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Abstract Debris flows are frequent natural hazards whose destructiveness is controlled by the dynamics of their flow fronts, surges behind the front and large boulders. Understanding the mechanisms underlying the spatiotemporal variations in flow depth and velocity is limited by a lack of catchment‐scale measurements. In this study, we present and analyze flow‐depth and velocity measurements from a new monitoring setup which consists of high‐frequency 3D LiDAR scanners installed at three different locations along the active debris‐flow fan of the Illgraben. For the event analyzed herein, we observe that (a) the LiDAR‐based velocities are in excellent agreement with measurements from a Pulse‐Doppler (PD) radar and with manually tracked feature velocities; (b) the flow front decelerates as it travels along the fan and a watery pre‐surge develops, likely due to a combination of segregation and vertical shear, which transport woody debris and small boulders to the front, as well as a horizontal velocity profile, required for transportation of large boulders through a mechanism we term “centerline advection”; (c) roll waves begin to develop on the lower part of the fan by coalescence of free surface instabilities and they exceed the front velocity by up to 2 to 3; (d) surges later in the event do not show an accumulation of boulders at the crest, but can accelerate individual boulders to velocities up to 2 the front. This study demonstrates that radar and LiDAR‐based measurements improve our understanding of debris‐flow processes, including front propagation and development of waves.
Spielmann et al. (2025) studied this question.