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

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.

Projected Responses of Tidal Dynamics in the North Sea to Sea-Level Rise and Morphological Changes in the Wadden Sea

“…Regarding the future morphological evolution of intertidal areas in the Wadden Sea, the availability of sediments from internal and external sources has to be considered. Tidal inlets, channels, and ebb-tidal deltas act as internal sediment sources in a tidal basin, while external sediments may be provided by foreshore areas, barrier islands, or remote sites (Wachler et al, 2020). Existing empirical (Hofstede, 2002) and schematized models (Van Goor et al, 2003) have shown that SLR may cause an internal sediment redistribution from tidal inlets, channels, and ebb-tidal deltas to the intertidal flats.…”

“…We assumed that sediments from external sources are fully accountable for the vertical accretion in the intertidal flats of the Wadden Sea for simplicity. In contrast to Wachler et al (2020), a deepening of local tidal inlets and channels was neglected in our model. Furthermore, we uniformly applied each considered rate of vertical accretion to the entire intertidal flats of the Wadden Sea.…”

“…Another numerical study by Jacob et al (2016) shows that recent historical changes in the morphology of the Wadden Sea have impacted the local and remote responses of tidal dynamics. It is thus mandatory to consider potential bathymetric changes in the Wadden Sea for projecting the impact of SLR on the regional tides, as highlighted by Wachler et al (2020). Concentrating on the responses of tidal current velocities in the German Bight, Wachler et al (2020) project that bathymetric changes in the Wadden Sea can even compensate the effects of SLR on the local tidal dynamics.…”

“…It is thus mandatory to consider potential bathymetric changes in the Wadden Sea for projecting the impact of SLR on the regional tides, as highlighted by Wachler et al (2020). Concentrating on the responses of tidal current velocities in the German Bight, Wachler et al (2020) project that bathymetric changes in the Wadden Sea can even compensate the effects of SLR on the local tidal dynamics. To-date, it is however only poorly understood how the combination of SLR and morphological changes in the Wadden Sea might impact the regional tidal dynamics.…”

Understanding the Impact of Bathymetric Changes in the German Bight on Coastal Hydrodynamics: One Step Toward Realistic Morphodynamic Modeling

“…The sediment budget analysis performed by Benninghoff and Winter (2019) for the period between 1998 and 2016 showed that the tidal flats in this region accumulate sediments at a rate higher than the observed mean rate of SLR, which ranges from 2.2 to 6.6 mm/yr for the German Bight (Wahl et al, 2013). Indeed, the SLR scenario of Wachler et al (2020) demonstrated that elevated tidal flats and deepened tidal channels tend to counteract the pure effects of SLR on tidal current velocities. In the following, we will refer to the change in the local depth as a function of time as a "bathymetric change.…”