Rheology of sediment transported by a laminar flow

Understanding the dynamics of fluid-driven sediment transport remains challenging, as it occurs at the interface between a granular material and a fluid flow. Boyer, Guazzelli, and Pouliquen [Phys. Rev. Lett. 107, 188301 (2011)] proposed a local rheology unifying dense dry-granular and viscous-suspension flows, but it has been validated only for neutrally buoyant particles in a confined and homogeneous system. Here we generalize the Boyer, Guazzelli, and Pouliquen model to account for the weight of a particle by addition of a pressure P-0 and test the ability of this model to describe sediment transport in an idealized laboratory river. We subject a bed of settling plastic particles to a laminar-shear flow from above, and use refractive-index-matching to track particles' motion and determine local rheology-from the fluid-granular interface to deep in the granular bed. Data from all experiments collapse onto a single curve of friction mu as a function of the viscous number Iv over the range 3 x 10(-5) <= I-v <= 2, validating the local rheology model. For I-v < 3 x 10(-5), however, data do not collapse. Instead of undergoing a jamming transition with mu -> mu(s) as expected, particles transition to a creeping regime where we observe a continuous decay of the friction coefficient mu <= mu(s) as Iv decreases. The rheology of this creep regime cannot be described by the local model, and more work is needed to determine whether a nonlocal rheology model can be modified to account for our findings.