Extreme sea levels on the Atlantic coastline in France: model and observations

By Etienne CHAUVEAU

L’Atlas Bleu / Exploring

Extreme sea levels, models and measurements, recent period, Atlantic coast of mainland France.

The article identifies the 10 most important storms in terms of marine submersion that have occurred on the Atlantic coasts of mainland France since 1984. It indicates the maximum sea levels recorded during these events. These values are compared with the extreme sea levels (with a 100 years period) derived from the SHOM/CETMEF predictive model. Several observations are drawn from this comparison.

carte, synthèse, NME, côte atlantique

As part of a preliminary study on the devastation caused by storm Xynthia, researchers assessed recurring incidences of flooding on the Atlantic coastline of France from the mid 1980s onwards (Feuillet et al., 2012). By taking into account the convergence of high tidal coefficients, very strong gusts of wind, and instances of high water occurring more or less synchronously with these gusts, ten events likely to have caused considerable flooding were identified.

In this study, we cross-referenced those results with extreme sea levels (100-year return period) and effective measures published by SHOM (the French Navy’s hydrography and oceanography department), to consider these flooding incidents in relation to these sea levels.

A number of points become clear from the graphs:

1) only five episodes resulted in sea levels bordering on extreme: in November 1984, October 1987, March 2008, February 2010 and February 2014;

2) in Brittany, the most significant episode was storm Johanna;

3) on the coastline between and including Saint-Nazaire and Arcachon Bay, storm Xynthia stands out everywhere as the event that generated the highest water levels, exceeding the mark for extreme sea levels everywhere between the Loire and Gironde estuaries;

4) in La Rochelle, the difference between the extreme sea level mark and the level reached during storm Xynthia is the most significant, at 65 cm, making it a noteworthy exception on the entire Atlantic coast;

5) the extreme water levels of 1 February 2014 occurred in moderate tidal surge conditions at high water (+ 40 cm), but at a tidal coefficient of 113.

Bibliography

FEUILLET T., CHAUVEAU E., POURINET L., 2012. Xynthia est-elle exceptionnelle ? Réflexions sur l’évolution et les temps de retour des tempêtes, des marées de tempête, et des risques de surcotes associés sur la façade atlantique française, Norois n°222. (2012/1), p.17-44

Etienne CHAUVEAU, géographe, Maître de conférences à l’IGARUN

Etienne CHAUVEAU, « Niveaux marins extrêmes (NME) sur les côtes atlantiques métropolitaines : modèle et observations », L’atlas Bleu, Revue cartographique des mers et des littoraux. Mis en ligne le 11 janvier 2020,

(version digitale adaptée d’après l’article paru dans L’Atlas Permanent de la Mer et du Littoral n°7 « Risques littoraux et maritimes ». Ed. LETG-Nantes, 2015. p.25)

URL : https://atlas-bleu.cnrs.fr/

DOI : 10.35109/atlasbleu-eng.1002