Morphological stability modelsShoreface-connected sand ridges

Resumen

In this thesis the generation and maintenance of shoreface-connected sand ridges has been investigated. These are elongated sedimentary structures with an alongshore periodicity. Such ridges are found, for example, in the Central Dutch coast or in the Atlantic coast of the Florida peninsula of the USA. They are found in water depths between 5 and 30m, with an alongshore spacing between 5 and 7 km, amplitude between 2 and 4 m and a length of approximately 10km and are found in a gently slopping bottom. The ridges have an upcurrent rotation with respect the dominant storm current and migrate downstream. Models for similar structures, the model of Trowbridge of 1995, together with several observations, suggest that shoreface-connected sand ridges have been develop as a free instability of the morphodynamic system form by the hydrodynamic processes and an erodible bottom. This is the main hypothesis of this thesis. For this propose the stability of a flat slopping bottom in the direction perpendicular to the coast and a uniform current parallel to the coast has been analysed. The basic current in forced by wind stresses, alongshore pressure gradients and the tides. The flow is derived by the depth-integrated shallow water equations, averaged in short time (waves) and length scales (bottom ripples). The sediment transport is proportional to the local velocities and takes in to account slope effects. Stability analysis shows that shoreface-connected ridges can be form on regions where storm currents are dominant. Moreover, sediment transport should be dominated by the waves. For this conditions a nonlinear models has been developed to study the long-term behaviour of the ridges. The main result is that the modelled ridges are similar in shape and amplitude to the observed ones.