What makes debris flows dangerous

The landslide that occurred in Blatten in the canton of Valais at the end of May 2025 and the one in the village of Brienz in Graub¨¹nden in June 2023 remind us of the potential for landslide hazards in the Alps. Debris flows are one such hazard. These flows of water, sediment and rock fragments typically occur after heavy rainfall in steep terrain, and rapidly travel down a channel, potentially destroying everything in their path.

In the past year, major debris flows have attracted attention, particularly in Sorte (canton of Graub¨¹nden), Fontana (canton of Ticino) and the Saas Valley (canton of Valais). There was also a major landslide in Bondo (Graub¨¹nden) in August 2017 which sent a 100-metre-wide flow of mud and debris hurtling through the Bondasca Valley. Eight people lost their lives that day.

Such debris flows occur repeatedly at exposed locations, at intervals of months or years. Scientists take advantage of this fact for the live monitoring of natural phenomena in areas with frequent debris flows.

Researchers from ETH Zurich and WSL were therefore prepared when on 5 June 2022 a debris flow came loose in Illgraben above the village of Leuk in the Valais, transporting 25,000 cubic metres of debris four kilometres through the bed of the Illbach river, before flowing into the Rh?ne at Susten. The team of scientists monitored the natural phenomenon at several measuring stations.

Up in the valley, the team monitored a two-metre high and rapidly-advancing front composed of large boulders of up to a cubic metre in size at the leading edge of the debris flows. Further down the valley, the debris flow was slower, but with frequent, fast-moving and powerful waves on the surface (see Video). During the half-hour event, the researchers recorded 70 such surge waves.

Surges occur spontaneously

¡°We¡¯ve known for a long time that these surges play a key role in the destructive force of debris flows,¡± says Jordan Aaron, professor of engineering geology at ETH Zurich¡¯s Department of Earth and Planetary Sciences. ¡°This is because surges make the flow particularly thick and fast.¡± According to Aaron, up to now little was known about the physical processes that give rise to these surges. Thanks to the measurements of the debris flow in June 2022 and modelling based on that, the researchers now know more: ¡°We were able to demonstrate that surges arise spontaneously on the surface of the flow. They stem from small irregularities, which grow over time, increasing in size and speed until they reach their maximum destructive force.¡±

This finding is at the centre of a study carried out by Aaron¡¯s team together with researchers from the Federal Institute for Forest, Snow and Landscape Research WSL and the University of Manchester. The study has just been published in the journal ¡®Communications Earth & Environment¡¯. ¡°Our analysis provides new insights into the dynamics of debris flows and will enable better hazard management in the medium term,¡± says Aaron. The destructive force of the surges is mainly what determines how dangerous a debris flow is.

On the basis of this study, in future scientists will be able to estimate whether a debris flow is expected to contain surge waves and how destructive they may be. From this they can work out the forces that exterior house walls and bridge pillars need to withstand in a hazardous area. This information could also be used to design dams and safety nets in the right dimensions.

High-resolution measurements

Illgraben above Leuk is known for the fact that several debris flows occur there every year. Since 2000 the valley has been equipped with measuring instruments that record debris-flow properties in their natural environment. However, the insights from the new study were only possible after the event in the summer of 2022 was measured with greater precision than ever before.

This was done using highly-accurate 3D laser scanners, known as LiDARs. These devices measure distance and speed and were originally developed for self-driving cars. In June 2022, five LiDAR scanners and six high-speed video cameras were used to record the debris flow in Illgraben. At three measuring locations, the surface of the debris flow was detected with a spatial resolution of 2 cm and a temporal resolution of 0.1 seconds. This was then used to calculate the power and speed of the debris flows.

Thanks to the measurement data, the researchers were able to work up a hypothesis about the underlying physical processes and develop a numeric model. This mathematical model is used to realistically simulate the progression of the debris flow (see video below).

Boulders influence flow dynamics

The measurement data further enabled the researchers to infer that large rock fragments have a significant influence on local flow dynamics. ¡°This phenomenon is not included in most previous predictions of debris flows,¡± says Aaron. ¡°Being able to observe and measure these effects in the field has allowed us to more accurately describe and better understand these natural processes.¡±