Two-Dimensional Hydrodynamic Modelling of Glacial Lake Outburst Flood Scenarios from Chubda Tsho: Breach Sensitivity and Downstream Flood Dynamics in the Chamkhar Chhu Basin, Bhutan

Abstract. Glacial lake outburst floods (GLOFs) are high-magnitude, short-duration hazards in glacier-fed Himalayan basins, where steep valley confinement can amplify downstream flood intensity. Despite the identification of Chubda Tsho as a potentially dangerous glacial lake in Bhutan, hydrodynamic assessments of potential outburst floods from this lake have not yet been reported. This study evaluates potential GLOF scenarios from Chubda Tsho using inflows generated by a calibrated and independently validated Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS), coupled with two-dimensional (2D) simulations in the Hydrologic Engineering Center's River Analysis System (HEC-RAS). The model was calibrated against the July 2007 flood (NSE = 0.90) and validated using the August 2015 event (NSE = 0.56). Hydrodynamic performance was assessed using an extreme-event hydrograph from Cyclone Aila (2009) and compared with observed inundation extent. Results show good agreement, with a Critical Success Index (CSI) of 0.67, a total area difference of 2.56%, and close correspondence between simulated (1.60 km²) and observed (1.56 km²) inundation extents within the 16.14 km² Chamkhar Valley model domain. The Chamkhar Valley contains approximately 8.41 km² of habitable area. Three breach scenarios representing 50%, 75%, and 100% lake-volume release were simulated using empirically derived breach relationships. Flood waves reach the Chamkhar Valley between approximately 1 h 50 min and 2 h 15 min after breach initiation. Peak discharge increases from 2,114 m³ s⁻¹ to 5,258 m³ s⁻¹, accompanied by nonlinear increases in peak depth (8.4–12.8 m) and velocity (5.0–7.6 m s⁻¹). While inundation extent remains constrained by valley geometry, flood depth and velocity increase disproportionately with breach magnitude. Results indicate that downstream hydraulic intensity is strongly influenced by breach magnitude, while warning time remains limited in confined Himalayan valleys. This highlights the value of validated 2D hydrodynamic modelling for GLOF hazard assessment in Himalayan catchments with limited observational data.