Dynamics of sand ridges in coastal seas: the effect of storms, tides and grain sorting

The work presented in this thesis concerns the dynamics of shoreface-connected ridges and tidal sand ridges. These large-scale bedforms are observed on the inner and outer shelf of coastal seas in water depths of 10-20m. The motivation of this work is to improve the understanding of the mechanisms related to their formation and the processes that determine their main characteristics. This is done with the use of idealised morphodynamic models. The basic assumption underlying these models is that large-scale sand ridges can solely form as free instabilities on a flat sea bottom. Mathematical methods based on a stability analysis are applied, whereas analytical and numerical methods are used to solve the equations. Existing models are extended with new physical processes, in particular including the role of grain sorting. An important part of this thesis concerns the unresolved question about the origin of the observed mean grain size pattern over the ridges. It explores the hydrodynamic processes that can lead to sediment sorting and the formation of large-scale sand ridges. The model results indicate that the dynamics for different forcing conditions strongly differ. Shoreface-connected sand ridges mainly form during storm conditions, whereas if fair weather conditions prevail the more offshore located tidal sand ridges develop. A probabilistic formulation of these two realisation of the model is used to find conditions for which both types of large-scale bedforms occur simultaneously, as is the case in the southern North Sea. These conditions turn out to be a low storm fraction and the presence of both tidal and storm-driven currents. The transport of non-uniform sediment is described by formulations for both bed load and suspended load, both of which account for dynamic hiding effects. A one-layer model for the bed evolution is used and two grain size classes (fine and coarse sand) are considered. The results of the model for storm conditions indicate that the observed phase shift between bed topography and mean grain size for shoreface-connected ridges (finest sand on seaward flanks) is due to the selective transport via suspended load of grains with different sizes. Parameter values are based on the sand ridges along the Atlantic coast of North America. A net stabilising effect on the initial growth and an enhanced migration is predicted. A physical explanation for the model results is given. During fair weather or tidally dominated conditions, when bed load transport of sediment is dominant, the results indicate an increase in initial growth and migration rates of tidal sand ridges for a bimodal sediment mixture. A symmetrical tidal current results in a grain size distribution, with the coarsest sand found at the crest of the ridges. Results are compared with the tidal ridges on the Belgian coastal shelf. The investigation of the long-term evolution of shoreface-connected ridges focuses on storm-dominated micro-tidal shelves. It is shown that, starting from an initial state without bedforms, a pattern of ridges with a finite height develops. The evolution of the spatial patterns in the mean grain size and bottom topography are shown and discussed in terms of physical mechanisms