Recently, several works have been devoted to the effect of confinement on the properties of granular flows: boundary conditions have a long-range effect within the flow. I will show here that this effect is also important before the flow - when inclined 3D granular packings are driven towards their stability limits - and at the other extreme: for high-speed flows down inclined channels. In rapid steady fully developed flows down inclines, new patterns emerge from the destabilization of unidirectional flows upon increase of mass holdup and inclination angle, and are characterized by complex internal structures, including secondary flows, heterogeneous particle volume fraction, symmetry breaking and dynamically maintained order. Interestingly, despite their overall diversity, these regimes are shown to obey a general scaling law for the mass flow rate as a function of the mass holdup.
These results open new perspectives for interpreting the features of geophysical granular flows. The scaling aforementioned could for instance explain, in a very simple and elegant way, the frictional velocity weakening in landslides on Earth and other planetary bodies observed in a recent publication (Antoine Lucas, Anne Mangeney, Jean Paul Ampuero, 2014, Frictional velocity-weakening in landslides on Earth and on other planetary bodies, Nature Communications 5, 3417).