Numerical modeling of extreme events of waves and water level for analysing coastal hazards on the coasts of Normandy (France)

Storms are some of the most powerful natural phenomena on Earth. From a climate perspective, they are driven by atmospheric perturbations that induce high waves and extreme storm surges. Coastal storms can have significant effects on coastal environments, including erosion of beaches, coastal infrastructure damage, or flooding, among other impacts. These impacts will likely increase with climate change and the rising human pressure due to the overexpansion of urbanization and infrastructure that takes place in these areas. Among all these risks, flooding in coastal and low-lying beach environments, is considered one of the major hazards that humanity must face, especially in recent decades. Investigating coastal flooding and its impact on coastal systems requires the consideration of the different natural drivers induced by marine forces with their specific return period. In this context, and in the framework of the Normandy coastal risk management, an integrative approach of numerical modelling has been developed to generate waves and sea-level hydrodynamics from the Atlantic Sea and the North Sea to the English Channel.   In this context, our work aims to examine the dynamics of coastal storms and their impacts close to Normandy coasts with gravel and composite beaches. Employing the outputs of the downscaling, extreme waves impacting on the coast have also been investigated at a local scale, simulating real storms on different profiles on the beach of Hautot-sur-Mer, France. This beach is an interesting location because of its morphology in the intertidal part of the profile. It is formed by a layer of sand with a mild slope covered by a layer of pebbles forming a steep slope. We have used the numerical model SWASH to obtain overwash and overtopping data in response to high energetic intervals of time for different sections of the beach. This allowed us to define the protective effect of the pebbles. The characterization of this effect has highlighted the importance of employing groins to slow down coastal transit in order to create natural morphologies that serve as barriers against extreme events produced by the action of the sea. In particular, the central part of this beach, sustained by groins, presented a much more dissipative response to high waves than the sides, where this kind of structures are no longer maintained.