Observations of rythmic features along the inner side of the Trabucador barrier beach are coupled to two numerical models to unravel the mechanisms of its formation. The Trabucador is a long (6 Km) narrow (125 m) barrier and microtidal beach at the SW side of the Ebro delta (Catalonia). Its inner side is a low energy beach with a sandy shallow terrace featuring an intricate alongshore rhythmic morphology. Sixteen aerial orthophotos from 1946 to 2014 have been analyzed and complemented with field observations from 1986 to present. This morphology is dynamic but it is usually characterized by: a) long finger transverse bars (LFTB) and b) large scale shoreline undulations (LSSU). The LFTB are thin and elongated with a length of the order of their spacing. They are intertidal and typically attach to the shoreline by a megacusp, commonly opening an anti-clockwise angle of 10°–40° with the shore normal. There can be many, up to 90, with both the mean and the most frequent alongshore spacing in the range 15–25 m. Spectral analysis always shows peaks in this range and sometimes additional peaks in the range 30–65 m that correspond to the spacing between the largest bars with smaller bars in between. The LSSU typically have wavelengths in the range 150–250 m. Their apexes sometimes coincide with the shore attachment of the largest bars but not always. Numerical modelling shows that both features could emerge out of feedbacks between hydrodynamics and morphology during the SW wind events involving a) deflection of the longshore current by the bars combined with the refractive wave focusing and b) gradients in total alongshore sediment transport rate triggering the high-angle wave instability.

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