The 2020 glacial lake outburst flood at Jinwuco, Tibet: causes, impacts, and implications for hazard and risk assessment

We analyze and reconstruct a recent glacial lake outburst flood (GLOF) process chain on 26 June 2020, involving the moraine-dammed proglacial lake – Jinwuco (30.356∘ N, 93.631∘ E) in eastern Nyainqentanglha, Tibet, China. Satellite images reveal that from 1965 to 2020, the surface area of Jinwuco has expanded by 0.2 km2 (+56 %) to 0.56 km2 and subsequently decreased to 0.26 km2 (−54 %) after the GLOF. Estimates based on topographic reconstruction and sets of published empirical relationships indicate that the GLOF had a volume of 10 million cubic meters, an average breach time of 0.62 h, and an average peak discharge of 5602 m3/s at the dam. Based on pre- and post-event high-resolution satellite scenes, we identified a large debris landslide originating from western lateral moraine that was most likely triggered by extremely heavy, south-Asian-monsoon-associated rainfall in June 2020. We back-calculate part of the GLOF process chain, using the GIS-based open-source numerical simulation tool r.avaflow. Two scenarios are considered, assuming a debris-landslide-induced impact wave with overtopping and resulting retrogressive erosion of the moraine dam (Scenario A), as well as retrogressive erosion without a major impact wave (Scenario B). Both scenarios are in line with empirically derived ranges of peak discharge and breach time. The breaching process is characterized by a slower onset and a resulting delay in Scenario B compared to Scenario A. Comparison of the simulation results with field evidence points towards Scenario B, with a peak discharge of 4600 m3/s. There were no casualties from this GLOF, but it caused severe destruction of infrastructure (e.g., roads and bridges) and property losses in downstream areas. Given the clear role of continued glacial retreat in destabilizing the adjacent lateral moraine slopes and directly enabling the landslide to deposit into the expanding lake body, the GLOF process chain can be plausibly linked to anthropogenic climate change, while downstream consequences have been enhanced by the development of infrastructure on exposed flood plains. Such process chains could become more frequent under a warmer and wetter future climate, calling for comprehensive and forward-looking risk reduction planning.

Landslide-GLOF cascade at the expanding Jinwuco in Tibet, 2020: a clear consequence of anthropogenic climate change

<p>Glacial Lake Outburst Floods (GLOFs) are amongst the most common and high-magnitude natural hydrological disasters in high-mountain regions that have resulted in severe casualties and socioeconomic losses over the last century. Here, we integrate various data and methods to analyse and reconstruct the GLOF process chain involving the moraine-dammed proglacial lake ‒ Jinwuco (30.356°N, 93.631°E) in eastern Nyainqentanglha, Tibet, China, which occurred on 26<sup>th</sup> June 2020. This lake underwent rapid expansion in area from 0.2 km<sup>2</sup> to 0.56 km<sup>2</sup> (1965-2020), and subsequently shrank to 0.26 km<sup>2</sup> after the GLOF. Topographic reconstruction and empirical relationships indicate that the GLOF had a volume of 10 million m<sup>3</sup>, an average breach time of 0.62 hours, and an average peak discharge of 5,390 m<sup>3</sup>/s at the dam. Pre- and post-event high-resolution satellite scenes reveal a large progressive debris landslide originating from western lateral moraine. This landslide which occurred 5-17 days before the GLOF was most likely triggered by extremely heavy, south Asian monsoon-associated rainfall in June. The time lag between the landslide and the GLOF suggests that pre-weakening of the dam due to landslide-induced outflow pushed the system towards a tipping point, that was finally exceeded following subsequent rainfall, snowmelt, a secondary landslide, or calving of ice into the lake. We back-calculate a part of the GLOF process chain, using the GIS-based open source numerical simulation tool r.avaflow, considering two scenarios: Scenario A - a debris landslide-induced impact wave with overtopping and resulting retrogressive erosion of the moraine dam; and Scenario B - retrogressive erosion due to pre-weakening of the dam without a major impact wave. Both back-calculated scenarios yield plausible results which are in line with empirically derived ranges of peak discharge and breach time. The breaching process is characterized by a slower onset and a resulting delay in Scenario B, compared to Scenario A. Our evidence, however, points towards Scenario B. The 2020 Jinwuco GLOF caused severe destruction of infrastructure (e.g. roads and bridges) and property losses in downstream areas (no fatalities were reported).</p><p>This study corroborates the clear role of continued glacial retreat in destabilizing the adjacent lateral moraine slopes, and directly enabling the landslide to deposit into the expanding lake body. As such, the GLOF process chain can be robustly attributable to anthropogenic climate change, while downstream consequences have been driven by recent development of infrastructure on exposed flood plains. Such glacial lake related process chains could become more frequent under a warmer and wetter future climate, calling for comprehensive and forward-looking risk reduction planning. We anticipate our findings will provide critical new process understanding on GLOF triggering mechanisms and these new insights will improve GLOF hazard and risk assessment frameworks, highlighting the need to consider both complex instantaneous and gradual process chains.</p><p> </p>

The 2020 glacial lake outburst flood at Jinwuco, Tibet: causes, impacts, and implications for hazard and risk assessment

We analyze and reconstruct a recent Glacial Lake Outburst Flood (GLOF) process chain on 26 June 2020, involving the moraine-dammed proglacial lake Jinwuco (30.356° N, 93.631° E) in eastern Nyainqentanglha, Tibet, China. Satellite images reveal that from 1965 to 2020, the surface area of Jinwuco has expanded by 0.2 km2 (+56 %) to 0.56 km2, and subsequently decreased to 0.26 km2 (‒54 %) after the GLOF. Estimates based on topographic reconstruction and sets of published empirical relationships indicate that the GLOF had a volume of 10 million m3, an average breach time of 0.62 hours, and an average peak discharge of 5,390 m3/s at the dam. Based on pre- and post-event high-resolution satellite scenes, we identified a large progressive debris landslide originating from western lateral moraine, having occurred 5–17 days before the GLOF. This landslide was most likely triggered by extremely heavy, south Asian monsoon-associated rainfall in June. The time lag between the landslide and the GLOF suggests that pre-weakening of the dam due to landslide-induced outflow pushed the system towards a tipping point, that was finally exceeded following subsequent rainfall, snowmelt, a secondary landslide, or calving of ice into the lake. We back-calculate part of the GLOF process chain, using the GIS-based open source numerical simulation tool r.avaflow. Two scenarios are considered, assuming a debris landslide-induced impact wave with overtopping and resulting retrogressive erosion of the moraine dam (Scenario A), and retrogressive erosion due to pre-weakening of the dam without a major impact wave (Scenario B). Both scenarios yield plausible results which are in line with empirically derived ranges of peak discharge and breach time. The breaching process is characterized by a slower onset and a resulting delay in Scenario B, compared to Scenario A. Evidence, however, points towards Scenario B as a more realistic possibility. There were no casualties from this GLOF but it caused severe destruction of infrastructure (e.g. roads and bridges) and property losses in downstream areas. Given the clear role of continued glacial retreat in destabilizing the adjacent lateral moraine slopes, and directly enabling the landslide to deposit into the expanding lake body, the GLOF process chain under Scenario B can be robustly attributable to anthropogenic climate change, while downstream consequences have been enhanced by the development of infrastructure on exposed flood plains. Such process chains could become more frequent under a warmer and wetter future climate, calling for comprehensive and forward-looking risk reduction planning.

Landslide-generated tsunami and particle transport in mountain lakes and reservoirs

Gravitational mass flows may generate tsunamis as they hit water bodies such as oceans, reservoirs or mountain lakes. Upon impact, they can generate tremendous particle-laden or debris flows and floods. Rapidly cascading waves down mountain slopes can trigger debris flows or floods, potentially causing huge damage to civil structures and endangering life. Here we apply a general two-phase mass flow model (Pudasaini, 2012), and present three-dimensional (3-D), high-resolution simulations for a real two-phase debris impacting a fluid reservoir. An innovative formulation provides an opportunity, within a single framework, to simulate simultaneously the sliding two-phase debris/landslide, reservoir, debris impact at reservoir, water-wave generation, propagation and mixing, and separation between solid and fluid phases. The results demonstrate formation and propagation of very special solid and fluid structures in the reservoir, propagation of submarine debris, turbidity currents, and complex interactions between the subaerial debris, surface tsunami and submarine debris waves. Our results reveal that the submerge timescaling for a deformable two-phase debris deviates substantially from the same for a non-deformable solid. These results substantially increase our understanding of 3-D complex multiphase systems/flows. This allows for the proper modeling of landslide/debris-induced mountain tsunami, dynamics of turbidity currents and highly concentrated sediment transports in Himalayan and Alpine slopes and channels, with associated applications to engineering, environmental and hazard-mitigation plans.