Tidal response to sea level rise and bathymetric changes in the German Wadden Sea

Caroline Rasquin,Frank Kösters,Benno Wachler,Rita Seiffert

doi:10.1007/s10236-020-01383-3

Abstract

Tidally dominated coasts are directly affected not only by projected rise in mean sea level, but also by changes in tidal dynamics due to sea level rise and bathymetric changes. By use of a hydrodynamic model, which covers the entire German Bight (South-Eastern North Sea), we analyse the effects of sea level rise and potential bathymetric changes in the Wadden Sea on tidal current velocities. The model results indicate that tidal current velocities in the tidal inlets and channels of the Wadden Sea are increased in response to sea level rise. This is explained by the increased ratio of tidal prism to tidal inlet cross-sectional area, which is due to the characteristic hypsometry of tidal basins in the Wadden Sea including wide and shallow tidal flats and relatively narrow tidal channels. The results further indicate that sea level rise decreases ebb dominance and increases flood dominance in tidal channels. This is, amongst others, related to a decreased intertidal area again demonstrating the strong interaction between tidal wave and tidal basin hypsometry in the Wadden Sea. The bathymetry scenario defined in this study includes elevated tidal flats and deepened tidal channels, which is considered a potential future situation under accelerated sea level rise. Application of these bathymetric changes to the model mostly compensates the effects of sea level rise. Furthermore, changes in current velocity due to the altered bathymetry are in the same order of magnitude as changes due to mean sea level rise. This highlights the significance of considering potential bathymetric changes in the Wadden Sea for regional projections of the tidal response to sea level rise.

Highlights

Sea level rise affects tides on all spatial scales ranging from global scale, i.e. oceans (Pickering et al 2017), to regional scale, i.e. continental shelves such as the European continental shelf (Pickering et al 2012; Pelling et al 2013; Pelling and Green 2014), and local scale, i.e. coastal waters such as the German Bight (Rasquin et al 2020) and its estuaries (Seiffert et al 2014)
The predicted compensational effect of the bathymetric changes applied to our model indicates that the morphology of the tidal basins is closer to morphologic equilibrium in comparison with the sea level rise simulation without bathymetric changes
A hydrodynamic model of the German Bight is used to analyse the effects of a sea level rise of 0.8 m on tidal current velocity in the Wadden Sea

Summary

Introduction

Sea level rise affects tides on all spatial scales ranging from global scale, i.e. oceans (Pickering et al 2017), to regional scale, i.e. continental shelves such as the European continental shelf (Pickering et al 2012; Pelling et al 2013; Pelling and Green 2014), and local scale, i.e. coastal waters such as the German Bight (Rasquin et al 2020) and its estuaries (Seiffert et al 2014). This is especially important in shelf seas like the North Sea and in extremely shallow coastal waters such as the Wadden Sea in the German Bight, where the tidal signal is strongly distorted and a large amount of tidal energy is Responsible Editor: Emil Vassilev Stanev. Based on an extensive literature research, we estimate potential, systematic morphologic changes in the Wadden Sea in response to an accelerated sea level rise within the twenty-first century. We transfer these findings into schematic changes of the model bathymetry (Section 2.3).

Objectives

Methods

Results

Discussion

Conclusion