Sediment dynamics driven by waves and currents in shallow-water estuarine environments impacts many physical and biological processes and is important to the estuary-wide sediment budget. Observational restrictions have limited our ability to understand the physics governing sediment entrainment and mixing in these environments. To this end, we use direct numerical simulation to simulate sediment transport processes in shallow, combined wave-and current-driven flows. Simulations are run with depth-averaged currents ranging from 0 to 9 cm/s, while wave conditions are held constant with a bottom orbital velocity and period of 10 cm/s and 3 s, respectively. Our results indicated that for wave-dominated conditions, waves reduce vertical momentum fluxes and the associated bottom drag, thereby accelerating mean currents. Conversely, currents do not significantly affect the wave velocity field. However, they increase the bed shear stress and change the timing and duration of sediment entrainment throughout the wave cycle. Counterintuitively, these effects lead to lower suspended sediment concentrations near the bed for a portion of the wave cycle. By analyzing sediment fluxes, waves are shown to drive near-bed sediment dynamics while currents control vertical mixing above the buffer layer, where downward settling is predominantly balanced by the current-generated vertical turbulent sediment flux. In the absence of currents, sediment concentrations are negligible above the wave boundary layer because mixing is weak. We show that the time-and phase-averaged sediment concentration profiles for wave and current conditions resemble the theoretical Rouse profile derived for equilibrium conditions in statistically steady, unidirectional turbulent channel flow. Plain Language SummaryThe transport of mass, such as nutrients and sediment, by fluid flows is fundamental to aquatic life and is crucial to many environmental and coastal engineering studies. Whether predicting the dispersion of shrimp larvae or assessing the mobilization of sediment-sorbed contaminants, the fluid mechanics governing the transport processes is the most important underlying physical phenomenon. Despite its importance, many mechanisms controlling the movement of sediment in estuaries are poorly understood. This is particularly true near the sediment bed where our ability to observe and measure properties relevant to the physics is limited. To this end, we apply state-of-the-art supercomputers to simulate sediment transport by fluid flow in environments with waves and currents. Contrary to popular belief within the fluid mechanics community, we find that currents can accelerate in the presences of waves. This acceleration can potentially affect how sediment and nutrients move within an estuary. Currents also affect the duration and magnitude of sediment erosion. Our results support the conceptual model that wind-generated waves strongly influence sediment erosion, but currents are required to mix sediment into the water column. Ultimately, our work gives b...
A suspended sediment transport model is implemented in the unstructured‐grid SUNTANS model and applied to study fine‐grained sediment transport in South San Francisco Bay. The model enables calculation of suspension of bottom sediment based on combined forcing of tidal currents and wind waves. We show that accurate results can be obtained by employing two‐size classes which are representative of microflocs and macroflocs in the Bay. A key finding of the paper is that the critical calibration parameter is the ratio of the erosion of the microflocs to macroflocs from the bed. Different values of this erosion ratio are needed on the shallow shoals and deeper channels because of the different nature of the sediment dynamics in these regions. Application of a spatially variable erosion ratio and critical shear stress for erosion is shown to accurately reproduce observed suspended sediment concentration at four‐field sites located along a cross‐channel transect. The results reveal a stark contrast between the behavior of the suspended sediment concentration on the shoals and in the deep channel. Waves are shown to resuspend sediments on the shoals, although tidal and wind‐generated currents are needed to mix the thin wave‐driven suspensions into the water column. The contribution to the suspended sediment concentration in the channel by transport from the shoals is similar in magnitude to that due to local resuspension. However, the local contribution is in phase with strong bottom currents which resuspend the sediments, while the contribution from the shoals peaks during low‐water slack tide.
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