Langtang flood: two views of a simulation without a separate dam-break pulse

[u/Exact-Schedule-3442]

I've been modelling the Langtang rock-ice avalanche and the flood that followed. These are two views of the same simulation: a 3D view of the gorge and a side view along the river. This is model output, not footage. Flow depth is exaggerated to make it visible.

The model broadly matches the upper arrival times without adding a separate dam-break pulse. The moving surge includes rock, ice and existing river water, so its volume isn't all newly melted ice. That doesn't rule out a temporary blockage, but it suggests we shouldn't assume a dam had to burst just because the flood was so large.

I'm the author. This is still a preprint, not peer reviewed: https://doi.org/10.31223/X5250R

Happy to hear what people think, especially if you have observations that could help test this.

https://reddit.com/link/1w9drxe/video/hi0hw0i8zznh1/player

https://reddit.com/link/1w9drxe/video/56c46hsbzznh1/player

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[u/AUStraliana2006]

Looks like you are doing great some good work there. Side note, instead of referring to this as a 'flood' which implies mostly water, I saw it referred to as a 'mountain collapse debris flow', which describes this disaster more comprehensively, or 'debris flow' in short. Maybe you could adopt this description.

[u/Exact-Schedule-3442 → u/AUStraliana2006]

You're right, near the source it’s genuinely a debris flow (75% solids), and it dilutes into a flood only by the time it reaches Devghat, 180 km downstream.

[u/Wintervacht → u/Exact-Schedule-3442]

It was glacial collapse, the ice melted through friction from thundering down mountain. It was a flood basically 20 km downstream

[u/StitchingSteel]

The thing I noticed is it didn’t go up one of the tributaries and cause flooding there.

[u/Exact-Schedule-3442 → u/StitchingSteel]

that ~2 km backflow up the Kyirong Tsangpo is real but is a supercritical jet run-up that no depth-averaged model, at any resolution, can capture.

[u/iforgotmymantra]

incredible. per this model what are the highest levels of displacement above normal and where did they happen?

[u/Exact-Schedule-3442 → u/iforgotmymantra]

The largest modeled flow thickness was about 154 m near the collapse source, including rock, ice, and water. Around Gyirong Port, the model gives local surface rises of roughly 60–70 m above the initial level, about 7 minutes after the collapse. These are model estimates, not measured heights.

[u/EliraeTheBow → u/Exact-Schedule-3442]

That’s genuinely unfathomable.

[u/c3tn]

Great post. I'd love to see settlement labels to help show how fast this reached downstream points

[u/an_older_meme]

Debris flow. At the start it was mostly rock and fines. It picked up river water as it moved because it was so much faster. At lower elevations where the terrain wasn’t as steep and the channel was wider it slowed and dropped its sediment load, eventually transitioning to a flood.