Sediment dynamics in tidally dominated environments controlled by transport and turbulence: A case study for the East Frisian Wadden Sea

Various tide‐related sediment transport mechanisms near a barrier island coast are addressed on the basis of analytical theory and numerical simulations carried out with a three‐dimensional (3‐D) numerical model. The theory proposed gives an explanation of the observed “twin peaks” in the concentration of suspended sediment; however, it does not adequately describe the whole variety of sediment responses. The major idea in this paper assumes that it is not only the change of the level of turbulence and advection individually but also the correlation between the two which controls the concentration of sediment. Because the horizontal gradients of sediment concentration are created not only by sources at the coast (e.g., fluxes from rivers) but also by the transport and turbulence, the system is highly nonlinear. Several simulations have been carried out aimed at revealing the nonlinear mechanisms in the sediment transport and individual contribution of turbulence and velocity field. An experiment in which the sediment model is driven only by turbulent kinetic energy fails to simulate most of the characteristic temporal and spatial sediment patterns. With another experiment it is demonstrated that the velocity gradient in the vertical gives an important mechanism shaping the patterns of temporal and spatial variability (and tidal asymmetries). Without shear diffusion, the sediment dynamics are largely governed by the “displacement mechanism”; that is, the system is more linear. One practical conclusion from the numerical simulations is that using simplified (2‐D) dynamics in sediment transport modeling along with externally prescribed turbulence characteristics could lead to large artifacts.