Cuspate shoreline relationship with nearshore bar dynamics during storm events - field observations at Sete beach, France.
Résumé
Cuspate shoreline or megacusps, having alongshore lengths of 100-1000 m, are features widely described, and several theories were proposed for their formation and evolution (edge waves, self-organization). Their dynamics is often related with rip-currents migration, crescentic nearshore bars evolution, even if the relationship between shoreline rhythms and inner-bar pattern appears to be extremely variable. An Argus video monitoring system was deployed in 2011 at Sète beach (French Mediterranean). This microtidal wave-dominated environment is characterized by the presence of a double crescentic nearshore bar and a cuspate shoreline with a 400 m wavelength. Wave climate is moderate at this site and most of the significant morphological evolution is observed during storm events. Monitoring of bar and shoreline evolution during two particularly energetic periods (autumn 2011 and spring 2012) permitted to evidence very different behavior in the coupling between bar and shoreline rhythms. Usually, a phase coupling is observed between bar shoals and a seaward bulge in the shoreline. However, during and just after an event, evolution and its timescale is variable for both morphologies, resulting in an apparent out-of-phase relationship. A storm group in autumn yields an important migration of the crescentic bar (200 m /day), due to an oblique wave incidence. However, shoreline cusps remained stable and migrated progressively during the storm fall. It took more than 10 days for the shoreline oscillations to recover a phased position with the bar shoals. In some occurrences, the bar displacement was rapidly reversed by a new event with opposite wave direction, and no shoreline migration was observed, probably because time was not long enough to observed a significant displacement. During the spring's storms, less energetic, a lower bar migration was observed (around 50m), and a very small shoreline movement is seen, mostly due to the erosion of the flank facing incident waves. After this event, wave conditions decreased rapidly preventing any morphological evolution of both bar and shoreline. These observations indicate that the coupling between crescentic nearshore bars and shoreline rhythms is time-dependent, and a given period with significant energy on the falling storm is needed to recover a phased position of both morphologies.