Effects of natural and human forcing on mesoscale shoreline dynamics of Saint-Michel-en-Grève bay (Brittany, France)

By monitoring shoreline changes and subaerial beach volume in Saint-Michel-en-Grève bay in northern Brittany over the past 20 years, the mesoscale sediment budget was assessed. Previous work has shown that both natural and human forcing have played an important role in the long-term changes of the bay, but the respective causes of each type of forcing have not previously been studied in detail. This study shows that between 1990 and 2009, a large volume of sediment input resulted in rapid accretion of dunes in the bay head. Sedimentation rates reached an average of 0.1m3/m2/yr over the whole study period. This positive sediment budget confirms the trend of sediment accumulation noted since the end of the 19th century. Nevertheless, temporal variation in the sediment budget can be related to changes in the frequency and magnitude of storms surges and resulting high tides. Shoreline erosion/accretion records dating from 1990 were compared with extreme water levels. Relatively high shoreline retreat rates during periods of 1999-2002 and 2006-2008 were associated with a relatively large number of storm surges. Slower erosion and relatively rapid accretion occurred between 1990 and 1999 and during a more recent period, when there have been relatively few major storm surges. Erosion phases were related to the occurrence of extreme spring high tides and wave action associated with storms. Storms and surges in the northern Brittany Channel are associated with Atlantic depressions whose direction and rate of movement have a strong influence on wind speeds, wave energy and the height of tides. However, sediment budget of Saint-Michel-en-Grève bay head beach, in terms of shoreline erosion/accretion changes, is not directly related to the North Atlantic Oscillation index. Tide condition leading to high or low water level plays a major role by controlling erosion or accretion processes. Human forcing was identified as feedback processes due to the modification of morphological and hydrodynamic conditions. These modifications occurred after coastal engineering works had been carried out on the foreshore to eliminate the confluence of two rivers. West-east longshore sediment transport induced the erosion of a dune of Grand Rocher, while the removed sand contributed to the growth of a dune of Tréduder.