Transboundary hazard and downstream impact of glacial lakes in Hindu-Kush Karakoram Himalayas.

Glacial Lake Outburst Floods (GLOFs) can generate catastrophic flash floods when the damming structure is breached or overtopped. Some of these glacial lakes are located in transboundary regions where floods originating from the lake in one country could inundate a neighboring country, devastating the population and infrastructure of both nations and influencing socio-political relationships. Therefore, assessing the lakes' hazard is crucial. This study investigates transboundary glacial lakes, considering their GLOF hazard, including potential mass movement intrusion, moraine's stability, upstream and downstream process cascades, downstream flood extents, and the exposure and vulnerability of the downstream infrastructure and affected population. GLOF exposure assessments were carried out to identify exposed buildings, bridges, and hydropower systems in transboundary regions. China currently has the highest number of transboundary lakes, with most of them potentially impacting India and Nepal. Most of the transboundary lakes in China, and many in India and Nepal, are susceptible to mass movements. Among the 230 transboundary glacial lakes in the Hindu Kush Karakoram Himalaya, 55 lakes can potentially impact other glacial lakes along their flow path, creating a cascade of events. Five transboundary lakes could potentially impact over 1000 buildings, and 16 lakes could impact over 500 buildings. A total of 35 lakes can impact at least one hydropower station along their flow path, and 4 lakes can impact two hydropower stations. This research emphasizes the critical importance of conducting comprehensive risk analyses of GLOFs in transboundary regions to inform policy-makers. It calls for investing in broad-scale assessments and data-driven decision-making for mitigating and adapting to GLOF risks effectively. Finally, by raising awareness among policy-makers, the study aims to drive actions that safeguard communities and infrastructure vulnerable to GLOF.

Transition of a small Himalayan glacier lake outburst flood to a giant transborder flood and debris flow.

Glacial lake outburst floods (GLOFs) are a great concern for the Himalaya, as they can severely damage downstream populations and infrastructures. These floods originate at high altitudes and can flow down with enormous energy and change the terrain's existing morphology. One such devastating event occurred on the night of 5 July 2016, from the inconspicuous Gongbatongsha Lake, located in the Poiqu basin, Eastern Himalaya. The Poiqu basin in the Tibetan Autonomous Region currently contains numerous big glacial lakes; however, this event originated from a small lake. The GLOF was triggered following heavy precipitation that led to a slope failure above the lake and deposition of debris into the lake, which breached the moraine dam and rapidly drained the entire lake. The flood damaged several downstream infrastructures, including the Arniko highway, the Upper Bhotekoshi hydropower plant, and several buildings as it made its way into the Bhotekoshi basin in Nepal. This study adopts a multi-model approach to reconstruct the GLOF trigger and the flood's transformation into a severe debris flow. Proxies including flow discharge, flow velocity, runout distances were used to calibrate the model and validate the results. Results reveal that a debris flow of volume ranging between 3000 and 6000 m3 from the headwall must have led to lake overfill, eventually leading to the GLOF event. The GLOF showed a significant increase in peak discharge from 618 to 4123 m3 s-1 at the Zhangzangbo-Bhotekoshi confluence. The average velocity of the flow is calculated to be ~ 5.5 m s-1. Reconstruction of the erosion and deposition dynamics show that maximum erosion occurred in the first 6.5 km, with maximum deposition occurring near the Upper Bhotekoshi hydropower station. The modeling indicates that the availability of the entrainable debris along the channel, likely from the previous landslides, amplified the event by three orders of magnitude-additional water ingested from the river. Overall, we demonstrate how the small-scale Gongbatongsha GLOF amplified downstream by incorporating pre-existing sediment in the valley and triggered damaging secondary landslides leading to an economic loss of > 70 million USD.

Recession of Gya Glacier and the 2014 glacial lake outburst flood in the Trans-Himalayan region of Ladakh, India.

This study assessed spatiotemporal changes at Gya Glacier, the associated development of a proglacial lake, and reconstructed the 2014 outburst flood that struck Gya Village in the Trans-Himalayan region of Ladakh, India. This study analyzed and for the first time modeled a Glacial Lake Outburst Flood (GLOF) event in the Trans-Himalayan region of Ladakh. Glacier and glacial lakes changes were quantified using remote sensing data supplemented with field observations. Glacier ice-thickness and glacier-bed overdeepenings were modeled using a shear-stress based model, GlabTop (Glacier-bed Topography). The reconstruction of the 2014 GLOF and the potential hazard assessment of Gya Lake were carried out using the hydrodynamic model HEC-RAS; results were validated against ground-collected data. Temporal evaluation of satellite data revealed a 45.6% loss in the total glacier area between 1969 and 2019. The earliest snow-free image available for the region shows that a proglacial lake existed as early as 1969 with an area of 3.06 ha. The lake has expanded to ~11 ha in 2019. Results from the GlabTop model suggest that the lake could grow further up to 12 ha in the future. Field-based geomorphic indicators suggest that the 2014 GLOF event resulted from a piping failure of the frontal moraine destroying numerous agricultural fields, some buildings, downstream infrastructure, and eroded natural channel embankments. The reconstruction of the event revealed that 25% of the lake waters drained out with a peak discharge of 470 m3s-1, inundating an area of ~4 km2 around Gya Village. However, a complete breaching of the terminal moraine could result in an event that would be 5.5 times larger than the 2014 GLOF. Therefore, this study could be useful not only in planning disaster-resilient infrastructure around proglacial lake environments in the cold-arid Ladakh but also in framing mitigation plans to reduce risk for vulnerable downstream communities.

Hydrodynamic moraine-breach modeling and outburst flood routing - A hazard assessment of the South Lhonak lake, Sikkim.

The presence of glacial lakes in the Himalaya makes it a potential mountain hazard, as catastrophic failure of such waterbodies may lead to high-magnitude glacial lake outburst flood (GLOF) events that can cause significant damage to the low-lying areas. The present study evaluates the hazard potential of the South Lhonak lake located in the state of Sikkim, using both one and two-dimensional hydrodynamic modeling approaches. Different breach parameters were calculated based on the lake bathymetry and moraine dimensions. The worst-case GLOF scenario is revealed during an overtopping failure of the moraine, producing a peak flood of 6064.6 m3 s-1 and releasing a total water volume of 25.7 × 106 m3. The GLOF hydrograph is routed to calculate peak flood (m3 s-1), inundation depth (m) and flow velocity (ms-1) along the main flow channel. The interaction of the flood wave with a major topographic obstruction located 15.6 km downstream of the lake, shows a significant reduction of the flow energy leading to a minimization of the South Lhonak GLOF impact. The flood wave reaches the nearest town Lachen, located at a distance of 46 km downstream from the lake, at 3 h 38 min after the initiation of the breach, with a peak flood of 3928.16 m3 s-1 and a maximum flow velocity of 13.6 ms-1. At Chungthang town, located at a distance of 62.35 km from South Lhonak lake, the flood wave potentially inundates settlements along the bank of the flow channel, where a peak flood of 3828.08 m3 s-1 is reached after 4 h of the initial dam breach event. The study also incorporates modeling of a framework to propose a potential flood remediation measure of the South Lhonak lake GLOF by demonstrating the effect of a lateral inline structure along the flow channel, to check the flow of the potential flood wave.