Field validation of debris-flow surge-wave equations at Illgraben, Switzerland

Debris flows are destructive mixtures of water and sediments. In mountain regions, debris flows are a relevant hazard as they threaten people and infrastructure. A critical yet understudied debris-flow characteristic are surge waves, which can occur throughout a debris-flow event. These waves travel faster than the bulk flow and often determine the maximum discharge and impact pressure, with important implications for hazard assessment and mitigation. Although surge wave kinematics have been studied experimentally and theoretically, the high-quality field data needed to validate these findings are missing. Here, we leverage recently developed LiDAR sensors and cameras to monitor surge waves high spatial (<2 cm) and temporal (10 Hz) resolution in the Illgraben channel, Switzerland. We use a neural-network-based object detection algorithm (YOLOv5) to identify and track surge waves, boulders and woody debris on 2D camera images. Object tracking was performed with the SORT algorithm. By projecting the tracks onto the LiDAR point clouds, we obtain precise data such as size and velocity of individual objects including the wave crest, the fluid downstream of the wave and small features such as woody debris interacting with the surge waves. This data builds the basis to validate the latest theories of surge wave dynamics. Preliminary results show that a representation of surge kinematics which treats the wave as uniform and progressive (Davies, 1997), as a wave traveling through still water, captures the velocity trend although only based on wave height and the depth of the fluid it travels through. In future, we aim to test more complex surge wave kinematic theories, which can solve space-time evolution of the wave and particles floating on the surface (Viroulet et al., 2018), such as the woody debris we detect and track. Therefore, the unique field data and methods we present will be helpful for better understanding surge wave kinematics and develop and test numerical models.

References

Davies, T.R., 1997. Large and small debris flows—Occurrence and behaviour, in: Armanini, A., Michiue, M. (Eds.), Recent Developments on Debris Flows, Lecture Notes in Earth Sciences. Springer Berlin Heidelberg, Berlin, Heidelberg, pp. 27–45. https://doi.org/10.1007/BFb0117760

Viroulet, S., Baker, J.L., Rocha, F.M., Johnson, C.G., Kokelaar, B.P., Gray, J.M.N.T., 2018. The kinematics of bidisperse granular roll waves. J. Fluid Mech. 848, 836–875. https://doi.org/10.1017/jfm.2018.348