Abstract

Beach cusps are sometimes present on the upper foreshore of sandy beaches along the northwest coast of France characterised by very large tidal ranges. These forms were observed on the high tidal zone of Luc-sur-Mer in the "baie de Seine" during morphodynamic surveys conducted between May and July 1996. Topographic surveys showed these cusps to be depositional features. The profile and incident wave characteristics do not favour the theory of cusp formation based on the presence of subharmonic edge waves, while the spacing between the horns is in disagreement with the theory of "self-organisation". The wave record on the lower beach shows, at the time of their appearance, two incident wave trains each with a specific direction and period. The observed beach cusps appear to be induced by the intersection of these two wave trains, thus confirming, for the first time in the field observations, experiments conducted in laboratory.

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