Observed Sediment Accretion Exceeded Sea Level Rise in a Large-Coherent Channel-Shoal System

The North Sea on the northwestern European continental shelf inherits the largest coherent channel-shoal system world-wide, the Wadden Sea. The system serves as an important sediment and carbon sink, offers unique biological niches, and natural coastal protection which is why it is essential to protect it from drowning under sea level rise. In the recent past, coastal protection and port authorities observed enhanced coastal siltation (fine material) which is not in line with a drowning hypothesis. Therefore, our research objective was the quantification of morphodynamics of the subtidal and intertidal bathymetry in the Wadden Sea in the period of 1996 to 2016 as a first step to understanding the recent development. We have merged high-resolution bathymetry data with modeled tidal high and low water to estimate annual intertidal zones and found a significant increase in lateral intertidal extent at the cost of a retreating subtidal zone. In addition, we identified significant vertical accretion by more than 1 cm/a which largely exceeded sea level rise in this period. At the same time, we observed increasing mean water depth in the subtidal zone. The development of the sediment volume indicated that recent intertidal accretion could be fed primarily from adjacent subtidal sediment sources as the net import in the study period was insignificant at most locations.