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A model is proposed to determine the sediment transport rate generated by the combined action of sea waves and tidal currents, the latter being modelled as a sequence of steady currents because of the large values of the Keulegan-Carpenter number of the tidal flows, which allow to neglect inertial effects in their modelling. The hydrodynamic module solves Reynolds-averaged momentum equations by introducing a two-equation turbulence model, which considers also the flow in the region closest to the sea bottom and enforces the no-slip condition and the vanishing of the turbulent kinetic energy at the bottom without the need to assume the existence of a logarithmic velocity profile and to fix the normal derivative of the turbulent kinetic energy close to the sea bed. Hence, the evaluation of the velocity profile in the near-bed region turns out to be accurate. The sediment transport module determines the sediment concentration and sediment transport for a mixture of non-cohesive and cohesive sediments. To validate the model, its predictions are compared with laboratory measurements and field data collected over rippled beds at two different tidal estuaries. Then, results are obtained for values of the parameters chosen to mimic a site close to Punta Umbría (Huelva, Spain) and considering different hydrodynamic conditions and different sediment mixtures. The influence of the waves on the suspended sediment concentration is lower for the cohesive fraction than for the non-cohesive fraction. For all the sediment mixtures, it turns out that the cohesive fraction is well mixed over the whole water depth, meanwhile the concentration of the non-cohesive fraction has significant values only near the bottom, thus showing that an accurate description of turbulence dynamics in the buffer layer and viscous sublayer (if it exists) is important to quantify the sediment transport rate. • Sediment transport module that reproduces the concentration of cohesive fraction is presented. • Tidal current is modelled as a sequence of steady-currents due to the large values of the Keulegan-Carpenter number. • Menter's turbulence model to predict the turbulence structure both in the wave bottom boundary layer and in the core region. • Turbulence dynamics description in the buffer layer and viscous sublayer is important to quantify the sediment transport. • Applicable to any type of sediment mixture. [ABSTRACT FROM AUTHOR]

Morphological evolution of large river deltas is highly vulnerable to extreme storm events due to insufficient sediment supply. As an abandoned delta lobe, the coasts along the northern Yellow River Delta (YRD) and Gudong Oil Field have recently suffered serious erosion due to extreme storm events and become increasingly vulnerable. In this study, a well validated and tested Delft 3D module by the observing hydrodynamic and sediment data to simulate the hydrodynamics and seabed erosion during a storm event in the littoral area of YRD. Observed wave, current and sediment data under both fair-weather and storm conditions were collected in the study area and used to validate the model. The results indicated that the model can reproduce well the hydrodynamic and sediment transport processes. A series of numerical experiments were carried out to examine the hydrodynamic changes and sediment transports. In the numerical experiment of normal condition, there is hardly any sediment transport off the YRD. The numerical experiment of storm condition showed that storms enhanced tidal residual currents, weakened tidal shear front, and significant wave heights up to 2 m, considerably intensified the sediment resuspension and dispersal. The local sediment resuspension due to the increased wave-induced bottom stress promoted the sediment plume to expand to the central area of Laizhou Bay, which seemed to provide sediment source for offshore and southward transport. During the storm, the active nearshore sediment resuspension provided sediment source for offshore and southward transport. The intensive dynamics and sediment transport under storm conditions caused significant changes in seabed erosion and siltation. The main erosion occurred off the Gudong and northern YRD, while the main siltation appeared in the central area of Laizhou Bay. No significant recovery after a storm and frequent strong winds have an accumulative effect on the erosion, which is very likely to dominate the erosive states of the YRD coast in the future. • Storm-induced energetic hydrodynamic forces intensify sediment resuspension and dispersal significantly. • Wave-induced bottom stress promotes sediment plume and enhances local resuspension. • Storms increase suspended sediment concentration and offshore sediment transport. • Storm-induced accumulative effect on seabed scour tends to cause long-term erosion. [ABSTRACT FROM AUTHOR]