Flow and sediment transport induced by tide propagation: 2. The wavy bottom case

A fully three-dimensional model for the generation and evolution of tidal sand waves and sand banks from bottom perturbations of a flat seabed subject to tidal currents is proposed. The basic flow, comprehensive of Coriolis effects, is completely resolved in the vertical direction from the sea bed up to the free surface and describes both clockwise and counter-clockwise rotating tidal ellipses. The flow regime is assumed to be turbulent and a Boussinesq's approach is adopted to model Reynolds stresses. The eddy viscosity depends on the distance from the bed and an accurate description of the flow close to the sea bed, where sediment motion is mainly confined, is obtained. Sediment transport is modelled in terms of both suspended load and bed load. The model is capable of predicting the conditions leading to the appearence of both tidal sand waves and tidal sand banks and to determine their main geometrical characteristics. As shown by previous works on the subject, the sand wave crests turn out to be orthogonal to the direction of the main tidal current and the sand bank crests are almost aligned with the main tidal current forming a small angle. While previous works always predict a counter-clockwise rotation, present results show that sand banks crests are clockwise or counter-clockwise rotated in comparison with the direction of the tidal current depending on the tide rotation. The predictions of the model are supported by comparing them with field data.