Lake volume and potential hazards of moraine-dammed glacial lakes - A case study of Bienong Co, southeastern Tibetan Plateau

The existence of glacial lakes in the southeastern Tibetan Plateau (SETP) is a potential hazard to downstream regions, as the outburst of such lakes has the potential to result in disastrous glacial lake outburst floods (GLOFs). In the present study, we conducted a comprehensive investigation of Bienong Co, a moraine-dammed glacial lake in the SETP. First, the lake basin morphology was determined, and the lake volume was estimated, showing that the maximum lake depth is g1/4181gm and the lake volume is g1/4102.3×106gm3. These scenarios included the possibility of GLOFs being triggered by ice avalanches (Scenarios A1-3) from the mother glacier or by landslides from the lateral moraines (Scenarios B1-3 and C1-3). Avalanche volumes of the nine trigger scenarios were obtained from the Rapid Mass Movement Simulation (RAMMS) modeling results. Next, the Basic Simulation Environment for Computation of Environmental Flow and Natural Hazard Simulation (BASEMENT) model was used to simulate the generation and propagation of the avalanche-induced displacement waves in the lake. With the model, the overtopping flows and erosion on the moraine dam and the subsequent downstream floods were also simulated. The results indicate that the ice avalanche scenario may cause the largest mass volume entering the lake, resulting in a displacement wave up to 25.2gm in amplitude (Scenario A3) near the moraine dam. Landslide scenarios with smaller volumes entering the lake result in smaller displacement waves. Scenarios A1, A2, and A3 result in released water volumes from the lake of 24.1×106, 25.3×106, and 26.4×106gm3, respectively. Corresponding peak discharges at the moraine dam are 4996, 7817, and 13g078gm3gs-1, respectively. These high discharges cause erosion of the moraine dam, resulting in breach widths of 295, 339, and 368gm, respectively, with the generally similar breach depth of approximately 19gm. In landslide scenarios, only overtopping flows generated by Scenarios B3 and C3 cause erosion on the moraine dam, with breach depths of 6.5 and 7.9gm and breach widths of 153 and 169gm, respectively. According to our simulations, GLOFs generated by Scenarios A1-3 all flow through 18 settlements downstream in 20gh, threatening more than half of them. Both Scenarios B3 and C3 produce GLOFs that flow through the first eight settlements downstream in 20gh and have a relatively small impact on them. Comparisons of the area, depth, and volume of glacial lakes for which the bathymetry data are available show that Bienong Co is the deepest known glacial lake with the same surface area on the Tibetan Plateau. This study could provide a new insight into moraine-dammed glacial lakes in the SETP and be a valuable reference for GLOF disaster prevention for the local government. © Copyright: