Abstract:
This study aims to hindcast and analyze the storm surge associated with Xynthia, a mid-latitude depression
that severely hit the French central part of the Bay of Biscay on the 27–28th of February 2010. The
main losses in human lives and damages were caused by the associated storm surge, which locally
exceeded 1.5 m and peaked at the same time as a high spring tide, causing the flooding of low-lying
coasts. A new storm surge modeling system was developed, based on the unstructured-grid circulation
model SELFE and the spectral wave model WaveWatchIII. The modeling system was implemented over
the North-East Atlantic Ocean and resulted in tidal and wave predictions with errors of the order of 3%
and 15%, respectively. The storm surge associated with Xynthia was also well predicted along the Bay
of Biscay, with only a slight underestimation of the surge peak by 3–8%. Numerical experiments were
then performed to analyze the physical processes controlling the development of the storm surge and
revealed firstly that the wind caused most of the water level anomaly through an Ekman setup process.
The comparison between a wave-dependant and a quadratic parameterization to compute wind stress
showed that the storm surge was strongly amplified by the presence of steep and young wind-waves,
related to their rapid development in the restricted fetch of the Bay of Biscay. In the central part of the
Bay of Biscay, both observed and predicted water level anomalies at landfall displayed 6 h oscillations,
with amplitudes of up to 0.2 m (10–20% of the surge peak). An analytical shelf resonance model and
numerical experiments demonstrated that the period of the observed oscillations corresponds to the resonant
mode of the continental shelf in the central part of the Bay of Biscay. It is concluded that these
oscillations originate from the interactions between the water level perturbation and the continental
shelf and this phenomenon is expected to be relevant at other places along the world’s coastlines.