Pocket beach hydrodynamics : the example of four macrotidal beaches, Brittany, France
Résumé
During several field experiments, measurements of waves and currents as well as topographic surveys were conducted on four morphologically-contrasted macrotidal beaches along the rocky Iroise coastline in Brittany (France). These datasets provide new insight on the hydrodynamics of pocket beaches, which are rather poorly documented compared to wide and open beaches. The results notably highlight a cross-shore gradient in the magnitude of tidal currents which are relatively strong offshore of the beaches but are insignificant inshore. Despite the macrotidal setting, the hydrodynamics of these beaches are thus totally wave-driven in the intertidal zone. The crucial role of wind forcing is emphasized for both moderately and highly protected beaches, as this mechanism drives mean currents two to three times stronger than those due to more energetic swells when winds blow nearly parallel to the shoreline. Moreover, the mean alongshore current appears to be essentially wind-driven, wind waves being superimposed on shore-normal oceanic swells during storms, and variations in their magnitude being coherent with those of the wind direction. We also found evidence for the contribution of infragravity waves to the formation of an intertidal sand bar (through prolonged duration of swash processes at the same beach level) and to the development of beach cusps (but not to their initiation), for which the observed standing edge wave pattern may have been enhanced by the enclosed pocket-beach setting. The paper highlights some of the specificities of pocket beach with regards to their hydrodynamics and morphodynamics. More detailed observations would be needed, however, prior to a more comprehensive review.