Granular bed consolidation, creep and armoring under subcritical fluid flow

We show that a freshly sedimented bed composed of spherical grains settles and creeps forward over extended periods under an applied hydrodynamic shear stress τ∗, which is below the critical value τ∗ c for bedload transport. The rearrangements are found to last at least over millions of times the sedimentation time scale of a grain in the fluid. Compaction occurs throughout the bed, but creep is observed to decay exponentially with depth, and decreases over time. The granular volume fraction in the bed is found to increase logarithmically, saturating at the random close packing value φrcp ≈ 0.64, while the surface roughness is on average essentially unchanged. Thus, we find that bed armoring occurs due to a deep shear-induced relaxation of the bed toward the volume fraction associated with the glass transition.