Morphodynamics of slightly oblique nearshore bars and their relationship with the cycle of net offshore migration

Abstract While slightly oblique nearshore bars seem to be present on most barred beaches worldwide, to date they have been poorly studied due to their great longshore extension that requires large spatial scale data sets. Here, we give a detailed report of the morphology and multi-annual dynamics of these bars using a large set of data (LiDAR, bathymetric profiles and aerial photographs) on two barred beaches of the Gulf of Lions (Mediterranean Sea, France). Slightly oblique bars are characterized by a down-drift longshore gradual increase of their distance from the shoreline over tens of kilometres, forming an angle of 3–4° relative to the shore. Thus, at any time, the bar is in a different phase of the net offshore migration (NOM) cycle according to its longshore extent. Alongshore, a single bar is in the emergence phase of the cycle at its proximal end near the coast, in the phase of seaward migration where it exhibits transition/switching state from on- to offshore position and finally in the decay phase at its distal end where its disappears. On a multi-annual timescale, net offshore migration (NOM) of slightly oblique bars results in an up-drift displacement of the bar pattern, with rates that can reach several metres per day. Thus, a longshore time lag appears in the NOM cycle when the bar is observed on separate cross-shore bathymetric profiles. A global worldwide inventory based on the literature shows that these bars are common to many barred beaches where NOM cycles have been identified. In almost all case, there is a gradual down-drift increase of distance of the bar from the shoreline. However, inter-site comparison of environmental parameters does not allow us to determine the factors controlling the formation of slightly oblique bars. The longshore displacement of the bars is directly related to the cross-shore dynamics. This study emphasizes the important role of slightly oblique bars in nearshore morphodynamics and the need to improve our knowledge of these features.

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