Threatened by rising sea levels and other climate change induced hazards, there is an increased need for coastal flood protection, such as storm surge barriers. However, the construction of such barriers may lead to unwanted coastal changes. This thesis examines the effect of the Eastern Scheldt storm surge barrier on sediment pathways and the implications for coastal erosion. In addition, possible sediment-based interventions to redirect the current with the aim of reducing coastal erosion were investigated. First, a literature study was performed to provide historical context on the natural and human induced development of the ebb tidal delta. A data analysis on the sediment budget of the ebb tidal delta was performed, which showed that from 1960 to 1987 the ebb tidal delta gained sediment, from 1987 to 2010 it lost sediment, and from 2010 to 2019 it gained sediment again. A numerical model in Delft3D FM was used to produce sediment transport vector fields over a morphologically representative tidal cycle, which were then visualized using SedTRAILS. The research found that the barrier decreased the strength of the ebb current on the entire ebb tidal delta. The Roompot Zuid has a predominantly ebb-dominant character before the barrier in 1976 but changes to a flood-dominant character due to the construction of the barrier, which weakened the ebb-current. In 2019, part of the Roompot Zuid regained an ebb-dominant character. The Schaar van Onrust also became flood-dominant due to the barrier, eliminating the sediment-retaining effect the ebb dominated current in this channel. This shift indicates that sediments that previously stayed in front of the coast of Noord-Beveland are now redistributed further along the coast in south western direction, which led to a sediment deficit and erosion at the coast. The tested interventions seem to induce little structural changes in the tidal currents. The removal of sediment causes the sediment transport patterns to converge into the dredged area, suggesting deposition. Therefore the tested interventions do not seem effective from a coastal management perspective. The approach used here may also be useful for the assessment of sediment transport impacts at other sites where the construction of similar barriers is being considered.

The Wadden Sea is a large system of tidal flats and barrier islands. Its individual features have been heavily researched, but sufficient knowledge on the sediment transport patterns on the scale of the Wadden Sea is not yet available. This thesis aims to indicate the dominant sediment transport patterns in the Dutch Wadden Sea and the hydrodynamic forcing mechanism the patterns can be attributed to. The Delft3D-FM model used to indicate these patterns is first adapted by implementing an improved Manning’s roughness field. To assess the patterns for each contributing hydrodynamic forcing mechanism, the tide, wind and waves are added to the model one by one. The results show that the Dutch Wadden Sea is a highly interconnected system, with the influence of each hydrodynamic forcing mechanism depending on the feature and inlet system.

A large part of the Dutch coast, the barrier islands in the Wadden Sea included, would be eroding if the deficit in the total sediment budget was not compensated for through nourishments. Ebb-tidal deltas have an important function within the coastal system that make them of interest for coastal defence as a source and transport path for sand to the back-barrier basins and the island coastlines. As part of the Kustgenese 2.0 project, a pilot nourishment of 5.5 million m3 has been placed on the Ameland ebb-tidal delta. Improved understanding of sediment bypassing processes and the interaction with a nourishment is needed for strategic placement of nourishments in the future. In this thesis we analyse the high-frequency bathymetric dataset available at Ameland inlet between 2005 and 2020 and model the tide- and wave-driven transport pathways using to SedTRAILS to determine how sediment bypassing works under natural circumstances and under the influence of a nourishment. A new series of conceptual models has been proposed. These show that the formation and growth of a series of ebb-shields on the western side of the ebb-tidal delta plays an important role in sediment bypassing. Their development gradually results in transport pathways connecting the western side of the ebb-tidal delta to the major transport pathway along the ebb-delta front, which forms a direct connection to the Ameland coast in 2017. The influence of the pilot nourishment on sediment bypassing processes is limited, adding volume to the system but not altering the existing transport pathways. Transport pathways show that sediment from the nourishment eventually reaches the Ameland coast and is unlikely to feed the Terschelling coast or the tidal basin. This is valuable knowledge for the future sustainable coastal management of Ameland inlet, which can also be extended to other inlets.

Ebb-tidal deltas play a key role in the morphology of barrier coastlines and tidal inlet systems as they serve as a natural source of sediment. They are typified by a dynamic morphology that interacts with the adjacent coastlines and sponsors a unique ecological habitat. Under increasing socio-economic, ecological and climate-induced constraints, it becomes imperative to obtain a better understanding of the evolution of ebb-tidal deltas in order to maintain and preserve these morphological features in the near-future time horizon. An increased interest has therefore been raised to understand, quantify and predict the development of ebb-tidal deltas.

In the context of developing a future-proof coastal management and maintenance strategy, the efficiency of ebb-tidal delta nourishments has been further investigated in research programmes such as Coastal Genesis 2.0. For the Ameland inlet, this entailed the construction of a 5 million m3 pilot nourishment over the course of March 2018 to February 2019 and subsequent monitoring in the years following. This research investigates the impact of this pilot nourishment on the natural behaviour of the Ameland ebb-tidal delta. The second goal is to build further knowledge on the modelling capabilities of present state-of-the-art models for the Ameland ebb-tidal delta.

To this end, a process-based Delft3D model is applied to hindcast the morphological development of the ebb-tidal delta with a particular interest in the evolution of ebb-shields and -chutes over the course of 2005 to 2020. It is identified that the representation of the wave-induced processes is key for capturing the development of ebb-shields and -chutes in the model predictions. Therefore, we applied and experimented with an updated nonlinear wave orbital velocity parameterisation and assessed its contribution to the modelling performance.
This thesis demonstrates that the evolution of ebb-shields and chutes on the outer delta is contingent on its initial presence in the initial bathymetry. Hence, the initiation of ebb-shield development is not inherited in the model response. These insights have been integrated and synthesised to assess the application of the present state-of-the-art model as a forecasting tool for the morphological development of ebb-tidal delta nourishments.

Ultimately, it is shown that a model using schematised boundary conditions and an efficient morphological updating scheme is able the predict the yearly-averaged development of the pilot nourishment on the ebb-tidal delta. It is demonstrated that the 2019 pilot nourishment only locally influences the behaviour of the Ameland ebb-tidal delta. Nourished sediment is likely to be redistributed along the ebb-shields, contributing on the long-term to the sediment exchange process with the downdrift coast of Ameland. The location of the ebb-tidal delta nourishment is thereby important for the sediment exchange process and the development of local features. Placing ebb-tidal delta nourishments to the south of the Westgat invokes a primary sediment exchange between the coast of Terschelling and surrounding morphological features. A secondary readjustment of the Westgat thereby influences the development of the ebb-shields and -chutes. Constructing an ebb-tidal delta nourishment north of the Westgat results in a sediment exchange between the local ebb-shields and the downdrift coast of Ameland. Our results also demonstrate that an increase in the pilot nourishment construction height enhances the redistribution of sediment along the ebb-shields.

Lastly, this study identifies further opportunities for improving medium-term morphodynamic models for the Ameland inlet by incorporating time-dependent boundary conditions including new transport formulations such as SANTOSS.