Modelling sand wave variation

Abstract

The sea floor of shallow seas is rarely flat and often dynamic. A widely occurring bedform type is the sand wave. Sand waves form more or less regular wavelike patterns on the seabed with crests up to one third of the water depth, wave lengths of hundreds of metres and a migration rate of metres up to tens of metres per year. Because of their migration speed and spatial dimensions, sand waves can interfere with anthropogenic activities. In relation to offshore activities especially the variation and extremes in sand wave characteristics are important. These variations can be caused by variation in environmental factors. In this thesis, we validate a non-linear idealized process-based model (Sand Wave Code, SWC) and include relevant physical processes, to relate them to variations and extremes in sand waves characteristics. Our specific interest is firstly to understand which environmental factors cause the sand wave shape and variation and, secondly, to predict this variation. First, a detailed comparison between the Golden Gate sand wave field and the SWC is carried out. The results of the SWC compare reasonably well with the observed sand waves when both an oscillating and a residual current are taken into account. Current velocity together with water depth, seem to be the most important factors influencing sand wave characteristics. Including physical processes shows that suspended sediment in general (1) shortens and lowers the sand waves, (2) increases the growth and migration rate, and (3) decreases the crest/trough ratio for the sand wave length and height. Smaller grain sizes, stronger currents, or more asymmetric currents, increase the quantitative suspended sediment effects. Surface waves, in general, lower the sand wave height and cause migration in the direction of the surface wave propagation. The shape of the sand wave changes to a broader crest, milder slopes and a smaller trough. Simulations with heterogeneous sediment indicate a coarsening trend towards the crest as observed in the North Sea. The heterogeneous sediment has no significant effect on the sand wave height and length.