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New research provides unprecedented insights into the hidden forces behind devastating Alpine debris flows, offering hope for better protection against future disasters.

The study, led by ETH Zurich and supported by The University of Manchester, sheds new light on how fast-moving mixtures of water, soil and rocks 鈥� known as debris flows 鈥� develop into a series of surges, destroying everything in their path. 

Using highly sensitive 3D laser scanners, the scientists collected measurements during a major debris flow in the Illgraben valley in Switzerland on 5 June 2022. Analysis enabled the scientists to pinpoint how small surface disturbances evolve down the channel into powerful large amplitude waves that concentrate the flow鈥檚 destructive power.

The findings, published in the journal , are among the most detailed measurements of a real-life debris flow ever recorded.

Debris flows are a recurring natural hazard in steep terrain throughout the world, and are triggered by heavy rainfall, and increasingly, glacial runoff and permafrost melt. Recent landslides in the Alps continue to highlight the risks posed by debris flows, such as the 2017 Bondo landslide in Graub眉nden, which triggered a debris flow that travelled 4km downhill into the Bondasca Valley. This emphasises the urgent need to better understand and predict these hazardous events.

Due to the frequency of debris flow occurence, the Illgraben valley has been equipped with measuring instruments since 2000. It has recently supplemented by five highly sensitive 3D laser scanners, called LiDAR, which can determine distance and speed, and six high-speed video cameras.

On the day of the June 2022 event, 25,000 cubic meters of water, earth and debris poured approximately seven kilometres down the bed of the Illbach before the muddy stream was absorbed by the river Rh么ne at Susten. The devices measured surface velocities and the evolving free surface of the debris flow at three measuring stations with a spatial resolution of 2 cm and a temporal resolution of 0.1 seconds.

The team of scientists from ETH Zurich, Swiss Federal Institute for Forest, Snow and Landscape Research (Birmensdorf) and The University of Manchester, were able to document how the waves grew along the channel and use the data to develop a new friction law that was used in a debris-flow model to realistically simulate the  genesis and growth of the waves.

They found that near the top of the (about 2km from the outflow into the Rh么ne river), the debris flow had a fast-moving wave front, but no surges, while further down the channel the flow became shallower and spontaneously developed a series of waves. During the 30-minute event, researchers recorded 70 of these surges, which emerge from a surface instability that allows the waves to grow and as they move downhill.

Lead researcher, Jordan Aaron, Professor of Engineering Geology at ETH Zurich, said: "It has long been known that waves play a central role in the destructive power of debris flows, because they concentrate the forces that are applied to structures in their path.

"Thanks to the measurements around the debris flow of June 2022 and the modelling based on them, the researchers now have a better understanding. Our analysis provides new insights into the dynamics of debris flows and enables improved hazard management in the medium term.鈥�

This research, which was partially funded by the UK鈥檚 Natural Environment Research Council (), has been published in the journal Communication Earth & Environment

Full title: Detailed observations reveal the genesis and dynamics of destructive debris-flow surges

DOI: doi.org/10.1038/s43247-025-02488-7

Link:  https://www.nature.com/articles/s43247-025-02488-7