Modelling the morphodynamic impact of offshore sandpit geometries

We investigate the hydrodynamic effects and morphodynamic impact of large-scale offshore sand extraction, for a variety of pit designs. We use a process-based idealized model for flow, sediment transport and bed evolution in a tide-dominated environment. Legislation and other practical considerations motivate our assumption that sandpits are both wide (horizontal dimensions of the order of kilometres) and shallow (the ratio of pit depth to water depth being small). This results in a semi-analytical tool that, unlike previous studies, enables a quick and extensive study into the effects of varying the physical characteristics as well as the pit design parameters. These parameters include pit length, width, and orientation with respect to the tide. The model results show that sandpits experience flow contraction, which is enhanced by friction–topography interactions and also affected by Coriolis effects. As a result, sandpits trigger the morphodynamic instability associated with the formation of large-scale bed features known as tidal sandbanks. It implies a gradual deepening and deformation of the pit itself, as well as the appearance of adjacent humps. The time scale of this behaviour is of the order of decades to centuries. A sensitivity analysis is then carried out to determine the effects of pit geometry on the model results. The morphodynamic response is found to be strongest for sandpits elongated in the preferred direction of sandbank formation (giving the largest area of morphodynamic influence), and weakest for pits perpendicular to this direction. The migration of the pit is shown not to depend on the pit geometry, but rather on the ambient flow conditions.