Video-derived velocity disribution along a debris flow surge

Abstract

Automated recording by videos of debris flows and the subsequent use of image processing techniques measures the surface velocity of two debris flows occurring in an instrumented basin of the Italian Alps. Mean front velocities obtained are consistent with and slightly less than average velocities derived from ultrasonic sensors measurements. A particular behavior was observed in debris flow surges: the surface velocity peak occurs behind the main front of the debris flow, whereas the velocity of the latter can be decidedly lower. This same behavior had been already revealed by other authors, through the use of laser doppler speedometers and time-lapse photography, and should be taken into proper account in the mathematical simulation of debris flow propagation. The trajectories of boulders in the debris flow front have also been examined: transversal movements toward the outer parts of the channel are mainly attributed to cross-sectional widening.