Changes in tidal asymmetry in the German Wadden Sea

Abstract

The recent morphological development of the German Wadden Sea (North Sea, Europe) has been characterized by expanding intertidal flats and deepening, narrowing tidal channels at declining subtidal volume. This study analyzes the effect of these changes on tidal asymmetry, based on numerical modeling with high-resolution bathymetry data, and discusses possible adaptations of the import and export behavior in intertidal systems. As common descriptors of tidal asymmetry may show a high spatial variability in bathymetrically complex intertidal systems, we develop a novel subregion averaging approach for a more robust trend estimation. Our data reveal a statistically significant decrease in flood and flood current duration in the period from 1996 to 2016 resulting in declining flood dominance or enhanced ebb dominance in most tidal basins of the German Wadden Sea. Mean and peak current asymmetry also indicate significant decreases in mean flood current magnitude. We relate decreasing flood dominance mostly to local bathymetric volume changes rather than tidal amplitude. However, it appears likely that the sum of local effects facilitates the adaptation of regional tidal dynamics which affects especially the northern German Bight. This regional shift is explained by the deceleration of rising tides due to increased friction on laterally expanded intertidal flats and decreased subtidal channel volume. The decrease in flood or increase in ebb dominance, respectively, indicates that the recent trend of sediment accretion in Wadden Sea areas may cease soon.

Exploring the Tidal Response to Bathymetry Evolution and Present-Day Sea Level Rise in a Channel-Shoal Environment

“…Changes in flood duration (Figure 4, d) further underlined different local and regional phenomena. The regional decline in flood duration turned into an increase at nearly all estuaries and tidal basins as previously identified by Hagen et al (2022). Our results emphasize that even seemingly small changes in basin bathymetry and geometry can cause local and regional changes in tidal characteristic patterns.…”

“…Our modeled changes in tidal characteristics were in line with long-term analysis of measured data. A tidal gauge assessment in the period of 1954 to 2014 for tidal range, high, and low water established similar spatial patterns with larger increases in tidal high water and tidal range in the Northeast of the German Bight, lower rates in the West and Southeast, and local effects near land reclamation zones (Ebener et al, 2020;Hagen et al, 2022;Jänicke, 2021). More importantly, a proportional relationship between SLR and tidal high water and a weaker relationship to the tidal low water was established.…”

“…Individual tidal inlets form a sediment-sharing system consisting of a tidal inlet, an ebb tidal delta, updrift and downdrift barrier island coasts, all of which strive towards dynamical equilibrium (Elias et al, 2012;Wang et al, 2018). We divided the coastal zone in 27 subunits (i.e., morphological units) with similar tidal properties (Hagen et al, 2022), e.g., a tidal basin, in all following analyses (see Figure 1). Unit boundaries were defined approx.…”

“…Consequently, SLR impact modelers still debate the flooding vs. noflooding option for the low-lying coastal terrain with an emphasis on tidal and wave energy and its dissipation. Recently observed increases in relative intertidal storage from such changes in the German Wadden Sea (North Sea) questioned the assumption that tidal flats will be lost under SLR (Hagen et al, 2022;Lepper, 2023). This emphasizes that a deeper understanding of the feedback between tides, SLR, and morphological changes is needed as the tidal flat height was shown to determine whether the tidal amplitude increases or decreases under SLR (Jordan et al, 2021;Pelling et al, 2013b).…”

Adaptation timescales of estuarine systems to human interventions

“…However, in the German Wadden Sea local closures (see Reise, 2005) such as the Meldorfer Bucht (48 km 2 ; completed 1978), the Lower Elbe (150 km 2 ; completed 1979), and the Nordstrander Bucht (33 km 2 ; completed 1987) may have had a larger impact on sediment availability. But independent of the mechanism causing higher sedimentation rates, it did lead to a decrease in flood-dominant sediment transport (or an increase in eb-dominant sediment transport) in the German Wadden Sea which will in time lead to a reduction in these sediment deposition rates (Hagen et al, 2022).…”