3D dynamics of debris flows quantified at sub-second intervals from laser profiles

We use pairs of parallel mounted laser profile scanners to measure main debris-flow variables in two debris-flow channels in central and southern Switzerland. The scanners measure the instantaneous cross-sectional geometry of debris flows at rates of 25–100 Hz, and we apply large-scale particle image velocimetery to estimate velocity. The scanners also provide direct measurements of flow depth. From these data, we were able to estimate debris-flow depth, velocity and discharge for 16 out of 17 events. These results are consistent with discharge estimated from a system of geophones and a radar gauge for two available datasets. We also investigated debris-flow geometry to quantify rheology-controlled cross-flow convexity and found that four events manifest strong surface convexity at their surge fronts where we expect the largest boulders and low pore-fluid pressures. The scanners provide a completely new view of debris-flow dynamics and channel morphology and present novel opportunities to measure discharge and investigate debris-flow geometries.