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

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.

Onset of sediment transport in mono- and bidisperse beds under turbulent shear flow

“…Our findings are consistent with observed segregation phenomena in bidisperse beds known as riverbed armoring: a slow process in which the bed segregates in a top region with large particles and a region below with smaller particles [5,19]. Recently, it has been found that such granular creep is very similar to kinetic sieving processes in dry granular flows [14].…”

Simultaneous Concentration and Velocity Maps in Particle Suspensions under Shear from Rheo-Ultrasonic Imaging

“…Furthermore, when such techniques can be used simultaneously for rheological characterization, they provide crucial insights into the coupling between particle dynamics and the macroscopic flow behavior of the suspension. For transparent suspending fluids and large enough particles, as in the case of geophysical flows, direct particle tracking is possible [7]. However, suspensions of smaller particles are generally opaque for volume fractions above a few percent, so the optical investigation of denser systems generally involves more advanced techniques such as confocal scanning light microscopy [8,9] or depolarized light scattering [10].…”

Scales of collective entrainment and intermittent transport in collision-driven bed load

“…While the role of turbulence has received the most attention, granular contributions to bed-load dynamics are increasingly being recognized (Frey and Church (2011a); Houssais et al (2015); Maurin et al (2016)). One of the defining features of granular systems is a continuous transition from flowing to static 5 regimes known as the jamming transition.…”

“…Experiments show that the onset of bed-load transport has the hallmarks of a jamming transition (Houssais et al (2015); Maurin et al (2016); Houssais et al (2016)). Near-threshold transport rates exhibit strong correlations and intermittency, while fluxes at rates far above threshold are uncorrelated and smooth (Singh et al (2009)).…”

“…This timescale may not be independent of driving rate, and it becomes impossible to measure when avalanches overlap in time. Another complicating factor at low driving rates is the influence of creep, which has been demonstrated to drive bursty bed-load transport in the viscous flow regime (Houssais et al (2015)). Movies of our experiments reveal the presence of slow creep also, but quantifying its significance is beyond the scope of the present paper.…”