AN HYDRODYNAMIC FIELD EXPERIMENT IN THE IROISE SEA, PORSMILIN AND BLANCS SABLONS STUDY BEACHES

摘要

Two field experiments were conducted in spring 2004 on Porsmilin and Blancs Sablons macrotidal beaches in the Iroise Sea (west Brittany, France). Aims were to qualify beach agitation and understand physical processes control on beach morphology. Hydrodynamic forcing was very variable during the experiment with successive storm events (Hs ≈ 1.5 m) and post-storm calm conditions during which a recurrent low amplitude sand bar formed on spring low tide levels at Porsmilin. 2DH flow structure was mainly characterized by a strong undertow in Porsmilin surf zone during storm events while Blancs Sablons currents were highly modulated by semi-diurnal and spring/neap tidal range in the shoaling zone. Morphological change at Porsmilin is expected to be related to infragravity wave energy : (1) spring low tide level swash bar enhanced by low frequency waves in shallow water depth during low energy conditions, and (2) occurrence of ponctual stationnary long waves (0.008 hz) at low and high tide levels after storm peak that could be involved in cusp morphology.