A Morphodynamic Mechanism for Transverse Bars in the Nearshore

Sand bars which are normal to the coastline have been described on sandy beaches from low to moderate wave energy. A generating mechanism for such bars is here proposed that is based upon the coupling between water motions and the evolving topography in case of normal wave incidence. Basically, the momentum carried by the incident waves is focused by the bars because of topographic refraction. This causes a strong onshore current on the bars and return flow at the troughs in between. Since wave stirring is typically decreasing shoreward, the onshore current over the bars produces accretion and the offshore current at the troughs produces erosion. A positive feedback is thus created between circulation and morphology and this explains the growth of the bars. A nonlinear numerical model of surf zone morphodynamics has been set up to show the growth of such bars from a number of different small amplitude initial perturbations of the alongshore uniform equilibrium.