Asymmetric morphodynamics of the Wadden Sea

Shallow coastal systems with tidal flats and barrier islands are valuable assets to coastal protection and unique habitats with thriving biodiversity. Sea level rise threatens to diminish these systems unless sediment accretion compensates submergence. To quantify these dynamics for the world’s largest channel-shoal system, the Wadden Sea, an observed geomorphological time series was created and assessed over three decades. We found how (i) accretion-erosion was asymmetric from deep to shallow, (ii) topographic steepening occurred, and (iii) the morphological evolution was systematic across tidal basins. Peak accretion was observed below tidal low and above tidal high water, while the greatest erosion was found at intermediate subtidal elevations. Most intertidal areas accreted faster than sea level rise, and accretion was prominent at the channel-flat and flat-marsh interfaces. The simultaneous deepening of tidal channels leads to topographic steepening. Overall, the current net sediment import of the Wadden Sea is 19.7 Mm³/yr.