are increasing disaster risks in High Mountain Asia. The 2023 South Lhonak Lake outburst illustrates a multihazard event with cascading and far-reaching impacts. Understanding the physical and societal drivers of these events is essential to reducing future disaster risks, requiring collaboration among scientists, administrative officials, nongovernmental organizations, and other stakeholders. RESULTS: The collapse of ~14.7 million m3 of frozen lateral moraine into the South Lhonak Lake at ~22:12:20 Indian standard time on 3 October 2023 generated a 20-m-high displacement wave. The wave overtopped and breached the frontal frozen moraine, causing the sudden drainage of ~50 million m3 of water, with a peak discharge of 48,500 m3 s−1. The GLOF eroded ~270 million m3 of sediment and triggered 45 secondary landslides. Most of the erosion occurred within ~67.5 km of the lake, with 66.5 million m3 of sediment displaced in the stretch between 40 and 45 km downstream. The flood cascade caused extensive damage—more than ~25,900 buildings were inundated, damaged or destroyed; 31 major bridges were lost; and ~270 km2 of agricultural land was affected. The Teesta III hydropower dam was destroyed, and several other dams along the Teesta River were damaged. A low-pressure system contributed to the heavy rainfall across Sikkim, West Bengal, and Bangladesh between 2 and 6 October 2023. The combined effects of the GLOF, eroded sediments, landslides, and subsequent rainfall exacerbated the human and economic toll. CONCLUSION: Our study highlights widespread impacts often overlooked in GLOF cascades that are associated with large volumes of mobilized material. The extreme water and sediment volumes underscore the risk to hydropower infrastructure in the Himalaya, revealing gaps in hazard assessment and risk management. The 3 October 2023 multihazard event highlights the need for a shift in modeling and observation methods to improve our understanding and quantification of cascading hazards. Moreover, there is an urgent need for improved risk-transfer mechanisms and better preparedness to mitigate future GLOF hazards in high-mountain regions. Enhancing early warning systems, infrastructure resilience, land-use management, and community preparedness is essential to reduce GLOF impacts and thus support sustainable development. South Lhonak multihazard cascade and impacts.: (A) Schematic illustrating the South Lhonak Lake outburst initiated by the collapse of frozen lateral moraine into the lake and the breaching of the frontal moraine. (B and C) Field photographs showing the destroyed 1200-MW Teesta III hydropower dam (B) and South Lhonak Lake after the GLOF event (C). [Photo credits: P.R. and ITBP] (D) Reconstructed GLOF dynamics from the lake to the Teesta III hydropower dam, showing the discharge at four cross sections (CS-3 to CS-6) along the flow channel. ITBP, Indo-Tibetan Border Police; PL, lake water; Q, discharge. (E) Maximum erosion depth and volume along the flow channel. [ABSTRACT FROM AUTHOR]
Copyright of Science is the property of American Association for the Advancement of Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: The Sikkim flood of October 2023: Drivers, causes, and impacts of a multihazard cascade.
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  Data: On 3 October 2023, a multihazard cascade in the Sikkim Himalaya, India, was triggered by 14.7 million cubic meters of frozen lateral moraine collapsing into South Lhonak Lake. The impact generated an ~20-meter tsunami-like impact wave, which breached the moraine and drained ~50 million cubic meters of the lake's water. The ensuing glacial lake outburst flood (GLOF) eroded ~270 million cubic meters of sediment, which overwhelmed infrastructure, including hydropower installations along the Teesta River. The physical scale and human and economic impacts of this event prompt urgent reflection on the role of climate change and human activities in exacerbating such disasters. Insights into multihazard evolution are pivotal for informing policy development, enhancing early warning systems (EWS), and spurring paradigm shifts in GLOF risk management strategies in the Himalaya and other mountain environments. Editor's summary: In October 2023, South Lhonak Lake in Sikkim, India, unleashed a devastating glacial lake outburst flood and ensuing sediment displacement that caused substantial loss of life and infrastructure damage. Sattar et al. report a multifaceted examination of the causes and consequences of this disaster, drawing from satellite imagery, seismic and meteorological data, field observations, and numerical modeling. Their results point to the pressing need to enhance prediction capabilities and early warning systems as melting glaciers raise the risk of such floods in the Himalayan region. —Jake S. Yeston INTRODUCTION: On 3 October 2023, a multihazard cascade began with a permafrost landslide into South Lhonak Lake at 5200 m above sea level in Sikkim, India. This triggered a glacial lake outburst flood (GLOF) that traveled 385 km along the Teesta River to Bangladesh. The disaster caused 55 deaths, and 74 persons were reported missing in Sikkim. This event caused extensive damage to infrastructure, making it one of the most disastrous multihazard events recorded in the region. The severity of these impacts stem from the large volume of water that was released, the massive amount of eroded material, and the recent construction of infrastructure and settlements along the Teesta Valley. RATIONALE: High mountain regions are experiencing rapid environmental and societal changes. The combined effects of the changing cryosphere and population growth, along with construction, are increasing disaster risks in High Mountain Asia. The 2023 South Lhonak Lake outburst illustrates a multihazard event with cascading and far-reaching impacts. Understanding the physical and societal drivers of these events is essential to reducing future disaster risks, requiring collaboration among scientists, administrative officials, nongovernmental organizations, and other stakeholders. RESULTS: The collapse of ~14.7 million m3 of frozen lateral moraine into the South Lhonak Lake at ~22:12:20 Indian standard time on 3 October 2023 generated a 20-m-high displacement wave. The wave overtopped and breached the frontal frozen moraine, causing the sudden drainage of ~50 million m3 of water, with a peak discharge of 48,500 m3 s−1. The GLOF eroded ~270 million m3 of sediment and triggered 45 secondary landslides. Most of the erosion occurred within ~67.5 km of the lake, with 66.5 million m3 of sediment displaced in the stretch between 40 and 45 km downstream. The flood cascade caused extensive damage—more than ~25,900 buildings were inundated, damaged or destroyed; 31 major bridges were lost; and ~270 km2 of agricultural land was affected. The Teesta III hydropower dam was destroyed, and several other dams along the Teesta River were damaged. A low-pressure system contributed to the heavy rainfall across Sikkim, West Bengal, and Bangladesh between 2 and 6 October 2023. The combined effects of the GLOF, eroded sediments, landslides, and subsequent rainfall exacerbated the human and economic toll. CONCLUSION: Our study highlights widespread impacts often overlooked in GLOF cascades that are associated with large volumes of mobilized material. The extreme water and sediment volumes underscore the risk to hydropower infrastructure in the Himalaya, revealing gaps in hazard assessment and risk management. The 3 October 2023 multihazard event highlights the need for a shift in modeling and observation methods to improve our understanding and quantification of cascading hazards. Moreover, there is an urgent need for improved risk-transfer mechanisms and better preparedness to mitigate future GLOF hazards in high-mountain regions. Enhancing early warning systems, infrastructure resilience, land-use management, and community preparedness is essential to reduce GLOF impacts and thus support sustainable development. South Lhonak multihazard cascade and impacts.: (A) Schematic illustrating the South Lhonak Lake outburst initiated by the collapse of frozen lateral moraine into the lake and the breaching of the frontal moraine. (B and C) Field photographs showing the destroyed 1200-MW Teesta III hydropower dam (B) and South Lhonak Lake after the GLOF event (C). [Photo credits: P.R. and ITBP] (D) Reconstructed GLOF dynamics from the lake to the Teesta III hydropower dam, showing the discharge at four cross sections (CS-3 to CS-6) along the flow channel. ITBP, Indo-Tibetan Border Police; PL, lake water; Q, discharge. (E) Maximum erosion depth and volume along the flow channel. [ABSTRACT FROM AUTHOR]
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  Data: <i>Copyright of Science is the property of American Association for the Advancement of Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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