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.

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.

The Wadden Sea serves multiple roles: it acts as a protective barrier against severe wave conditions, is a natural habitat for diverse flora and fauna and has recreational purposes. This research provides a comprehensive analysis of the intricate morphological dynamics of the Wadden Sea. By implementing various model schematizations, the study aims to enhance understanding of complex interactions between tidal inlets and basins, sediment transport dynamics, the influence of forcing actors and the importance of grid resolution. The ultimate goal is to improve the process-based models that are used to perform morphological forecasts, serving as a valuable tool for efficient coastal management and ensuring the preservation of this unique tidal ecosystem. The study incorporates several schematizations, including the spatial extent of the modelling grid (domain schematization), grid resolution and various forcing components applied to the model boundaries and grid. Analysing water and sediment exchange between basins and through the Ameland tidal inlet has revealed distinct differences in hydrodynamics and morphodynamics across the model schematizations. The single-inlet domain schematization encompassed only the Ameland tidal inlet region, while the more expansive multi-inlet schematization captured the entire Wadden Sea area. The single-inlet schematization does not account for hydrodynamic interactions between the Ameland basin and neighboring basins. This impacts sediment transport and morphological development in the Ameland inlet. A multi-inlet approach, encompassing the entire Wadden Sea, grants a more comprehensive model. The model with Wadden Sea domain schematization is essential for obtaining accurate results, as it showed significant differences in sediment transport compared to the single-inlet model. Modeling practices that exclude transport dynamics between tidal basins risk misrepresenting the natural processes in play. Around the Ameland inlet, ebb dominance is observed in most regions, but there are exceptions in the marginal flood channel and at the outer side of the ebb-tidal delta. The Ameland model shows more ebb-dominance than the Wadden Sea model. Sediment transport during ebb periods exceed than during flood periods and differences in sediment transport between model schematizations are more pronounced during ebb than during flood. Regarding the drivers of change, the spring-neap tidal cycle is particularly indicative of periods with meaningful residual sediment transport. Wind, especially from southwesterly directions, has a significant impact on water transport dynamics and a limited impact on sediment transport through the inlet. Two grid schematizations are used, one with a base resolution of 60 by 60 meter and another with a high-resolution grid of 30 by 30 meter near the inlet. An enhanced grid resolution results in marginal changes in simulation outcomes, offering limited advantages for the conducted short-term morphological forecasts. Overall, enhanced model schematizations can significantly improve short-term morphological forecasts of the Ameland tidal inlet, providing a more accurate and comprehensive representation of the complex hydrodynamic and sediment transport processes. Leveraging high-performance computing (HPC) for executing Delft3D-FM models has been central in this research, offering possibilities to enhance model resolution and reduce computational time. The benefits of HPC come with the added challenge of mastering new software, handling larger models and interpreting more extensive data sets. A deep understanding of the physical processes and careful cost-benefit consideration is crucial, as increased computational power does not automatically resolve all modeling limitations. As such, the effective use of HPC in coastal modeling requires a balance between its sophisticated capabilities and the complexity it introduces.

In 2019, a 5 million m³ sand nourishment was placed at the Ameland ebb-tidal delta. This pilot nourishment aimed to investigate the feasibility and effectiveness of ebb-tidal delta nourishments and to understand the dynamics in this system. To assess how this nourishment has dispersed through the ebb-tidal delta system, SedTRAILS was used. SedTRAILS is a particle-based sediment transport model currently in development. A new process for interaction with the bed and burial of sediment was recently added to the model. The impact of this process on dispersal and sediment pathways needed to be analysed, as not much is known about the effects of its inclusion on transport and dispersal. Several schematisations of wave- and tidal forcing were tested. The wave forcing selection had little effect on the overall model results. Storm conditions included in the more detailed schematisation were underrepresented in the model results. The implementation of wave-driven bed velocity had little influence on result as well due to small magnitudes that were calculated for this. A significant difference was found between using a morphological tide and an artificial spring-neap tidal cycle however, requiring further investigation. Investigation of the burial formulation gave insight into the response of the model to changes in the different parameters. Direct dependence of erosion probability on deposition probability presents a major limitation in the current implementation of burial. Additionally, deposition probability in its current form is positively dependent on the maximum bed shear stress. Inclusion of burial does provide significant room for calibration of results. Through modelling these various configurations of SedTRAILS, results consistently showed a recirculation of sediment from the nourishment on the ebb-tidal delta together with bypassing around the edge of the ebb-tidal delta, as seen in Figure 1. However, the degree to which bypassing occurs depends heavily on the chosen burial configuration. The bypassing is largely a result of flow around the periphery of the ebb-tidal delta and differs from known bar migration and shoal attachment pathways. Transport into the Wadden Sea basin was not found, likely due to the underestimation of influence of storm conditions. To improve dispersal modelling with SedTRAILS, the impact of storm conditions needs to be more accurately included in the model. An under-representation of energetic wave conditions leads to a consistent underestimation of wave-related transport. Additionally, more investigation into the effects of the spring-neap tidal cycle is required. The significant difference presented here is an early indication that this is an important factor for SedTRAILS. Validation of dispersal modelled with SedTRAILS can be achieved through quantification of transport, assigning a volume of sediment per particle.

In 2019, a 5 million m³ sand nourishment was placed at the Ameland ebb-tidal delta. This pilot nourishment aimed to investigate the feasibility and effectiveness of ebb-tidal delta nourishments and to understand the dynamics in this system. To assess how this nourishment has dispersed through the ebb-tidal delta system, SedTRAILS was used. SedTRAILS is a particle-based sediment transport model currently in development. A new process for interaction with the bed and burial of sediment was recently added to the model. The impact of this process on dispersal and sediment pathways needed to be analysed, as not much is known about the effects of its inclusion on transport and dispersal. Several schematisations of wave- and tidal forcing were tested. The wave forcing selection had little effect on the overall model results. Storm conditions included in the more detailed schematisation were underrepresented in the model results. The implementation of wave-driven bed velocity had little influence on result as well due to small magnitudes that were calculated for this. A significant difference was found between using a morphological tide and an artificial spring-neap tidal cycle however, requiring further investigation. Investigation of the burial formulation gave insight into the response of the model to changes in the different parameters. Direct dependence of erosion probability on deposition probability presents a major limitation in the current implementation of burial. Additionally, deposition probability in its current form is positively dependent on the maximum bed shear stress. Inclusion of burial does provide significant room for calibration of results. Through modelling these various configurations of SedTRAILS, results consistently showed a recirculation of sediment from the nourishment on the ebb-tidal delta together with bypassing around the edge of the ebb-tidal delta, as seen in Figure 1. However, the degree to which bypassing occurs depends heavily on the chosen burial configuration. The bypassing is largely a result of flow around the periphery of the ebb-tidal delta and differs from known bar migration and shoal attachment pathways. Transport into the Wadden Sea basin was not found, likely due to the underestimation of influence of storm conditions. To improve dispersal modelling with SedTRAILS, the impact of storm conditions needs to be more accurately included in the model. An under-representation of energetic wave conditions leads to a consistent underestimation of wave-related transport. Additionally, more investigation into the effects of the spring-neap tidal cycle is required. The significant difference presented here is an early indication that this is an important factor for SedTRAILS. Validation of dispersal modelled with SedTRAILS can be achieved through quantification of transport, assigning a volume of sediment per particle.

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.

The Wadden Sea is the largest system of tidal flats and barrier islands in the world extending from the northern Dutch coast to the coast of Denmark. Such a large natural area is ofgreat importance to bothhuman beings and ecosystems. Over 10,000 species of flora and fauna are found in theWadden Sea being a perfect habitat due to the relatively calm environment and high food availability. However, accelerating sea-level rise and human interventions, such as gas extraction, may induce an unwanted morphological change in theWadden Sea posing a threat to the natural habitats. This study investigates the morphological response of the Wadden Sea to SLR and subsidence induced by gas mining. A new hybridmodel whose aggregation level is between a process-based model (Delft3D) and a aggregatedmodel (ASMITA) is applied in this thesis. The two-dimensional hybrid model applies depth integrated shallow water formulations and the advection diffusion equation for the transport ofsediments like Delft3D, but calculates the exchange ofsediment betweenbed andwater columnbymeans ofan equilibriumbathymetry concept under scenarios disturbing these equilibrium conditions. The Ameland inlet is chosen as the research area because it is a relatively autonomous and undisturbed basin. The high robustness of the hybrid model makes it possible to apply a high morphological scale factor and a coarser grid compared to the processbased model, which shortens the computation time by orders of magnitude. However, the equilibrium concept also fixes the shoal-channel structure and suppresses channel migrations. Three different scenarios of sea-level rise rate (4, 6, 8 mm/year) are applied over a simulation period of 100 years. The general morphological response simulated by the hybrid model shows that the channels and the ebb-tidal delta erode acting as the main source ofsediment for accretion of the intertidal flats. The erosion/sedimentation is more pronounced with a higher sea-level rise rate. Sensitivity analysis shows a significant influence of the sediment diameter on the channel erosion and sediment supply to the intertidal flats. Diffusivity plays an important role in the horizontal sediment exchange between the channel and the flat but only slightly influences the sediment import. Global equilibrium concentration and power n are similar to diffusivity in affecting morphological activity. The adaptation time scale is inversely proportional to both of these two parameters. The possibility of using more than one sediment fraction is proved and it can reproduce a more realistic sediment distribution as the observation. The hybrid model is also applied to simulate the morphological response to local subsidence and the restoration after subsidence stops. The center of the subsidence circle dosen’t lower as much as the subsidence rate, which indicates that sediment is transported to the center. Sediment is supplied to the area ofsubsidence by the adjacent main channel. The result proves the sediment transport principle underlying the hybrid model that sediment is always transported along the gradient of the sediment demand.