Continental Shelf Ridges

In the sandy bottom of coastal seas large-scale rhythmic bed forms are usually observed. Shoreface-connected sand ridges and tidal sand banks are the largest ones with spacing in the order of kilometres and heights of several meters. The shoreface-connected ridges have been observed in many the inner shelves, e.g. eastern American continental shelf, Argentinean shelf and central Dutch coast. In these areas significant net longshore currents driven by wind, tides and longshore pressure gradients are present. The shoreface-connected sand ridges are upcurrent rotated forming an angle of 10 to 30 degrees with respect their shoreface attachment, migrates downstream with a characteristic velocity of 1-10 meters per year. The spacing between successive crests is in the range of 2-10 km, the height varies from 1 to 6 m and the length of individual crests is between 10 and 20 km. Observations suggest that shoreface-connected ridges started to form several thousands years ago and that they are active under the present hydrodynamics conditions. The tidal sand banks forms on the continental shelf where tidal currents are stronger than 0.5 m per second, as the midshelf of the North Sea. The general characteristics are similar to the ones of the shoreface-connected sand ridges, with the exception that the tidal sand banks do not migrate and that they are cyclonically rotated (in the Northern Hemisphere) with respect the dominant tidal currents.

In order to gain fundamental understanding on the basic physical mechanisms involved in the initial formation, develop and long term evolution, together with other general characteristics of these bedforms, numerical morphodynamic models have been developed. The models are based on two-dimensional shallow water equations supplemented with a bottom evolution equation and parameterisations for the sediment transport. In the model, the underlying hypothesis is that these large scale bedforms are formed as an inherent free inability of the coupled system form by the currents and a erodible bottom. The initial formation of these features has been investigated with linear stability analysis. Several aspects have been considered: storm conditions, tidal currents (M0, M2, M4), suspended and bedload transports, and sorting effects. To model the long-term evolution a non-linear model was developed to address aspects like the saturation times, saturation processes and to study the effect of large-scale human interventions.