A new morphodynamic instability associated with cross-shore transport in the nearshore

Abstract

The existing theory for shore-transverse rhythmic sand bars relies on morphodynamic instabilities involving the wave-driven longshore current and rip currents. Intriguingly, transverse finger bars are common on coasts with sediment excess, something not related to those currents. Here we show that, if the actual beach profile is above the equilibrium profile, cross-shore transport can induce an instability triggered by onshore transport together with wave refraction by the emerging bars. We use a numerical model that filters out the dynamics associated to longshore and rip currents but includes a simplified version of cross-shore transport and is able to reproduce the formation of shore-transverse bars. The alongshore spacing scales with the wavelength of the incident waves and the cross-shore extent is approximately equal to the distance from shore to the depth of closure. The modeled bars compare qualitatively well with observations at El Trabucador back-barrier beach (Ebro delta, Western Mediterranean Sea).