Dynamics of the rhythmic morphology on the low-energy Trabucador beach (Ebro delta)

Abstract

Observations of rhythmic morphology along the Trabucador beach are contrasted with two numerical models to unravel the mechanisms of their formation. The Trabucador is a long narrow barrier at the SW side of the Ebro delta (Catalonia). Its inner side is a microtidal low energy beach with a sandy shallow terrace featuring an intricate rhythmic morphology. Sixteen aerial orthophotos from 1946 to 2014 have been analyzed and complemented with field observations from 1986 to present. The morphology is dynamic and it is usually characterized by: a) long transverse finger bars (LTFB) and b) large scale shoreline undulations (LSSU). The LTFB are thin and elongated, commonly opening an anticlockwise angle of 10°–40° with the shore normal. Their alongshore spacing is in the range 15 - 25 m, with a secondary wavelength in the range 30 - 65 m. The LSSU typically have wavelengths in the range 150 - 250 m. 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.