Numerical modelling of sediment transport in shelf seas and estuaries. Case studies: the Kwinte Bank and the Ijzermonding

The increasing economical and environmental attraction towards coastal and offshore areas has put relevant pressure into the understanding of the different processes taking part in this zone. Sediment transport, in particular, affects directly several human activities and has a strong impact on marine biology. From a physical point of view, bottom morphology and dynamics play a key role with respect to hydrodynamics and wave processes. The interaction with the bottom modifies the current velocity profile, the energy content of the wave field and their shape. Despite its importance, sediment transport prediction still contains very high margins of uncertainty. Sediment characteristics are often scarcely known, the interaction with waves and currents are difficult to model and the number of field measurements for a validation of the calculations very limited.In this work, three different numerical models were set-up in order to simulate the hydrodynamic flow, the wave field and their effects on sediment transport. The possibility of running the different models as separate applications or in coupled mode, allowed assessing the relative importance of waves and currents and their joint effect.Two different study areas were considered for a validation of the numerical models: a typical sandy environment represented by a sand bank in the North Sea (Kwinte Bank) and a prevalently muddy environment represented by an estuary of the Belgian coast (IJzermonding). The application of the models to two different environments allowed testing different approaches and formulations typical for non-cohesive and cohesive sediments. Model verification involved a comparison with hydrodynamic, wave, sediment transport and bottom data.At the Kwinte Bank both, waves and currents were considered as driving forces of sediments at the bottom. Two issues were investigated: the consequences of sand extraction on the sediment transport at the sand bank and the effect of wave activity on the morphodynamic evolution of the bank. Numerical modelling indicates that the Kwinte Bank stability is not endangered by the intense sand extraction. On the other hand, wave activity seems to play a central role in enhancing the magnitude of sand transport and changing its direction.In the modelling work at the IJzermonding, wave activity was neglected. The high complexity of sediment dynamics in this area and the strong anthropogenic impact necessitated the use of a variety of measurements in the interpretation of model results. The amount of deposited sediment was calculated and compared to the volumes yearly dredged. Erosion and deposition areas calculated by the model were verified by the use of airborne images