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

Abstract

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.

Glacial lake outburst flood hazard under current and future conditions: worst-case scenarios in a transboundary Himalayan basin

“…At the border with Nepal (Zhangmu), our simulations reveal potential peak discharges in the range of 35 000-170 000 m 3 s −1 under worst-case scenarios, which is more than 15 times larger than indicated by earlier modelling studies (Shrestha et al, 2010), suggesting that previously estimated potential property losses of up to USD 197 million in downstream communities of Nepal are far lower than what could feasibly occur. In comparison with past events, the 1981 outburst from Cirenmaco, resulting in around 200 fatalities and up to USD 4 million in damage, had an estimated peak discharge of around 10 000 m 3 s −1 in Zhangmu (Cook et al, 2018;Wang et al, 2018), while the 2016 event from Gongbatongshacuo lake was about half this magnitude again but resulted in economic losses of > USD 70 million but no loss of life (Sattar et al, 2022). Despite the threat the lake poses, the focus at Jialongco on hard engineering strategies to reduce GLOF risk could prove both costly and inefficient if not complemented by a more comprehensive and forward-looking strategy that considers large process chains and appropriate response actions.…”

Geomorphic process chains in high-mountain regions - A review and classification approach for natural hazards assessment

“…The second event in 1970 was triggered by an earthquake. In the Himalayas, Cook et al ( 2018) and Sattar et al (2022) assessed an outburst flood at the Gongbatongshacuo Lake, which produced many slope processes along the Zhangzangbo River and subsequent damages to buildings and infrastructure 40 km downstream. Ruiz-Villanueva et al (2017) provided an overview of landslide-dammed lake outburst floods across the Hindu Kush-Karakoram-Himalayas.…”

Modeling Potential Glacial Lake Outburst Flood Process Chains and Effects From Artificial Lake‐Level Lowering at Gepang Gath Lake, Indian Himalaya

“…Glacial lakes are often dammed by loosely consolidated glacier deposits that form terminal moraines. Catastrophic failure of these dams can cause far‐reaching glacial lake outburst floods (GLOFs), impacting livelihood and infrastructure up to several tens or hundreds of kilometres downstream (Carrivick & Tweed, 2016; Li et al., 2022; Sattar et al., 2019, 2022; Zheng, Allen, et al., 2021). In the Himalaya, several devastating GLOFs were reported (e.g., S. K. Allen, Rastner, et al., 2016; Cook et al., 2018; Majeed et al., 2021), but they do not show a definite pattern regarding failure timings and locations (Veh et al., 2018).…”

“…By far, the most dangerous of these processes are direct impacts on the lakes from large, high‐energy rock‐ice avalanches that originate from surrounding slopes (Haeberli et al., 2017). GLOFs are often associated with a chain of processes, including a triggering mechanism, displacement waves, overtopping at the dam, erosion at the dam site, erosion and deposition along the flow path, and downstream flood or debris flow propagation (Sattar et al., 2022). These process chains can differ depending on the (a) nature of the trigger, for example, ice and/or rock avalanche, rockfalls, landslides, or internal failure of the damming moraine; (b) trigger magnitude, which is a function of total volume, flow type, drop height, and average slope; (c) lake characteristics, including lake dimension, bottom topography, and distance of the lake from the external trigger source; (d) morphology and composition of the damming moraine; and (e) downstream topography and the nature of the flow, for example, the transition of floods to debris flows or hyperconcentrated flows.…”

“…These extreme flows could also transform into debris or hyperconcentrated flows as they propagate downstream, causing severe impacts. For example, (a) the 11 April 2010 GLOF event in the Cordillera Blanca where an ice‐rock avalanche triggered the GLOF from the Laguna 513 that impacted the city of Carhuaz (Schneider et al., 2014), (b) the 20 April 2017 event in the Barun valley, Nepal Himalaya, where an outburst of the Langmale glacial lake resulted in a GLOF and debris flow that blocked the Arun River (Byers et al., 2019), (c) the 1941 GLOF event of Lake Palcacocha in Peruvian Cordillera Blanca that claimed at least 1,600 lives and devastating parts of the Huaraz city (Carey, 2005; Carey et al., 2012; Mergili, Pudasaini, et al., 2020), and (d) the 2016 Gongbatongsha GLOF in the Poiqu basin, eastern Himalaya damaged the Araniko highway, the Upper Bhotekoshi hydropower plant, and other infrastructure in Nepal (Sattar et al., 2022).…”