High-resolution measurements of debris-flow surges in a natural channel

Debris-flow hazard assessment relies on the accurate estimation of (peak) discharge and volume. However, traditional methods for inferring these hazard-related parameters often  encounter significant limitations, especially in natural, dynamic channels without mitigation measures. Furthermore, existing assessment methods often overlook the characteristic surging behavior of debris flows and the influence of material composition. While laboratory experiments have demonstrated that mixtures with larger grain sizes produce more pronounced levees and extended runout distances, field evidence remains largely qualitative and anecdotal. Consequently, the combination of measurement uncertainties and the omission of flow composition continues to result in substantial uncertainties in debris-flow hazard assessments. Therefore, high-resolution and accurate measurements are needed to better understand debris-flow hazards.

Here, we present high-resolution measurements of over 130 debris flow surges, which occurred in the natural debris-flow channel of the Oeschibach in Kandersteg,Switzerland,in 2024. The sediment source is a rock slope instability in permafrost (Spitze Stei), which is closely monitored. Its recent acceleration has also led to increased debris-flow activity downstream. In 2024, we installed a high-resolution camera and 3D LiDAR sensor, which recorded several debris flows at 10 fps. Using a set of processing algorithms including Particle Image Velocimetry (PIV) on hillshade images, point cloud differencing, and a deep-learning based boulder detection model on camera images, we derived spatially distributed flow velocity, depth, discharge, and material properties (grain count and grain size).

Our key findings include that coarser surges tend to be faster, deeper, levee-forming and erosive. These findings are in line with laboratory experiments, whereas the levee-formation likely also causes surges to be more confined and therefore faster and deeper. Furthermore, we observed that while all events consisted of a series of surges, the bigger the first surge, the more surges were to follow. As traditional monitoring techniques cannot capture these dynamics in sufficient detail, we provide a comprehensive and novel data set in a natural channel, which helps bridging the gap between laboratory experiments and field evidence to reduce uncertainties in debris-flow hazard assessment.