To investigate the dominant sediment transport and trapping mechanisms, a semi-analytical three-dimensional model is developed resolving the dynamic effects of salt intrusion on sediment in well-mixed estuaries in morphodynamic equilibrium. As a study case, a schematized estuary with a converging width and a channel-shoal structure representative for the Delaware estuary is considered. When neglecting Coriolis effects, sediment downstream of the estuarine turbidity maximum (ETM) is imported into the estuary through the deeper channel and exported over the shoals. Within the ETM region, sediment is transported seaward through the deeper channel and transported landward over the shoals. The largest contribution to the cross-sectionally integrated seaward residual sediment transport is attributed to the advection of tidally averaged sediment concentrations by river-induced flow and tidal return flow. This contribution is mainly balanced by the residual landward sediment transport due to temporal correlations between the suspended sediment concentrations and velocities at the M2 tidal frequency. The M2 sediment concentration mainly results from spatial settling lag effects and asymmetric bed shear stresses due to interactions of M2 bottom velocities and the internally generated M4 tidal velocities, as well as the salinity-induced residual currents. Residual advection of tidally averaged sediment concentrations also plays an important role in the landward sediment transport. Including Coriolis effects hardly changes the cross-sectionally integrated sediment balance, but results in a landward (seaward) sediment transport on the right (left) side of the estuary looking seaward, consistent with observations from literature. The sediment transport/trapping mechanisms change significantly when varying the settling velocity and river discharge.@en

A semianalytical three-dimensional model is set up to dynamically calculate the coupled water motion and salinity for idealized well-mixed estuaries and prognostically investigate the influence of each physical mechanism on the residual salt transport. As a study case, a schematized estuary with an exponentially converging width and a channel–shoal structure is considered. The temporal correlation between horizontal tidal velocities and tidal salinities is the dominant process for the landward residual salt transport. The residual salt transport induced by residual circulation is locally significant, but the induced salt transport integrated over the cross section is small. The impacts of the estuarine geometry, Coriolis force, and bathymetry on the salt dynamics are studied using three dedicated experiments, in which the impact of each of these factors is studied separately. To assess the impact of width convergence, a convergent estuary without bathymetric variations or Coriolis force is considered. In this experiment, the temporal correlation between tidal velocities and salinities is the only landward salt transport process. In the second experiment, Coriolis effects are included. This results in a significant residual salt transport cell due to the advection of the tidally averaged salinity by residual circulation, with salt imported into the estuary from the left side and exported on the right (looking seaward). In the last experiment, a lateral channel–shoal structure is included while the Coriolis effects are excluded. This results in a significant landward salt transport through the deeper channel and a seaward salt transport over the shoals due to the advection of the tidally averaged salinity by residual circulation.@en

Estuaries are important ecosystems accommodating a large variety of living species. Estuaries are also important to people by their demand of freshwater for drinking, irrigation, and industry. Due to natural changes and human activities, the estuarine water quality, influenced by both salinity and turbidity (the cloudiness or haziness of water), has been greatly changed in many estuaries and may continue to change in the future. To predict and control the salt intrusion and the occurrence of high turbidity levels, it is essential to understand the physical mechanisms governing the estuarine dynamics. To that end, this thesis provides a systematical investigation of the dominant physical processes which result in salt intrusion and the formation of the Estuarine Turbidity Maxima (ETM’s) in well-mixed estuaries.@en

Understanding salt dynamics is important to adequately model salt intrusion, baroclinic forcing, and sediment transport. In this paper, the importance of the residual salt transport due to tidal advection in well-mixed tidal estuaries is studied. The water motion is resolved in a consistent way with a width-averaged analytical model, coupled to an advection–diffusion equation describing the salt dynamics. The residual salt balance obtained from the coupled model shows that the seaward salt transport driven by river discharge is balanced by the landward salt transport due to tidal advection and horizontal diffusion. It is found that the tidal advection behaves as a diffusion process, and this contribution is named tidal advective diffusion. The horizontal diffusion parameterizes processes not explicitly resolved in the model and is called the prescribed diffusion. The tidal advective diffusion results from the correlation between the tidal velocity and salinity and can be explicitly calculated with the dominant semidiurnal water motion. The sensitivity analysis shows that tidal advective diffusivity increases with increasing bed roughness and decreasing vertical eddy viscosity. Furthermore, tidal advective diffusivity reaches its maximum for moderate water depth and moderate convergence length. The relative importance of tidal advective diffusion is investigated using the residual salt balance, with the prescribed diffusion coefficient obtained from the measured salinity field. The tidal advective diffusion dominates the residual salt transport in the Scheldt estuary, and other processes significantly contribute to the residual salt transport in the Delaware estuary and the Columbia estuary.@en

Along-channel and cross-channel sediment transport in tidal estuaries is usually driven by tides, density gradients, Coriolis’s force, wind stress, channel curvature and bathymetric variations. Since the water motion is influenced by density-induced gravitational circulation which in turn affects the salinity distribution, the coupled water motion and salinity has a potentially strong effect on the residual sediment transport, and thus the trapping of fine sediment. To better understand the dynamical effects of water motion and salinity on sediment transport, the salinity field has to be computed consistently. In this work, the water density is assumed to be a function of salinity only, thus ignoring the influence of temperature and assuming the sediment concentration to be low. To obtain the coupled water motion and salinity, the three-dimensional shallow water equations and the salinity equation are solved simultaneously using a perturbation method together with an iterative finite element method (Kumar et al., 2016; Wei et al, in preparation), resulting in a consistent water motion and salinity field. This information is then used to calculate the sediment concentrations, so that the influence of the salt dynamics on sediment transport is prognostically calculated. Owing to the adopted perturbation method, the contribution of various physical processes to residual sediment transport can be studied separately, which allows for a full investigation on individual contribution of each process to longitudinal/lateral transport of salinity and sediment. Moreover, as wind is another important forcing of estuarine circulation (Chen et al., 2009, de Jonge and van Beusekom 1995, Ridderinkhof et al., 2000), the influence of wind stress on estuarine sediment transport will be studied. The present work will bring insights into sediment transport and trapping mechanisms in real estuaries, for example, the Delaware estuary. @en