Understanding coastal dynamics at an ebb-tidal delta in the Wadden Sea

Abstract

Schiermonnikoog is a Dutch barrier island with wide beaches and dunes which are both of importance for recreation as for safety. However, after decades of coastal accretion, in the last years, a strong coastline retreat is observed at the island head in the northwest. In tidal inlets, various forcing conditions of tides, waves, winds and estuarine circulation can all influence the pathways and magnitude of sediment transport. The dominance of any one of these processes can have a controlling influence on the morphology of the delta, inlet and island coast. In the case of Schiermonnikoog it is unclear what drives the recently observed coastal erosion. Therefore, the relation between the ebb-tidal delta development and the coastal transgression has to be studied. With knowledge on the behavior of the system, well-founded decisions can be made about the policy and the management in the area. In this thesis, the newly developed Delft3D Flexible Mesh (FM) modeling suite has been used to set-up and calibrate a morphostatic model. This model consists of the coupled D-Flow FM (Kernkamp et al., 2011) and D-Waves modules (SWAN, Booij et al., 1999), and can adequately hindcast measured water levels, wave heights and the expected sediment transports in the investigated area at Schiermonnikoog. Model uncertainties are overcome by incorporating the entire Dutch Wadden Sea in the model schematization. The unstructured grid allows for a local increase of the resolution in the area of interest. By analyzing the influence of individual physical processes on the system, it is found that wave action is the main driver of the migration of the approaching sand shoal. With the present-day bathymetry, the flood channel in front of the island coast no longer experiences significant tidal currents due to its hydraulic inefficiency. This indicates its upcoming abandonment, which would allow for the attachment of the sand shoal to the island coast. However, wave-driven currents and related sediment transport through the channel obstruct a closure. Similar to the situation at a the neighboring barrier island Ameland in 2014, a divergence point in the channel is visible at the location where the largest waves reach over the approaching sand shoal. The resulting gradient in the wave heights and consequent gradient in the alongshore sediment transport can be linked to the observed coastal erosion. This shows the importance of the shape of the sand shoal for the coastal dynamics in the system. To better understand the final stage before the abandonment of a flood channel, a conceptual model is presented.