Spatial variability of extreme wave height along the Atlantic and channel French coast

The knowledge of wave climate, and more particularly of the extremes and historical large wave events, is crucial for offshore infrastructure design as well as coastal applications such as defences design or submersion and erosion risks assessment. When it comes to analysing the spatial variability of extremes, a key issue is to ensure a uniform approach to get spatially comparable results. The present paper describes a new wave extreme values database for the French Atlantic and Channel coasts (BoBWA-X) relying on: (1) the wave hindcast BoBWA-10 kH (1958-2002; Charles et al., 2012. J. Clim. 25 (6), 2020-2039. doi:10.1175/JCLI-D-11-00086.1); (2) a POT/GPD method adapted to reduce the operator subjectivity in the threshold choice so as to ensure reproducible and comparable results along the coasts. The obtained extreme wave heights of 43 points distributed along the coast, exhibit a significant spatial variability delimiting 4 relatively homogenous areas, with 100-year return wave heights ranging between 3 m (East Cotentin) and 16 m (Western Brittany). These spatial distributions are analyzed in terms of spatial variability of the statistical parameters, using a depth-independent analysis and 7 quite homogeneous coastal segments are identified. The delimited segments are directly related to the wave climate and the exposure to classical storm waves. Therefore, they show similar repartition frontiers with the delimited areas by the Hs100 spatial variations but with a higher degree of precision. The analysis of past events over the 1958-2002 period of the BoBWA-10 kH dataset shows 7 events characterized by wave heights with return periods larger than 50 years. The extent and intensity of these events vary greatly from one zone to another. For instance, the 1979 event affected 950 km of coast. Brittany is a particularly exposed region, with two events (1958, 1990) whose Hs return period (Rp(Hs)) ranges between 70 and 100 years. The highest return period is detected in the Dover Strait area (Rp(Hs)=107 years) during the Daria storm (January 25th 1990). The spatial variability of these large wave events is discussed regarding the atmospheric conditions and their similarities with classical weather types. Both databases (BoBWA-10 kH and BoBWA-X) are available at http://bobwa.brgm.fr.