Cohesive sediment affects sand wave formation

Sand waves, with wavelengths of hundreds of metres and a height of several metres, are bedforms commonly observed in shallow shelf seas. Literature suggests that small amounts of cohesive sediment have a great impact on sand wave occurrence, but it is unclear how cohesive sediment and sand waves interact exactly. To investigate the impact of cohesive sediment on sand wave formation, we developed a new 2DV model in which cohesive sediment locally increases the critical bed shear stress for erosion, and is transported independently from the non-cohesive sediment, but can only be entrained alongside non-cohesive sediment. Application of linear stability analysis to the model demonstrates that different regimes exist, depending on the background mud content: (1) for low mud content (<10% for our parameters), sand waves form and mud accumulates in their troughs; (2) for intermediate mud contents (10 to 25%), sand waves form with mud accumulating at the crests; (3) for high mud contents (>25%), no sand waves form. This transition results from the competition between two mechanisms. The availability contribution, i.e. the fact that more mud erode from locations where more mud is present, dominates in the first regime (“mud repels mud”). The critical shear contribution, i.e. the fact that less sand and mud erodes from locations with more mud due to the increased critical shear stress, dominates in the second regime (“mud attracts mud”). In the third regime, the critical shear stress exceeds the tide maximum bed shear stress, preventing any sediment transport and thus bedform formation. The existence of these different regimes underline the potential impact of cohesive sediment on sand wave dynamics.