Book of Abstracts
NCK Days 2018
Geo-logic in coastal and shelf research: a matter of multi-disciplinarity
March 21-23
Teylers Museum – Haarlem
Sponsored by:
Organized by:
77
NUMERICAL MODELLING OF THE MIGRATION DIRECTION OF OFFSHORE SAND WAVES USING
DELFT3D
S. Leenders1,2*, J.J. Schouten 2, R. Hoekstra2, B.W. Borsje3 1 Delft University of Technology, 2 Deltares, 3 University of Twente
* Sjoerd.Leenders@deltares.nl
Rhythmic large-scale bedforms are present in shallow seas with sandy beds (Figure 1). Depending on the length, height, propagation speed and formation timescale a spatial and temporal distinction can be made between these large-scale rhythmic forms (Table 1).
Owing to their dynamic behavior, sand waves can interact with human offshore activities. Their large dimensions are relevant for engineering and research purposes. Presently the most commonly used engineering tool to investigate sand waves is data analysis. Data analyses are based on seabed surveys over preferably more than 10 years. Though most reliable at the moment, they are costly. Modelling the dynamics of sand-wave fields has become a relevant alternative for the offshore wind industry, which will grow intensively in the coming decades. However, not all relevant processes regarding sand-wave dynamics are understood well enough to predict sand-wave fields fully over the lifetime of an offshore windfarm. Research so far has focused on the length and height of sand waves. To add a step in determining full development of sand-wave fields, the migration direction of sand wave is essential. Understanding the governing processes of the migration direction of sand waves, the focus of this research, uses the model of Borsje et al. (2014) as a starting point.
An idealized (3D) model is set up to investigate the most prominent processes governing the migration of sand waves, being characteristics of tide constituents, sediment transport modes, seabed topography and waves. The model results are compared with bathymetry data analyses over several decades of windfarm areas in the North Sea. To model the presence of larger rhythmic forms under sand-wave fields, a slope is added to the idealized model, representing a sand bank (Figure 2).
Underlying topography in the form of sand banks influences the local hydrodynamics (Figure 3) and thereby sediment transport loads and sand-wave migration. Models indicate the possibility of migration towards opposite directions on a small scale, as also observed in the field.
In this ongoing research, the first results indicate that the topography is an important candidate for deviations in migration direction on small spatial scales. Results of the idealized model are tested regarding the data from the field. The physical mechanisms regarding these small-scale deviations will be further explained.
Borsje, B.W., Kranenburg, W.M., Roos, P.C., Matthieu, J., Hulscher, S. J.M.H. (2014). The role of suspended load transport in the occurrence of tidal sand waves. Journal of Geophysical Research: Earth Surface, 119(4), 701-716.
