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.

The Mekong Delta in Vietnam is facing several challenges as a result of climate change. Among others, the effects include an increase in river discharge during the wet season, leading to river floods, and a decrease in river discharge during the dry season. The decrease in discharge results in a shortage of fresh water required for irrigation and drinking water. Besides that, the combination of sea-level rise, land subsidence, and decreased river discharge during the dry season results in saltwater intrusion. This threatens freshwater supply even more. Furthermore, there is an increasing risk of floods from the sea due to low land elevation and the rising sea level in combination with the occurrence of storm surges. The scope of this research is the area around the Ham Luong estuary, which is a branch of the Mekong River. The partial closure of this river branch is considered by the Vietnamese government as a measure to reduce the effect of the above-mentioned effects of climate change. However, not enough research has been conducted yet on the impact of a partial closure on the Ham Luong estuary. This has lead to the following research question: “What is the impact of various closure scenarios on the hydraulic characteristics and social activities in the Ham Luong estuary, considering a 75-year forecast?” The region of the Ham Luong estuary is characterised by its intensive agri- and aquaculture. More than 60% of the inhabitants is directly active within the agri- or aquaculture. As these activities are strongly dependent on the salinity of the estuary, they are highly affected by the effects of climate change. The region is densely populated with more than 125,000 inhabitants living near the Ham Luong estuary. It is clear that the effects of climate change are threatening the region in hydraulic aspects, as well as socio-economic aspects. A partial closure could reduce these effects, but will influence the region in several ways. In order to estimate the impact, a combination of hydraulic and socio-economic aspects is assessed based on a criteria set. This criteria set contains the criteria of freshwater supply, agricultural and aquaculture adaptation, biodiversity, stable riverbanks, and navigability. These criteria will be tested on a total of four alternative interventions in the Ham Luong estuary. Three alternatives with a storm surge barrier and one alternative without a storm surge barrier. All alternatives include heightening of the existing dyke system, as this seems to be inevitable when aiming for long-term development in the region. The extend of dyke heightening is subject to the choice of alternative. As a part of the impact analysis, a Delft3D model was built to analyse the hydrodynamic and morphodynamic processes in the Ham Luong estuary. The model was restricted to the chosen spatial scope, which only covers the Ham Luong estuary, without any upstream bifurcations. The model gave insights in processes like salt intrusion, sedimentation rates, and water levels. However, due to model simplifications and assumptions, the outcomes of the model where not useful for quantitative assessments. Still, the results are used to compare the impact of the different alternatives to each other. As expected, the alternatives that include a storm surge barrier will provide more possibilities to retain fresh water than the alternative without a barrier. From the results, it followed that the limited spatial scope excludes the redistribution of upstream discharge. It is recommended to look at a larger scale of the Mekong Delta when assessing hydro- and morphodynamic processes. Forming a flood protection system, the structural design of such a storm surge barrier, together with a quick estimation of a dyke system. The dyke system is different for each alternative, depending on the presence and the location of a barrier. The barrier design includes a thorough analysis on feasibility of gate types, technical requirements, load combinations, design of dimensions, and the operation. The load combinations take hydrostatic, hydrodynamic, wind, and soil loads into account. The design of the dimensions is done for the gates, sill, lifting structure, pier, foundation, and the bed protection. By assessing the above-mentioned criteria, a preferred solution is identified. This preference is based on a Multi-Criteria Analysis, which includes weighted scores for all alternatives. The outcome of the Multi-Criteria Analysis appears to be very sensitive to the rating and weights of the criteria, which makes it difficult to identify one of the alternatives as the preferred solution based on only the score on the different criteria. For this reason more research is needed. However, when including a cost estimation of the four alternatives, it can be stated that the alternative of no storm surge barrier and only the corresponding extensive dyke heightening could be considered as most cost-beneficial alternative and therefore as the preferred solution. It is expected that with or without closure of the Ham Luong estuary the system will change. The availability of fresh water will be improved by the presence of a closure, although more research is needed to specify this further. The increasing salt intrusion, as a result of Relative Sea-Level Rise (RSLR) will lead to agricultural and aquaculture adaptation in all alternatives. Either due to the construction of the barrier, or due to the gradual RSLR. A closure also has effect on the biodiversity, stability of the river banks, and navigability in the river. When implementing a closure these effects should be further investigated to assess the effect quantitatively.