Tidal flats are complex coastal environments where tides, waves, and sediment dynamics interact to shape morphology and sustain ecological functions. Understanding these interactions is essential for managing estuarine systems such as the Western Scheldt, where port activities, sea level rise, flood protection and ecological conservation increasingly compete for space. Although previous studies have highlighted the importance of waves in mobilising fine sediments, many have relied on monochromatic representations of wave fields. These approaches reduce irregular wave fields to a single characteristic height and period, which conceals the influence of long waves and multi-modal spectra. This thesis reviews the potential of a spectral approach to improve the understanding of wave and sediment dynamics on muddy tidal flats by explicitly considering the full wave spectrum and its cross-shore transformation.

The research investigates whether a spectral approach can provide a better interpretation of wave-driven sediment dynamics than a monochromatic description. Field measurements were conducted on the Gat van Borsele tidal flat in the Western Scheldt during a month-long campaign in spring 2025. Three instrument frames were deployed along a cross-shore transect, each equipped with acoustic Doppler velocimeters (ADV), suspended sediment sensors (STMS), and pressure sensors. These instruments recorded 3D velocities, suspended sediment concentrations (SSC), and surface elevations to capture the evolution of both short and long wave components under varying water levels, tidal currents, and wind forcing. The resulting dataset allowed the construction of wave spectra and the analysis of corresponding near-bed orbital velocities, bed shear stresses, and sediment responses.

Short waves, defined within frequencies f = [0.1 - 1.5]Hz, were found to dominate surface elevation variance and to behave largely linearly, confirming that local wind-generated waves account for most of the tidal flat’s wave energy. The mild wind conditions were unable to generate depth-limited waves which are typically identified on tidal flats. Instead the effect of wind direction and fetch was clearly observed, where more onshore winds drove larger short waves. Long waves, defined as $f = [0.003 - 0.1] Hz$, were generally less energetic but contributed a significant portion of near-bed velocity variance, particularly at large water depths which attenuate short wave velocities. Large long waves could also be identified which originated outside of the local wave field. These are suspected to be formed by seiches or container vessels passing through the navigation channel. These more pronounced long waves reached orbital velocities similar to the short wave field, highlighting their capacity to influence sediment dynamics.

The SSC analysis revealed strongest responses to wave forcing within shallow water. Suspended sediment concentrations did not respond strongly to changes in wave driven bed shear stress. Instead, peaks in SSC were observed within a suspected turbid fringe which traveled up and down the tidal flat within shallow water between the middle and low measurement frames. At the high measurement frame, SSC had minimal response which contradicted the largest wave driven bed shear stresses at that location. Observing time series of SSC and long wave velocities reveals fluctuations of SSC similar to the long wave time scale. Yet this would require a co-spectral analysis to be confirmed.

The study concludes that a spectral approach enhances understanding of wave and sediment dynamics on muddy tidal flats by distinguishing between short and long wave processes that would otherwise be obscured. Under calm wave climates such as those observed during the measurement period, a monochromatic approach remains adequate. However, during more energetic events or at sites influenced by seiches, vessel traffic, or infra-gravity waves, the spectral approach holds great potential to correctly interpret cross-shore sediment transport by waves. Future research should therefore target more exposed or navigationally influenced flats to capture the full variability of wave-driven processes. Here it is critical to measure for a sufficient burst duration as to fully capture the potential long waves. By applying spectral analyses in such settings, this work contributes to a more complete understanding of estuarine morphodynamics and supports sustainable management of tidal flat systems.

Previous studies show that climate change effects and anthropogenic disturbances are having an increasingly strenuous effect on the performance of the coastal system of Sisal, a rural fishing village in the state of Yucatán, Mexico. Resilience has been a crucial component for these studies, as it presents the system's ability to react and adapt to environmental hazards and human interventions. In recent years, several research initiatives have aimed to assess the resilience of Sisal's coastal system through the use of previously identified resilience index calculations. However, these indices lack the ability to quantify disparities of resilience on a local scale. Furthermore, previous studies focus solely on the technical aspects of resilience, and therefore fall short on defining other relevant aspects of the term.

Consequently, this research project aimed to understand Sisal's coastline by understanding the multi-faceted definitions of resilience by analysing the socio-economic and technical factors impacting the coastal system. The study involved examining both its historical context, as well as potential future trends and developments.

This was done through a thorough evaluation of different aspects of the coast, a storm impact study and a social analysis. The findings of these studies are subsequently incorporated into a comprehensive web-tool, providing experts, policymakers, and community members with clear-cut and valuable information on the current resilience of Sisal's coastline.

The findings of the research present that the coastal resilience of Sisal is highly negatively impacted by human interventions along the coast, predominantly causing inadequate resilience performances on profiles with significant anthropogenic perturbations and generally along the coastline westwards of Sisal's port. Current coastal management policies are considered ineffective to deal with these developments, which is among others caused by exclusive decision-making processes and ambiguous governmental policies. Changes in coastal management strategies are therefore needed to effectively deal with future threats of environmental hazards and human interference. Ultimately, resulting in improved coastal and community resilience.

This research study has investigated the effects of sills as a nature-based countermeasure to reduce saltwater intrusion within Rijnmond Drechtsteden. The effects of this measure have been assessed regarding its impact on port logistics and freshwater supply in the region. The heavily dredged New Waterway provides the vital open connection to the sea for port operations. Meanwhile this same waterway allows salt to intrude landwards into the estuary, placing pressure on the freshwater network. With climate change leading to rising sea levels and extreme variations in river discharge, the impacts on both stakeholders are likely to increase. Previous research into bed shallowing indicated detrimental effects for port operations, therefore alternative nature-based solutions need to be investigated such as underwater sills.

The impact of a sill has been assessed using the Frame of Reference approach (Van Koningsveld, 2003). To determine the relationship between each stakeholder, a quantitative trade off has been defined using the comparison tool as per the method of Iglesias (2022). The tool facilitates the comparison of individual effects created by (nature-based) interventions using design parameters and performance indicators.

Via an extensive literature study, critical design parameters were defined for underwater sills. It was concluded that sill height and sill length would have the greatest impact on both the port and the freshwater supply in the region.
Then, the impact on the stakeholders was defined through choosing specific performance indicators and modelling techniques. The performance of port logistics was represented via the percentage of accessible tides and modelled using a simplified case in the nautical traffic model OpenTNSim. The performance of the freshwater supply was quantified using the percentage of freshwater availability, this was modelled using the concept of an idealised estuary in Delft Flexible 3D Mesh (DFM).
The research method takes the operational objective of the port as a base to select specific sill designs. This objective depends on tidally bound inbound vessels having access for 99% of all tides (de Jong, 2020). To comply with this requirement, sill heights of 0.1m, 0.2m, 0.3m and lengths of 2km, 5km, 10km were chosen. The model results show that sill height has a large impact on accessible tides, therefore severely limiting the range of heights that could be considered. Sill length has a lesser impact, allowing for a wider range to be assessed.

The effect of the sill designs on freshwater availability is negligible, with responses varying between 0.01% and 0.8%. This lack of response indicates that the chosen sill designs are too small to have a significant impact on salt intrusion in the channel. The sill designs led to access percentages between 100% and 70%, a wider range of percentages was investigated to see the effect of a more flexible port.
The resulting trade off curves all exhibit linear functions, indicating the lack of any trade off relation between the performance indicators and the sill designs. Since the sill designs fail to achieve both operational objectives, the overall strategic objective is not met. It can therefore be concluded that underwater sills are not feasible in the case study of the Rotterdam waterways.
The research results indicate that other nature-based solutions should be investigated to better suit the actively dredged channels in the Rotterdam waterways. Alternative methods which can synergise with this active dredging characteristic may pose to be significantly effective.