How do directionally varied flows influence the development of stress history in granular beds?

Bedload transport occurs when the shear stress, or non-dimensional Shields stress, imparted by a fluid exceeds a critical Shields stress (τ_*c) for sediment motion. Previous studies have shown that the history of fluid stress imparted onto a sediment bed influences this critical Shields stress­ – particularly that sustained subcritical flows will strengthen the bed through a combination of processes such as compaction, topographic adjustment, shear jamming, and the development of a preferred orientation that can resist fluid shear. Recent studies have also shown that this strengthening effect is strongly directional; bed mobility increases when the direction of an above-threshold flow is opposite to prior subcritical flows (Galanis, et al., 2022; Cúñez, et al., 2022). In this study, we examine directional strengthening and weakening in a granular material for multiple fluid stress orientations by utilizing a rotating bed of sand within a laboratory flume. After subjecting the granular bed to a subcritical conditioning flow, we then changed the orientation of the bed within the flume by 0˚, 45˚, 90˚, 135˚, or 180˚ and applied an erosive flow. We use Lagrangian particle tracking to measure the behavior of a population of grains and quantify the effect of directional conditioning flows. Using this approach, we aim to better understand how the history of subcritical and critical fluid-imposed stresses can modify the contacts and force networks within granular materials.