Strain hardening by sediment transport

The critical fluid-shear stress for the onset of sediment transport, $\theta_c$, varies with the history of applied shear. This effect has been attributed to compaction, but the role of shear jamming is unexplored. We examine the response of a granular bed to fluid-shear stress cycles of varying magnitude and direction, and determine isotropic and anisotropic contributions. Creep and bed-load transport result in direction-dependent strain hardening for $\theta/\theta_c < 4$. Dilation-induced weakening, and memory loss, occurs for larger stresses that fluidize the bed. Our findings provide a granular explanation for the formation and breakup of hard packed river-bed 'armor'.