A three-dimensional model of sand bank formation

The results of a fully three-dimensional model for the generation of tidal sand banks are discussed. The model is based on the linear stability analysis of the flat sea bed configuration subject to oscillatory tidal currents. 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 and bed loads. As discussed in Besio et al. (2004), where model predictions are compared with field data, the model can reliably predict the conditions leading to the appearance of tidal sand banks. Presently, attention is focussed on the prediction of the geometrical characteristics of sand banks. While previous works on the subject always predict sand banks with crests counter-clockwise rotated with respect to the direction of the main tidal current, the present results show that sand bank crests are clockwise/counter-clockwise rotated depending on the counter-clockwise/clockwise rotation of the velocity vector induced by the tide. Only when the tidal current tends to be unidirectional, sand banks are always characterized by crests which are counter-clockwise rotated with respect to the direction of the tidal current. Moreover, no preferred direction is selected by the analysis when the tide tends to be circular. The geometrical characteristics of sand banks (wavelength and angle of rotation) are computed as function of the ratio between the minor and major axes of the tidal ellipse for both clockwise and counter-clockwise rotating tidal velocity vectors.