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Onset of sediment transport is a continuous transition driven by fluid shear and granular creep

M. Houssais,C. Ortiz,D. Durian,D. Jerolmack

Published 2015 in Nature Communications

ABSTRACT

Fluid-sheared granular transport sculpts landscapes and undermines infrastructure, yet predicting the onset of sediment transport remains notoriously unreliable. For almost a century, this onset has been treated as a discontinuous transition at which hydrodynamic forces overcome gravity-loaded grain–grain friction. Using a custom laminar-shear flume to image slow granular dynamics deep into the bed, here we find that the onset is instead a continuous transition from creeping to granular flow. This transition occurs inside the dense granular bed at a critical viscous number, similar to granular flows and colloidal suspensions and inconsistent with hydrodynamic frameworks. We propose a new phase diagram for sediment transport, where ‘bed load’ is a dense granular flow bounded by creep below and suspension above. Creep is characteristic of disordered solids and reminiscent of soil diffusion on hillslopes. Results provide new predictions for the onset and dynamics of sediment transport that challenge existing models. The onset of sediment transport is commonly believed to be an abrupt process, which influences our understanding of river erosion and landscape dynamics. Houssais et al.show instead a continuous transition from a creeping to a granular-flow regime by monitoring internal motion in a sediment bed.

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