Abstract
The Wadden Sea is a unique and important intertidal coastal zone under the pressure of changing driving forces (i.e. sea level rise, storm surges and increasing tidal range). In this study, we characterize the recent morphologic evolution of the German part of the Wadden Sea for the time period 1998 to 2016 based on a large dataset of available digital elevation models. A sediment budget analysis reveals that the Wadden Sea is accumulating sediment. Changes in the ratio of intertidal to subtidal surface area indicate an extension of the intertidal zone. Most of the intertidal flats accumulate sediments with rates higher than the observed mean sea level rise in the German Bight, while simultaneously the subtidal mean depth increases. For the period of investigation this Wadden Sea steepening is quantified to averaged values of +7.9 mm/yr for the tidal flats and −20.0 mm/yr for the channels.
Highlights
The majority of the different Wadden Sea basins show opposing trends of the subtidal and intertidal evolution: Almost all of the intertidal areas show an accretion of sediments, whereas the majority of subtidal areas deepen over time
Tidal flat accretion is assumed to be driven by the observed increase in mean sea level (MSL), which induces feedback mechanisms in the Wadden Sea tidal channel/flat hydro- and morphodynamics[15]: An increase in the accommodation space over tidal flats leads to an increase in time of tidal inundation, favouring the settling of fine suspended sediment
In the case of a more pronounced flood dominance this results in sediment export to the intertidal areas and with it an increase in height of the flats until a new dynamic equilibrium is established, constituting a negative feedback loop[15,28,29]
Summary
The accelerated rise of the mean sea level may pose a threat to the stability of the Wadden Sea with its tidal channel-shoal systems[11,24]. It is of significant importance to investigate whether and how the different Wadden Sea elements are keeping up with sea level rise. Conceptual and numerical models are available, there is a gap of knowledge considering the observed recent morphologic adaption of the German Wadden Sea. In this study, we assess the recent evolution based on a set of annual digital elevation models (DEM) for the period 1998 to 2016 and set the findings into perspective to common schematic models of tidal system morphodynamics
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