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.