How does substrate roughness affect geophysical granular flows ?

In nature, the propagation and deposition dynamics of geophysical flows - such as debris flows, rock avalanches, and pyroclastic flows - are governed by the rheology of the flowing material itself, but also by the interaction with its environment. In particular, the interaction between the flow and the substrate plays a key role in the frictional dissipation process at the base, whereas it can vary considerably in natural situations. In fact, favorable conditions could even partly explain the high mobility of geophysical flows usually reported in relation to laboratory experiments. To tackle this question, we investigate the role of substrate roughness on the dynamics and deposition of concentrated, dry granular flows by combining small-to-large scale experiments and numerical simulations. We reveal that substrate condition can significantly affect the propagation and deposition of geophysical granular flows. Specifically, we show that the substrate type can be characterized as smooth, rough and macro-rough, based on the grain-to-roughness size ratio for a wide range of materials (i.e., glass beads, sand, volcanic materials). We then characterize the macroscopic properties of the flow in each of these configurations. Finally, this study offers guidelines for improving the modelling of geophysical granular flows.