| Summary: | From 2002 to 2009, a morphosedimentary survey of trailing accumulations forming comet tails was conducted on Trielen and Lez ar Chrizienn Islands in the archipelago of Molène (Brittany, France). In addition, an analysis of hydrodynamics, based on tidal and wave data obtained from modelling and field measurements, was carried out. These trailing beaches are no longer actively supplied with sediments from the nearshore platform. Therefore, their morphosedimentary evolution consists in a reworking of the current stock mainly dependent on hydrodynamic conditions. When storm waves occur in conjunction with high spring tides, sediment is transported from east to west. Offshore waves diffracted when passing Ouessant Island take a meridian direction and attack the north-eastern tip of the comet tails. Sediment transport in the direction of western incident waves occurs only during periods of low wave energy. From 2002 to 2009, for both accumulations, the net longshore transport of sediment has been heading in the opposite direction to the one that prevailed during their construction. This type of morphosedimentary dynamics corresponds to a process of “cannibalisation” peculiar to all trailing beaches that are no longer supplied with sediments. Analysis of hydrodynamic conditions shows a lack of relationship between the morphosedimentary changes and the North Atlantic Oscillation. The conjunction of storms and high spring tides seems to be much more relevant. Entre 2002 et 2009, un suivi morphosédimentaire des formes fuyantes en queue de comètes a été réalisé sur les deux îlots de Trielen et de Lez ar Chrizienn dans l’archipel de Molène (Bretagne, France). Dans le même temps, l’analyse des conditions hydrodynamiques, basée sur l’acquisition des données de houle par modélisation et à partir de mesures in situ, et des données marégraphiques a été effectuée. Ces formes fuyantes ne sont plus alimentées ceci s’explique par le tarissement du stock sédimentaire disponible accumulé sur la plateforme continentale. L’évolution ...
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