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Authors
Sunim Acharya 
Abstract
An ice-rock mass detached from the northern flank of the Langtang massif, Nepal, at 08:37:10 Nepal Time on 26 August 2026. It blocked the Lhende Khola and released a cascade that killed more than 500 people along the Bhote Koshi-Trishuli-Narayani corridor. This study measures the warning chain end to end, timing the front at six stations from ten-minute stage series recovered from the river-watch service of Nepal's Flood Forecasting Division. The mass public alert followed the trigger by 38.8 min while the wave reached the uppermost gauge within 12.8 min, giving lead times of -26 to -16 min at the first inhabited settlement and +4 to +14 min at the next. Three destroyed gauges were lost at 27, 69 and 87% of their own warning stages, so threshold logic never fired. Fatalities concentrated where lead time was negative or negligible, while districts warned hours in advance became the body-recovery zone. A closed-form energy budget rules out frictional meltwater as the source. Supplying the measured 19.96*10^6 m^3 excess volume would require melting four times the ice in the entire 2021 Chamoli detachment. The constraint on survival was the time the institutional chain consumed, not flood magnitude.
DOI
https://doi.org/10.31223/X5HN5H
Subjects
Earth Sciences, Environmental Monitoring, Hydrology
Keywords
Bhote Koshi, Rasuwa, flash flood, ice-rock avalanche, cascading hazard, flood early warning, lead time, Langtang, Nepal, transboundary risk, Himalaya
Dates
Published: 2026-08-29 15:18
Last Updated: 2026-09-01 21:29
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License
CC BY Attribution 4.0 International
Additional Metadata
Conflict of interest statement:
The author declares that there is no conflict of interest.
Data Availability:
The data package underlying every figure, table and number in this paper is archived at 10.5281/zenodo.22153582 under CC-BY-4.0.
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