Retrospective analysis and hazard assessment of Gega glacial lake in the eastern Himalayan syntaxis

Abstract

The frequency of glacier related hazards like Glacial Lake Outburst Flood (GLOF) is increasing in the High Mountain Asia due to global climate warming. The extreme floods generated by glacial lake outburst aggravate the risk to people, towns and infrastructure in downstream regions. However, little attention is paid to the destructive potential of the lakes in the southeastern margin of the Tibet in spite of recent rapid increases in population, infrastructure, and economic investment. With retrospective investigation of the Gega lake, we found historical overtopping flood(s) impacted by landslides based on the lake dam consists of landslide deposits on both sides and bedrock in the middle, manifesting the GLOF risk in future. BRCH-J model is used to simulate the flow hydrographs after dam break, then, the HEC-RAS software is used to simulate the propagation of outburst floods. Finally, the GLOF risk is evaluated with a semi-quantitative approach considering overtopping and piping failure scenarios. The peak discharge after dam failure would reach up to 17,000 ​m3 ​s−1 (at dam site), which is about two times greater than the average seasonal flood peak near Gega village in the trunk river. Piping failure attains a larger peak discharge and quicker peak time (∼0.13 ​h) compared to the overtopping failure (∼0.5 ​h). The sensitivity analysis of materials properties (like internal friction angle and porosity etc.) shows geotechnical properties of the dam material affect the magnitude of peak discharge. The 1D steady flood routing of the peak flow indicates the flow depth is larger than 35 ​m at the Yarlung Tsangpo - Gega confluence. The extreme flow through narrow and steep terrain could even trigger debris flow along the Gega stream. The results can be useful for mitigating risk of GLOFs in the eastern Himalayan syntaxis and neighboring alpine areas.