A rock-ice avalanche and the 2026 Bhote Koshi-Trishuli flood, Nepal

hyoseok park 

On 26 August 2026 a rock-ice avalanche in the Langtang Himal produced a debris flow and flood that destroyed the Rasuwagadhi-Gyirong border crossing within minutes and reached Devghat about six hours later. We reconstruct the cascade with a depth-averaged mixture-flow model constrained by the seismic origin time, a CCTV frame, gauge outages in the gorge and downstream stage records. The model distinguishes rock, ice and water densities and conserves mass during melting. For a release of 35.4 million cubic metres with 20% ice, added at rest, the front reaches Gyirong Port at 390 s and Syabrubesi at 1050 s, within the working observation windows. Parameter comparisons favour low effective resistance within the tested source and terrain scenarios. Downstream routing selected using waveform errors gives stage rises of 8.94 m at Galchhi and 4.98 m at Kali Khola, compared with observed rises of about 8.8 and 5.6 m. Their model rise times of 1.85 and 4.77 h also fall within the observation windows. Without prescribing an additional outburst from a temporary landslide dam, the reconstruction reproduces the upper timing windows and broadly follows these two downstream hydrographs. The results support an avalanche-derived surge followed by valley-controlled redistribution and identify low effective resistance and lateral storage as controls within the adopted model.