The exceptional influence of storm ‘Xaver’ on design water levels in the German Bight

Designwater levels for coastal structures are usually estimated based on extreme value statistics. Since their robustness depends heavily on the sample size of observations, regular statistical updates are needed, especially after extreme events. Here, we demonstrate the exceptional influence of such an event based on storm ‘Xaver’, which caused record breakingwater levels for large parts of the southwesternGermanNorth Sea coastline on 6December 2013.We show that thewater level estimates for a 1 in 200 years event increased by up to 40 cmdue to the update after ‘Xaver’, a value twice as large as the estimated regional sea level rise for the entire 20th century.However, a thorough analysis of different independentmeteorological (winds and pressure) and oceanographic components (tides, surges,mean sea level (MSL) anomalies) driving the event reveals that their observed combination does not yet represent the physically possible worst case scenario. Neither tides, nor surges norMSL anomalies were at their observationalmaximum, suggesting that there is a realistic risk of a storm like ‘Xaver’ to cause even higher extremewater levels by a few decimetres under current climate conditions. Our results question purely statistical design approaches of coastal structures, which neglect the physical boundary conditions of individual extreme events.