Scientists finally pinpoint how debris flows become so destructive

Debris flows are sudden, fierce, and often deadly. They can flatten forests, sweep away buildings, and catch entire villages off guard.

Events like those in Blatten (Valais) in May 2025 and Brienz (Graubünden) in June 2023 are reminders of how vulnerable mountainous regions can be after heavy rainfall.

These destructive flows don’t happen just once. In many parts of the Alps, they are repeat offenders.

In the past year alone, Sorte in Graubünden, Fontana in Ticino, and the Saas Valley in Valais have all experienced major debris flows.

One of the worst happened in Bondo (Graubünden) in 2017, when a river of mud over 300 feet wide surged down the valley, killing eight people.

Following the path of destruction

Because these flows occur in the same spots again and again, scientists use these active locations to monitor how they form and behave.

And in June 2022, researchers were ready when a major debris flow surged down Illgraben, a steep ravine above the village of Leuk in Valais.

A team from ETH Zurich, the Federal Institute for Forest, Snow and Landscape Research WSL, and the University of Manchester captured the entire event using a network of advanced sensors.

In just 30 minutes, 32,700 cubic yards of debris traveled roughly 2.5 miles down the Illbach riverbed into the Rhône at Susten.

Why debris flows turn deadly

The researchers observed something striking. At the top of the valley, the front of the flow surged forward, rising over 6.5 feet high and filled with massive boulders – some as large as a cubic yard.

Farther down, the debris moved more slowly but showed intense surface activity. The scientists recorded 70 powerful surge waves rippling across the surface in rapid succession.

“We’ve known for a long time that these surges play a key role in the destructive force of debris flows,” said Jordan Aaron, professor of engineering geology at ETH Zurich. “This is because surges make the flow particularly thick and fast.”

Until now, though, there wasn’t a solid explanation for how these surges form. Using their field data and computer models, the research team discovered that the surges don’t need any external trigger. They form on their own, right on the surface of the flow.

“We were able to demonstrate that surges arise spontaneously on the surface of the flow,” Aaron said. “They stem from small irregularities, which grow over time, increasing in size and speed until they reach their maximum destructive force.”

Turning science into safer cities

Understanding these surges isn’t just academic. The findings make it possible to assess how strong a debris flow might be before it strikes.

This means engineers can better calculate how much force structures – like houses, bridges, and dams – might need to resist in high-risk areas.

“Our analysis provides new insights into the dynamics of debris flows and will enable better hazard management in the medium term,” said Aaron.

LiDAR maps a moving threat

Illgraben is no stranger to debris flows. Since 2000, it has been equipped with instruments to track debris movements in real time. But the 2022 event gave scientists their best look yet.

The team used LiDAR scanners – laser-based sensors originally developed for self-driving cars – along with high-speed video cameras. Five LiDAR units and six cameras captured the debris flow in incredible detail.

At three points along the ravine, the researchers recorded the surface of the flow with a spatial resolution of 0.8 inches and a temporal resolution of 0.1 seconds. This allowed them to precisely measure the speed and force of each surge.

Using this data, they developed a detailed numerical model to simulate how the debris flow evolved – and what made it so dangerous.

Big boulders steer debris

Another important takeaway from the study: big boulders can change how debris flows behave. These massive fragments alter the way material moves within the flow, especially at the local level.

“This phenomenon is not included in most previous predictions of debris flows,” Aaron noted. “Being able to observe and measure these effects in the field has allowed us to more accurately describe and better understand these natural processes.”

By simulating real debris flow behavior, the new model offers a better way to plan for and respond to future disasters. It gives communities, engineers, and planners a better shot at protecting people and infrastructure from one of nature’s most destructive forces.

The full study was published in the journal Communications Earth & Environment.

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