The Eastern Scheldt region in the southwestern Netherlands depends on various dams and dikes for coastal protection, including the storm surge barrier (OSK) at the seaside. The OSK was built for both coastal protection and ecological preservation. Initially constructed as a compromise to avoid completely closing off the Eastern Scheldt, the OSK allows tidal exchange flows in normal conditions while it closes during storms. However, the barrier has disrupted the natural dynamics that normally govern a tidal basin, causing erosion of tidal flats as the channels ’demand’ sediment from the flats. These flats are critical for the ecosystem, supporting migratory birds that forage there and playing a crucial role in wave attenuation, which reduces the wave load on dikes. Preserving these tidal flats is therefore beneficial for both flood risk reduction and ecology. Currently, this is achieved by carrying out sediment nourishments on the flats of the Eastern Scheldt to maintain them.

With rising sea levels, the OSK will need to close more frequently, potentially up to 66% of the time with a 2-meter sea level rise. This would disrupt water exchange, threatening food availability for ecosystems and mussel farming while increasing the demand for sediment nourishments on the flats. Frequent closures and ecological pressures raise concerns about the long-term viability of the OSK. Exploring alternatives, such as removing the OSK, could restore sediment exchange, reduce channel sediment demand, and promote tidal flat recovery.

However, uncertainties about the long-term morphological effects of OSK removal persist, making further research essential to evaluate its potential as a sustainable solution for both ecological preservation and coastal protection. This study aims to investigate the long-term morphological development of the Eastern Scheldt following the potential removal of the OSK. To achieve this objective, the following research question will be addressed: What is the long-term morphological development of the Eastern Scheldt, focusing on the tidal flats, when removing the storm surge barrier and under the influence of sea level rise?

To investigate the long-term morphological development of the Eastern Scheldt without the OSK, this study adopts the equilibrium concept for tidal basins. This approach was chosen because it avoids the complexities of process-based models, in which it remains challenging to balance wave erosion and tidal sedimentation over the long term in the Eastern Scheldt. The equilibrium approach predicts aggregated volumes of the flats, channels, and ebb-tidal delta without providing spatial or depth-specific details. The methodology consists of two parts:
1. Data Analysis: Measured volumes of flats, channels, and the ebb-tidal delta are compared with equilibrium volumes derived from empirical equations to assess their alignment. Equilibrium volumes for the scenario without the OSK are then calculated to predict the likely morphological state of these elements.
2. ASMITA Model: To predict dynamic changes influenced by factors such as sea-level rise, the ASMITA numerical model is used. ASMITA simulates sediment exchange among flats, channels, and the ebb-tidal delta by also using the equilibrium concept and estimates morphological trends over time.

The results from the data analysis show that the channels will be closer to their theoretical equilibrium if the OSK is removed, which will reduce the sand demand of the channels and is expected to result in less sediment being drawn from the flats. It is also expected that, due to the removal of the OSK, sediment blockage will disappear between the channels and ebb-tidal delta. Furthermore, from the large deviations between equilibrium volumes and measured volumes of the flats and ebb-tidal delta suggest that the relations used in this research to
calculate the equilibrium volumes should be refined. A good comparison should first be made between the basin on which the empirical equilibrium volumes are based and the Eastern Scheldt. Identifying these differences could lead to more specific equilibrium relations for the Eastern Scheldt.

Results modeled with ASMITA indicate that the flats will import sediment in all scenarios. Sensitivity analysis on these results show that the rate and magnitude of this sediment import largely depend on the sediment concentration entering the basin without the OSK. A reliable estimation of this sediment concentration requires further research (see Recommendations). The ASMITA results also show that the increase in flat volume depends on the balance between sediment import and sea-level rise. Most scenarios indicate an increase in total flat volume. However, for scenarios with extreme SLR rates, the flats in the Eastern Scheldt may eventually
drown, even with the OSK removed.

The expected long-term morphological response of the Eastern Scheldt to removing the OSK is sediment importation to the flats. This can explained by the disappearance of the sediment blockage and the current volume of the channels matching the new equilibrium volume, therefore demanding significantly less sediment of the flats.
The total increase in the volume of the flats depends on the balance between sediment import and sea-level rise. Further research is required to provide an absolute estimate of the total increase in the volume of the flats.

In response to the escalating coastal erosion at the Weg naar Zee region in Suriname, this study explores the beneficial reuse of dredged sediment from the Suriname River. The integral question is, ”What is the feasibility of the beneficial reuse of dredged sediment through sediment nourishment to rehabilitate the growth of mangrove forest on the Weg naar Zee, Suriname coast?” To address this, literature research provides a basis for understanding imperative areas related to mud coast dynamics, sediment dynamics, mangroves, and the Suriname coastal system. This culminates in a system analysis and develops into a conceptual model of the Suriname River Estuary’s dynamics. Further development of a hydrodynamic numerical Delft 3D model of the Suriname River Estuary system ensues. This analysis forms the foundation for further assessment of potential sediment input locations throughout the estuary. Simulations are utilized to verify the sediment transport mechanisms, primarily focusing on two transport types: initial transport through the water column as suspended particulate matter (SPM), and migration over the bed influenced by wave forcing. These simulations offer valuable data on the impact of timing and placement of sediment input on the transport and deposition process. Employing recent survey data of the Weg naar Zee shoreline’s foreshore, a schematisation was established to ascertain the total infill volume necessary for a convex-up profile favourable for mangrove rehabilitation conditions. The results demonstrate the feasibility of achieving this profile by strategically inputting sediment, hence, capitalising on both transport types in the estuary. The study concludes by reiterating that the strategic placement of sediment nourishments can be a viable means of restoring the Weg naar Zee coastline. Finally, possible subsequent studies to address the existing uncertainties and streamline the implementation strategy are discussed.