A New Morphodynamic Instability Associated With Cross‐Shore Transport in the Nearshore

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). Plain Language Summary Beaches sometimes exhibit sand ridges (bars) nearly perpendicular to shore that tend to be quite regularly spaced alongshore. Their spacing and cross‐shore extent range from tens to thousands of meters. Intriguingly, these bars develop preferably at beaches with an abundant supply of sand such as delta barrier beaches, barrier islands and estuaries. Here we provide a possible explanation. Due to the sand excess, the bed in these beaches is very flat, the tendency for the sand to move downslope is very weak and the waves push the sand onshore. On the other hand, waves refract, that is, their crest tip on deeper water propagates faster than the tip on shallower water. As a result, they turn toward shallower areas and, thus, the onshore movement of the sand is deflected toward incipient shoals and accumulates there. This causes more intense wave refraction, which in turn brings more sand to the shallows, and so on. In this way, bars can form out of small random irregularities in bed level. Key Points Cross‐shore sediment transport in the nearshore can be unstable in the alongshore direction The morphodynamic instability can develop only for beach profiles above the equilibrium profile This instability could explain transverse finger bar formation at beaches with sediment excess