Multi-decadal shoreline mobility of a managed sandy tidal coast (Normandy, France): Behavioural variability in a context of sea-level rise and increasing storm intensity

A unique dataset of 27 years of shoreline changes along the western coast of Cotentin Peninsula (Normandy, France) with a high temporal resolution (about 60 fields surveys) provides an opportunity to analyse the space and time variability of a tidal sandy wave-exposed shoreline and to identify the main drivers of this variability. Spatially, as often observed around the world, this coast shows a clear balance between erosional and accretionary sectors. Maximum shoreline retreat and mean annual erosional rates are, however, greater by a factor of two than maximum and mean accretion rates, with hotspots mainly observed in the vicinity of tidal inlets. A cluster analysis highlighted nine groups of shoreline behaviour that show important contrasts, depending on locations, between erosional and accretionary trends. Linear trends over the nearly three decades of monitoring are uncommon and reverse trends are frequent. Clear divergences in behaviour were observed over the same time periods depending on location, e.g., erosion and accretion can be contemporaneous as can accretion and stability, independently of the antecedent multi-annual trends. Ebb delta zones show heterogeneous patterns, and survey locations belonging to the same clusters appear spatially scattered. The temporal shoreline behaviour can be linked to combinations of high wave energy and high spring tides only for at best 37.5% of the survey locations, and this only over the last decade. No obvious link with mean sea-level rise and the 18.6 y lunar tidal cycle was observed. However, as in the case of wave activity, these external drivers can often be masked by local factors such as inlet dynamics or/and anthropogenic influences (essentially shoreline engineering). Further insight on sand supply and beach elevation changes is necessary to improve the understanding of the main factors driving space and time variability in shoreline mobility, and it is expected that sea-level rise in the future will be of increasing concern, with important, potentially negative, implications for shoreline mobility on the low sandy sectors of the Normandy coast.