Coral reef ecosystems are declining due to climate change and other anthropogenic factors. Studies have shown that the presence and integration of reef fish are essential for the health and resilience of coral reefs. Consequently, RoffaReefs made them an integral part of their program to safeguard and improve the health of coral reef ecosystems. With this goal, they developed a floating breeding system designed to increase the survival rate of reef fish. Defining an optimal location for this breeding system requires knowledge of spawning locations and egg dispersal patterns in nearshore waters. However, spawning conditions are species specific and not known beforehand. Furthermore, current dispersal studies often opt for global or regional models, which do not resolve local scale abiotic drivers. The objective of this research is to identify possible spawning grounds near Bachelor's Beach based on the sampling data and varying abiotic drivers in nearshore conditions.

The sampled reef fish eggs are pelagic, exhibiting slight positive buoyancy, which keeps them within the surface layers. Given the focus on abiotic dispersal mechanisms, the eggs are modelled as passive Lagrangian particles that remain near the surface throughout the simulation. The literature review identifies tidal, wind-driven and wave-induced currents to be the key abiotic drivers near Bachelor's Beach. Through Delft3D-FLOW and -WAVE a hydrodynamic model is developed to simulate different scenarios, entailing the conditions of each abiotic driver and possible combinations. The obtained horizontal flow velocities and accompanying dispersal patterns indicate that wind-induced currents are the primary abiotic driver of dispersal near Bachelor's Beach, under both uniform and time-varying conditions. The impact of wave-induced currents only become significant under the presence of wind, whereas tidal forcing seems to play a limited role.

Using Delft3D-PART, dispersal patterns are simulated based on the hydrodynamic output. Under time-varying conditions, a consistent north to north west dispersal pattern was observed, moving eggs away from Bachelor's Beach. These results suggest that spawning grounds that cause eggs to pass Bachelor's Beach should be located within close proximity of the sampling area. It is recommended to RoffaReefs to position their breeding system in southern curve of Klein Bonaire, as many eggs tend to pass this location. Future research should prioritise local hydrodynamic measurements for model calibration and validation. To add to this a more detailed assessment of the spawning release conditions are advised to move from simplified to more biologically realistic scenarios. ... Read more

This thesis presents a comprehensive study of Lagrangian and Eulerian mass transport in a wave flume experiment, aiming to improve understanding of mass transport mechanisms and their role in wave-current interactions. This has important implications for nearshore particle transport and pollutant dispersion. The research utilizes state-of-the-art Particle Tracking Velocimetry (PTV) to directly measure the Lagrangian velocity of individual particles, providing higher temporal resolution and precision in capturing unsteady flow phenomena compared to traditional methods. The study compares experimental mass transport data with existing mathematical models, including the irrotational and conduction solutions, and qualitatively discusses the potential importance of the convection model.
The focus of the study is on the temporal evolution of vertical velocity profiles influenced by waves and wave-induced currents, exploring how these profiles change under varying relative water depths (kh)
and wave steepness (ka). Additionally, the temporal evolution of vorticity profiles is examined to identify the underlying mechanisms driving these changes.
To extract the net Lagrangian drift from particle tracking data, three averaging methods—time-averaging, wave-by-wave, and low-pass filtering—are compared. The wave-by-wave method is found to be the most suitable for this study.
The velocity profile transitions from an initially irrotational state, characterized by uniform motion near the surface, to a more complex structure resembling the conduction solution as vorticity diffuses. However, the observed profiles do not quantitatively align with the conduction solution. After approximately 60 minutes, the profile stabilizes but continues to exhibit discrepancies from both the irrotational and conduction models. At equilibrium, while the velocity profiles qualitatively align with the theoretical predictions of the conduction solution, significant quantitative differences remain, particularly in surface drift velocities an the negative peak velocity in the middle of the water column.
The study also shows that as wave steepness increases, deviations from theoretical predictions grow, indicating that higher steepness disrupts the assumptions underlying the conduction solution, leading to greater discrepancies between observed and predicted velocities. The evolution of the velocity profile is further explained through vorticity transport, where a more uniform vorticity distribution is observed compared to predictions from the conduction solution. This complex behavior is influenced by factors such as sidewall interactions and vorticity convection along the direction of wave propagation. ... Read more

Land subsidence poses significant damage risks to residential real estate, including pile rot, differential settlements, pluvial flood risk, and dewatering risk amounting tens of thousands of euros. In the Netherlands, the number of affected houses has surpassed one million and is estimated to reach two million, accounting for a quarter of all houses in the country. Without an action perspective homeowners are left to their own devices which this research shows can lead to postponing action until risks materialize. To mitigate these risks, multiple adaptation strategies are available. This thesis focuses on determining the cost-effectiveness of two general pathways for residential real estate constructed with either a wooden pile foundation or a shallow foundation: renovating existing houses or replacing them entirely. The study also investigates whether incorporating additional measures aimed at improving overall benefits such as increasing the amount of living space or transitioning towards a more sustainable house with energy label A, is cost-effective by comparing the costs and benefits associated with the adaptation strategies. When the benefits outperform the costs the strategy is labelled favourable. Through a comprehensive analysis, this research provides valuable insights into economically viable approaches for addressing land subsidence-related risks in residential real estate in relation to the location specific characteristics. It proves additional measures are required to make either renovation or replacement a cost-effective adaption strategy. Two options are shown to be cost-effective. Firstly, renovation including additional investments to
increase square meters of living space and transitioning to an energy performance A-label. Secondly, replacement including densification by building back more square meters of living space, increasing the amount of houses or a combination of the two. Both require large investments resembling around 75% to 150% of the current housing price respectively. Increasing the amount of square meters even further improves the result however this increasingly affects the character of the neighbourhood and its social composition. With these results, this thesis hence provides an action perspective to homeowners and stakeholders including policy makers and financial institutions by means of a solution space to address
land subsidence in an economically favourable way, whilst simultaneously reflecting on the differences in implications for the parties involved.
... Read more

This thesis evaluates flood insurance solutions for the Netherlands, focusing on the repercussions of primary flood defence failures. Recognizing the amplified need for a solid safety system due to climate change, the study uses a comprehensive approach, including literature review, expert opinion, and numerical modelling, to assess existing insurance systems and suggest novel ones. It begins by identifying seven insurance failure mechanisms, vital for appraising potential solutions. International practices are examined for context, and experts from various sectors provide quantitative analysis of these mechanisms on reimbursement schemes. The study proposes seven solutions, narrows them down to four, and constructs an insurance premium model reflecting Dutch housing risks to evaluate the financial implications for homeowners. It then scrutinizes each solution's framework, efficacy, and financial impact. A final reflection considers the preferences of key stakeholders—government, individuals, insurers—and recommends four tailored flood insurance solutions for the Netherlands. The thesis aims to guide policy formulation and enhance the collective understanding of flood insurance management. ... Read more

Due to river training in the past centuries, the bed level of the Waal started and is still decreasing by several centimetres a year. Erosion of the riverbed causes problems for objects underneath the riverbed like cables, pipelines and foundations of structures. Furthermore, some parts of the river are eroding at a higher velocity compared to other parts due to a difference in the composition of the bed material. This can result in a jump in the bed level while the water level cannot follow this jump. This results in less available draught for inland vessels. If no mitigation measures are applied and the bed level keeps decreasing, more problems are likely to occur resulting in less transport over the Waal. A possible solution to reduce bed erosion inside the Waal locally is the use of groyne field nourishments.
The objective of this research is to obtain more knowledge about the flow in a real-life groyne field and the effect of vessels on this flow. The research question formulated for this research is, therefore: \textit{what is the influence of vessels on the flow properties inside a groyne field and in what way do the characteristics of the vessels influence this?} To answer this research question a literature study is done first. Secondly, a measurement campaign is performed and finally, the results are analysed to investigate the effect of the characteristics of the vessels.
The literature study is done to obtain more knowledge about the flow properties inside a groyne field. The distinction is made between emerged and submerged groynes and the scenario with and without vessels. The situation where the groynes are emerged is dominant for the erosion inside the groyne field. In this scenario, the flow inside the groyne field consists out of one or two circulation patterns. The first circulation pattern is a large eddy in the downstream part of the groyne field, referred to as the primary eddy. When the groyne field is long enough, a second eddy is present in the upstream part of the groyne field, the secondary eddy. This eddy has a flow direction opposite to the primary eddy and a smaller flow velocity. When a vessel passes a groyne field, the flow pattern inside the groyne field changes due to the primary waves created by the vessel. Firstly, the water level inside the groyne field is raised due to the bow wave. Secondly, due to depression in the water level caused by the primary wave the water level inside the groyne field is lowered. Finally, the water level inside the groyne field is increased again due to the stern wave.
To investigate the effect of vessels on a groyne field, a measurement campaign is performed. The flow direction, flow velocity and water level inside the groyne field are measured with the use of several measurement instruments for two weeks. The obtained data by the measurements is analysed and visualised. The results show that the primary eddy remains partly intact when a vessel sails past the groyne field. Since the primary eddy remains partly intact, the water is guided towards the upstream part of the groyne field. In the upstream part of the groyne field, the secondary eddy does disappear and the flow is directed out of the groyne field. This results in the water, and sediment, flowing out of the groyne field mainly in the upstream part of the groyne field. This has resulted in a scour hole present in the upstream part of the groyne field.
In this research the effect of multiple vessel characteristics on the water level and flow velocity inside the groyne field is investigated. The vessel characteristics investigated are the draught, the sailing speed, the vessel length, the vessel width and the distance of the vessel towards the measurement instrument. The effect of vessels on the water level and flow velocity inside the groyne field differs for each vessel. Information about the vessels has been obtained by using AIS (Automatic Identification System) data. During the processing of the AIS-data, irregularities and errors were found and mostly removed from the AIS-data. According to the final results, the draught of a vessel does not influence the water level and flow velocity inside the groyne field. An increase in the sailing speed of a vessel and a decrease in the distance of a vessel towards the groyne field does have an enlarging effect on the water level difference and the flow velocity inside a groyne field. The combined effect of increasing the length and the width of a vessel also has an enlarging effect on the water level difference and the flow velocity inside the groyne field. It should be noted that the results show a large variance and it is impossible to predict the effect of a single vessel based on the vessel characteristics.
The measurement campaign showed that a constant water level fluctuation is present inside the groyne field even when no vessel is affecting the flow inside the groyne field. Multiple explanations for this fluctuation are given with transverse oscillations between both riverbanks being the most likely.
The measured flow pattern inside the groyne field without vessel is according to the literature. When the flow is affected by a vessel, the flow pattern differs from the flow pattern described in the literature. Furthermore, the effect of a vessel is likely not only depending on the characteristics of a vessel but also on the flow mechanism inside the river system such as the helical flow, the angle of the groyne field with respect to the main channel and the constant water level fluctuation inside the groyne field.
This research adds to the already existing knowledge about the flow in groyne fields. To investigate the effectiveness of groyne field nourishments, a pilot is planned where the actual nourishment will take place at the same location as the measurement campaign. Therefore, this research can be used to better prepare the planned nourishment pilot to investigate the effectiveness of groyne field nourishments. Furthermore, this research contains data about the flow pattern inside the groyne field without nourishment which can be compared to the situation during and after the nourishment pilot. In the end, this research can be a part of the answer to whether groyne field nourishments can reduce or stop the erosion inside the Waal river.  ... Read more

Large morphological features, so-called sand ridges, with an average height and length in the order of 10 meters and 3 kilometers respectively are located at future offshore wind farm locations in the Dutch North Sea. Recent literature, considering relatively deep water conditions (kh 1, with k the wave number and h the water depth), has shown that a wave field propagating over a shallowing depth transition shows an increase in kurtosis. Kurtosis is a measure for the vertical deviation from the commonly applied linear ocean wave model, and therefore describes a higher probability of occurrence of "rogue" waves. These "rogue" waves are known to the general public from sailors' tales and thrilling videos. Scientifically, rogue waves are described as statistically abnormal waves and are generally classified as waves that exceed twice the significant wave height. Because these waves have an abnormal height, they are associated with extreme forces. Considering the sand ridges at future offshore wind farm locations, and possible increased damages to wind turbines due to rogue waves, this thesis researches if the increased probability of occurrence of rogue waves due to the local bathymetry as observed in the relatively deep water literature also applies to relatively shallow water conditions and the Dutch North Sea. To achieve this objective, physical experiments have been performed in the Hydraulic Engineering laboratory of the Delft University of Technology. Also, an existing statistical model that describes a rogue wave mechanism due to the presence of depth transitions is evaluated. The model describes the rogue wave mechanism as the interaction between the transmitted wave field and second-order components: the free sub- and super-harmonic components generated by the depth transition. As the model assigns a prominent role to these sub- and super-harmonics, it was ensured that these components are generated correctly during the physical experiments. Following, this thesis also assesses and contributes to the second-order wave generation at the TU Delft laboratory facility. ... Read more