High-frequency video observation of a geologically-constrained barred-beach: La Grande Plage de Biarritz (France)

The Basque Coast (SW France) is a major international and national tourism destination that has been facing serious erosion issues over the last decades. Among all the embayed beaches of this section of coastline, La Grande Plage de Biarritz is one of the most attractive and busy location. In this paper 2.5 years of daily time exposure images are examined to study cross-shore sandbar behavior and location preferences of rips. Results show that the surfzone sandbar behavior strikingly contrasts with that commonly observed along open beaches as a result of geological constraints (headlands, offshore emerged rocks and rocky substratum). Non-surprisingly the sandbar rapidly migrates offshore during storms and slowly migrates onshore during subsequent lower wave energy conditions. But conversely with the general observation along open beaches, here sandbar three-dimensionality increases with increasing wave energy. Three distinct types of rip channels are found along the beach: (1) a topographic rip channel against the southern headland, (2) one or two controlled rip channel(s) that are presumably forced by both alongshore gradients in wave height caused by the offshore emerged rock and the rocky substratum in the intertidal domain and (3) a transient rip channel that forms a the southern end of the beach and subsequently migrates northward to merge with a controlled rip. Our high-frequency video-observation therefore goes with more general existing studies suggesting that geological inheritance is a major factor influencing beach type and surfzone sandbar dynamics.