Effect of sea level extremes on the western Basque coast during the 21st century

Rising global sea level increases the vulnerability of coastal regions to storm surge flooding. The impact of extreme high waters resulting from the combination of tidal oscillations and changes in mean sea level and in storm surges during the 21st century has been explored in the Bizkaian coast (northern Spain). Mean sea level variations due to temperature changes were estimated from an ensemble mean of global atmosphere-ocean general circulation models in the Bay of Biscay under A1B and A2 climate change scenarios. Changes in the frequency and intensity of storm surges were obtained from the output of a barotropic regional ocean model forced by greenhouse gas concentrations during 2000-2100. The storm surge model was calibrated using available tide gauge observations. Based on the above estimations, return levels of total sea level extremes are expected to increase by up to 40 cm with respect to present-day values. The likely impacts of resulting floods on coastal and estuarine habitats of the Bizkaian coast were assessed applying the return levels to a high precision light detection and ranging (LiDAR) based Digital Terrain Model. Results show that during the second half of the 21st century up to 202 ha are under risk of flooding, of which 50% correspond to urbanized land including industrial and residential areas. This represents a more than 3-fold increase compared to the area at risk from present sea level extremes. This study provides insight into both regional sea level variability and local flood-risks, and thus provides inputs for the formulation of effective mid-term adaptation measures to sea level extremes.