Morphological feedback between nearshore bars and shoreline dynamics during storm events – video observations at Sète beach, France

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 years (from March 2011 to March 2013) 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. Most of the time, both morphologies are in phase and their evolution on the medium-term is very comparable, and consists in a Net Longshore Migration, quite similar to the well-known Net Offshore Migration. The cusps migrate alongshore, followed by the seaward bulges in the shoreline, and when reaching a certain position, a new cusp is formed downstream and reinitiates the cycle. However, during oblique storms, the cuspate inner bar migrates very rapidly (reaching 200 m in a few hours), while the shoreline oscillations remain quite stable or show low migration due to the erosion of the flank facing incident waves. This results in an out-of-phase position of cusp and shoreline bulges. During the falling storm conditions and just after, the evolution seems to be driven mostly by the remaining wave energy. If the significant wave height decreases to rapidly, the morphologies are “frozen” and remain out-of-phase. If the wave climate remains moderate after an event, keeping the same incidence, a longshore migration of the shoreline is observed, and the seaward bulges recover a phased position with the bar horns after 10 to 20 days. These observations indicate that the coupling between crescentic nearshore bars and shoreline rhythms is timedependent, and a given period with significant energy on the falling storm is needed to recover a phased position of both morphologies.

Y. Balouin, M. Giusti, J. Tesson, M. Gervais - Coastal Dynamics