The Wadden Sea is a dynamic coastal ecosystem characterised by broad intertidal flats that are exposed at low tide and submerged at high tide. Exposure time is an essential input parameter for ecotope mapping; the classification of ecological zones based on abiotic factors.

While the interplay of wind, tides, and sediment dynamics has been studied, there remains limited understanding of detailed exposure times, areas and flow patterns under varying wind conditions, particularly in shallow intertidal zones of the Wadden Sea.

This study quantifies how variations in wind direction and magnitude affect exposure characteristics and flow patterns on the Pinke Wad, located between Ameland and Holwerd. A 3Di-calibrated subgrid model, which is well suited for very shallow areas, is used to simulate twelve wind scenarios in addition to a base case without wind. The effects on one tidal cycle are analysed.

The scenarios result in different hydrodynamic responses on flooding and drying. Easterlies cause a non-linear increase in mean exposure time, from 3.67 h in the baseline to 5.51 h under an East-9 storm (≈ 23 m s-1). By contrast, westerly and northerly winds shorten exposure almost linearly for the simulated scenarios: the mean exposure time decreased to 0.95 h (North-9) and 0.77 h (West-9), with exposed area decreasing by 70–82%. A northerly wind of 6 Beaufort decreased exposure on higher flats by up to 49% (∼6 h), whereas an easterly of the same force increased it by 20% (2.5 h). Residual currents responded accordingly, easterly winds reversed the net flow westward from Beaufort 3 onwards. Westerly winds generated the strongest eastward residual currents. Northerly winds also induced westward flow starting from Beaufort 3, though with smaller directional shifts. Wind-driven changes in exposure time and changing flow patterns have clear implications for ecotope mapping, because there is a large variability over time.

The results highlight the substantial and spatially important role of wind in changing hydrodynamics on the Wadden Sea’s intertidal flats. By linking variations in wind speed and direction to changes in residual flow and exposure patterns, the findings provide a better understanding of hydrodynamic-ecological interactions. The use of a (well-calibrated) hydrodynamic subgrid model may offer a promising way forward for calculating exposure times as input for ecotope mapping compared to the current method, which relies on interpolation on water levels between gauge stations.

Salt marshes are complex and highly productive ecosystems in coastal areas. They play crucial roles in providing habitats for diverse species, sequestering carbon, and serving as natural buffers against storms. However, salt marshes have been threatened by human activities (e.g. reduced sediment supply) and climate change (e.g. sea-level rise), leading to a potential loss. This loss of salt marshes highlights the importance of an influx of sediment to salt marshes for maintaining their structure and resilience. Mudflats, located adjacent to salt marshes, facilitate sediment transport to these marshes under certain conditions. Therefore, advancing our understanding of sediment transport between mudflats and salt marshes is important, as it can provide valuable insights into effective salt marsh management.

This research aims to unravel the varying sediment transport processes between mudflats and salt marshes under different hydrodynamic and sediment dynamic conditions. Chongming Saltmarsh from the Yangtze Estuary and Paulina Saltmarsh from the Western Scheldt Estuary have been selected as study cases. The distinct differences in hydrodynamic forcing and sediment availability between these two estuaries contribute to differing environments and states of their intertidal systems. These differences enable us to compare the sediment transport processes across divergent systems and explore the mechanisms governing the long-term evolution of salt marshes.

Saltmarsh creeks are recognized as efficient conduits that actively facilitate the exchange of water and sediment between mudflats and salt marshes. To identify the role of marsh creeks in sediment transport between two different intertidal systems, the sediment transport processes in a main creek and on the adjacent mudflat in Chongming Saltmarsh (China) and Paulina Saltmarsh (the Netherlands) have been investigated (Chapter 2). Our findings revealed notable differences and common patterns in sediment transport between the two systems. In Chongming, SSC exhibited significant asymmetry between flood and ebb tides, with large SSC peaks occurring during most flood periods. This asymmetry in SSC caused the marsh creek in Chongming to function as a conduit for sediment import. Furthermore, distinct overbank and underbank tides were observed in Chongming. During underbank tides, sediment was trapped and retained within the creeks, only to be eroded and transported to the marsh during subsequent overbank tides. Additionally, the mudflats in Chongming showed a relatively rapid recovery after erosion events. These mechanisms were not observed in Paulina Saltmarsh, where a net export of sediment through the marsh creek was recorded during calm weather. In both systems, the SSC in marsh creeks showed a slight increase due to local erosion of the creek bed but responded more significantly to the erosion of mudflats, indicating that the main sediment sources of the high SSC result from the sediment advection rather than local erosion. These comparative findings suggest that the role of marsh creeks in sediment import and export is closely linked to the availability of sediment from adjacent mudflats, highlighting the importance of mudflats for the growth of salt marshes.

After recognizing the role of main creeks in sediment transport within turbid systems, the role of creek tributaries in sediment delivery still remains poorly understood. Therefore, field measurements were conducted in a main creek and in a secondary creek within Chongming Saltmarsh. These measurements revealed the dual roles of saltmarsh creek systems in drainage and sediment transport, as well as the mechanisms driving residual sediment flux within saltmarsh creeks (Chapter 3). The results indicated that the main creek played a dominant role in sediment delivery, while the secondary creek, influenced by the presence of vegetation, was more effective as a drainage conduit and contributes less to sediment transport. Additionally, the direction and magnitude of residual sediment flux are influenced by the relative importance of asymmetries in net discharge and sediment concentration. Overbank tides primarily result in an ebb-dominant flow asymmetry, which tends to drive sediment export along with the net outflow. However, the abundance of sediment during flood tides can occasionally counteract this export tendency, mitigating the impact of flow asymmetry on sediment export.

Sediment can be imported from mudflats to salt marshes through marsh creeks and marsh edges. To address how varying tidal and wave conditions affect sediment transport within marsh creeks and over marsh edges (Chapter 4), a two-month field campaign was conducted in Paulina Saltmarsh. Field data revealed that tidal ranges determine the direction of residual sediment flux in the marsh creek, while wave intensity determines its magnitude. Conversely, wave intensity determines the direction of residual sediment flux over the marsh edge, whereas tidal ranges determine the magnitude. Specifically, sediment was imported through the marsh creek during tidal cycles with small tidal ranges and strong waves, whereas sediment was imported through the marsh edge during tidal cycles with large tidal ranges and weak waves. These findings offer deeper insights into sediment transport through marsh creeks and marsh edges under different tidal and wave conditions, which is crucial for effective salt marsh management.

This dissertation explores sediment transport between mudflats and salt marshes in two different systems, providing insights into the roles of marsh creeks and marsh edges in facilitating or impeding sediment import to salt marshes under varying conditions. The findings offer guidance for developing conservation and management strategies to support salt marsh growth in response to decreasing sediment supply and accelerating sea-level rise.

Sand waves are rhythmic seabed patterns which develop as a result of the interaction between tidal flow and perturbations in the seabed. Due to their size and dynamic behaviour sand waves can pose a threat to offshore infrastructure such as cable trenching. To ensure the safety of these cables, it is crucial to understand the interactions between sand waves and trenches. Residual currents, currents that are not induced by the tide, can exert a significant influence on sand wave dynamics due to the non-linear relationship between flow velocity and sediment transport. Hence, this thesis aims to investigate the effects of residual currents on the morphological interactions between sand waves and trenches. A Delft3D FM model is used to simulate a case study of a trench within a group of sand waves in the Belgium Continental Shelf. The influence of residual currents, mostly induced by wind and atmospheric pressure, is studied during a period of a storm and a period with representative residual currents. It is found that during the tidal cycle at the peak of the storm, the residual currents cause an increase of 66% in sediment infilling into the trench compared to tide-only conditions. Since this is in the same order of magnitude as the background infilling due to tidal dynamics, this is less than expected. The residual currents are however strong enough to reverse the sediment transport patterns of the sand wave and trench system. During the representative residual currents, the contribution of residual currents is found to be insignificant, especially compared to the observed influence of the spring-neap tide. This study concludes that the overall effects of the residual currents, induced by wind and atmospheric pressure, can alter sediment transport patterns but the magnitudes are insignificant in the case study under investigation. It is recommended, for further research and for effective design and maintenance of offshore infrastructure procedures, to keep in mind that morphodynamics of sand wave and trench systems are highly subject to the variations in tidal dynamics induced by the spring-neap tide.

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.

The Hollandsche IJssel plays an important role in the freshwater provision of the province Zuid-Holland. Consequently, for Rijkswaterstaat it is key that salt intrusion is minimal in the Hollandsche IJssel. Recent studies noted that salt intrusion in the Hollandsche IJssel is limited due to a phase difference between tidal velocities in the main channel, the Nieuwe Maas, and the side channel, the Hollandsche IJssel. Earlier research investigated the impact of phase differences between branches and found it can lead to increased dispersion in the main channel, through a process known as tidal trapping. At the same time, this phase difference can prevent the saltiest water from entering the side channel, as was found at the Hollandsche IJssel. Because of this role, it is relevant to find out how this phase difference may be influenced by sea level rise, more extreme river discharges and particularly how it depends on the geometry of the main and the side channels. Especially the latter could help Rijkswaterstaat to minimize salt intrusion at locations relevant to freshwater intake, such as the Hollandsche IJssel.

The main objective of this thesis is to investigate how the geometry of the side and main channel influences the tidal phase difference between these two channels, and how this may impact the salt dispersion in the side channel. For this, an analytical model is developed describing harmonic wave propagation in multi-branch systems and this is used next to results from a 3D numerical model for the Rhine Meuse Delta (RMM3D). First, the influence of changes in geometry and forcing is systematically investigated for a network containing a single junction. This shows that the length and depth of the side channel are the most significant variables. The depth is one of the main variables impacting friction, which governs the type of wave which can form in the system. A decrease in friction allows a wave to transform into a standing wave pattern as the return wave becomes more important, while increased friction transforms it into a propagating wave. The length also controls the type of wave which can form as it determines the distance along which the friction can work. Additionally, the length also governs potential resonance in the side channel.

Next, the phase differences of the M2, M4 and M6 tide are determined for the junction with the Hollandsche IJssel in the Rhine Meuse Delta (RMD) based on the RMM3D model. The main tidal constituent regarding tidal trapping was found to be M2. However, this does not fully represent the time difference between flow reversal at the Hollandsche IJssel and the Nieuwe Maas, which was found to be around 75 minutes. Additionally, the phase difference at the Lek was investigated. For the M2 tide at the Hollandsche IJssel and Lek, a phase difference of 55⁰ and 31⁰ was found, respectively. These phase differences prevent salt intrusion in the respective side channels. The inflow of the side channels starts while the main channel still flows to the sea during the ebb. At this moment, the salt concentrations in the main channel have already returned to background levels...

In response to the sharp decline of seagrass globally, various restoration techniques have been developed and applied; many at limited scale and with limited success. This research focuses on the fluid mechanics and behaviour of seagrass seeds in a sand mixture through physical laboratory experiments. The objective is to establish a technique that incorporates seagrass seeds with sand-capping to develop seagrass restoration methods suitable for upscaling. Due to the limited amount of available seeds, laboratory experiments were conducted to find a suitable proxy for seeds from both intertidal and subtidal Zostera marina populations by tracking and analysing the particle's terminal settling velocity. The average terminal settling velocity of the seeds was 5.97 cm/s, slightly larger than that of sand particles with a diameter of 0.35 mm with a settling velocity of around 5.2 cm/s. Subsequently, the distribution of proxy seeds in the settled sediment of the "sand-capped layer" was determined after dispersal in five sediment-water mixtures with a volumetric concentration ranging from 5 % to 30 %. Mixtures with three-grain sizes grain sizes were released (d_{50} = 0.175, 0.35, and 0.52 mm) in a column of 1m and subsequently in a column of 1.5 m, where only the top 0.5m had a sediment-water mixture, while the area below consisted of a column of still water. Results show a general trend: seeds segregate to the top of the settled layer, further enhanced by larger grain sizes and higher sediment concentrations. Tests using the lowest concentrations (5-10 %) showed the best result, for which the largest number of proxies were located at an optimal depth (1 cm-5.5 cm) for protection and germination when using medium coarse and fine-sized sand grains. The results provide valuable insights into the dynamics of seed segregation in sediment mixtures and the hindered settling effect of a bimodal mixture, showing that seeds move differently to sand in a mixture, experiencing more rotation and drifting within a mixture not observed when following trajectories in a still water column. The larger seed particles segregated to the top of the settled layer due to the difference in behaviour with the sediment mixture. Even though the terminal velocity of the seeds in a still water column was larger.

Intertidal flats and salt marshes are critical ecosystems offering significant services but face degradation due to anthropogenic pressures. To mitigate this, restoration efforts like the "Mud Motor" have been introduced, where fine sediments dredged from ports are strategically relocated to nourish intertidal areas and salt marshes. This approach enhances ecological development while reducing maintenance dredging. This dissertation, conducted within the Mud Motor Pilot Project (2016–2017) in the Dutch Western Wadden Sea, investigates the role of wind in the hydro-morphodynamics of intertidal flats, focusing on sediment dynamics and morphological evolution.

Field campaigns across three years (2016–2018) in the Dutch Wadden Sea provided comprehensive datasets on water levels, sediment concentrations, currents, waves, and bed-level changes. Analysis revealed wind's significant influence on hydrodynamics. Opposing winds to tidal currents could reverse tidal flows, especially in higher intertidal zones. A newly developed analytical model validated with field data quantified the nonlinear interactions between wind- and tide-driven flows.

The findings emphasize the pivotal role of wind direction in sediment transport. Low to moderate winds in alignment with tidal residual transport facilitate sediment accumulation in low-energy zones, while short periods of opposing winds resuspend and redistribute this sediment. These wind-driven sediment fluxes critically shape short- and long-term sediment dynamics in systems like the Wadden Sea.

Moreover, the research identifies a "window of opportunity" for tidal flat accretion, driven by temporal sequences of sediment deposition and over-consolidation under favorable wind conditions. Sediment gains sufficient strength to resist erosion only through prolonged drying processes influenced by wind-driven water level set-down.

This study underscores the complexity of wind's impact on intertidal ecosystems, offering insights for restoration projects to better integrate natural processes. By accounting for wind effects, these projects can improve predictions and identify new restoration opportunities.

In the Netherlands, the coastlands act as the principal protection against the sea. Without human interventions, the Dutch coast would be eroding due to an imbalance in the sediment budget of the coastal zone. The preferred coastal management strategy is sand nourishment, which have become larger in scale and more complex over the last 70 years. Additionally, besides sand, large quantities of mollusc shells can also be found on the ocean floor. This study aims to gain insight into the current-driven bed load transport of a sediment-shell bed composition using bivalve shells of different shapes and sizes.

Flume experiments were conducted to measure the influence of bivalve shells on the threshold of motion of sand grains and current-driven bed load transport. The sediment-shell bed composition was altered in both experiments by varying the volumetric percentage of shell content. The bed compositions consisted of two distinct bivalve shell species: Ensis leei (elongated) and Spisula subtruncata (rounded) bivalve shell species.

It is proposed that the presence of shells initially disrupts the flow, leading to an increase in turbulence intensities. As shell content increases, a second effect becomes increasingly prevalent, which is sediment stabilization. The smaller sediment grains are hidden from the flow by larger, more exposed shell valves and shell fragments. Consequently, a higher bed shear stress is needed to mobilize the sand grains. The relative importance of these processes varies depending on shell content, shell species, and the potential of the shell to enhance turbulence intensities. The elongated shape of the E. leei bivalve shell enhanced turbulence intensities significantly and thereby influencing the threshold for sand grain motion. Bed load transport rates are reduced with increasing shell content. Initially, at a shell content of 10%, the rates only decreased marginally compared to a situation without shells, and no clear difference between shell species was observed. At a shell content of 20%, bed load transport rates decreased further. Thus, when predicting sediment transport, considering the presence of shells alone is insufficient, and consideration of shell shape and size is crucial.

The Wadden Sea region is characterized as a diverse and contradictory area. On the one hand, it stands as the largest intertidal area globally, supporting a rich and diverse flora and fauna habitat showcasing nature’s power and allure. On the other hand, it is a landscape shaped by human habitation and extensive transformations, including establishing numerous polders, reclaimed land, and imposing dikes. The region’s history reveals the enduring struggle of its inhabitants against the sea over the past millennium, leaving behind a valuable cultural heritage.

One particular area exemplifying this dichotomy is the Ems estuary, which represents the Wadden Sea’s most ecologically compromised section, often referred to as the “yellow river” of Europe. The industrial development occurring along the estuary, including activities like waterway deepening and riverbank embankment, has resulted in substantial alterations to the morphology, hydrology, and ecological composition of the area. The major challenges are the loss of brackish and freshwater marshes and the conflicting objectives of ecological preservation and further waterway deepening. Additionally, the area’s cultural significance often remains overlooked due to its designation as a natural heritage site. Consequently, the region lacks sufficient allure, resulting in a declining population and an inferior economic state compared to the southern Netherlands and the surrounding islands.

This master’s thesis addresses the crisis in the Ems estuary by focusing on the transformative potential of mud as a catalyst. Through a comprehensive study analyzing the status and role of mud and integrating various strategic options, the project explores opportunities for revitalizing the estuary. As the muddiest estuary within the Wadden Sea and the last naturally connected river in the Netherlands, mud plays a vital role in the overall estuarine system, significantly influencing the economy, ecology, and cultural memory formation. The thesis proposes a spatially dynamic transformation of the Ems estuary through small-scale design interventions and pilot projects by conducting research and analysis in these three domains.

In the Nieuwe Waterweg an ongoing pilot takes place in one of the groyne areas. This pilot is a part of the “De Groene Poort” project. The idea for this project is to create intertidal areas by modifying and nourishing groyne areas. These artificially formed intertidal areas will create a new habitat for wildlife and attract different vegetation. For this project, several modifications have been made to the groyne area: a dam has been placed in front of the groyne area, the groynes on either side have been partially heightened and a nourishment has taken place. To better understand the effects of these modifications to the groyne area, research has been done on the effects of the tide and waves caused by shipping on the flow and sediment behaviour inside the groyne area.

Two different measurement campaigns have been carried out for this research. In the first measurement campaign six ADVs were placed in the groyne area between March 24 and April 12 ,2022. These ADVs measured the velocities and pressures with a frequency of 8 Hz. Also, sediment samples were taken during this campaign, which have been analyzed using laser diffraction. In the second campaign, ADCP measurements were carried out with a floating ADCP attached to a jet ski. With this ADCP the vertical velocity profiles have been obtained for several cross-sections at different stages of the tide.

It was found that the flow in the groyne area has changed due to the dam and the nourishment. Typical flow in an emerged groyne field with the eddies is unrecognizable during lower water levels in the modified groyne area. The exchange of water between the main channel and the groyne area is through the gap in the dam, which removes the mixing layer that separates the groyne and the main channel during submerged conditions. The flow in this groyne area is lateral and divided into two main flows along either side of the nourished island. Rocks and a higher entrance in the downstream groyne compared to the entrance in the other groyne, gives the groyne area different flow velocities between ebb and flood. The neap and spring tidal cycle enhances Ebb and flood differences in the groyne area even more. These velocities differences also impact the sediment transport and its absolute direction in the groyne area. The dam also helps reduce the wave influence on the groyne area, although these waves still significantly influence the velocities. Waves also propagate in a lateral motion from groyne to groyne through the groyne area. The water can move material from the nourishment, but the spreading of this material is limited to a number of locations in the groyne area. For the largest part, the nourished material is still at the location where it is deposited.

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.

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.

Low-lying, densely-populated coastal areas across the world are under threat of hurricane-induced floods. This is the case in, among others, the Galveston Bay Area. In response to this threat the USACE (United States Corps of Engineers) recommended a plan to reduce the risk of flooding, through a range of measures to form a resilient coast of Texas. This plan includes a storm surge barrier at Bolivar Roads in Galveston Bay, whilst the other inlet at San Luis Pass remains open for environmental reasons. Leaving the San Luis Pass open still results in a significant rise in the water level of Galveston Bay, so finding a way to close the San Luis Pass is preferred. Therefore, this thesis researches a barrier for the San Luis Pass to contribute to a resilient Texan coast whilst also taking the local natural habitat into account. The methodology combines the concept of 'Building with Nature' and the civil engineering design method. The methodology incorporates the flood safety, ecological values, and socio-economic aspect into the design process. A conventional storm surge barrier does not fulfill all the requirements and criteria for the San Luis Pass. This research thus proposes a new, innovative type of storm surge barrier: The Shade Curtain Barrier. The shade curtain barrier functions by being rolled down in extreme conditions and stored under the bridge under normal conditions. The two main advantages are that no bottom protection is required and the view is hardly disrupted. Concluding, the shade curtain barrier is a promising solution for a storm surge barrier at the San Luis Pass to contribute to a resilient Texas coast.

A managed realignment is the landward relocation of a flood defence to re-establish tidal exchange on formerly reclaimed land. In a managed realignment, the newly formed intertidal area acts together with the realigned dike as a nature-based flood defence system. In this thesis, the focus is on the managed realignment of Perkpolder, a former polder located in the south of Zeeland (The Netherlands). This realignment serves as nature compensation for the dredging activities in the Western Scheldt. The realigned area contains an intertidal flat which facilitates ecological services.

A combination of different data sources (sediment samples, flow velocities, turbidity measurements and bathymetric data), together with a hydrodynamic model of the Western Scheldt and a morphodynamic model of Perkpolder are used to interpret the hydrodynamic and morphodynamic response of Perkpolder after the managed realignment.

Bathymetric data composed of LIDAR, multibeam and single-beam measurements shows three main developments in Perkpolder: (1) erosion of the frontal entrance area, the inlet and the seaward side of the creeks, (2) infilling of the creeks landward of the first bifurcation and (3) sedimentation on the intertidal area and even more sedimentation of the pond. By comparing the temporal evolution of the bed levels of the frontal entrance area with measured concentrations at the inlet of Perkpolder, it can be concluded that the frontal entrance area acted as a finite source of sediment that eroded quickly in the first years and increased sedimentation within Perkpolder during these years.

New realignment projects can profit from the research at Perkpolder. Bed shear stresses of the initial proposed layout of Perkpolder gives a clear indication if the creeks are setup correctly. Furthermore, an estimate of the initial sedimentation can be based on the accretion rate of other intertidal areas within the estuary. Since realigned areas are often more sheltered and have a lower elevation, accretion rates may be higher. Moreover, not only the forcing of the estuary, but at the same time the availability of different sediment sources and the initial bathymetry shape the morphodynamic response of the realignment site. Therefore, it is recommended to measure the bathymetry for a sufficient long period (e.g. 10 years after the realignment) to get grip on the mechanisms shaping the morphodynamics at these areas.

In an era of rising seas and other challenges posed by climate change, coastal regions like the Netherlands are facing ever graver threats. Strategic sand nourishments could mitigate the threat of coastal erosion and sea level rise on barrier island coasts while limiting ecological impacts. However, insufficient knowledge of sediment transport pathways at tidal inlets and ebb-tidal deltas prevents an informed response in these areas.

The main goal of this project was to describe and quantify the pathways that sediment takes on an ebb-tidal delta. To reach this goal, we focused our analyses on Ameland ebb-tidal delta in the Netherlands. Before we could begin to tackle this challenge, we needed to develop new tools and techniques for analyzing a combination of field measurements and numerical models. These include a method for analyzing the stratigraphy and mapping the morphodynamic evolution of ebb-tidal deltas, a new metric for characterizing suspended sediment composition, and innovative use of sediment tracers. We also established a quantitative approach for looking at and thinking about sediment pathways via the sediment connectivity framework, and developed a Lagrangian model to visualize and predict these pathways efficiently.

The techniques developed here are useful in a wider range of coastal settings beyond Ameland, and are already being applied in practice. We foresee that the main impacts of this project will be to improve nourishment strategies, numerical modelling, and field data analysis. This dissertation also points forward to numerous opportunities for further investigation, including the continued development of the connectivity framework and SedTRAILS. By managing our coastal sediment more effectively, we will set the stage for a more sustainable future, in spite of the challenges that lie ahead.

An increasing amount of the worlds coastal regions suffer from structural erosion due to anthropogenic influences. A prime example for this phenomenon is the Oosterschelde, Netherlands, where the partial closure of an estuary lead to a decline of the local intertidal flats, threatening the valuable functions they provide as an ecosystem and for shore protection. A sustainable measure to slow down the decline, is the usage of wave disrupting oyster reefs, which reduce the wave energy on the flat, thus reducing erosion. As of now, the conditionality and extend of the wave attenuation effect of such reefs is still widely unknown. This is thwarting a more extensive usage of oyster reefs for erosion management, since the actual effects of such a reef are highly uncertain prior to construction. This thesis was designed to investigate and quantify wave attenuation capabilities of an artificial oyster reef in a macrotidal environment, improving the understanding of the impact of such structures on their physical environment. A wave analysis was carried out on a reef using field data, allowing for the quantification of attenuation, the identification of processes, a parameterization of the results and an insight on the predictability of attenuation. Extensive field data was acquired on seven locations on a transect over the investigated reef, including pressure and velocity data. The reef was built using shell filled wire gabions and is orientated perpendicular to the main wind direction. It has dimensions of approximately 120 m length, 8 m width and 0.8 m height. The reef crest is located 0.5 m above mean low water level and 2.3 m below mean high water level, being emerged approximately 30 % of the time. Incoming waves are characterized by maximum significant wave heights of 0.8 m and maximum mean periods of 3 seconds. Wave attenuation was quantified by comparing the energy flux in front of (seaward) and behind (shoreward) the reef under different hydrodynamic conditions. The conditionality of attenuation was found to be best represented by the relative submergence ds /Hm0,i , which represents the ratio of the freeboard on top of the reef and the incoming wave height. Attenuation was found until a ds /Hm0,i ratio of approximately 4. Compared to the bare tidal flat in front of the reef, the reef attenuated energy up to 40 times more effectively per distance. The spatial footprint of the reef was found to vary strongly with different ds /Hm0,i ratios, but being roughly of the same order of magnitude as the reef length. Reflection, overtopping, wave breaking and friction were identified as the main mechanisms behind energy attenuation. An attempt to parameterize these was done using dimensionless numbers, which can be used to predict attenuation by using incoming wave, and reef characteristics. The determined processes were best represented by the parameters ds /Hm0,i and L01 /ds. Three empirical relations describing wave attenuation over breakwaters were used to investigate whether existing formulas can predict wave attenuation over the observed oyster reef. The measured and calculated results show a strong correlation of R ~ 0.9 but need to be tuned to reach reasonable root mean square errors. This implies that such existing formulas can be adjusted to represent oyster reefs, making them a possible tool to predict oyster reef performance in the future. The findings show that the present oyster reef is effective in attenuating waves and contribute to the overall understanding of how oyster reefs can reduce erosion. A better insight on the quantity of attenuation and the spatial footprint of the reef's effects was achieved. The footprint of the reef's effect is hereby limited to an area close to it, meaning that the reefs impact on erosion should not be overestimated. Such a reef should thus rather be seen as a complementary measure against erosion, stabilising the local area. As a next step, setting up a numerical model is recommended, where the wave attenuation can be linked to actual erosion quantities. Quantitative research on additional oyster gabions can further be used to refine existing empirical relations, creating some predictability of transmission values over oyster reefs.

The Rhine-Meuse estuary is part of the Southwestern Delta area in the Netherlands. The nature present in the estuary suffered a great impact due to urbanization, as a natural delta transformed into an industrialized river, mainly due to port activities. Various parties decided to collaborate in order to return the tidal nature in the Rhine Meuse estuary through constructing tidal parks. Although the idea of reintroducing tidal nature into an industrialized estuary seems promising, problems arise when the projects are being executed. Human interventions necessary to complete the design of the tidal parks often lead to undesired morphological changes. These either consisted of an excessive amount of cohesive sediment import, or hardly any import. Consequently, maintenance in form of dredging or nourishing activities is required. When introducing tidal nature, little maintenance is desired, both from a financial perspective and from an ecological perspective. The undesired morphological changes result from limited knowledge on the impact of human interventions on the hydro and morphodynamics in tidal parks. This research therefore primarily focuses on gaining new knowledge on the hydrodynamics and morphodynamics inside tidal parks and assessing the effect of different elements such as groynes and longitudinal walls on the latter. This is done through a case study, the Groene Poort, a collective name for the tidal parks located in the Nieuwe Waterweg.

A depth-averaged (2DH) Delft3D model is used to research the hydro and morphodynamics of the tidal park. A constant tidal forcing and a constant fresh water discharge are used as boundary conditions. The effects of wind, waves and density driven currents are neglected. The effects of changes in hydrodynamic forcing, including a morphological tide, a spring tide and a spring tide in combination with a surge on the morphodynamics are determined. This research showed that the tidal flow results in relatively small velocities inside the studied tidal park with mean velocities around 5 cm/s. The critical velocities for transport of noncohesive sediment transport are only reached at the west entrance of the studied tidal park. At this location the sediment deposits, further transport towards the river bank is not possible resulting from the magnitude of velocities. A spring tide is responsible for an increased import of noncohesive sediment at the river bank area.

This research provides a guide on the effect of relatively small adaptations in geometry which will enhance or decrease the natural processes of sedimentation and erosion in the tidal parks. Tables are included providing the important parameters both with respect to the hydro and morphodynamics of various adaptations in geometry. These tables can be used in future design and adaptations of already constructed tidal parks. However, the effect of the surrounding area including the location in the river greatly impact the hydrodynamics and morphological development of the tidal parks.

The overarching aim of this research is to contribute towards a best practice in modelling meso-scale O(10km), medium-term O(5yrs.) morphological features in Tidal & Estuarine environments. Therefore, various modelling aspects and stages are reviewed. The first pillar argues the need for numerical models in the Geomorphological Engineering discipline on the basis of a brief overview of the historic advances in numerical modelling. Next, the foundation of a numerical model is considered by means of a concise overview of numerical discretisation techniques and their characteristics. We assess the representation of the typical set of physical processes that one may encounter in present-day morphodynamic models and compare their (mathematical) representation following from the underlying choice of discretisation technique. Hereto, we refrain ourselves to the two commonly used methods - finite element and finite difference method. The second pillar gives an overview of the methods that geomorphologists may employ to reduce the model input, and methods to accelerate morphological computations. The last pillar sheds light on different methods that may be applied by geomorphologists to give more insight into the performance of morphodynamic models. These results can, in turn, be of considerable interest in the pursuit to quantitatively validate model predictions. Lastly, this research is concluded with a set of interviews wherein esteemed applied morphodynamic experts were asked about their perception on a possible best practice in modelling skillful meso-scale features in the timeframe of reference. This research hints, therefore, towards an overarching systematic strategy that could be applied by geomorphologists to acquire accurate morphodynamic model predictions on the temporal scale of 5-10 years and for meso-scale features in the order of tens of kilometres.