2023 Sikkim Flood: Drivers, Causes, Impacts

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.