Reply on RC1

Abstract

The existence of glacial lakes in the Southeastern Tibetan Plateau (SETP) is a potential hazard to the downstream regions, as the failure of some lakes has potential to result in disastrous glacial lake outburst flood (GLOF) events of high-magnitude. In the present study, we conducted a comprehensive investigation for Bienong Co, an end moraine-dammed glacial lake in SETP, including its area evolution analysis, basin morphology simulation, water volume estimation, possible outburst triggers analysis, and one- and two-dimensional hydrodynamic simulation. The results show that the area of Bienong Co was 1.15 ± 0.05 km2 in August 2020, which has remained generally stable over the past four decades. However, it exhibits the high risk of GLOFs due to its larger area, the steep and high moraine dam, the close distance to its mother glacier, and the surrounding steep slopes. The lake basin is relatively flat at bottom and steep on both flanks, and the slope near the glacier (16.5°) is steeper than that near the moraine dam (11.3°), with a maximum lake depth of ~181.04 m and a water volume of ~1.02 × 108 m3 in August 2020. Four scenarios of GLOFs based on different breach depths, breach widths and failure times were simulated using the hydrodynamic model of MIKE 11 and MIKE 21 to predict the potential impacts on downstream areas. An extreme-magnitude GLOF would have a catastrophic impact on the downstream region, with most of the settlements, all bridges and majority of Jiazhong Highway along the flow channel being completely submerged. However, in a low-magnitude GLOF, most settlements would be safe or partially inundated. It means that most of the residents in the flow channel of Bienong Co can avoid the damage caused by a low-magnitude GLOF (smaller breach depth of dam). Although three settlements in the downstream area are at risk of being completely submerged in a low-magnitude GLOF event, the flooding arrives late and people have enough time to escape. Finally, the maximum depths of glacial lakes with similar areas were compared for 16 glacial lakes with measured bathymetry data in the Himalayas and Bienong Co in SETP, according to the regional division of maritime and continental zones. The results show that glacial lakes located in maritime regions have larger depths than those in continental regions, and Bienong Co is the deepest glacial lake comparing others in the Himalayas. Therefore, a huge amount of water could be discharged by a potential GLOF event of Bienong Co, creating a serious hazard, which should be taken seriously in the future. Overall, this study of Bienong Co could provide a new understanding of the moraine-dammed glacial lakes in the SETP and a reference of GLOFs to the local government.