Effects of sub-critical fluid flows on sediment strength

Granular-fluid interactions are prevalent on Earth. Fluid flow over granular materials imparts a shear stress, which above a critical strength causes particles to become entrained in the fluid flow. Studies have shown that granular beds subjected to a sub-critical fluid flow gives rise to strengthening in the same direction as the sub-critical flow. In contrast, flows in the opposite direction cause particle mobilization at a lower flow strength. To investigate the physical mechanisms that control directional strengthening and the angular dependence of the sub-critical flow, we perform discrete element method simulations of granular beds subjected to model fluid flows in two and three dimensions. We find that the sub-critical fluid flows do not cause compaction, which is one of the common mechanisms for strengthening of granular beds. Instead, we find that the directional strength of the granular beds is highly correlated with the stress and fabric anisotropy of the grains on the bed surface. We find that inter-particle friction influences the bed strength, but is not necessary for bed strength anisotropy. Finally, we find that the local anisotropy is associated with a time scale, beyond which granular beds do not exhibit direction-dependent strength when subjected to fluid flow. This research enhances our understanding of the erosion of granular beds caused by fluid flows in different directions, underscoring the importance of tracking the history dependence of granular beds.