Numerical modelling of the migration direction of tidal sand waves over sand banks

Tidal sand waves are large-scale bed forms found in shallow sandy seas, which show a migration of several meters per year. Field data from the Dutch part of the North Sea revealed a migration pattern causing bidirectional migration of sand waves over a sand bank, resulting in sand wave migration uphill from both sides of the sand bank. In order to understand the physical mechanisms behind this migration behaviour, we study the inclusion of a sand bank under a sand wave field using the numerical model Delft3D. First, the schematized model set-up (i.e. schematized bathymetry and simple tidal forcing) showed that the alteration of the tidal flow by a sand bank resulted in tide-averaged horizontal flow towards the top of the sand bank, causing the bidirectional migration of sand waves over the sand bank. Second, a case study was performed in which a more complex model set-up was used (bathymetry and tidal forcing identical to a study site in the North Sea where an offshore wind farm is developed). Here, the model results revealed migration directions comparable to field observations. The results open opportunities to explore modelling migration patterns of sand wave fields for offshore wind farm development in areas with complex bathymetric environments.