Cascading hazards from two recent glacial lake outburst floods in the Nyainqentanglha range, Tibetan Plateau

Glacial lake outburst floods (GLOFs) are a major natural hazard in high mountain regions and have caused significant loss and damage. In the past decade, two lakes (Ranzeria Co in 2013 and Jiweng Co in 2020) burst out within a single river basin in the Nyainqentanglha range of the Tibetan Plateau. These GLOFs destroyed local infrastructure and vegetation, and eroded river banks. In this study, field measurements, satellite remote sensing, and hydrodynamic modelling were used to reveal the causes, mechanisms, and flow dynamics of both events. Field surveys in 2020 and 2022 including lake bathymetry, lake shorelines, high-resolution aerial photographs, geomorphological data, and downstream damage provided evidence to analyze the triggers and mechanisms involved in both cases. The results show that Ranzeria Co expanded by 123% in area between 1976 and 2012, and contracted by 53% in 2013 due to the outburst. The area of Jiweng Co grew by 54% from 1976 to 2020, and shrank by 57% after the dam breach. Lake levels decreased by 20.2 +/- 0.3 m in 2013 for Ranzeria Co and by 25.8 +/- 0.3 m in 2020 for Jiweng Co, corresponding to water volume losses of about 7.9 x 106 (68%) and 1.1 x 107 m3 (87%), respectively. Using the hydrodynamic model HEC-RAS, it was found that the GLOFs reached peak discharges of about 7,500 m3/s with an average velocity of 8.8 m/s at Ranzeria Co, and 5,100 m3/s with 3.7 m/s at Jiweng Co. The Ranzeria Co outburst involved a cascade of processes, including the collapse of the parent glacier, a displacement wave in the lake, and the overtopping of the moraine dam. The processes of the Jiweng Co outburst flood were more complex, including an unstable lateral moraine, landslides entering the lake, rising water level, dam erosion, and finally dam failure. The study provides important insights into the mechanisms of two GLOFs, which are key to assessing and managing the hazards of critical glacial lakes in the Tibetan Plateau and its surroundings.