Simulating avalanche-triggered lake overspill and downstream impacts at Birendra Lake using RAMMS and HEC-RAS
Abstract. The study presents the first comprehensive quantitative assessment of avalanche-triggered GLOF hazards at Birendra Lake using integrated RAMMS-HEC-RAS modelling to evaluate cascading risks from avalanche release to downstream flood propagation. Three scenarios representing small (5.1 × 104 m³), medium (5.3 × 105 m³), and large (1.2 × 106 m³) avalanche releases from steep slopes (30°–48.8°) surrounding the lake were simulated. The modelling framework demonstrates that all scenarios reach Birendra Lake with substantial mass retention (62–86 %), generating maximum velocities of 33.8–72.8 m/s and flow heights of 11.2–36.8 m. The displacement-driven overspill mechanism displaces 0.01–0.18 % of total lake volume (4.7 × 10⁶ m³), producing peak discharge rates of 615.7–3,151.8 m³/s. HEC-RAS flood modelling reveals rapid downstream propagation, with flood arrival times of 0.15–0.43 hours at Samagaon and 4.6–19.76 hours at Jagat, accompanied by maximum flood depths of 0.96–12.69 m and velocities of 1.94–15.62 m/s. The modelling results demonstrate strong qualitative alignment with the April 2024 event, validating the overspill mechanism. Medium to large avalanche scenarios pose severe threats to downstream communities, with the large scenario producing catastrophic conditions at Samagaun, where depths exceed 12 m with velocities above 15 m/s. The findings establish Birendra Lake as an imminent high-risk system where steep avalanche-prone terrain, lake proximity to unstable glacier zones, and significant downstream exposure create catastrophic cascading hazards. This research provides essential quantitative foundations for early warning systems and risk reduction strategies in avalanche-prone glacial lake environments across High Mountain Asia.
Each subsection should clearly describe its role in the study, including the input parameters used and references where applicable. A similar structure should also be followed in the Results and Discussion section for consistency. Subsections can be further divided as needed to highlight important components.
The Results and Discussion section should focus solely on the outputs and findings of the present study. Currently, this section often reads like an extended version of the methodology. The descriptive elements already covered in the methodology should not be repeated. Instead, this section should include clear interpretation and analysis of the model outputs, supported by relevant figures and tables.
These need to be formally integrated into the manuscript structure. Additionally:
The following revisions are suggested for the figures:
-Why was roughness calibration considered limited or not performed in detail for this model?
-How can sensitivity analysis be incorporated within HEC-RAS modeling to improve flood hazard assessment?