The Wadden Sea is of great ecological importance. Key elements in the Wadden Sea are the extensive intertidal flats. These contain vast amounts of life, serve as feeding grounds for millions of birds, and provide ecosystem services such as coastal protection. It follows that it is important to be able to understand and predict how these vital elements of the highly dynamic Wadden Sea change over time.

The existence and evolution of intertidal flats rely on the intricate balance between erosion and sedimentation caused by the forcing of wind, waves, and the tide. Exact horizontal flow patterns on intertidal flats are not fully understood, especially under the influence of wind. This study aims to systematically investigate the tidally- and wind-driven horizontal flow patterns on the scale of an intertidal flat in the western Dutch Wadden Sea. To do so, existing literature is assessed and a subgrid-based hydrodynamic model of the Dutch Wadden Sea is created. Most hydrodynamic models require a wetting-and-drying algorithm to resolve the flow patterns near the wet/dry interface. Several types of wetting-and-drying algorithms exist, each with unique advantages and limitations. Subgrid modeling technique does not require a wetting-and-drying algorithm as wetting and drying is automatic. The adequacy of using a subgrid-based hydrodynamic model to achieve the aim of the research is investigated through a comparison of simulations with different settings. Computational grid resolutions are manipulated on the scale of the intertidal flat, along with whether subgrid modeling technique is applied. The tidally and wind-driven horizontal flow patterns are assessed using simulations in which the phase of the spring-neap tidal cycle is manipulated, along with whether wind forcing is applied. These scenarios are applied to two non-fringing intertidal flats with different geometries.

Simulations show that subgrid modeling technique is beneficial for representing the impact of small-scale bathymetric features on the flow pattern while using a coarser computational grid. Subgrid modeling technique offers an increase in how well the flow pattern is represented compared to simulations using interpolated bathymetry and the same computational grid resolution. The automatic wetting and drying is crucial for representing shallow flows, especially at depths where measurements and other hydrodynamic models face difficulties.

Results indicate that the horizontal flow patterns on intertidal flats follow the water level gradient between surrounding channels and are influenced by the intertidal flat geometry (i.e. size, bed elevation, and degree to which the intertidal flat is intersected by tidal channels), hydraulic boundary condition (i.e. phase of the spring-neap tidal cycle, asymmetry of the tidal wave), and wind forcing (which can completely alter hydrodynamics at several scales).

This report contributes to the knowledge on horizontal flow patterns on intertidal flats, benefiting studies into ecology, navigational maintenance, flood protection, and the ability of intertidal flats to keep up with sea level rise. However, the flow pattern is not solely responsible for morphological change. Moving forward, continued research into the drivers of the morphological change of intertidal flats will be crucial for informed management of ecologically valuable estuarine areas like the Wadden Sea.

In recent years, there has been an increasing need for solutions against the threats that SLR and climate change represent for coastal systems, especially in low lying areas like the SW Dutch coast. Simultaneously, awareness of the impacts that human activities have on natural environments has surged. Integrated and sustainable solutions are necessary. The Delta21 plan proposes an integrated plan for flood-protection, energy storage, and natural ecosystem restoration for The Netherlands, specifically for the Haringvliet estuary. This large-scale intervention will produce large disturbances in the system. Despite the resilient character of tidal basin systems \parencite{Wang2009}, the evaluation of the intervention's effects, in terms of the initial response and the variations in the mechanisms driving morphological changes is compulsory. Assessing the bed level changes is especially important, considering the valuable intertidal and subtidal ecosystems present in the Haringvliet mouth. It is essential to guarantee their preservation despite the large disturbances. The evaluation of the Haringvliet mouth response is performed employing a long-term morphodynamic 2DH computational simulation to forecast the response of the system. The models implemented for the simulation are the Delft3D-FLOW and Delft3D-WAVE. To reduce computational times, the modeling approach applies input reduction and acceleration techniques. The hydraulic forcing is schematized (morphological tide, waves, and discharges) while maintaining the seasonal character of the input. A variable morphological acceleration factor (MF), depending on the wave condition is applied. The variable MF allows us to employ low values during strong-episodic conditions and larger during more regular conditions. The approach is especially beneficial in the presence of oscillating forcings that are also enhanced by the MF. Finally, the impact of the D21 plan on the Haringvliet mouth is assessed by studying the alterations to the hydrodynamic regime, the related net sediment transport pathways, and the observed bed level changes. The analysis of the resulting morphological evolution focuses on the variations of the intertidal and subtidal zones and the mechanisms behind them. We showed that the tidal regime shifts again towards a long-basin regime once the D21 is implemented. The intervention also causes the emergence of new subtidal and intertidal areas, mostly within the Tidal Lake, resulting from the redistribution of the material from the main channel. The original intertidal and subtidal areas, formed by the Hinderplaat and Slikken van Voorne, are almost undisturbed. This behavior guarantees the preservation of the intertidal and subtidal ecosystems. The results also show that despite the initial dredging activity necessary for the D21 plan implementation, which decreases the extent of shallow areas, the emergence of intertidal and subtidal features balances the negative effect as long as no further dredging is performed. The TL shows a net sediment export of material at both control cross-sections (the Haringvliet Dam and the new TL inlet). The findings of the morphological development agree with the known morphology and processes of mixed-energy tidal inlet systems, despite the large-scale intervention. This study provided the opportunity to evaluate the emergence of typical tidal features in a highly disturbed system, where the tidal inlet is shifted further offshore. To achieve an accurate forecast, anthropogenic forcing signals also had to be considered and extrapolated to long-term morphodynamic simulations. The forecasting of large-scale anthropogenic interventions in sensible systems, such as tidal estuaries, is performed successfully by adapting an existing model. The development of this type of study will be increasingly relevant in the future for the evaluation of measures against SLR during conceptual project stages.

The Bollen van de Ooster, in this report referred to as the Ooster, is a sand bar in the outer delta of the Grevelingen. The Ooster is separated from the coast by a relatively deep channel, called the Schaar. Closure of the Grevelingen in 1971, with the construction of the Brouwersdam, initiated vast changes in morphology. Damming of the estuary mainly affected the tide-induced flow patterns and therefore the relative influence of the waves at the outer delta. In this study the main focus is on the morphological development near the coastline of Goeree-Overflakkee. During the past years an erosion trend of locally up to 27 m/year, just south of the Flaauwe Werk, led to concerns with Rijkswaterstaat. This erosion is caused by the migration of the Ooster along the coast. Out of precaution a beach nourishment has been planned in this area due to the potential danger for the Flaauwe Werk. However, uncertainty about the future morphological development and therefore the necessity to take measures remains. The aim of this study is to provide a better understanding in the changes that have occurred, bring more certainty about the future and therefore contribute to informed decision-making. The coast of Goeree-Overflakkee has had a long history of coastline erosion. Closure of the Grevelingen estuary led to an increase of this erosion trend at the Westhoofd due to a combination of increased tide-induced flow velocities and morphological development of the Ooster. The latter was characterised by an eastward migration which forced the Schaar into the coastline. This process initiated multiple nourishments in the period 1969-1985. The attachment of one of the shoals of the Ooster resolved this problem, resulting in a large accumulation of sediments on the beach. In the years thereafter these sediments have been eroded as the Ooster migrated along the coast and transported further north. The latter resulted in beach widening along the coast of Goeree-Overflakkee. Due to the rapid elongation of the Ooster the erosion problems of the 80’s returned. The Schaar shows a continuous decrease in depth and flow surface since 2003, according to data analysis. Model results show that the channel mainly plays a role during low tide when the Ooster is emerged. Significant tide-induced flow velocities occur in the channel. In general, the decreasing channel dimensions lead to an increase of the magnitude of the flow in the channel. However, this occurs for a limited time duration during the full tidal cycle. A striking observation is the small influence of the wave angle on the wave-induced flow velocities near the channel. Waves coming from the north are refracted considerably due to the extension of the Haringvliet outer delta. The result is a large net longshore sediment transport rate at the seaward side of the Ooster. This could explain the pace of the migration in eastern direction. Moreover these sediments are a source for the channel, contributing to decreasing the channel dimensions. In general the presence of wind decrease the magnitude of the flow in the channel. Based on these findings it can be concluded that the waves are the dominant forcing mechanism in shaping the morphology and attachment of the Ooster is expected to occur in the near future. Attachment of the Ooster implies the disappearing of the eroding currents. The duration at which this attachment can be expected cannot be deduced from this study. The erosion of the coastline will continue as long as the channel is present. Based on the width of the dune row and the presence of beach groynes the potential treat of the morphological development for the primary flood defences seems minor.