Impact of mean sea level rise on storm surges and tide-surge interaction in the German Bight

Abstract. Changes in mean sea level not only directly contribute to extreme water levels (EWLs) but also modulate tidal dynamics, storm surges, and their nonlinear interactions, particularly in shallow shelf seas. This study investigates how sea level rise (SLR) affects storm surges and tide-surge interaction (TSI) in the German Bight, with the aim of assessing the validity of linear approaches commonly used to estimate EWLs in coastal risk studies. A hydrodynamic model was used with a set of controlled experiments for the period 1981‒2010, explicitly separating tidal and atmospheric contributions and their interactions, additionally comparing present-day conditions with a +1 m SLR scenario. We demonstrate the importance of tidal phase in modulating the magnitude and timing of storm surges via TSI. The strength of this interaction increases with storm surge height, while its dependence on high tide magnitude and tidal range is comparatively weak. Under SLR, TSI weakens mainly because increased water depth reduces bottom friction, diminishing its dampening effect by about 25 %. Moreover, storm surge heights show a slight decrease, primarily due to phase shifts caused by faster wave propagation in deeper water and reduced wind stress efficiency acting upon a thicker water column. Overall, the nonlinear effects of SLR on storm surges and TSI are small, on the order of a few centimeters for 1 m SLR, relative to the direct contribution of SLR. This supports the use of linear superposition for many practical applications, while highlighting that nonlinear interactions introduce systematic, process-based adjustments relevant for precise EWL estimates.