Modelling Erosional Dam Breach & Downstream Flood Exposure from Cascading Multi-Glacial Lake Outburst Processes in the Eastern Himalayas

Rapid retreat of glaciers due to global warming has led to the expansion of glacial lakes, heightening the risk of glacial lake outburst floods (GLOFs), which pose severe threats to lives and infrastructure in downstream regions. In the Sikkim Himalaya, the Gurudongmar Lake Complex (GLC) consists of four lakes holding approximately 148 × 10⁶ m³ of water, with an enlargement rate of 74 ± 3%. This study integrates a two-dimensional erosion-based moraine-dam breach model using TELEMAC 2D and SISYPHE with one-dimensional inundation modelling in HEC-RAS to assess the impacts of multi-lake GLOFs under varying scenarios. Simulations based on remote sensing and field data revealed that in the most extreme case – an 80% overtopping breach – peak flood discharges could reach up to 8882.0 m³/s, releasing a total water volume of 59.4 × 10⁶ m³. Flood heights under these scenarios could significantly exceed those observed during the South Lhonak GLOF event of October 2023, intensifying risks for downstream communities. The assessment of 19 settlement sites using a 15×15 m fishnet revealed that Thangu Valley and Chungthang town are most vulnerable, with potential inundation levels and infrastructure exposure highest in these areas. Combined breaches, such as the sequential failure of lakes GL-2 and GL-1 or GL-3 and GL-1, further amplify the flood risk, underscoring the complex dynamics of multi-lake outbursts. This research provides critical insights into moraine-dam erosion processes and downstream flood impacts, offering a robust framework for hazard mitigation in the Eastern Himalayas and similar glacierized terrains worldwide.

Keywords: Glacial lake outburst floods (GLOFs); Erosion dam breach model; Hydrodynamic flood modelling; HEC-RAS; Exposure assessment; Sikkim.