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Existence of the estuarine turbidity maximum (ETM) zone is ubiquitous in partially mixed estuaries. It is characterized by high suspended sediment concentration. The position of the TM usually corresponds to the head of the salt instruction. In the Yangtze Estuary, the ETM zone is extremely large in spatial scale, spreading over the entire mouth zone downstream the South Branch and inside the 10 m isobaths. The ETM is aslo accompanied by a board shallow area (basically around 6 m) in the mouth zone, namely the mouth bars. The presence of the mouth bars hampers the navigation significantly. Thus extensive engineering works are implemented to achieve deeper water depth with the help of dredging activities in the North Passage in the Yangtze Estuary. However after the completion of the engineering works, high siltation appears in the middle segment of the North Passage. The hypothesis is that the location of high siltation (expressed in the morphology) falls in the range of ETM zone modified by the completion of the engineering works. The formation and development of ETM and consequent morphodynamic development is focused on.A three-dimensional z-layer model Delft3D is employed. This study clearly shows that salinity gradients can be better simulated using the z-layer model, where the stratification at the upper limits of the salt wedge is better resolved. Afterwards, the consequent morphological changes are qualitatively demonstrated. The study shows the capability of this z-layer model to exploit the mechanism of generation and development of ETM, thus to explain the complex phenomenon of the high siltation in the North Passage of Yangtze estuary

Coastal 3D nearshore processes were considered with an emphasis on the effects of oceanic forcing and beach characteristics on sediment transport in both cross- and alongshore directions, as well as on foreshore bathymetry changes. In our numerical experiments, we combined the FLOW module of the Delft3D model with the WAVE solver of Xbeach models. k-ε turbulence closure was used to resolve the three-dimensional Navier-Stokes equations for incompressible flow and the beach morphology. The sediment transport module simulates both bedload and suspended load transport of non-cohesive sediments. A series of numerical experiments was performed for a range of control parameters. For each case, the general morphological response was determined in the shore-normal and shore-parallel directions. The simulations confirmed that the sole wave forcing is sufficient to drive a sediment circulation pattern that results in bar and berm formation. The wave characteristics have a considerable effect on the cumulative erosion/deposition, cross-shore distribution of longshore sediment transport, and the sediment transport rate across and along the beach face. For the same oceanic forcing, beach morphology exhibits different erosive characteristics depending on grain size. Fine beach sands were transported offshore, whereas coarse sands moved onshore-wards. Sediment movement increases with wave energy, which was shown to be the most dominant factor controlling the beach face shape. In the surf zone, the sediment transport rate increases towards the shore until the wave collapses whereas in the swash zone it decreases. The present model is able to reproduce complicated flow and sediment transport processes and estimation of beach face dynamics.

Two major parameters that determine the beach type are sediment grain size and beach slope. Intermediate beaches normally have steep slopes and are associated with coarse-grained sands and narrow surf zones, while dissipative beaches generally have mild slopes and are related to fine sands and wider surf zones. In numerical experiments, the Delft3D and Xbeach models were combined to resolve the 3D Navier-Stokes equations for incompressible flow and beach morphology. The sediment transport module supports both bed-load and suspended load transport of non-cohesive sediments. Numerical simulations were run for different hydrodynamic conditions, but with a focus on different beach slopes and grain sizes, and considering hydrodynamic processes, sediment transport in cross- and alongshore directions, as well as foreshore bathymetry changes. Larger grain sizes tend to generate more complex nearshore hydrodynamic patterns. The transformation of incoming waves as they reach shallow water occurs closer the shoreline for steeper profiles. Consistently, the peaks in eddy viscosity, turbulence dissipation rate (TDR), turbulent kinetic energy (TKE) and wave set-up are shifted onshore for steeper slopes. High values of eddy viscosity, TKE and wave set-up are spread offshore for coarser grain sizes. The TDR is an order of magnitude smaller for the coarsest grains compared with other cases. The numerical results showed that TKE, sediment concentrations and sediment transport rate are greater on steep beaches than on mildly sloped beaches. The beach morphology exhibits different erosive characteristics depending on grain size (e.g., foreshore profile evolution is erosive and accretionary on fine and coarse sand beaches, respectively). The results confirmed that wave energy, beach grain size and bed slope are the main factors influencing sediment transport and beach morphodynamics.