Limitation of High Water Levels in Bays and Estuaries During Storm Flood Events

During storm events inducing extreme sea levels, coastal floods may occur. Water volumes that flood low-lying coastal zones may be important and significantly impact the sea levels along the coast. This effect is expected to be particularly significant in tidal bays or estuaries. This phenomenon was recently illustrated during storm Xynthia (February 2010) on the French Atlantic coast; volumes that flooded Charente-Maritime and Vendée departments corresponded to a non-negligible fraction of the tidal prisms of the Pertuis Charentais Sea (area between Ré and Oléron Islands and the continent); the order of magnitude is 10 %. The impact of the flooding associated with Xynthia is investigated by means of numerical modelling in the Pertuis Charentais carried out by LIENSs. Within the framework of risk mapping, CREOCEAN conducted flood studies in two sites of the eastern English Channel: Authie Bay and Dives Estuary, for which simulations of assessed future extreme events were run. Although these studies used different modelling systems at sites displaying distinct features, they all suggest that flooding can impact water level along the coastline up to more than 0.5 m. Limitation of high tide levels, related to flooding phenomena, are mapped and analyzed. Recommendations are provided to define a relevant methodology, accounting for such a limitation process, for submersion and risk studies.