Modelling the Impact of Residual Currents on the Interaction Between Sand Waves and Trenches in Delft3D FM

Abstract

Sand waves are rhythmic seabed patterns which develop as a result of the interaction between tidal flow and perturbations in the seabed. Due to their size and dynamic behaviour sand waves can pose a threat to offshore infrastructure such as cable trenching. To ensure the safety of these cables, it is crucial to understand the interactions between sand waves and trenches. Residual currents, currents that are not induced by the tide, can exert a significant influence on sand wave dynamics due to the non-linear relationship between flow velocity and sediment transport. Hence, this thesis aims to investigate the effects of residual currents on the morphological interactions between sand waves and trenches. A Delft3D FM model is used to simulate a case study of a trench within a group of sand waves in the Belgium Continental Shelf. The influence of residual currents, mostly induced by wind and atmospheric pressure, is studied during a period of a storm and a period with representative residual currents. It is found that during the tidal cycle at the peak of the storm, the residual currents cause an increase of 66% in sediment infilling into the trench compared to tide-only conditions. Since this is in the same order of magnitude as the background infilling due to tidal dynamics, this is less than expected. The residual currents are however strong enough to reverse the sediment transport patterns of the sand wave and trench system. During the representative residual currents, the contribution of residual currents is found to be insignificant, especially compared to the observed influence of the spring-neap tide. This study concludes that the overall effects of the residual currents, induced by wind and atmospheric pressure, can alter sediment transport patterns but the magnitudes are insignificant in the case study under investigation. It is recommended, for further research and for effective design and maintenance of offshore infrastructure procedures, to keep in mind that morphodynamics of sand wave and trench systems are highly subject to the variations in tidal dynamics induced by the spring-neap tide.