A graphic depicting the different elements of a mudslide and their different speeds.

The different surges of a mudslide. The arrows show the speed of its individual elements. | Graphic: R. Spielmann et al. (2025)

A mudslide transports debris comprising water, stones and wood downhill. This happens several times a year in Illgraben in Valais. In order to measure the speed of the debris flow (see arrows), ETH Zurich has now installed 3D lidar scanners there. During an event in 2022, wave-like surges (pink) moved up to three times faster than the front of the mudslide (red), with some pieces of rock (purple) travelling at twice that speed. These data can help in designing protective structures in the future.

R. Spielmann et al.: Sorting and Surging: 3D LiDAR and Pulse-Doppler Radar Analysis of a Natural Debris Flow. JGR Earth Surface (2025)

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