The impact of sea level rise on storm surge water levels in the northern part of the German Bight

Abstract

The impact of mean sea level rise (SLR) on extreme water levels is investigated using a numerical model that covers the entire North Sea, but has its highest spatial resolution in the northern part of the German Bight. A 40-year hindcast covering the period 1970 to 2009 is conducted using observed mean sea level (MSL) changes, tides and atmospheric forcing as boundary conditions. The model reproduces the observed water levels well for this control period. A second 40-year run is then conducted considering the same atmospheric forcing but adding +0.54m to the MSL to explore the effects of sea level rise on storm surges in the investigation area. At most locations, the second model run leads to changes in the storm surge water levels that are significantly different from the changes in MSL alone. The largest increases of the order of 15cm (in addition to the MSL changes) occur in the shallow water areas of the Wadden Sea. These increases in storm surge water levels are caused by nonlinear changes in the tidal constituents which are spatially not coherent. The response of the tidal propagation to SLR is investigated based on the results from a tidal analysis of each individual event. These analyses point to an increase in the M2 amplitude and decrease in the amplitudes of frictional and overtides accompanied by less tidal wave energy dissipation. Attributed effects are changes in phase lags of individual constituents leading to a different tidal modulation, thus additionally increasing tidal water levels. Finally, we estimate how SLR affects return water levels in the northern part of the German Bight, with the result that relevant design water levels increase due to the non-linear relationship between SLR and changes in extremes.