Debris Flows in the Mountains: Why They’re More Deadly Today

When a landslide struck Blatten in Valais at the end of May 2025, it caught many residents off guard. Just two years earlier, another major event hit Brienz in Graubünden. These disasters are part of a growing pattern in the Alps, more frequent and more intense debris flows following heavy rain.

Each time it happens, questions follow. Why did it move so fast? Could it have been predicted? What makes these flows so dangerous?

What is Debris?

Debris means loose material like rocks, mud, soil, and tree parts. After heavy rain, this debris can mix with water and rush downhill in a powerful flow. These flows can destroy roads, homes, and forests in their path.

Scientists now have better answers, thanks to a high-precision study of a debris flow in Illgraben. A team from ETH Zurich, the Federal Institute for Forest, Snow and Landscape Research (WSL), and the University of Manchester used laser scanners and high-speed cameras, tools originally developed for self-driving cars, to track every movement of the flow in real time.

“This was a chance to observe a full-scale debris flow in rare detail,” said Jordan Aaron, professor of engineering geology at ETH Zurich. “It’s helping us understand not just when these flows occur, but how they actually behave.”

How Debris Flows Gain Power

Debris flows are sudden, fast-moving mixtures of water, sediment, and large rocks. They usually happen after heavy rain hits steep mountain slopes. Once in motion, they can tear down valleys at speeds of up to 60 kilometers per hour, taking trees, roads, and buildings with them.

The 2022 Illgraben event carried about 25,000 cubic metres of debris. But what made it especially dangerous was the way it moved, not in one steady wave, but in a series of violent surges.

“We recorded nearly 70 surge waves during that single event,” Aaron said. “Each one was like a mini-disaster in itself.”

These surges form on the surface of the debris flow and grow larger over time. Aaron and his team found that they didn’t come from sudden changes in terrain or outside triggers. They developed on their own from small bumps or disturbances in the flow.

“Surges arise spontaneously,” Aaron explained. “Tiny surface irregularities grow into fast, thick waves that hit with maximum force.”

How 2025 Study Changed Understanding

The 2025 study helped scientists better understand how debris flows become so powerful as they move downhill. A team used detailed field data and computer models to track a debris flow in Switzerland. They found that large boulders in the flow change how it moves. These rocks increase friction at the base, slow parts of the flow, and break its smooth motion. This creates sudden surges that can double or triple the flow’s force.

The study also showed that small slope changes matter. A shift of just one degree can change the flow from fast, shallow waves to slower but thicker ones that do more damage. These surges don’t need outside triggers. They form on their own from small bumps on the flow’s surface and grow stronger as they move.

What stood out was that the team could model this behaviour using a simple simulation. Even without adding details like water pressure or changes in materials, the model still matched what they saw in the field. That means scientists could build faster and easier tools to predict debris flows. With more data from other events, these tools could help warn mountain communities before disaster strikes.

What the Data Can Show

The team’s findings, published in the journal Communications Earth & Environment, explain why debris flows are so destructive, and why surge waves matter most when assessing the danger.

The force of each surge can be intense. Large boulders, some the size of a small car, often travel at the front of the flow. At one measuring station in Illgraben, researchers saw the front surge moving at 5.5 meters per second.

The scientists used five 3D laser scanners (LiDARs) and six high-speed cameras to measure flow speed, depth, and shape. The tools scanned the surface every 0.1 seconds with two-centimeter accuracy. This experience helped the team build a detailed picture of how the flow evolved from start to finish.

This level of detail allowed them to create a mathematical model of the flow. It also gave them a better sense of how forces shift across space and time, something earlier field studies could not capture.

The new findings have practical uses. Engineers can now estimate how much force buildings and bridges in risk areas must withstand. The data can also guide the design of protective structures like barriers, nets, or retention dams.

Why This Research Matters Now

The study also highlighted the role of boulders in changing the flow’s behaviour.

“Large rocks change the way the waves move and build up,” Aaron said. “This wasn’t fully included in older prediction models.”

Debris flows like this one are not rare. The Alps see several each year. Some, like the 2017 event in Bondo, end in tragedy. That flow killed eight people in a remote part of Graubünden.

With climate change bringing more intense rainfall, the risks are growing. More homes and roads now sit in areas vulnerable to such flows.

The goal, Aaron said, is not to stop debris flows, they’re part of the Alpine environment, but to better predict their force and build smarter protections.

“The more we understand how debris flows work,” he said, “the better we can protect the people living in their path.”

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