Coastal flooding and dune breaching in the central part of the Bay of Biscay

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Xavier Bertin, Gael Arnaud

Chair(s): dr. Makropoulos

Tuesday 30 june 2015

16:00 - 16:15h at Central America (level 0)

Themes: (T) Special session, (ST) Coasts at threat in Europe

Parallel session: 7H. Special session: Coasts at threat in Europe

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European shorelines are exposed to extra-tropical storms that can induce large surges, rough sea states and significant material damages and human losses. This study presents a synthesis of results obtained in our Lab over the last 5 years in the scope of several research projects, including the ongoing FP7 project RISCKIT. Our research was initially motivated by the catastrophe associated with Xynthia, a mid-latitude storm that severely hit the central part of the Bay of Biscay in February 2010. Despite displaying wind fields classical for a winter storm, Xynthia induced an exceptional surge that peaked at a high spring tide and induced the flooding of extensive low-lying coastal zones including inhabited areas. The unusual storm surge was explained by the presence of a particular sea-state, which strongly increased the wind effect. The analysis of a numerical hindcast of the flooding associated with Xynthia revealed that massive flooding can lower water levels seaward compared to a situation where the flooding is prevented. This phenomenon has serious implications for coastal management and engineering practices. Historical archives combined with numerical modeling revealed that, over the last century, 5 events led to coastal flooding comparable to that induced by Xynthia. Our study showed that accounting for these forgotten storms would strongly impact the determination of the return period for extreme water levels and surges, which presently rely on incomplete tide gauge data. Finally, the winter 2013-2014 was characterized by an exceptionally rough sea-state in the Bay of Biscay, which led to severe dune erosion and several washover developments. Field measurements carried out at La Faute-sur-Mer during one of these events suggest that infragravity waves are the dominant process in dune breaching. A numerical hindcast of the washover development is being performed and will allow for a better understanding of the mechanisms controlling such phenomenon.