Four representative sectors encapsulating the morphological and hydrodynamic diversity of the French coast are used to illustrate the morphodynamics of beach/dune systems at various timescales. These beach/dune systems cover the four shoreline sectors composing the French coast, i.e., the southern North Sea, the English Channel, the Atlantic Ocean, and the Mediterranean, and range from storm-dominated short-fetch to swell settings, and from micro- to macrotidal. The short-term processes affecting the beach/dune systems are strongly hinged on wind incidence and storm intensity, while sediment supply, modulated by marine processes, notably storms and tides, and by the regional wind directions, constitutes the primary control on long-term patterns of development. Beach morphology, notably the presence of bar/berm-trough systems, and water levels attained during storms, constitute key elements in the process of sand supply to the dune systems, which vary from low, poorly developed forms in the Mediterranean, to the well developed, but largely presently stable to erosive systems characterising the other three sectors. The comparative study of these systems shows that there is no linear relationship between storm intensity and the rate of dune erosion (except on Vougot site, English Channel), because of the influence of various parameters, notably the antecedent beach state.

In this paper, we examine the development of an extensive hummocky incipient foredune field of approximately 100,000 m² that developed along a short stretch of coastline on the macrotidal coast of northern France. Analysis of historical maps and documents, of a series of aerial photographs as well as recent LiDAR and topographic data allowed us to depict the formation and evolution of this incipient foredune field. Our results show that this coastline stretch is prograding seaward since at least the 16^th century, this shoreline advance being related to the onshore welding of subtidal sand banks. During the early 20^th century, isolated sand islets, surrounded by the sea at high tide, formed at the landward edge of a wide convex-shape sand flat. They progressively merged to the main shoreline during the mid-20^th century, inducing a shoreline advance of up to 300 m between 1938 and 2012. Once merged to the main shoreline in the 1960s, a field of incipient foredunes, facing the dominant winds began to develop, inducing a shoreline progradation at a rate of 5 m.y^-¹ that resulted in a seaward widening of the hummocky dunefield. Although the field of incipient foredunes continued to expand since its initiation, the morphology of individual mounds, colonized by a sparse vegetation cover, tended to remain remarkably stable through time. In this area of high sediment supply, the formation of a hummocky dunefield is probably due to the fact that the incipient foredune do not have enough time to coalesce and to merge into a continuous foredune ridge as they rapidly become disconnected from their main sediment source.

Tidal sand banks are common along the coast of northern France facing the North Sea, where they form linear shore-parallel or slightly oblique sand bodies from shallow coastal areas to depths of tens of meters. Hydrographic surveys have been carried out since the 1830s for mapping the seabed of the coastal zone. An analysis of the bathymetry evolution shows significant morphological changes have occurred across the shoreface since the early 19th century, largely due to cross-shore and longshore sand bank migration. Our results show that nearshore sand banks mainly migrated onshore and gained sediment, especially during the 20th century; acting as temporary sediment sinks, which can in turn serve as sand sources for providing sediment to the coast. Alongshore, the migration and elongation of sand banks can be related to tidal asymmetry that is mostly directed to the east-north-east in the region. Shore-perpendicular movement can likely be explained by the action of shore-normal storm-waves in the nearshore zone after their refraction over shallow offshore sand banks. A seaward displacement of sand banks was also observed. This may be related to the combined action of waves and tidal currents which can induce erosion on one side of the bank, decreasing its width, and eventually leading to its seaward migration. Our observations point out that some nearshore sand banks respond to the action of currents and waves, and interact between each other via feedback morphodynamic processes induced by sand bank morphological changes. The substantial morphologic changes that affected the nearshore zone of northern France during the last centuries probably had large impacts on coastal hydrodynamics and associated shoreline evolution.

Crapoulet, A., Héquette, A., Levoy, F. and Bretel, P., 2016. Using LiDAR topographic data for identifying coastal areas of northern France vulnerable to sea-level rise. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1067 - 1071. Coconut Creek (Florida), ISSN 0749-0208. A major portion of the coast of northern France consists of wide macrotidal beaches and coastal dunes protecting low-lying backshore areas (mostly reclaimed lands) from marine flooding. Although the shoreline was stable or even prograded seaward in places during the last decades, several coastal areas underwent severe erosion during the same period, while flooding sporadically occurred locally during major storms. A study of the potential impacts of sea-level rise on the coast of northern France was conducted based on airborne LiDAR topographic data collected from 2008 to ...

Le littoral du Nord-Pas-de-Calais consiste en grande partie en de larges plages sableuses et en dunes côtières, localement interrompues par des estuaires, des agglomérations côtières et quelques falaises rocheuses. Comme toutes les côtes basses, les littoraux meubles du Nord-Pas-de-Calais sont sujets à des phénomènes d’érosion et de submersion marine, certains secteurs de dunes côtières étant caractérisés par un recul du front dunaire depuis plusieurs décennies. Les dunes littorales constituent souvent un rempart naturel protégeant des zones de polders de la plaine côtière des attaques de la mer et leur érosion ne fait qu’augmenter les risques de submersion marine pendant les tempêtes. La zone côtière du Nord-Pas-de-Calais étant densément peuplée, le recul du trait de côte et les submersions de tempête représentent une menace pour les riverains dans plusieurs secteurs de ce littoral. Le caractère macrotidal (marnage > 4 m) du littoral du Nord-Pas-de-Calais résulte toutefois en une atténuation des effets des tempêtes, car les fluctuations du niveau d’eau dues à la marée limitent le temps pendant lequel les phénomènes d’érosion et de submersion marine peuvent se produire. L’élévation récente du niveau de la mer dans la région suggère cependant que les phénomènes d’érosion littorale et de submersion marine risquent d’augmenter pendant les prochaines décennies.