Directional strengthening and weakening in hydrodynamically sheared granular beds

When a shear flow delivers sufficient stress to a granular bed, bed particles will start to erode. It has long been known that the critical stress required to initiate sediment transport depends on the shear history of the bed, and that beds tend to strengthen in response to weak flow. We find here that this strengthening effect is highly directional, in that beds only become stronger against flows oriented in the same direction as the conditioning flow. In fact, we find that conditioned beds are actually significantly weaker against flows oriented in other directions, with significant implications for predictions of sediment transport.