Hypsometric characterization of the Dutch Wadden Sea tidal systems and development of intertidal flats under relative sea-level rise

Abstract

The Wadden Sea contains the largest contiguous area of intertidal flats in the world, stretching along the coasts of the Netherlands, Germany, and Denmark, and is marked as a UNESCO World Heritage Site due to its unique ecological value. With rising sea levels, the intertidal habitats are at risk of drowning. However, it remains unclear how the morphology within the tidal basins will react. In this study, detailed hypsometric characterization of the Dutch Wadden Sea is carried out to gain a better understanding of the dynamics within these tidal basins. Hypsometry is the distribution of elevation over area within a morphological unit. The shape of this curve can be parameterized and area, volume, and height of depth zones can be calculated. Therefore hypsometric characterization allows for strong data reduction. In estuary research, predictability to the pattern in concavity-convexity of the curve has been found. This raises the question whether similar predictability is present for tidal basins. Results show that curve convexity and presence of intertidal habitats increase from west to east and from the inlet to the land. The mean height of the flats decreases away from the inlet. Furthermore, the pattern of intertidal flat area shows a negative exponential relationship with basin size between and within tidal basins. These patterns provide insight into the distribution of intertidal habitats in the DutchWadden Sea and can be linked to changes in tidal range, sediment availability, wave energy, and basin size. This is a starting point for further temporal analysis and for finding predictability in the shape of hypsometric distributions. Moreover, the proportion of tidal flat area is predominantly dependent on basin size, suggesting a certain degree of self-organization in the underlying formation processes leading to a scale-dependence of this proportion. This argues against the assumption that the Marsdiep and Vlie basins would grow to exhibit a proportion of intertidal flat area similar to that of the small basins in the easternWadden Sea under sufficient sediment supply. This is often considered an equilibrium condition and strongly influences the magnitude of predicted sediment deficits and drowning of tidal flats. Predominant scale-dependency implies lower sediment deficits and less drowning in the western Wadden Sea than assumed in previous studies.