Dynamics of Debris Flows: Reproducible Initial and Boundary Conditions in Scaled Laboratory Experiments to Determine Velocity Distributions

Debris flows turn into natural disasters if they take place in vulnerable areas. There, they can cause deaths of humans and animals as well as economic damage. The catastrophic effect is intensified by the increasing land use of deposit areas and the poor predictability of the events. Debris flows are one of the most common mass movements and a global phenomenon that occur everywhere with steep relief and at least occasional precipitation falls in the form of rain. Especially in arctic and alpine regions, strong slopes and large amounts of loose boulders increase the potential for flows.

Today, protection against debris flow prevention is realized through a combination of direct and indirect precautionary measures. These include, e.g., risk prevention, prediction and monitoring as well as indirect precautionary measures in the debris-flow-channels or in infrastructure elements in the hazard zones.

Reliable risk quantification requires a comprehensive understanding of the process. The task of experts from the fields of geology, geotechnical engineering and hydrology is the prediction of the frequency, magnitude, propagation paths, forces and velocities of debris flows as well as the associated destruction potential. Of particular importance for the process understanding and the conception of countermeasures is the estimation of velocity distributions and shear stresses of the flow cross-section, as they have an impact on the propagation of debris flow.

The aim of our investigation was hence the analysis of the velocity profiles and shear stresses of granular debris flows with variable but reproducible input and boundary conditions. To meet the requirements we built up a 4 m long, 0.3 m wide and 0.3 m high teeterboard-Plexiglas-model in the laboratory. It is the first model with the particular feature of repeatedly usable material for several tests, thereby comparable initial conditions. We used material from the Swiss Illgraben as extensive field measurements already exist and validation is possible. The model’s slope is variable as well as the roughness. In addition, the water or solids content of the material was modifiable. As measurement instruments we used ultrasonic probes and LSPIV.

The measurements show the velocity distribution and water levels of the debris flows. With these results, the shear stresses were calculated. The results for the different variants lead to a better process understanding. Thereby, they will serve as basis for a numerical model, which we plan. The combination of both models are of central importance for the development of protection and countermeasures.