A rock–ice avalanche without yield strength caused the 2026 Bhote Koshi–Trishuli flood, Nepal

hyoseok park 

On 26 August 2026 a rock–ice avalanche from the north face of Langtang Lirung, Nepal, produced a debris flow and flood that destroyed the Rasuwagadhi–Gyirong border crossing within eight minutes and reached Devghat, 180 km downstream, six hours later. We reconstruct the cascade with a depth-averaged mixture-flow model on the observed terrain, constrained by the seismic origin time, a CCTV frame, the gauge outages in the gorge, satellite-mapped trimlines and the stage records of four downstream gauges. The arrival times in the gorge are reproduced only if the moving mass had no yield strength (basal friction below 0.005), which we attribute to ice at the grain contacts; the energy of the fall then melted most of the ice within minutes. The gorge flow heights, the gauge records and the melting constraint jointly constrain the moving volume to 27 (18–35) × 10⁶ m³, consistent with the mapped detachment, and the ice fraction to 0.1–0.3, whereas the seismic force alone requires a three- to fivefold larger source. Storage on the valley floor, about 25 × 10⁶ m³, controlled the flood wave below Syabrubesi. No glacial lake, landslide dam or stored water is required to explain the flood.