A morphodynamic mechanism for transverse bars in the nearshore

Abstract

Sand bars which are normal to the coastline have been described on sandy beaches Irom low to moderate wave energy (Niedoroda and Tanner, 1979; Konicki and Holman, 2000). A generating mechanism lor such bars is here proposed that is based upon the coupling between water motions and the evolving topography in case 01 normal wave incidence. 6asically, the momentum carried by the incident waves is locused by the bars because 01 topographic relraction. This causes a strong onshore current on the bars and return Ilow 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 erosiono A positive leedback is thus created between circulation and morphology and this explains the growth 01 the bars. A nonlinear numerical model 01 sur! zone morphodynamics has been set up to show the growth 01 sueh bars Irom a number 01 dillerent small amplitude initial perturbations 01 the alongshore unilorm equilibrium.