Characterization of Sea-level Variations Along the Metropolitan Coasts of France: Waves, Tides, Storm Surges and Long-term Changes

Dodet, G.; Bertin, X.; Bouchette, F.; Gravelle, M.; Testut, L., and Wöppelmann, G., 2019. Characterization of sea-level variations along the metropolitan coasts of France: Waves, tides, storm surges and long-term changes. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 10–24. Coconut Creek (Florida), ISSN 0749-0208.With 5853 km of coastlines facing the North Sea, the English Channel, the Atlantic Ocean and the Mediterranean Sea, France displays littoral zones exposed to a wide spectrum of wave climates, tidal ranges and storm surges. This study aims at characterizing in a systematic way sea level variations along the coasts of France. Wave climates are first characterized using state-of-the-art high resolution hindcasts validated against available observations from the national network CANDHIS. This analysis reveals a contrasting situation between coastal zones located along the Bay of Biscay, exposed to energetic and moderate-to-long period swells, and coastal zones located in the English Channel and the Mediterranean Sea, exposed mostly to wind seas and shorter period swells. Tides are characterized using available tide gauge measurements retrieved from the national REFMAR database and display large contrasts, between microtidal regimes along the Mediterranean Sea and macrotidal regimes along the English Channel. The analysis of storm surges computed from the same data reveals that they are controlled not only by storm tracks but also by the width of the continental shelf. Thus, during the studied period 1998-2018, storm surges derived from tide gauges measurements hardly reach 1.0 m along the coastlines of the southern Bay of Biscay and the eastern Mediterranean Sea but can exceed 2.0 m in the English Channel. The analysis of long-term sea level trends reveals significant variabilities, with an accelerated sea level rise on average from 1.2 mm/yr over the 20th century to 2.4 mm/yr over the two last decades, and ranging locally from 1.23 mm/yr (Roscoff) to 4.25 mm/yr (Nice). Finally, these values are compared against vertical land motions, computed from a state-of-the-art GPS reanalysis. A moderate land subsidence (<1 mm/year) is found for the majority of the stations.