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

Abstract:

Design water levels for coastal structures are usually estimated on the basis of extreme value statistics. Since their robustness heavily depends on the sample size of heuristic observations there is an urgent need of regular statistical updates especially after the occurrence of record high extreme events. Here we demonstrate the exceptional influence of such an event based on storm Xaver which brought record high extremes for large parts of the southwestern German North Sea coastline on December 6^th 2013. We show that the estimates of an event occurring once in 200 years increased by roughly 30 cm due to the update after Xaver, a value 1.5 times larger than the entire 20^th century sea level rise in the region. However, a thorough analysis of different independent meteorological (winds and pressure) and oceanographic components (tides, surges, mean sea level anomalies) driving the event also indicates that their observed combination still does not represent the physically possible worst case scenario. Neither tides nor surges and mean sea level anomalies were at their observational maximum, suggesting that there is a realistic risk of storms bringing even up to a few decimeter higher extremes just under present day conditions without any influence of future global warming. The results question purely statistical design approaches neglecting the physical boundary conditions of individual extreme events.