The Wadden Sea is a unique intertidal wetland area, forming an important hub for migratory water birds. A feared effect of accelerated sea-level rise (SLR) is the gradual loss or even disappearance of the ecologically valuable intertidal flats. To date, the effect of SLR on the time-evolution of the intertidal areas in the Dutch Wadden Sea has not been studied. To explore the sensitivity of the intertidal flats to SLR and the spatial differentiation of the response, simulations are carried out with the reduced-complexity model ASMITA for four sea level rise scenarios: one with a stable rate of 2 mm/yr (current rate), and three with accelerated sea level rise rates to respectively 4, 6 and 8 mm/yr. In addition, a scenario with a linearly increasing rate to 17 mm/yr in 2100 has been added to get an impression of what may happen under more extreme SLR-rates. The results show that the intertidal flats in the larger basins are most vulnerable to drowning. Due to differences in tidal flat geometry, the intertidal flats in the smaller basins mainly reduce in average height, while the intertidal flats in the larger basins mainly reduce in surface area. Within the basins, largest losses are expected to occur just off the land reclamation works and along the western part of each tidal watershed. The intertidal flats are sensitive to the rate of SLR. With doubling the rate of SLR, losses nearly double as well. Complete drowning is not predicted for any of the considered scenarios, but for the larger basins volume losses of nearly 50% by 2100 are predicted for the highest considered scenario. This will transform these basins into more lagoon-like basins, which is expected to have major consequences for the ecology.

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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.