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Sand waves are found on shallow, sandy seabeds throughout the world and their dynamics may pose an imminent threat to offshore construction. Therefore, there is a pressing need to understand bed level dynamics in sand wave areas. These bed level dynamics lead to variations in sand wave shape and migration rate over time. However, these variations cannot be explained with the present-day process-based sand wave models, which all include a purely periodic tidal forcing. To explain these fluctuations a more intricate description of the hydrodynamics is necessary. The aim of this study is to explore the importance of time-varying, non-tidal currents for sand wave dynamics in the North Sea. We adopted the three-dimensional Delft3D-Flexible Mesh model, and were able to reconstruct time-varying, non-tidal currents on top of the periodic tidal forcing, while significantly reducing computation times. The simulated currents and water levels showed a good agreement with in-situ measurements. Compared to the situation with only tidal forcing, the simulated sedimentation and erosion rates were amplified up to 15 times due to time-varying, non-tidal currents. Additionally, periods of net erosion were found at locations in the sand wave transect where tidally forced models only showed net-sedimentation. It is therefore important to consider time-varying, non-tidal currents when predicting future sand wave dynamics in the field. • The reproduction of observed hydrodynamics is improved significantly compared to previous sand wave modelling approaches. • Effects of time-varying, non-tidal currents on sediment transport in sand wave fields are quantified. • Net sedimentation rates due to time-varying, non-tidal currents are be up to 15 times larger than the tidally induced rates. • First application of Delft3D FM software to study sand wave dynamics leads to a significant decrease in computation time. • Excluding time-varying, non-tidal currents can lead to underestimation of sand wave migration and trench infill rates. [ABSTRACT FROM AUTHOR]

Large perturbations in the coastline, such as the 'Sand Motor' nourishment ( ∼ 21 million m 3 ) at the Holland coast, can initiate considerable spatial and temporal changes in the median grain size ( D 50 ) of the sea bed on the lower shoreface. The relevance of hydrodynamic conditions for the development of the heterogeneity in D 50 at large-scale nourishments was assessed with a numerical model (Delft3D), which required a validation against 2.5 years of D 50 measurements. A good representation of the observed spatial pattern of D 50 was obtained independent of a 2DH or 3D approach and initial condition for the D 50 of the bed. Five sediment size fractions and a multi-layer administration of the bed composition were used. The extent and magnitude of the coarsening of the bed is related to the velocity of the horizontal tide, while a far less pronounced coarsening takes place during energetic conditions (i.e. H m 0 ≥ 3 m). Differential suspension behaviour between the size fractions, which are all mobilized at the bed, causes a preferential transport of fine sediment (in alongshore direction) away from the Sand Motor at the lower shoreface (i.e. seaward of MSL -6 m). Storm conditions may induce a partial removal of the coarse top-layer due to mobilization of all of the size fractions and mixing with the relatively fine substrate material. Simulations also show that transport of the fine sand fraction extents to much deeper water than for the medium and coarse sand fractions. Models with multiple sediment fractions are therefore required for the assessment of environmental impacts of large-scale coastal structures or land reclamation's and sediment transport on the lower shoreface. [ABSTRACT FROM AUTHOR]