Secular evolution of sandy coasts of Normandy (NW France)

In the present context of climate change and rising sea level, sandy coasts are particularly vulnerable environments. Many studies around the world deal with the problem of coastal erosion of sandy beaches, but few still consider their evolution at the scale of the entire sediment coastal wedge (e.g. Certain et al., Mar. Petrol. Geol., 2005; Denny et al., Cont. Shelf. Res., 2013; Schwab et al., J. Coastal Res., 2013), i.e. taking into account the volume and dynamics of sand accumulated in the shoreface, which can potentially supply the foreshore. We are developing this global approach at the regional scale of the Normandy coastline in order to better understand the sandy beach behaviour since the 19th century and evaluate their stability.

The Normandy coasts show a high diversity in relation with the variability in the geological substratum, tidal range and currents, wave energy and incidence, and sediment nature. Our study aims at providing knowledge on the global behaviour of this composite coastal system at the scale of the whole coastal sedimentary wedge. The study is based on the quantification of sediment volumes accumulated in the subtidal (shoreface) domain. The mobility of the stocks as well as the evolution of the adjacent coastlines are also estimated over the last centuries. Longshore and cross-shore sediment transfers are studied. The purpose is to define the relationships between subtidal and intertidal domains and to discuss the synchronism/diachronism of observed evolutions at a regional scale.

A consistent database, processed and integrated under GIS, has been compiled, including old and recent maps, aerial photographs, geophysical data (seismic, sonar, multibeam echosounder, LiDAR) and sediments samples. We acquired new seismic and multibeam data in sectors that had not been already investigated.

The results obtained from the comparison of the numerous historical cartographic documents since the 18th century, allow illustrating the movements of the coasts including their progressive management. Due to the low accuracy of old charts, only high amplitude changes are identified. Seismic data enable to characterize the spatial distribution of the sediment cover thickness over the geological substratum. In the study area, the sedimentary cover is related to the last post-glacial transgression (Holocene) and may comprise several depositional units. The most basal unit corresponds to the infill of paleo-valleys. It is overlain by one to two units forming the sediment reliefs, such as banks or dunes fields or sand sheets, and representing the sediment stocks we quantify. The mobility and dynamics of the stocks are monitored over the two last centuries from historical bathymetric data. Significant differences in volumes are evidenced locally. Bedform morphology and size enable specifying sediment transport direction and intensities.

The first results show direct relationships between the behaviour of subtidal and intertidal domains, allowing to better understand the distribution of stability, erosion or accretion areas.