A modeling-based analysis of the flooding associated with Xynthia, central Bay of Biscay

• High resolution hindcast of sea-state and water levels associated with storm Xynthia.
• Realistic reproduction of flooding extension.
• Massive flooding can limit water levels seaward.

Storm-induced coastal flooding is among the most destructive natural disasters, as seen recently in the Bay of Bengal, the Gulf of Mexico and the Philippines. This study presents a high resolution hindcast of the flooding associated with Xynthia, a mid-latitude storm that severely hit the central part of the Bay of Biscay in February 2010. A 2DH fully coupled modeling system is applied to the North-East Atlantic Ocean, with a resolution locally reaching a few meters along the coastline of the study area. Such a fine resolution was required to adequately represent the dikes and the barriers that usually prevent the area from flooding, but results in a > 1,700,000 element unstructured grid. The comparison with the available data reveals that waves and water levels are reproduced with normalized errors of the order of 10% and 5%, respectively. The extension of the flooding is also well reproduced, although with some underestimations along the coastline and overestimation in the inner part of large marshes. These limitations are explained by a lack of spatial resolution locally and the absence of several processes in the model such as infragravity waves and wave runup. The comparison between our baseline simulation and a simulation where the flooding is disabled by increasing the dike height reveals differences in maximum water levels locally reaching 1.0 m. This result is of key importance for coastal management strategies and also questions classical engineering approaches relying on one-way nesting.