Modelling the morphodynamics of the Kwinte Bank, subject to sand extraction

The North Sea is, to an increasing degree, subject to human activities and interests; a particular example of a user function of this environment is sand extraction. In order to satisfy the demand for sand, tidal sandbanks in some sectors of the North Sea act as a source of sediment, this may lead to the creation of large-scale pits on these bedforms. Sandbanks, which provide protection to adjacent stretches of coastline, are therefore worthy of investigation, especially when their potential as a source of aggregates is taken into account. To investigate seabed dynamics, in general, and to predict the long-term morphodynamics of tidal sandbanks subject to sand extraction, in particular, process-based modelling is a commonly-used method. Different approaches can be considered: (i) based on complex numerical simulations; (ii) or applying an idealised model, designed specifically to describe sandbank dynamics. Herein, the first approach is applied to a case study of the Kwinte Bank; this is a tidallymaintained sandbank, located on the Belgian continental shelf. The modelling is set up using the complex processbased model Delft 3D – Online, the waves were not taken into account. Numerical results and experimental data from a field campaign (undertaken in March 2004) are compared and show good agreement. Analysis of residual currents indicates a predominance of ebb flow over the bank. However, the residual sediment transport pattern is flood-dominated. The predicted residual sediment transport pattern shows that the Kwinte Bank should be regarded as part of a system of swales and sandbanks. Finally, the long-term evolution is discussed, considering complementary approaches that combine the benefits from the complex numerical modelling with the idealised one. The evolution of a tidal sandbank, after removing an amount of sand, is difficult to predict. No clear tendency is evident in the evolution of the depression area on the basis of the long-term full process-based modelling. However, on the basis of idealised modelling, the anticipated long-term trend of an excavated area is the recovery of the depression, resulting in an equilibrium of the sandbank, over a time-span of a few centuries.