Mangrove forests are coastal ecosystems that provide a wide range of ecosystem services. Among these, their role in coastal protection is especially relevant, as mangroves have the potential to reduce storm-driven flooding by slowing down surge, dissipating short-wave energy, and retaining water. With tropical storms intensifying due to climate change and mangrove ecosystems being increasingly under threat, understanding the capacity of mangroves to reduce flood extent under tropical storms has gained greater urgency. Yet, the complex interactions between ecological, geomorphological, and hydrodynamic variables that shape the capacity of mangroves to reduce flood extent make this difficult to assess. Hydrodynamic flood models face challenges in incorporating such complexity and are often constrained in their validation by the scarcity of observational flood data in tropical regions where mangroves are most prevalent. Remote sensing presents a promising yet underutilized opportunity to improve understanding of mangrove flood attenuation and support the validation of hydrodynamic models.

This research presents a novel approach consisting of (i) a conceptual guideline that structures the key variables influencing mangrove-induced flood attenuation and (ii) a remote sensing tool that maps flood extent using Sentinel-1 SAR C-band data in Google Earth Engine and visualizes relevant variables. Together, they form an approach that not only maps flood extent in mangrove areas following tropical storms, but also integrates key ecological, geomorphological, and hydrodynamic variables to contextualize the factors influencing flood attenuation by mangroves.

The approach was applied to a case study of Hurricane Irma (2017) in the mangrove region of the Everglades National Park, where the remote sensing tool detected a flood extent of 168,811 hectares, showing spatial resemblance to flood maps reported in literature. The flood detection model and its settings were refined through dry tests and a threshold sensitivity analysis. Applying the conceptual guideline to the case study area, guided by the outputs of the remote sensing tool, provided insights into elements affecting flood attenuation by mangroves in the region, including mangrove extent, storm track, precipitation, elevation, mangrove biomass and zonation, and channel patterns. Furthermore, comparing the flood extent derived from the remote sensing tool with that of a bathtub inundation model enabled the identification of a high-potential area where mangrove impact on flood extent appears most likely, providing a targeted basis for future research.

These results highlight the value of remote sensing as an accessible and globally applicable tool for generating flood extent maps and identifying areas with high potential for flood reduction by mangroves following tropical storms. The developed remote sensing tool represents a first step toward demonstrating how remote sensing can strengthen the validation of hydrodynamic flood models. Since such models can isolate the influence of mangroves on flooding, integrating remote sensing-derived flood extents as validation can enhance their accuracy and support future research into the flood attenuation capacity of mangroves. Ultimately, combined with the insights provided by the conceptual guideline, this can contribute to the wider goal of improving the understanding of the role of mangroves in coastal flood protection during tropical storms in the face of climate change.

Shallow lakes are systems where wind-driven circulation affects water quality and sediment transport. This study investigates how bathymetry, lake planform, and artificial islands influence hydrodynamics in Lake Markermeer, a wind-driven, non-tidal shallow lake (~ 4m depth). Using Delft3D-FLOW hydrodynamic depth-averaged simulations, this study examines the effects of depth variation, shoreline complexity, and the presence and shape of islands on horizontal circulation patterns and vorticity generation. In this study, vorticity is considered as a proxy for fluid mixing, which, together with horizontal circulation, plays an important role in determining water quality.

A multi-step modeling approach was used: (1) idealized simulations assessed bathymetric and shoreline effects on horizontal circulation and vorticity; (2) an artificial island (Marker Wadden) was placed in the lake to evaluate its impact. A high-resolution numerical model quantified local vorticity for different island shapes, which were generated using Fourier transformation applied to the island boundaries.

Bathymetry plays the dominant role in large-scale circulation, with depth variations producing pressure gradients that drive gyre formation. In particular, depths over 4 meters are responsible for creating gyres in Lake Markermeer, while lakes with uniform depth show little horizontal flow. Lake planform has limited influence, mainly affecting local vorticity near shorelines. Although limited in influence, an irregularly shaped lake like Lake Markermeer can have vorticity values up to 70% higher than a circular lake of similar size.

Artificial islands influence local hydrodynamics but have minimal effect on large-scale horizontal circulation within the basin. Circular islands lead to more uniform and predictable beach erosion, while irregularly shaped islands induce flow separation and vortices that enhance mixing but increase the risk of localized and less predictable erosion. For optimal erosion control, an elliptical island shape aligned with the dominant large-scale circulation is recommended.

These findings provide insights for lake management, demonstrating that bathymetric modifications are more effective than shoreline modifications for influencing horizontal circulation. The results suggest that in artificial island design, streamlined shapes are preferable for erosion control, while complex shapes enhance local mixing, promoting nutrient distribution and improving water quality. The methodology developed in this study is applicable to other shallow lakes worldwide, providing a structured approach for hydrodynamic assessments in lake management.

Coral reefs play a crucial role in coastal protection and ecosystem sustainability. However, they face increasing threats from sea-level rise, ocean warming, and acidification. While coral reef restoration is often proposed as a means to reduce coastal hazards, there remains insufficient field evidence or large-scale laboratory data to validate this claim. A key aspect in assessing the effectiveness of reef restoration is understanding the flow structure within artificial reefs, as wave energy dissipation is closely tied to these dynamics. Not accounting for in-canopy flow in numerical models can lead to inaccuracies in predicting wave dissipation and water level changes.

This thesis investigates wave transformation and dissipation through large-scale wave flume experiments conducted at Deltares' Delta Flume. The experiments utilized a 1:3 scale model of a Maldivian fringing reef, tested under five wave conditions and two water levels. The experimental setup included a 45-meter-wide reef flat, where a 10-meter section was equipped with either 91 or 153 3D-printed artificial reef elements (0.25m high, 0.40m wide). Data were collected using pressure sensors and flow meters to measure wave transformation, while two ADV arrays analyzed in-canopy flow. These CREST experiments, carried out in collaboration with Plymouth University, Deltares, Boskalis, and Coastruction, contribute to the ARISE project, which explores atoll island adaptation to rising sea levels.

The research addresses four primary objectives: (a) evaluating the impact of artificial reefs on wave transformation, frequency dependence, and water level response; (b) identifying in-canopy flow characteristics; (c) linking these flow characteristics to wave dissipation; and (d) assessing the accuracy of an existing in-canopy flow model.

Findings indicate that as water levels rise, the relative height of the artificial reef decreases, leading to a decline in wave height reduction capacity. The reduction ranges from 8.5% to 15.2% at low water levels and from 4.9% to 5.9% at high water levels. Despite reducing incoming wave heights, artificial reefs can, under certain conditions, increase rather than decrease extreme water levels onshore due to the artificial reef-induced drag force amplifying wave setup.

Analysis of streamwise velocity variance revealed significant spatial differences. Velocity variance decreased more in the ADV array located behind the reef elements compared to the ADV array positioned between them, where velocity variance increased. Flow attenuation was found to be more pronounced at lower water levels, with longer and higher waves attenuated more effectively than shorter and lower waves. The modeled canopy wave dissipation rate, was found to be 43-87% lower than observed dissipation, though still within the same order of magnitude. The discrepancy is partly due to the model not accounting for breaker dissipation and non-linear energy transfers. Flow convergence corrections significantly increased flow attenuation and reduced canopy dissipation rates, while ADV selection had minimal impact.

According to canopy flow regime classification, the tested conditions fell between inertia-dominated and general flow. Reducing element spacing could shift the flow further into the general flow regime, enhancing frequency dependence in flow attenuation and wave dissipation. Given its limited wave height reduction capacity, artificial reefs like these would be most effective when integrated into hybrid coastal protection strategies. Such combinations could enhance both coastal resilience and ecological benefits. Ultimately, the CREST experiments provide a valuable dataset for model calibration and validation, contributing to a deeper understanding of reef hydrodynamics and in-canopy flow dynamics.

Low-lying coral islands are highly vulnerable to sea-level rise and climate change, with coral reefs, essential for coastal protection, facing significant decline due to climate change and anthropogenic stressors. Artificial reefs present a potential solution by restoring ecology and mitigating coastal flooding, but their effectiveness and impact on wave dynamics remain unclear. This study investigates the effects of an artificial reef on spectral wave transformation over fringing reefs, combining numerical modelling with SWASH and large-scale experiments conducted in the Deltares Delta Flume. The experiments involved a scaled reconstruction of a fringing reef island with a geometric scaling factor of 3. Numerical modelling was used to optimise the laboratory experiments by adjusting reef placement and element density for maximum wave height reduction.

The results of this thesis show a limited impact of an optimised artificial reef on the reef flat, with effectiveness expected to decrease further as sea levels rise. However, the smaller scale of the reef elements prioritises ecological benefits, and when combined with larger structures, these reefs can still offer valuable benefits for biodiversity and wave mitigation. The experiments provide a valuable new dataset on the hydrodynamics of complex canopies, and can be used to calibrate several numerical models and methods of schematising artificial reefs.

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.

Lake Bardawil is a hypersaline shallow micro-tidal double-inlet lagoon on the northern coast of the Sinai Peninsula in Egypt. The inlets, named Boughaz 1 and Boughaz 2, connect Lake Bardawil to the Mediterranean Sea. The local population relies on the fishing yields from the lagoon, however, in the current situation, the lagoon’s inlets are unstable, needing constant maintenance dredging to stay open. Additionally, due to the hypersaline situation (limiting fish attraction) and high fish catch demand, the current fishing situation is unsustainable. The Weather Makers propose a solution to the problem using dredging interventions. Enlarging the inlets of Lake Bardawil is expected to result in increased fish migration, decreased salinity in the lagoon and increased stability of the inlets. This research aims to investigate the short-term response of Lake Bardawil and specifically its inlets to the dredging interventions proposed. To investigate the response to the interventions, two numerical models are used; a hydrodynamic model in D-flow FM and a coupled morphological model using D-flow FM and D-waves. The model results are analysed in terms of the hydrodynamic tidal response, combined hydrodynamic response to tide and weather and finally the morphological response to tidal and weather effects. From the hydrodynamic model, it can be concluded that the interventions result in a larger tidal prism for the lagoon by a factor of 1.6. This is expected to result in increased fish migration and reduced salinity in the lagoon. As the tidal prism increases, so does the discharge through the inlets. The increased discharge through the inlets results in inlet velocity amplitudes that exceed the theoretical critical value for convergence to a dynamically stable inlet cross-section for both inlets. The interventions affect more than just tidal currents in Lake Bardawil. Weather effects in the lagoon are amplified by the interventions. This is demonstrated by increased interaction between the inlets in terms of net import and export of water. During winter storms, the wind-induced flow from Boughaz 1 to Boughaz 2 is greater in magnitude. The wind waves caused by these winter storms result in sediment resuspension. This results in larger sediment transport from the basin to Boughaz 2. Using the coupled model, the morphological response in both inlets can be predicted. In Boughaz 1, the present sediment-importing character is amplified. Whereas the similar summertime sediment-importing character of Boughaz 2 is reduced substantially. The sediment export of Boughaz 2 during winter is larger in the new situation. The interventions result in increased sedimentation in the dredged channel of Boughaz 1. This does not hinder the increased tidal discharge at Boughaz 1, which is the main intended function of the interventions. This means the increased sedimentation does not influence the intended functioning of the interventions in the short term. Due to the increased sedimentation in the inner dredged channel, there is no indication of increased stability in Boughaz 1. Boughaz 2 shows less morphological activity in the situation after interventions. As is the case with Boughaz 1, there is no hindrance to the functioning of the interventions during the short simulation period. There is reduced deposition of sediment in the inner flood delta region of the inlet and the updrift side of the inlet channel compared to the known unstable situation during both winter and summer simulations. This indicates improved stability for Boughaz 2. The short-term response to the interventions in the inlets of the Lake Bardawil system is as intended, the tidal prism is increased and while there is only an indication of increased stability in Boughaz 2, there are no short-term impediments to the intended primary functioning of the interventions in either of the inlets.

Living Breakwaters have evolved from traditional breakwaters to create multi-purpose structures that provide environmental and social benefits. These breakwaters promote a healthy habitat for flora and fauna, while achieving the same structural capabilities as conventional structures. Reefy has developed a modular living breakwater that can serve both as an artificial reef and a stable breakwater. This study aims to model the impact of a Reefy breakwater on the transmitted wave height, using a process-based numerical model, namely XBeach.

This study builds upon a previous one conducted by van den Brekel [2021], where scaled experiments were performed in a physical wave flume, to investigate the hydrodynamic and ecological functionalities of the Reefy breakwater. In the wave flume experiments a total of 15 structure configurations together with 35 wave conditions, both regular and irregular, were tested. The first 7 of the configurations were 2D structures with a relatively simple shape and a porosity of 20%, and the rest were complex 3D structures with a porosity above 20%. The wave flume was recreated within the numerical model. The influence of the breakwaters on the wave heights are examined through the comparison between the calculated transmission coefficient of the wave flume and the numerical model. However, XBeach, and more specifically XBeach non hydrostatic+ mode which was used, does not resolve the complex interaction between waves, friction and porosity of the structures. As a result, a simple method had to be found to compensate for the loss of these phenomena, while successfully calculating the transmission coefficient.

The findings of this analysis demonstrate that treating the breakwater as an impermeable change in bathymetry, coupled with a 15% reduction in structure height, produces the desired outcomes. From the 35 wave conditions only the 8 irregular wave conditions were chosen to be of interest due to time constraints. Among these 8 wave conditions that were used as an input in XBeach, only 5 of them were satisfying the criterion that describes the range for which the numerical model should produce accurate results (kd<2). The transmission coefficient was defined as the ratio of the transmitted wave height behind the breakwater, and incident wave height in front of the structure. The wave decomposition method used was a modified Guza split method. In pursuit of finding a valid solution, the breakwaters were modeled firstly as impermeable structures with a decreased height, secondly as impermeable structures with a decreased width, thirdly as vegetation with the help of the vegetation module, and lastly the maximum wave steepness criterion as applied in the model was increased (maxbrsteep up to 1.4), without modifying the structure height.

Among the suggested solutions and after being validated for 13 experiments with both simple and complex structures, decreasing the width did not achieve the desired results while the vegetation module failed to produce consistent outcomes. As far as transmission coefficient is of interest, a 15% decrease in the structure height had the smallest mean absolute percentage error of almost 10% and a root mean square error of 8.2%. On the other hand, choosing to increase the maximum wave steepness (to a value of 1.4) is not a valid option for the complex 3D structures, but illustrates even better behavior than a change in the structure height when simple structures with 20% porosity are concerned. To summarize, this thesis provides alternatives to replicate in a simplified way the impact of porous structures, such as Reefy breakwaters, without the need of an extensive and time expensive numerical model.

Mangrove forests are important (sub)tropical intertidal ecosystems providing coastal protection and other ecosystem services like carbon storage and biodiversity. In view of the loss of these forests many rehabilitation projects have started, but unfortunately these efforts often fail. A main cause for coastal erosion is a disrupted (fine) sediment balance due to human interventions. This imbalance prevents establishment of propagules (floating mangrove seeds) at intertidal areas and the early growth of planted mangroves. Coastal solutions such as permeable dams have aimed to stop the erosion, but we need additional systematic methodologies to understand the coastal system.

The objective of this thesis is to extend the physical and ecological understanding of coastal mangrove systems, especially of currents, sediment pathways and propagule pathways, to improve rehabilitation strategies using Demak (Central Java, Indonesia) as study area. Demak's coastline suffers from erosion problems, originating from extensive land use by fish ponds and land subsidence. An idealized morphostatic 3D model was set up in Delft3D-4 to simulate the hydrodynamics of the area, influenced by the interaction between flows due to river discharge, tides and wind. The simulations are used as input for the SedTRAILS particle tracking model to compute sediment and propagule pathways.

The model results support that sediment supplied by alongshore currents and riverine sediment could serve as source for the coastline. The river plume is at the same time obstructing as well as trapping sediment and propagules, because stratification prevents mixing of the sea and river water. However, sediments suspended near the bed could be transported underneath the river plume towards the coast by estuarine circulation. Another lesson learnt is that sediment (suspended in the water column) and propagules (floating around the water surface) do not follow the same pathways, as for propagule dispersal the wind direction is of much larger influence than tidal currents. The simulations confirmed that seasonal variability in wind direction could significantly influence the establishment area and availability of propagules, and old fish pond bunds can increase the establishment area for propagules. Sensitivity analysis show that the timing of propagule release within one tidal cycle (flood-ebb) influences the probability to successfully establish on the nearby coast. A propagule survival module is developed (based on inundation-free period and bed shear stress) as stormy seasons may increase the sediment establishment, but the larger bed shear stresses may lower the survival of propagules.

Although this modelling study is focusing on Demak, there are many other similar mangrove coasts around the world where an idealized model would enable to understand where the sediment and propagules are going and the travel time from source to sink. Permeable bamboo dams would be recommended to add to the model in further research, because they could increase the survival of propagules by reducing bed shear stresses and increase the establishment of propagules by trapping them. In conclusion, this research has made a start in counteracting coastal erosion and improving the success of mangrove forest rehabilitation by studying sediment and propagule pathways, and developing a propagule survival module.

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.

Lighthouses have guided seafarers for centuries and still play an essential role in navigating marine vessels and bringing them to safe harbours. With climate change, the already harsh ocean environment may become more severe. It is expected that future wave conditions can compromise the structural integrity of the lighthouses with their violent impact. The hydrodynamic processes of wave transition before, at, and after the point of impact are complex, highly non-linear, and turbulent. There is no exact description of these processes. Therefore, there is a need to further study them in order to better understand the broken wave load and its effect on the structural integrity of lighthouses. From the very beginning, wave breaking processes are complex. For instance, prior to impact with shoal-mounted cylindrical structures, waves undergo a rapid transition from deep to shallow water. During this transition the waves shoal, start to break, and once broken they hit the cylindrical structure of the lighthouse. It is therefore necessary to approximate the hydrodynamics. Since, it is difficult to measure the processes using only physical models. Consequently, this research focuses on broken wave load analysis using a process-based numerical model. An approach widely used for analysing wave forces on slender cylindrical structures is calibrated for breaking wave loads. Critical factors for determining these loads are the curling factor and slamming coefficient. However, there is still no scientific consensus regarding the exact values for these parameters. Further research, like this study, can help in determining these values. The main goal of this thesis is to assess the pressure distribution on the structure and quantify the values for the curling factor and slamming coefficient for a shoal-mounted slender cylindrical structure. The STORMLAMP (STructural behaviour Of Rock Mounted Lighthouses At the Mercy of imPulsive waves) project's experimental data of Dassanayake et al. (2019) is used as a benchmark case to validate the proposed computational framework. In addition, the performance of two different Volume of Fluid methods, isoAdvector and MULES, are assessed. This study demonstrates that the curling factor and slamming coefficient are significantly different for the broken wave load compared to the breaking wave load. The curling factor is found to be twice as large in the case of a broken wave load. On the other hand, the peak slamming coefficient is five times smaller. Moreover, the isoAdvector solver in comparison to the MULES solver produces results that better resemble the benchmark case results. Although the two solvers do not produce the same peak impact forces, they do both produce similar values for the aforementioned critical parameters. Also, it was found that the isoAdvector solver is faster and more efficient than the MULES solver in the case of a 3D simulation. From the results it can be concluded that the chosen numerical framework represents the benchmark case well, and that the methodology used for breaking wave loads can be applied for broken wave loads with adjustment of the curling factor and slamming coefficient.

Many coral reef islands are low-lying, which in combination with population growth, sea level rise and possibly more frequent extreme weather events is likely to result in increased coastal risk (e.g. Storlazzi et al., 2015). On smaller scales of O(10 km) wave-driven coastal inundation can be accurately predicted with advanced models such as XBeach (Roelvink et al., 2009), at already high computational costs. For larger scales, larger number of islands, for scenario modelling, and for implementation in early warning systems, computationally faster methods are needed. Reduced physics models, which neglect some of the processes (e.g. non-hydrostatic pressure gradient term and viscosity), are a potential solution. However, their accuracy and the best method to force them has not been established. In this research we propose a new methodology to model wave-driven flooding on coral reef-lined coasts. A look-up-table (LUT), composed of XBeach model runs, is combined with a reduced-physics model, SFINCS (Leijnse et al., 2021), to achieve high accuracy predictions at limited computational expense. The LUT consists of pre-run 1D XBeach simulations for several reef profiles from Scott et al. (2020), forced with different offshore wave and water level conditions. Wave conditions close to the shore as predicted by the LUT are used to force SFINCS which then simulates the wave runup, overtopping and flooding. These are forced in SFINCS using random wave timeseries from an interpolated parameterized wave spectrum following Athif (2020). The accuracy of the method is investigated for 6 distinctive cross-shore profiles from Scott et al. (2020), for two wave scenarios (gentle swell and stormy conditions). Results of complete XBeach simulations are compared to LUT-SFINCS simulations with different boundary forcing locations. The sensitivity analysis shows that the most optimal boundary location to initialize the SFINCS model is at a water depth of 0.5 m, shore-ward of the reef edge. Interpolation of the forcing conditions at the boundary is investigated with 9 different interpolation methods. Results reveal that the most accurate method to interpolate spectral parameters (the amount of high frequency wave energy, the amount of low frequency wave energy and the frequency peak of high frequency part of the spectrum) and wave setup at the boundary is the Inverse Distance Weighting method with a power of -2. Errors introduced by the interpolated parameterized boundary conditions lead to runup estimation errors of 10-20% on average with the maximal errors up to 60%. Simulating wave runup with XBeach LUT – SFINCS couple leads to about 50-times higher computational speed compared to the XBeach model simulating hydrodynamic processes along the entire reef profile.

Ebb-tidal deltas play a key role in the morphology of barrier coastlines and tidal inlet systems as they serve as a natural source of sediment. They are typified by a dynamic morphology that interacts with the adjacent coastlines and sponsors a unique ecological habitat. Under increasing socio-economic, ecological and climate-induced constraints, it becomes imperative to obtain a better understanding of the evolution of ebb-tidal deltas in order to maintain and preserve these morphological features in the near-future time horizon. An increased interest has therefore been raised to understand, quantify and predict the development of ebb-tidal deltas.

In the context of developing a future-proof coastal management and maintenance strategy, the efficiency of ebb-tidal delta nourishments has been further investigated in research programmes such as Coastal Genesis 2.0. For the Ameland inlet, this entailed the construction of a 5 million m3 pilot nourishment over the course of March 2018 to February 2019 and subsequent monitoring in the years following. This research investigates the impact of this pilot nourishment on the natural behaviour of the Ameland ebb-tidal delta. The second goal is to build further knowledge on the modelling capabilities of present state-of-the-art models for the Ameland ebb-tidal delta.

To this end, a process-based Delft3D model is applied to hindcast the morphological development of the ebb-tidal delta with a particular interest in the evolution of ebb-shields and -chutes over the course of 2005 to 2020. It is identified that the representation of the wave-induced processes is key for capturing the development of ebb-shields and -chutes in the model predictions. Therefore, we applied and experimented with an updated nonlinear wave orbital velocity parameterisation and assessed its contribution to the modelling performance.
This thesis demonstrates that the evolution of ebb-shields and chutes on the outer delta is contingent on its initial presence in the initial bathymetry. Hence, the initiation of ebb-shield development is not inherited in the model response. These insights have been integrated and synthesised to assess the application of the present state-of-the-art model as a forecasting tool for the morphological development of ebb-tidal delta nourishments.

Ultimately, it is shown that a model using schematised boundary conditions and an efficient morphological updating scheme is able the predict the yearly-averaged development of the pilot nourishment on the ebb-tidal delta. It is demonstrated that the 2019 pilot nourishment only locally influences the behaviour of the Ameland ebb-tidal delta. Nourished sediment is likely to be redistributed along the ebb-shields, contributing on the long-term to the sediment exchange process with the downdrift coast of Ameland. The location of the ebb-tidal delta nourishment is thereby important for the sediment exchange process and the development of local features. Placing ebb-tidal delta nourishments to the south of the Westgat invokes a primary sediment exchange between the coast of Terschelling and surrounding morphological features. A secondary readjustment of the Westgat thereby influences the development of the ebb-shields and -chutes. Constructing an ebb-tidal delta nourishment north of the Westgat results in a sediment exchange between the local ebb-shields and the downdrift coast of Ameland. Our results also demonstrate that an increase in the pilot nourishment construction height enhances the redistribution of sediment along the ebb-shields.

Lastly, this study identifies further opportunities for improving medium-term morphodynamic models for the Ameland inlet by incorporating time-dependent boundary conditions including new transport formulations such as SANTOSS.

Atolls and coral islands in and around the Pacific and Atlantic Ocean largely rely on coral reefs for their coastal protection. On these reefs a large roughness is created by different coral species. The dissipative character of these species reduces the incident wave height, thereby reducing the amount of run-up on the coast. In our current society we are confronted with large issues due to climate change such as global sea level rise and temperature rise. Because of these issues, coral reefs are more prone to bleaching and degradation. Research has also shown that the wave climate will intensify due to sea level rise. As the sea level will rise globally, the water level above the rough coral elements will increase leading to less wave dissipation and more extreme wave conditions at the coast. It is therefore of vast importance to gather more knowledge about the dissipative character of corals to quantify the risks coral reef islands and atolls will face in the coming decades due to climate change.

For this master thesis, an experimental study was performed in the wave flume of the Fluid Mechanics Laboratory of the Technical University of Delft, investigating dissipation rates due to complex corals. 384 corals were reproduced and exposed to 21 different wave conditions. It was found that the presence of corals significantly enhanced dissipation rates. Shallower water depths, higher wave frequencies, and higher wave heights positively influenced the dissipation rates due to the corals. By increasing the amount of corals in flumewise direction and over the cross section, rates also increased.

Lowe et al. (2005b) and Lowe et al. (2007) developed a canopy model that is able to estimate in-canopy flow velocities and dissipation rates due to coral canopies. In the final part of this thesis, this canopy model was assessed and its output was compared with the experimental results. In general, the model predictions were in good agreement with the experimental results.

The research presented in this thesis is a contribution to the understanding of hydrodynamic processes around coral reefs. The good performance of the analytical model of Lowe et al. (2005b) and Lowe et al. (2007) can be of value for quantifying risks atolls and coral reef islands will face in the coming decades. The model can also be used to estimate the impact of coral restoration projects, when coastal protection is of the essence.

At the edge of Vancouver and the Strait of Georgia the Point Grey cliffs experience marine and subaerial erosion. The retreat of the cliffs will eventually result in losing university assets, infrastructure and archaeological treasures. Wave-induced sediment transports are supposed to be the main marine erosion mechanism. This report presents other potential marine erosion mechanisms (e.g. direct wave attack, sediment budget deficits, sea-level rise and increased storm intensity) over different time scales and studies the sediment transport and budgets in more detail. The study is performed with the help of a numerical hydrodynamic model with representative scenarios of the hydrodynamic conditions. The model and scenarios are set-up by means of a site visit, literature, water level records, wind and wave records and sediment samples. The resulting scenarios and results provide an hydrodynamical overview of flow- and wave conditions. Subsequently, the sediment transports, indeed, seem to be initiated by waves. Results also opened up a new potential sediment transport path not yet mentioned by sources. However, the sediment transports does not seem to be the only erosion mechanism. A potential human-induced sediment budget deficit is likely as well. Additionally the direct wave attack and the combined sea-level rise and intensified storms are also either present or becoming more relevant in the future. The contents of this report forms a basis for developing design criteria for a coastal solution to the marine erosion of the Point Grey cliffs. An engineered beach, together with a foreshore nourishment, out of locally dredged material seems like the most logical option. By further improving the numerical model and the overall study with presented recommendations, this study can inspire further design steps for a solution to save the Point Grey cliffs.

Low-lying islands are highly vulnerable to wave-induced flooding, with low-frequency waves (frequency <0.04Hz) being one of the main drivers. The impact of these inundations can increase due to wave resonance over coral reefs, which has been observed in the range of low-frequency waves. This study aims to understand the reef resonance phenomenon along with processes that could limit its resonant amplification over wave height and wave run-up. A numerical experiment was carried out based on a 1D SWASH numerical model. A cross-shore profile of a schematized fringing coral reef was built, and resonance was forced over this bathymetry for the first two resonant modes and two water depths. The offshore forcing was designed as a simplified wave climate, with small amplitude regular low-frequency waves. Resonance was found to occur in a bandwidth of periods for each resonant mode, generating two resonant amplification peaks (one for each resonant mode). The periods leading to the maximum resonant amplification inside each resonant bandwidth are the modeled resonant periods, which were found to be longer than the theoretical resonant periods (based on reef flat width and water depth). The resonant amplification over wave height and run-up were found to be more significant for the fundamental mode than for the first mode, decreasing for both resonant modes when increasing the water depth. Moreover, for the wave conditions modeled in this experiment, the relative resonant amplification was found to be stronger for smaller wave heights than larger wave heights. Reef wave resonance presented a build-up behavior, needing a minimum number of waves to reach a maximum resonant amplification, which varied depending on the wave period, wave height, and water depth. Resonant amplification was found to increase for a larger amount of trapped wave energy over the reef and lower friction dissipation. Frictional dissipation was found to be the most effective process to counteract wave resonant amplification. Thus, increasing coral reef bottom friction is essential for enhancing low-lying island coastal safety.

Sandy barriers comprise 12% of coastlines around the world, and most of these barriers enclose tidal bays and lagoons. These systems accommodate human settlements vulnerable to climate change, which offer enough economic, social, and environmental utility to require further research on the impact of climate change and subsequent best management practices.

The present work aims to analyze how climate change impacts the hydrodynamics and morphodynamics of two barrier inlet systems: Katama Bay (United States of America), and the Santa Lucia Estuary (South Africa). The goal is to estimate future changes in forcing variables (e.g., sea level rise, wave climate, river discharge, tides), implement them in process-based models (coupled SWAN and Delft3D), and identify changes in the dynamics of both systems by comparing present and future state simulations.

This thesis develops a replicable and flexible methodology that can be used as a systematic tool to assess the impacts of climate change on the overall dynamics of tidal inlet systems. A novel approach (copula analysis) was used to derive the wave climate implemented in Delft3D, which was then qualitatively validated for both sites. Model results were used to compare changes to inlet stability, inlet geometry, and sediment pathways for present and future hydrodynamic conditions.

Results show that sea level rise is the primary contributor to the overall morphodynamics at both sites, whereas changes in wave direction strongly impact the rate of inlet migration. Other changes (e.g., significant wave height, wave period, and river discharge) play a secondary role in the dynamics of both systems. Comparisons with previous studies suggest that wave direction impacts each system differently. These impacts must be specifically addressed for each tidal inlet, as the results from one site should not be used to determine a general behavior for the assessment of CC impacts in tidal inlet systems.

San Francisco Bay is the largest estuary on the west coast, and it supports a surrounding population of over 7 million people. Human developments have reduced the amount of fine sediments delivered to the Bay-Delta system in recent years, which threatens habitat restoration initiatives and suitable water quality in the southern reach of the Bay (South Bay), where an ample supply of fine sediments is needed to support tidal flat development and to maintain conditions that limit the excessive growth of algae populations. A better understanding of how sediments enter and are redistributed within South Bay is needed to anticipate how the dynamics and supply of fine sediments will change under future conditions.  

In this study, a pair of process-based numerical models was applied to investigate fine sediment pathways in San Francisco Bay. An offline coupling between a DELWAQ water quality model and a Delft3D FM hydrodynamics model provides an efficient method to study suspended sediment transport in large, complex systems.   The DELWAQ model was applied to various hydrodynamic scenarios to better understand the effects of wind, density-driven flows, and freshwater discharge on sediment dynamics in San Francisco Bay. The results suggest that sediment import and export at the mouth of South Bay is linked to density-driven circulation induced by horizontal salinity gradients between South Bay and Central Bay. During years with particularly high volumes of freshwater discharge from the Sacramento and San Joaquin Rivers, South Bay is more likely to import sediments, and extreme pulses of freshwater were shown to enhance sediment connectivity between North Bay and South Bay. Sediment exchange between the channel and the flats in South Bay is facilitated by the tide, and wind wave resuspension influences the magnitude of sediment fluxes into and within the southern reach.  

This study advances our understanding of sediment dynamics in the complex San Francisco Bay estuary. With further validation, the DELWAQ model developed in this research can be used as a tool to predict system response to future conditions brought on by climate change and human-induced alterations to the watershed. Large-scale management decisions regarding the Bay Area and the Central Valley could alter the long-term trajectory of the estuary and determine whether it will maintain ecological viability while adapting to changing hydrodynamic and environmental conditions. Insights learned from sediment transport modelling can adequately capacitate the Bay's leadership as well as countless population-dense estuaries around the world confronting analogous challenges.

Drifters equipped with GPS-trackers have been widely used to investigate surface velocities in a wide range of coastal and oceanic settings. A detailed description of a new, cell-phone based, budget drifter system is given, along with an overview of the potential error sources and the magnitude of these sources. Drag related “windage” errors were found to be significant. As a part of the SEAWAD research project, the surface velocities on the ebb tidal delta of the
Amelander Zeegat (the Netherlands) are measured over a period of two weeks. The drifters performed well in the field (no drifter losses) but the results showed relatively frequent data gaps and filtering resulted in significant data loss. The signal quality in the area appeared to significantly influence the data loss.
The velocities on the ebb tidal delta vary around 0.7 [m/s], depending on location, depth, tidal phase and wind. The spatial structure of the velocities shows generally a uniform flow structure. Different wind conditions caused the most significant changes in the spatial distribution of the velocities, followed by the effect of the tidal range. Waves were found to have an insignificant effect
on the flow velocities. A number different deployment configurations were investigated in order to shorten the measurement interval as much as possible. Due to the absence of spatial structure, spatial “resolution” can be exchanged for a shortened measurement interval by deploying the drifters in two or more groups across the research area. Methods for measurement verification using nearby ADCP-equipped measurement points are presented but not performed.
Overall, the described drifter measurement method provides a cheap way of measuring the spatial distribution of velocity in an area. However, the accuracy of the measurement has not been verified. Until then, the accuracy of the results is uncertain.

In the coastal defence of the Netherlands, sand nourishment is a common practice. Part of the Dutch coastal system, the Wadden Sea coast, is a very complicated coastal system in terms of hydrodynamics and morphodynamics. Many different processes are playing a role in the coastal dynamics in the Wadden Sea area. The execution of a mega nourishment in this area is intended. To make this possible, the dynamics of the system must be understood better than they are today.

Part of the coastal dynamic system is the intrawave sediment transport. To improve understanding of this phenomenon, research is done on the wave conditions in a tidal inlet of the Wadden Sea. In the accompanying field campaign use is made of unanchored WaveDroids, used as wave resolving drifter. This is the first time unanchored use of WaveDroids is carried out. The use of a moving measurement device gives rise to differences in measurement results compared to the known approach with fixed measurement devices.

This research shows the difficulties that must be overcome to process raw measurement data in such a way that the data becomes suitable for analysis and interpretation. The data is filtered on frequency and on wave height to give a useful representation of the wave field. The processed data is assessed on the energy density spectrum, the wave height distributions and the time series of wave heights and surface elevation. Subsequently, in the interpretation of the selected data, insight is given in the measured wave field.

From this research is concluded that the use of WRD’s is suitable to measure the vertical component of wave displacement. The effect of a Doppler shift, originating from the use of different frames of reference, is not significant is common sea states. The measurement of the horizontal component of wave displacement requires more research on correcting processing steps.

Sea level rise poses challenges to the tidal inlet systems of the Wadden Sea, including the barrier islands. If no measures are taken, the shorelines of the islands will retreat and the basins may drown. In the Ameland Inlet, the Boschplaat at the eastern tip of Ameland and Bornrif at the western tip of Terschelling are currently shrinking.
This thesis evaluates different nourishment strategies to counteract the coastal retreat, with the ultimate goal of improving our understanding of how nourishment sand of different grain sizes is shared between the different morphological units of the Ameland Inlet.
The process-based model Delft3D is used as a means of predicting the transport pathways of nourishment sand on a time scale of 10 years. In this thesis an existing model of the Ameland Inlet is improved, in order to make it suitable for medium term nourishment calculations. The Mormerge (parallel online) approach is applied in combination with a phase shift in the tidal signal for the different wave conditions. As a result the computational time is significantly reduced. The bathymetry is updated based on the 2016 Vaklodingen and Bed Composition Generation is used to create a suitable bottom composition. The composition of the nourishments is based on the particle size distribution of the borrow area.
The model results suggest that different objectives, such as feeding the Terschelling or Ameland coast, feeding the basin, or reinforcing the ebb tidal delta, can be met by varying the nourishment location and composition.
The main findings regarding the transport pathways within the inlet system are as follows: Near the Terschelling beach import via Boschgat is dominant. Further offshore, bypassing via the ebb tidal delta is the most important transport pathway. Boschgat feeds the western part of the basin and Borndiep feeds the eastern part, but interaction between the two is limited. The model predicts some transport from Boschgat to Borndiep via Westgat under the influence of the tide, or driven by waves over the shallow area in between the channels. There is no connection in the other direction. Borndiep connects the basin to the ebb tidal delta, resulting in transport in and out of the system. Bornrif is the only location where a net westward transport into the inlet is observed.
Finer fractions are imported further into the basin by the tidal flow. When bypassing the inlet, they are transported further offshore, since they are more influenced by the tidal currents flowing out of the inlet. As a result coarser nourishment fractions end up closer to the Ameland shoreline.
The model has several limitations, mainly since insufficient data is currently available for a sound calibration and validation. However important phenomena observed in the field are reproduced by the model, such as the coastal retreat at Boschplaat and the bar migration at Bornrif. This gives confidence in the model performance regarding the predictions of the sediment transport patterns.