The transport of cohesive sediment is known to be impacted by salinity and consolidation, which control the electro-chemical force and microscale structures of clay. However, such impacts remain poorly understood, due to a lack of direct visualization and characterization methods. Here, we combine a transparent clay, fluorescent dye, and laser-based technologies to directly visualize the transport of clay and its microstructure and identify the critical shear stress for clay erosion, Tau_crit. We show that as salinity increases, Tau_crit increases by over one order of magnitude under low salinity (<1.52 ppt) and then decreases under high salinity (>1.52 ppt). This non-monotonic dependence of Tau_crit on salinity can be attributed to a change in clay microstructures. In addition, we show an increase in Tau_crit after clay consolidation and attribute it to a change in clay microstructures. Our results demonstrate the important role of salinity, consolidation, clay microstructure, in controlling clay erosion.

Aquatic vegetation modifies hydrodynamics, turbulence structure, sediment transport, and ecological processes in marine ecosystems. Recent turbulence models for vegetated flows have focused on open channel unidirectional flows. However, the unsteadiness and turbulent structure of oscillatory flows often prevent the direct application of such models in wave-dominated environments. We investigate Turbulent Kinetic Energy (TKE) connected to the flow structure in oscillatory flows through aquatic vegetation. Using an oscillatory tunnel, we test vegetation densities up to $\phi=0.10$ with wave periods between 2.1-5.3 s and wave amplitudes between 2-10 cm. Our measurements show a nonlinear relation between the TKE inside the canopy and vegetation density due to the change from the stem- to canopy-scale dominated regime. We observe that $ah\geq 0.8$ marks a threshold for this transition: a reduction of wake TKE inside the canopy and an increase of shear TKE at the top of the canopy. This transition is characterized by increasing frequency and intensity of sweeps and ejections near the bed and at the canopy top. We developed a two-equation predictor for TKE at the top of the canopy using the “short-cut” TKE transfer first proposed by \citeA{finnigan2000turbulence} where canopy-scale eddies convert TKE into stem-scale eddies via the work against vegetation drag. For near-bed TKE, we adapt \citeA{tanino2008lateral}’s model to predict the maximum TKE values on oscillatory flows. These two predictors provide easy-to-use tools suitable for wave-dominated environments to accurately estimate TKE levels inside the canopy for estimating sediment transport rates and mass exchange across the canopy.