In this thesis, the loading characteristics of a crown wall on a rubble mound breakwater are investigated using a 2D physical model. The study examines both overall stability (through sliding) and the temporal and spatial distributions of pressure along the face and base of the crown wall. A range of wave conditions is tested, with particular emphasis on the influence of foundation level on the loading characteristics. Additionally, a second breakwater core is used to assess the impact of permeability on the wave loading characteristics. The findings highlight the significance of both parameters and analyze the discrepancies between predicted and measured maximum horizontal and vertical forces after wave impact. Furthermore, a novel two-step method is proposed to improve existing design methods, enabling more accurate estimation of wave loads.

The key parameters governing collision energy calculation are the velocity of the vessel, its mass, and the angle of approach. The conventional method outlined in the Recommendations of the Committee for Waterfront Structures Harbours and Waterways (EAU, 2012), which was specifically developed for berthing ships, relies on an overly simplistic model that fails to capture the dynamics of ship­to­guiding structure collisions. Nevertheless, in the Netherlands, the Guidelines for Civil Engineering Structures (Richtlijnen Ontwerp Kunstwerken, ROK) suggest that the EAU 2012 method be applied unmodified for guiding structures. This report introduces a methodological approach aimed at refining the calculation of collision energy by addressing specific deficiencies in the existing framework, particularly regarding its application to guiding structures.

The kinetic energy approach used in the EAU 2012 method requires, as inputs, the mass of the ship, its velocity vector normal to the structure, a virtual mass coefficient accounting for water mass moving with the ship, and an eccentricity coefficient accounting for energy loss due to rotation when the ship does not approach the structure parallel.

The literature review identifies critical gaps in the EAU 2012 approach to calculating collision energy for guiding structures. An unsuitable application of the eccentricity coefficient and assumptions about the velocity vector direction, which do not align with actual conditions in ship­to­guiding structure colli­ sions, are discovered. Moreover, the review reveals that the framework does not differentiate between berthing and guiding approaches, resulting in a one­size­fits­all approach that fails to account for the unique characteristics and requirements of guiding structures. The framework only partially includes the ship’s angle of approach to the structure in the kinetic energy calculation. Although the recommen­ dation is to calculate the kinetic energy using the normal ship speed to the structure—thus indirectly accounting for the approach angle—when it comes to the eccentricity coefficient, which relies on ap­ proach geometry, the recommendation is to consider the total velocity vector. This practice results in exaggerated collision energy when a ship maintains forward speed near guiding structures while passing through the lock.

The theoretical refinement proposed in this thesis focuses on the scenario where the ship approaches a guiding structure with purely longitudinal speed. Specifically, a new definition for the angle variable in the eccentricity coefficient expression is proposed, which directly reflects the effect of the approach angle of the ship linked to the contributing velocity vector and not only laterally through the magnitude of the contributing vector. This proposed change is validated through comparative analysis.

The comparative analysis is performed using historical data cases from model research published by Ri­ jkswaterstaat (Ministry of Infrastructure and Water Management). The data consist of experiments with ship approaches to structures that have characteristics fitting guiding structure approaches. The ships are of lengths commonly found in lock passages, maintain forward speed, and approach structures similar to those at lock entrances. The historical data provide the maximum force exerted on the tested structure and the resulting deflection. This information is used to derive the energy absorbed by the structure. The published reports valuable to this research resulted in the Virtual Water Mass method. This method is reproduced to provide benchmark values for the kinetic energy absorbed, which are compared to both the EAU 2012 collision energy calculation and the proposed refined calculation for the historical data cases.

The findings of the comparative analysis suggest that the proposed refined calculation provides results closer to the benchmark values than the EAU 2012 calculation. However, a consistent improvement was not discovered. Although the refined calculation is significantly closer to the benchmark value in every case compared to the EAU 2012 calculation, this improvement cannot be quantified as a universal percentage, as it varied for each case. Despite this, the comparative analysis still supports the main objective of demonstrating the exaggeration in the EAU 2012 collision energy calculation.

A case study, as part of this research, explores the potential effect of the refined calculation on an actual structure design. In the case study, the design vessel, velocity, and angle of approach are sourced from the Waterway Guidelines (2011, 2020) and ROK 2.0 (all documents published by Rijkswaterstaat). After calculating the impact energy using the two methods compared in this research, the design of the mono­ pile follows the Blum method. The structure is modeled as a series of piles connected by a main girder, with the girder having the same diameter as the piles to ensure uniformity and strength.

The case of the Volkerak locks is chosen, and calculations using both the standard EAU 2012 method and the proposed refined one are executed. Two designs are produced and compared. With the refined calculation, a design with a smaller pile diameter of 1,620 mm and a longer in­between pile distance of 6.5 m is achieved, compared to the EAU 2012 design, where the pile diameter is 2,020 mm and the distance between piles is 3 m. These results demonstrate the potential for material savings if the refined calculation is adopted.

The study concludes that a refined approach is indeed necessary and that focus should be placed on specific definitions and descriptions in the guidelines. Ambiguity in such definitions leaves decision­ making subject to individual interpretations, creating inconsistency among the designs of structures with the same purpose. Although this inconsistency may not be inherently dangerous, as it usually results in an overly conservative design, it contributes to excess material usage and resource waste.

Known for its low-lying and flood-prone geography, the Netherlands has a long history of flooding. Flood events in the 1990s, which led to large-scale evacuations, shifted the country's focus from simply strengthening dikes to creating more space for rivers to flow and storing water to prevent similar disasters. With changing climate conditions, rising water levels, land subsidence and sediment supply issues, the safety of primary flood defences needs to be regularly reassessed. The latest safety assessment, using a new methodology that assesses dikes based on the probability of flooding, found that many dikes do not meet the required safety standards. These dikes will need to be redesigned and reinforced, and there are opportunities for innovative and environmentally friendly solutions.
Previous research has extensively investigated the potential of different ecosystems, such as salt marshes, mangroves and willow forests, to reduce the impact of waves on coastal and riverine structures. Studies have shown that vegetation can significantly dissipate wave energy, thereby reducing the load on flood defences. However, the dynamic and unpredictable nature of vegetation has made it difficult to incorporate it reliably into defences. While there is general agreement on the benefits of vegetation for wave attenuation, its reliability in reducing failure probabilities requires further investigation.
This thesis tests the potential of riparian forests for wave attenuation and dike design optimization. The main objective is to investigate how wave attenuation by riparian forests can improve dike safety and to identify the critical variables that most significantly affect the probability of flooding. Key research questions include how wave attenuation can contribute to dike design and safety assessment, and which variables most influence the probability of flooding.
Vegetated foreshores can reduce the height of incoming waves, with positive implications for dike design and safety assessment. A model was developed to test this by combining current wave impact and run-up models with a wave energy balance that includes energy dissipation by vegetation. The model was subjected to a Monte Carlo simulation to determine the probability of dike failure due to erosion of the outer slope by wave impact and run-up. The model analyzed a dike section near Kampen and showed that riparian forests can significantly reduce the impact of waves on dikes. The results showed that the forest length, frontal-surface area and stiffness of the forest must be large enough to be effective. Strategically placing trees with the largest frontal-surface area at critical points can significantly reduce the probability of dike failure.
In conclusion, this thesis highlights the potential of riparian forests to improve dike safety and contribute to a more resilient and environmentally friendly flood protection strategy in the Netherlands.

With a setback storm wall impact wave loads can occur which cause much higher loads than with a straight vertical wall. Empirical formulas for a first estimation exist, but they do not consider the geometry of the structure (height storm wall, length quay) or are not applicable for negative freeboard, or both. Due to this limitation, the main research question of this research is “What is the influence of the geometry of a retreated storm wall on top of a quay wall on the wave forcing exerted on the storm wall?”. The main method used for this research is numerical modelling using DualSPHysics (Smoothed Particle Hydrodynamics). This model is validated using a laboratory experiment. The result of the thesis is are formulae for rough first approximations of the horizontal force for the conceptual design.

In 2022, the Eastern Scheldt barrier was assessed regarding its compliance with the requirements in the Environment and Planning Act. The result of this assessment, mentioned in HKV et al. (2022), was that the barrier barely met the standards in failure probability: a value of 1/13.057 years while the minimum requirement is 1/10.000 years and the signal value is 1/30.000 years. This leaves a margin of a factor 1.3 increase for the current failure probability. The upper trusses of gates Scharen 13 (S13) untill Scharen 16 (S16) proved to be the weakest link in the barrier and contributed most to the total failure probability. This study only focuses on the upper truss of gate S13, and the results are assumed to be a rough indication of how the barrier responds as a whole.

This research investigates the usage of alternative wave load related model choices and their effect on the structural failure probability of the upper truss of gate S13. Combining these two aspects provides an estimate of the uncertainty in the estimated 1/13.057 years. Four areas within the current wave load related framework were considered.

- Dynamic wave pressure model. This aspect considers how the waves translate to dynamic pressures on the gates. Sainflou, quasi-regular linear wave theory (LWT), New Wave LWT η_recalculated and a spectral LWT are considered suitable alternatives compared to SainflouOSK, which is the currently used method. The linear models, especially the spectral variants, have the most complete representation of the physical processes. The spectral methods need the complete wave spectra as input, which is not yet accurately reproduced by the wave propagation model. New Wave LWT η_recalculated is particularly recommended because it provides a spectral calculation that accounts for the effects of steep waves. Sainflou corresponds best with a small, but high quality dataset of average pressures during regular waves. The linear theories perform best with a larger, but lower quality dataset of pressures during six hours in one storm. However, as no wave conditions were measured during this specific storm, wave propagation model output was used to employ this dataset. It is advised to solve the inconsistency between the data sources by performing scale model tests or SWASH simulations. Especially the performance under steep regular waves and bimodal spectra is crucial, as this study shows that these are the conditions that prevail during relevant storms. The structural failure probability changes significantly when these other dynamic wave pressure models are employed. Sainflou results in a factor 10 increase in failure probability for the upper truss of gate S13 and spectral LWT in a factor 100 decrease.

- Wave height distribution. This distribution is applied to move from a significant wave height to a maximum wave height within a storm. Two alternative distributions were considered, the Rayleigh distribution and the Battjes-Groenendijk distribution. The Rayleigh distribution, which is currently used in the assessment, is demonstrated to be conservative compared to six hours of wave pressure measurements in one storm. The Battjes-Groenendijk distribution could be an alternative, even though not validated in this study, leading to a factor five decrease in failure probability for the upper truss of S13. Given the significant impact on the failure probability and the indication of room for improvement, it is advised to perform further research in which wave height distribution is most suitable. SWASH simulations could be used to validate an alternative distribution. 

- Wave propagation model. Wave propagation is computed with SWAN considering wind-induced wave generation, whitecapping, depth-induced breaking, bottom friction, triad wave-wave interactions and quadruplet wave-wave interactions. From these source terms, adjustments to the formulations of wind-induced wave generation, whitecapping, depth-induced breaking and triad wave-wave interactions are considered. Modifying the CDS2 KOMEN whitecapping parameter within the current SWAN settings, which increases the whitecapping dissipation, improved the bias correction for both the wave height and period. However, this SWAN setting is not considered a better alternative. This is due to the lack of physical substantiation and its inability to reproduce measured spectral characteristics, especially during high water levels in storm conditions. The DCTA triad wave-wave interaction formulation showed most promising results in reproducing the bimodal spectra measured during severe storms. Additional research to improve the calibration of the DCTA formulation is recommended over employment of the adjusted CDS2 parameter. Using four other variations in SWAN settings with their corresponding bias corrections resulted in a spread of failure probabilities for the upper truss of S13 ranging from a factor 1.2 decrease to a factor five increase. 

- Wave load-related uncertainties and biases. This concept relates to the methodology of how uncertainties in parameters (probabilistic or deterministic) and models (bias corrections) are taken into account. Using energy interpolation over wave height interpolation between SWAN outcomes is considered an improvement for estimating the bias corrections. It leads to a factor 1.1 reduction in failure probability for the upper truss of gate S13. Similarly, bias estimation of the wave height and period by using measurements with an additional restriction on the water levels resulted in a factor 1.1 decrease. However, incorporating the entire distribution of wave heights, not only the mode of the chosen distribution, is considered an advancement that increases the failure probability by a factor two.

When combining the evaluations of alternative wave load related models and their effect on the structural failure probability of the upper truss of gate S13, it can be concluded that there is a large scatter around the 1/13.057 years. This uncertainty is a consequence of the inability to draw definite conclusions on which SWAN setting, dynamic wave pressure theory and wave height distribution to choose. There are well evaluated choices that decrease the failure probability below the signal value, but also other interesting options that increase it beyond the minimal requirement. Further research and data collection is necessary to determine which of these two scenarios is most likely. This study illustrates the starting point and direction for these next steps, while also providing the motivation and urgency to take them. 

Building with Nature is a sustainable approach to design multifunctional flood defences (e.g., accommodating recreational services, nature development, etc.) that harness natural forces as much as possible. Building with Nature therefore extensively involves coastal dunes. These dynamic sandy barriers naturally grow because of aeolian processes. Building with Nature may harness these aeolian processes to build, maintain and reinforce coastal dunes. In that case, aeolian processes are coupled to the variable flood risk of sandy barriers. To facilitate sustainable coastal management, it is crucial to understand the relation between wind speed and aeolian sand transport. Aeolian sand transport can either be capacity- or supply limited. When sand is abundant and available, the transport rate is generally capacity limited; and is determined by the wind speed. However, if the supply of sand is limited (e.g., due to moisture content, vegetation, etc.), the capacity of the wind cannot be reached. In this supply limiting case, the transport rate is not governed by the wind speed anymore. Many models are still based on capacity limited transport; and therefore, overestimate measured coastal transport rates. Additionally, supply limitations are still not fully quantified. Many supply limitations are posed by the bed’s erodibility, i.e., its ability to supply sand grains to the air flow. Exploring the erodibility of the sand surface can improve this situation. To this end, a specialized mobile wind tunnel system was developed. This system is called Mobi-Gust 2 and also includes a wind gauging system. This research involves the design and performance of Mobi-Gust 2; as well as testing its viability to conduct applied aeolian research. In this process Mobi-Gust 2 experimentally verified some supply limitations. The outcomes demonstrate that Mobi-Gust 2 is able to conduct applied aeolian research. The next step concerns the usage of Mobi-Gust 2, so that coastal aeolian sand exchange can be quantified. In this way Mobi-Gust 2 can improve the predictive power of numerical models (e.g., Aeolis).

Salt Marshes are a coastal ecosystem which have numerous benefits as coastal defense such as wave attenuation and can adapt to sea level rise. Wave flume tests, using a scale model of a cross-section of a salt marsh adjacent to a dike, were conducted in the Hydraulic Laboratory, to quantify the effectiveness of such a salt marsh system as flood defense (part of the Living Dikes research program).

This thesis focuses on the reduction in wave run-up due to salt marshes on the adjacent dike, with a focus on high water levels. The wave run-up was measured using video processing, using a newly created algorithm to track the water movement on the dike slope. The results show a significant reduction in wave run-up due to wave attenuation over the salt marsh, further dependent on the presence of vegetation and the water depth on top of the salt marsh. The measured wave run-up values show some differences with values acquired using the TAW/EurOtop wave run-up formula. There is a correlation found with the wave steepness, where waves with a lower wave steepness do match the equation, and show a larger deviation for increasingly higher wave steepnesses.

Coastal structures such as breakwaters can mitigate the erosive effects of sea level rise by protecting shorelines from wave impact. The Reef Enhancing Breakwater (REB) is a modular, permeable structure designed to dissipate wave energy and boost marine biodiversity by providing suitable habitats. This research investigates the hydraulic stability of the REB under wave loading using a physical model.

 

Experiments were conducted in the Scheldt flume of Deltares with a 1:20 scale model. Both 2 and 3 level structures with simple and complex forms were tested on gravel underlayers of varying roughness. Irregular waves with significant heights up to 12.5 cm and wave steepness ranging from 2% to 4% were used, with varying water depths. As failure was not reached on the horizontal foreshore (main location), the structure was moved closer to the transition slope (alternative location) where plunging breakers can emerge. Smart ReefBlocks with integrated mobility sensors measured accelerations and angular velocities induced by wave motion.

 

Five main types of movements were observed: sliding, rocking, shaking, tilting, and lifting. Sliding was the most common, occurring mainly in the top layer, with a maximum of 4 sliding events per block per 1000 waves. Three limit states were defined: start of motion, start of damage, and failure. Start of motion is when blocks move for the first time, damage is when a block loses interlocking, and failure is defined by a damage level Nod=0.4Nod=0.4.

 

Expected and characteristic values of the stability number (Hm0/ΔDnHm0/ΔDn) were determined for each limit state. Only one displacement occurred at the main location, while failures were reached near the transition slope due to plunging breakers. The 2 and 3 level structures were stable on bed slopes without plunging waves. For non-plunging waves, the start of movement limit state (Ns,mNs,m) ranged from 0.39 to 0.51. When plunging waves occurred, the start of damage limit state (Ns,dNs,d) was 0.94, and the failure limit state (Ns,fNs,f) ranged from 0.94 to 1.11. The start of motion was not determined for setups at the alternative location.

 

For setups at the main location, sliding was analyzed in relation to wave height, wave steepness, water level, and structure height. Higher water levels allowed for higher waves, leading to more instabilities, but excessive submersion reduced movements. Waves with 2% steepness caused more movements than 4% steepness due to higher energy in longer waves. Other factors like underlayer irregularity and increased drag from epifauna were also considered. Most sliding movements occurred for Hm0/ΔDn>0.8Hm0/ΔDn>0.8. The model blocks, made of PLA with lower friction than concrete, started sliding under smaller forces, making the model conservative for sliding movements.

 

The impact of sliding movements on the REB's structural performance was investigated by estimating stresses in the protrusions from data collected by smart ReefBlocks. A conservative model was used to calculate impact forces and check for protrusion rupture. The resulting tensile stresses exceeded the concrete strength, but the model's representativeness is questionable as it differs from the actual protrusion. Further research is needed for a definitive answer.

This study focuses on the infragravity (IG) waves at 5 measurement sites (Brouwershavense Gat 2 (BG2), Oosterschelde 4 (OS4), Cadzand, Hansweert and Bath) in the Scheldt region, examining their characteristics in these waters as well as to what extent they can be modelled using SWAN (Simulating WAves Nearshore). To facilitate this, data collected during the occurrence of four storms (Corrie, Dudley, Eunice and Franklin) that traversed the North Sea in January and February 2022 has been used.

A spectral analysis has been used to provide insight into the IG and sea-swell (SS) wave field. To this extent, the significant wave height for the IG (0.005 – 0.04 Hz) and SS (0.04 – 0.33 Hz) frequency bands have been derived, in addition to the storm averaged wave period. Measurement device BG2 documented the largest storm averaged significant wave height for both the IG and SS frequency bands (0.256 m and 3.46 m during storm Corrie, respectively). The measurement device with the smallest storm averaged significant waveheight values was Hansweert, which measured 0.024 m and 0.217 m for the IG and SS wave frequencies, also during storm Corrie. Most of the wave energy for the measurement devices in the Western Scheldt, Hansweert and Bath, can be attributed to waves with frequency values larger than the upper boundary for SS waves (T < 3 s). The fraction of the total wave energy contained within the IG and SS frequency bands ranged from 0.1 to 0.56 for these two measurement devices. Conversely, the IG and SS frequency bands contain 0.81 to 0.96 of the total wave energy for measurement device Cadzand, BG2 and OS4. The correlation between the IG and SS significant wave heights is mostly strong for the BG2, OS4 and Cadzand measurement devices (0.657 to 0.956) and generally moderate for Hansweert and Bath (0.474 to 0.87), with one outlier equivalent to 0.128). A bispectral analysis was used to offer more insight regarding the components contributing to the total IG wave field. The results at Bath proved to be erroneous for all storms except Corrie, as the derived free IG (FIG) contribution was negative. The origin remains unclear, although it appears to be related to unexplained spurious bursts in the wave elevation signal. The bispectral analysis for the remainder of the locations lead to maximum TIG wave heights ranging from 0.05 m to 0.12 m for Hansweert, 0.15 m to 0.39 m for BG2, 0.11 m to 0.35 m for OS4 and 0.21 m to 0.34 m for Cadzand for the 4 storms. The contribution of the bound IG (BIG) and FIG energy at Bath and Hansweert appears to fluctuate heavily, undergoing rapid changes on an hourly basis. For BG2, OS4 and Cadzand, the FIG energy contribution is strongly dominant as it generally amounted to > 0.75 of the TIG energy.

The capabilities of the SWAN model were evaluated by simulating FIG waves in the North Sea basin. The model relates incident SS wave energy to reflected FIG wave energy with FIG source lines based on the reflection parametrization of Ardhuin et al. (2014). The hourly SS wave inputs were provided by Copernicus Marine Service. The predictive skill was used to qualitatively assess to what extent the model is able to reproduce FIG waves in the Scheldt region. The model was unable to correctly predict the FIG waves Hansweert and Bath, where the predictive skill values for all storms ranged from 0.0003 to 0.0009 for Hansweert, and were 0 for Bath. Better predictive skill values were obtained for BG2 (0.5044 to 0.6565), OS4 (0.3327 to 0.4293) and Cadzand (0.6466 to 0.7510), but there is still room for improvement. The lacking skill values can be attributed to not enough FIG wave energy being able to penetrate the estuarine waters. Reflective lines were implemented along the coastline of the Western Scheldt in an attempt to force more FIG energy into the estuary. This only led to a minor increase for the predictive skill values (O(0.002) for Hansweert, 0 for Bath and O(0.0001) for BG2, OS4 and Cadzand) but from a spatial standpoint, it appears that there are locations in the estuary where the absolute increase was O(0.02), which is significant as there are areas in the estuary where the significant FIG wave height has doubled as a consequence of the implementation of the reflective lines.

Sea dikes have long been one of the main coastal defense infrastructures in the Netherlands, protecting against flooding and storm surges. As climate change intensifies severe phenomena and sea level rises, dikes must be heightened to protect the coast efficiently against future threats. Meanwhile, nature-based solutions are emerging as viable alternatives. One of these solutions is the use of salt marshes as coastal protection in front of dikes instead of heightening the dike, which could potentially lead to a more cost-effective solution. A salt marsh vegetated foreshore not only dampens the incoming waves but also adapts to the advancing waters of the sea level rise by trapping sediment.
This study’s main objective is to assess the efficacy of salt marshes in mitigating wave run-up and overtopping on a real-scale sea dike with a vegetated foreshore.
To carry out these large-scale experiments were carried out at the Deltares Delta Flume. Experiments were carried out for low (0.75 m), medium (1.5m) and high (2.5m) water depths above the foreshore, designed to simulate the extreme storm scenarios experienced by the Friesland dikes in the northern Netherlands. In addition to these conditions, the tests were performed for three different qualities of vegetation, good damaged, and mowed vegetation. First, the experiments ran for a fully vegetated foreshore, then after several runs the vegetation was assumed damaged as an important part of the biomass had eroded. Finally, the vegetation is mowed to leave a bare foreshore which is the baseline scenario.
The acquired run-up results are compared with empirical equations from literature. In addition, the results were compared with those of similar research on salt marshes. Finally, the other important objective of this study is to quantify the reduction in wave loads according to vegetation conditions by introducing certain damping ratio parameters.
To measure the important parameters for the completion of this study, three wave gauges offshore and three wave gauges close to the toe of the dike were used for the measurement of the wave characteristics. For the wave run-up a camera was placed above the dike slope to capture the run-up events over time, and at the same time a LIDAR laser scanner recorded the same slope to collect data on run-up and overtopping.
From the wave analysis, the spectral parameters offshore and at the toe of the dike were calculated. From this, it was possible to have a first estimate of the wave attenuation by comparing the incident significant wave height at these two locations. The results reveal a 14 to 30 % decrease in wave height for low conditions. For medium and high storm conditions the attenuation was decreased to a range between 5 to 15% and 4 to 19% respectively.
The video process run-up measurements were in good agreement with the literature with RMSE = 0.18 - 0.19 m. An additional method was used to validate the results of the run-up. The detection of the run-up was completed visually, by tracking the waves that exceed the markers on the slope, for part of three experiments to compare their signal with the signal derived from the video process. The bias between the signals ranged from 0.017 to 0.022 m, indicating a strong agreement between the methods. One of the most important findings from this study is that the reduction in run-up is not directly attributed to the presence of vegetation itself, but rather to the role played by the significant wave height at the toe of the dike. The run-up was reduced between 2 and 16% for low storm conditions and a reduction between 4 and 24% for medium storm conditions, with an average value of 10.3%. At the same time, the reduction of run-up from damaged vegetation to fully vegetated foreshore is up to 9% with an average of 7.7%. The laser scanner was also used to measured the wave run-up. The signal obtained from the laser is compared with the signal of the visual detection method and reveals a bias between 0.032 and 0.037 m. The run-up 2% results were also compared with the same results from the video camera, revealing a 0.14 m (7%) deviation which may be due to the accuracy of the laser.
The laser scanner also obtained the overtopping results using the virtual overtopping method. From this method, the virtual volume and the virtual overtopping discharge can be calculated and then compared with the equations from the literature. The comparison reveals a strong agreement between the calculated and predicted values, which proves that the equations predict accurately the overtopping discharges for the case of a living dike. For low storm conditions, a decrease of 2 to 50 % of the maximum virtual overtopping volumes is possible, while for medium conditions a maximum volume decrease of 5 to 54% according to the crest height. For the highest storm conditions, this volume decrease is calculated to be between 20 to 28%.

In the field of offshore engineering, subsea rock installation has emerged as a pivotal technique. As offshore operations extend to greater water depths, the deployment of a versatile fallpipe has become a cornerstone for precise rock particle placement. A comprehensive determination of the overall rock material volume required for installation entails accounting for geometric factors, operational losses, and seabed displacements. To ensure effective project management and cost estimation, a precise grasp of volume loss during installation proves indispensable. Against this backdrop, the current thesis delves into the intricate domain of volume loss stemming from rock penetration within sand substrates.

The core endeavor of this research is the development of a model that quantifies volume loss due to rock penetration within sandy seafloors. The investigation combines theoretical modeling with empirical validation through lab testing. Through analysis, it is revealed that the complex behavior of granular media necessitates an in-depth understanding of the theory, together with assumptions and simplifications to create an effective penetration depth model. The literature study outlines the various forces exerted on a rock as it impacts the seafloor and commences its penetration into the sand. Within the summation of forces, one component to consider is the bearing force exerted by the sand. This force can be calculated using two distinct approaches: the Terzaghi formula for shallow foundations and the Brinch Hansen formula, also applicable to shallow foundations. In this thesis, both formulas are employed to assess their applicability to the dynamic penetration of spherical particles in a sandy seafloor. Once these forces have been aggregated within the equation, two distinct methods to resolve the equation and determine the penetration depth of a spherical rock in sand are employed. The first method is rooted in the principle of a work-energy balance, stating that the cumulative work done by the sum of forces matches the change in the rock's kinetic energy. In contrast, the second method is based on the impulse-momentum balance, underlining the concept that the summation of forces multiplied by a specific duration in time, is equivalent to the change in the rock's momentum during that defined time interval. The work-energy method is preferred over the impulse-momentum method because of the specific objective of the single stone model, which centers on determining penetration depth, a distance measurement. The work-energy method inherently incorporates the distance variable within its formula, aligning directly with the desired outcome. In contrast, the impulse-momentum method necessitates calculating penetration depth by multiplying time with velocity for each discrete time step, introducing a less efficient computational process. \\
Following a comparison between the results derived from laboratory tests and the computed penetration depths using both the Terzaghi and Brinch Hansen-based models, it becomes evident that the Terzaghi formula offers a closer approximation to the behavior of the bearing force.

Further insights are drawn from multi-stone penetration testing, wherein assumptions and simplifications are strategically employed. The intersection of the multi-stone model with a three-dimensional normal distribution delineates the positions of rocks post-fallpipe discharge, bridging theoretical modeling with practical subsea rock installation scenarios. Although a somewhat unrealistic scenario of centric rock-rock collisions and perfect stacking of rocks on top of each other is assumed, the model provides a valuable worst-case scenario in terms of volume loss.

Though the model's validation is confined to sandy seafloors, its implications extend beyond. The study underscores the need for real-world data integration to refine the model.

Wave overtopping of coastal structures is generally expressed in terms of average discharge and maximum overtopping volume. While substantial research can be found on the relationship of such variables with incident spectral characteristics and other geometrical dimensions, limited research has been done to identify the conditions for which severe individual overtopping events occur. The objective of the present work is to further investigate the statistical dependencies between individual overtopping volumes and incident wave characteristics and to identify the conditions which result in maximum overtopping events. To this aim, we statistically analyse timeseries of waves and overtopping events for the case of a vertical wall breakwater on a horizontal bed under non-breaking wave conditions. The timeseries are generated using a validated numerical model for 6 sets of boundary conditions. The analysis of the data shows that, among the examined variables, the incident wave crest level at the toe of the structure has the highest rank correlation with the individual overtopping volumes. Conversely, the incident wave heights show a lower rank correlation while the wave periods show no correlation at all. Additionally, by leveraging the tool of copulas we find that the incident wave crest levels have strong upper tail dependence with the individual overtopping volumes while the wave heights show no tail dependence. Subsequently, we examine possible dependencies with the wave group periods and wave group energy. We find that a moderate correlation exists between wave group periods and overtopping volumes but no upper tail dependence. Moreover, our results show that the fraction between the wave group energy and the group period has a high correlation and upper tail dependence with the overtopping volumes. Finally, we discuss possible practical applications of our findings as well as points for future research.

This thesis explores the potential of a fixed artificial bypass system for a sustainable and eco-friendly approach to coastal management in IJmuiden. The presence of the IJmuiden Port disrupts the natural processes, causing significant morphological changes to the coastal area. Current management practices require frequent dredging and nourishment to mitigate the erosion of the downdrift coast and maintain the depth necessary for navigation in the channel and port. These activities result in significant emissions and adverse ecological impacts. Particularly affecting benthic life, organisms living in or on the seabed.


Implementing a fixed artificial bypass system, already proven successful in similar projects globally, emerges as a potential solution to reduce dredging and nourishment activities. This system aims to restore the natural sediment transport by pumping sediment from the updrift to the downdrift side. In addition, its more continuous discharge of sediment is anticipated to be less disruptive for benthic life compared to traditional nourishment methods.

The best-case scenario calculation performed in this thesis presents that an artificial bypass system at IJmuiden could potentially reduce dredging activity by 3.5% and nourishment activity by 37%. To assess whether these reductions can be achieved, this thesis introduces a newly developed framework for assessing the effectiveness of sediment bypass concepts based on four performance indicators: (1) Dredging Activity of Channel and Port, (2) Sediment Demand of Downdrift Coast, (3) Impact on Benthic Community and (4) Feasibility.



This method includes simulating the response of the coastal system after implementing varying bypass concepts using a Delft3D model. The Delft3D model’s applicability and predictive skill are assessed via hydrodynamic and morphodynamic validation. Concluding that the model can reproduce the general trends but introduces numerical errors in the exact quantification of the morphological development. Despite this limitation, the output from these Delft3D simulations was used to evaluate the response of different artificial bypass concepts based on the four performance indicators. The first two indicators are based on the simulated sediment transport values and assessments of the development of the bed. The evaluation of the third indicator is based on a calculation performed using the developed benthic evaluation tool, named the 'Benthimeter'. This newly developed tool provides a method that intends to visualize and quantify the impact on the benthic community induced by nourishment activity. Although the Benthimeter requires further calibration and validation, it marks a good first step towards integrating ecology into coastal management.


The results of this thesis demonstrate that the coastal system of IJmuiden allows for sediment withdrawal, where allready 10% of the required annual sediment trap was observed within one simulated month. Also northward sediment dispersal towards the downdrift coast was observed at simulations, indicating that such a system could reduce the sediment demand. These findings provide confidence that the principles of bypassing sediment around the port of IJmuiden hold. Consequently, it is anticipated that an artificial bypass system would, to some amount, reduce the need for dredging and nourishment activity. Also, the calculated impact on the benthic community confirms the hypothesis that a more continuous nourishment approach reduces the impact on the benthos. While the findings of this study provide an initial indication of the potential effectiveness of an artificial bypass system at IJmuiden, they do not provide long-term effect estimates. Further research is suggested to examine the primary drivers of dredging and nourishment activity, along with efforts to simulate the equilibrium state to evaluate the long-term effects.


The most important contribution of this thesis is the introduction of innovative tools, guidelines, and effective methods. This framework can be used in future research to improve our knowledge of sustainability and ecology in coastal practices.

Groynes are hydraulic structures commonly applied in the Dutch rivers. Depending on the water depth, groynes can be emerged or submerged. When a groyne is emerged, it has two main functions: maintaining the water depth in the main channel and preventing the river from eroding its bank. When the discharge in the river increases, the groynes become submerged. A submerged groyne acts as an obstacle to the flow, resulting in additional resistance and an increase in the upstream water level.

To increase the flow capacity of a river during extreme conditions, a couple of measures can be considered: increasing the height of the dikes and embankments, reducing the overall resistance of the river cross-section and increasing the area of a river cross-section.

This thesis focuses on reducing the overall resistance of the river cross-section, and more specifically, on investigating the effect that streamlining the downstream slope of a groyne has on the groyne-induced resistance. Hypothetically, streamlining the groyne by decreasing its downstream slope would decrease the intensity of flow processes like downstream flow separation, which could increase the discharge capacity of the groyne, and thus of the river cross-section, as a result of the decrease in groyne-induced resistance. An increase in discharge capacity would decrease the flood risk during extreme conditions.

To investigate the effect of streamlining a groyne by decreasing its downstream slope, a physical model experiment was set up in the 5 x 40 m^2 flume at the Hydraulic Engineering Laboratory of Delft University of Technology. The physical model represents part of a river cross-section. From the total 5 m width, the main channel and groyne field both took up 2 m and the floodplain took up 1 m. In streamwise direction, the flume contained six groynes and five full groyne fields. The groyne fields had transverse bed slope of 1:25. Electromagnetic flow meters (EMF meters) were used to measure the flow velocity at many points in both streamwise and transverse direction. The water level was measured by laser altimeters.

Decreasing the downstream slope of a groyne to 1:8 compared to a reference situation with a downstream slope of 1:3 resulted in a 4 % increase of the discharge capacity over the crest of the groyne. This increase of discharge capacity was not uniform over the transverse axis. Near the groyne tip, this increase of discharge capacity was more than 6 %, whereas at the end of the groyne, the discharge capacity decreased slightly compared to the reference situation.

A more detailed analysis of the data gave insight in why decreasing the downstream slope of a groyne increases the discharge capacity of a groyne. Streamlining the groynes resulted in a significant decrease of the relative turbulence intensity along the downstream slope of the groyne. This was consistently around 50 % lower for the streamlined groynes compared to the reference situation, indicating less intense turbulent structures. Moreover, auto-correlation functions of the flow velocity signals showed a shorter correlation length in conjunction with lower amplitudes and smaller periods of the fluctuations for the streamlined groynes. This indicates a signal with less correlation and higher frequencies. These results explain why the discharge capacity of a streamlined groyne is larger than of a reference groyne.

This work seeks to expand on the research done previously by the WOODY group on the ability of vegetation as a form of wave damping. Specifically in this work, we are looking at pollard willow trees (Salix Alba) and the relationship between the motion of the object and the forces it can induce on the wave flow. This was done by testing a range of stiff and flexible conical mimics of three different sizes and varying moduli of elasticity across multiple water levels. The results were compared to commonly used relationships for predicting object motion such as Cauchy, KC, and CAL/KC (Jacobson). Due to the non-constant cross-sectional area of a conical object. However, the relationships could not fully capture the motion. As such a new equation was fitted and derived from the relationships that better captured the motion of the mimics under wave flow. This equation relates the peak forces felt by the objects to the peak deflection. Which can be used as an indicator of wave damping. Along with this number thresholds for motion were also derived from the data. It was found that when an object tip has deflected 2 degrees or less it can be considered completely stiff. If deflecting between 2 and 5 degrees it can be considered relatively stiff, however, high-frequency flow effects can cause discrepancies in the forces between tests.

The dunes along the Dutch coast serve as a critical component of flood defense. Storms and a shortage of sediment supply offshore lead to dune and shoreline erosion, posing a hazard to flood safety. To combat coastal erosion, the Dutch government decided in 1990 to proactively nourish the coast. It is expected that as an indirect effect of the sand nourishments, the volume of the dunes will increase. This thesis aims to determine if the influence of the nourishments and storms along the Dutch coast at decadal timescales can be observed.

A conceptual model is presented to address the impact of storms and nourishments. With the model it is argued that storms have little influence whilst nourishments will likely significant influence the long term dune development.

To investigate the impact of storms yearly coastal profile measurements along the entire Dutch coast (JarKus dataset) and storm profile measurements from the storm in 1976 are used. Following the analysis, no correlation is found between the magnitude of the storm impact and the post storm dune development. However, a difference in horizontal dune toe translation (DTT) trend before the storm (1965-1975) and after the storm (1976-1981) is observed. For all coastal regions except the Delta regions, a higher post storm DTT is found.

To investigate the impact of nourishments yearly coastal profile measurements from 1965 to 2022 of more than 1000 transects along the entire Dutch coast are used. Fitting a continuous bi-segmented piecewise linear function showed an average dune volume change (DVC) of 4.5 m3/m/y before the break point year 1993 and 9.3 m3/m/y after 1993 for the entire Dutch coast. These findings align with the fact that large-scale nourishments commenced in 1990.

Furthermore, the nourishment volume is compared to the change in DVC per coastal region to investigate the impact of the nourishments. A Pearson correlation coefficient of r = 0.78 is found between the change in nourishment volume and the change in DVC between the period before and after the breakpoint year. The ratio between nourishment volume and dune growth is 0.24, meaning that if the correlation is causal, 1m³ of nourished volume contributes to 0.24m³ of dune growth.

Based on the data analysis, we conclude that there is no evidence based on the storm measurements in 1976 to suggest that the size of the storm influences dune growth post-storm. However, the measured DTT trend before and after the storm supports the idea that the dune toe tends to return to the pre-storm trend after a storm. Therefore, on a long-term scale, there seems to be no lingering influence of the storm on dune development. Additionally, the data analysis revealed a distinct change in the trend of dune growth in 1993, three years after the start of large-scale nourishments. A strong relationship is found between the amount of nourished sediment and the DVC. These findings supports the idea that nourishments along the Dutch coast have a significant impact on dune development on a decadal timescale.

The development of floating islands is an attractive option to provide living/working spaces at sea, as it would be a more economical solution for many locations at sea where the water depth is high. However, large second-order wave forces cause the floating island mooring system to become expensive as the wave height increases. So, in many locations with an intermediate water depth, land reclamation is still a more economical solution. A breakwater connected to the wave-ward side of the floating island can reduce the mean wave drift force of the entire structure by attenuating wave energy and thereby lowering the mooring costs. In this thesis, the optimal geometry of this breakwater is designed.

An optimisation is performed to find the ideal geometry of this breakwater such that it attenuates the maximum amount of wave energy while experiencing a low mean wave drift force. A parametric design of a breakwater is made, with six varying factors/parameters that define its shape, such that the geometry could range from a flat plate just below the water line, a large box-type breakwater at the water surface, a deeply submerged wedge-type breakwater, or anything in between. ComFLOW (a simulation method for free-surface flow) is used to determine the mean wave drift force and wave attenuation performance of all the breakwaters in the design space for two different regular wave conditions: Wave Condition 1 with a wave height of 3.0 metres and a peak period of 6.0 seconds, and Wave Condition 2 with a wave height of 9.0 metres and a peak period of 10.4 seconds. After all ComFLOW results are gathered, an optimisation method Design of Experiments is used to map the dependency of the six input factors that define the geometry, on the performance of the breakwaters, to come up with the geometry of the optimal breakwater.

To obtain the most accurate representation of reality in the numerical environment of ComFLOW, a validation of the programme is performed in two phases. Firstly, ComFLOW results are compared to analytical formulas, derived from linear wave theory. And secondly, ComFLOW dissipation characteristics are compared with the results of a physical experiment involving a regular wave plunging over a barred beach profile. From this followed, a minimum of twenty cells is required at the height of the structure to give reliable results of the mean wave drift forces on the structure. ComFLOW results for the wave transmission, wave reflection and forces on the structure followed the same trend as linear wave theory, with a converging offset. The wave energy dissipation characteristics of ComFLOW of all different grid sizes used in comparison with the experiment in which waves were plunging over a breaker bar, showed results that were in compliance with the experiment.

In Design Iteration 1, 94 different geometries were simulated in ComFLOW, of which 46 were completely submerged. Around 80\% of all submerged breakwaters experienced a mean wave drift force in the opposite direction of wave propagation. An explanation for this negative mean wave drift force is sought by the set-up and set-down of the water level, which occur around breaking waves. This results in a difference in hydrostatic pressure, and if the breakwater is positioned in between, it will experience a mean wave drift force in the opposite direction of wave propagation. The length of the sloping beach of the breakwater needs to be shorter than the wave length; otherwise, the set-up will be above the beach, and this results in a contribution to the mean wave drift force in the same direction of the wave propagation. The optima found by \acrshort{doe} based on minimising the mean wave drift force were either a shallow box-type structure relatively far below the water surface or a large structure closer to the waterline with a sloping beach on its wave-ward side, which induces the waves to break and thus dissipate wave energy. For maximal wave attenuation performance, the height and length of the breakwater must be maximised.

A cost function is developed that computes the reduction in captical expenditures (\acrshort{capex}) the breakwater provides to the floating island. Therefore, the construction costs of the breakwaters and the reduction in the mooring costs of the floating island are calculated. The difference between the two is the total cost reduction that the breakwater provides. An optimisation is done on maximising this total cost reduction for Wave Condition 1, which resulted in an optimal design of a breakwater with a length of 10.0 metres, a depth of 9.6 metres, a draught of 6.2 metres and its wave-ward side is a sloping beach that induces the incoming waves to break. It can provide a cost reduction of 24.4 k\texteuro per unit width. Compared with a floating island designed by the 3-year Space@Sea project for a location in the North Sea, this breakwater would deliver a 51% mooring cost reduction and a 7.7% reduction in total costs.

A dam-break wave can result in considerable damages and casualties. Events that can lead to such waves include dike and dam breaches, storm surges and impulse waves. Dutch history is familiar with a number of coastal dike breaches but also river dike breaches have occurred, resulting in dam-break waves reaching far inland. Beyond the Netherlands, propagating walls of water could occur when dams of water reservoirs collapse. More specific knowledge of the hydrodynamic behaviour of dam-break waves is necessary in the battle against water.
The impact of a dam-break wave can be reduced through a better understanding of the currently poorly understood link between bed roughness and wave hydrodynamic properties. The objective of the present study is therefore to study experimentally the effect of bed roughness on the hydrodynamic properties of dam-break waves, on both dry and wet beds. The approach was to generate dam-break waves through a lift-gate and a reservoir with a depth d0 = 0.4 m and to measure the water levels over time in the downstream channel. 7 different bed roughness configurations were made (range of ks = 0.6 to 48 mm) in addition to smooth bed reference test. Dry bed tests were performed, 5 wet bed tests with an initially still water level (h0) (range of h0/d0 = 0 to 0.125) were performed. The analysis focused on wavefront celerity, inundation depths and roller length which are used to characterise the hydrodynamic properties.
Visual observations showed substantial differences in wavefront behaviour between surges and bores as well as between smooth and rough beds. Validation of the generated waves was done by successfully comparing the reconstructed water levels over the flume length with a new approach.
Results showed that an increase in bed roughness resulted in a decrease in wavefront celerity for surges and that the initially still water level can work as a lubricant for bores. The initial still water level is dominant over the bed roughness when h0 >0.5ks.
The maximum water level increased with increasing bed roughness for surges, while bores showed an opposite behaviour. Increasing the initially still water level reduced the relative maximum water level for rough bed configurations.
In the present study, only some test configurations revealed the presence of a plateau height. If there was one, an increase in bed roughness showed a decrease in plateau height for bores with h0/d0 = 0.125. Three sections in the h0/d0 range with different plateau height behaviours were noticed.
The roller lengths were determined with the wavefront celerity and the plateau height. No trivial relations were found for the influence of bed roughness or initial still water level.
Overall, this project revealed some interesting behaviours that need to be further investigated in the future. It is recommended to conduct a full quantitative analysis once precise values of the bed roughness configurations are determined. Nevertheless, these results present some preliminary, yet interesting, findings that will contribute to a better understanding of the behaviour of dam-break waves over rough bed.

Mangroves trap sediment and are the first coastal defence line for many coastal areas. Unfortunately, mangrove coasts worldwide are eroding due to deforestation, exposing local communities to flood hazards. In Demak, Indonesia, a Building with Nature approach was chosen to counteract erosion. Permeable dams were built on the foreshore to attenuate waves and restore the sediment balance. The dams have increased the bed level locally and mangroves expand after construction. A new project called MuMaCo (Mussels as Mangrove facilitators for Coastal defence) is launched to integrate mussel aquaculture into the mangrove rehabilitation project in Demak. Mussels increase the roughness and diameter of the bamboo poles, which could increase their wave attenuation capacity. Moreover, mussel culture gives the local communities an economic incentive to sustain the structures. This thesis describes and quantifies how mussels' growth influences the forces acting on bamboo poles. Small-scale experiments are performed in the wave flume of the Delft University of Technology on a scale of 1:3 and 1:6. Physical models were used for experiments with steady flow and waves. The steady flow experiments cover a range of Reynolds (Re) numbers between 10^3 and 10^4 to seek the drag crisis. Single mussel covered cylinders for both scales are compared to smooth cylinders and cylinders with an equivalent diameter. For the wave experiments, cnoidal waves with Keulegan-Carpenter ($KC$) numbers ranging from 3 to 113 that correspond with measured waves in Demak are chosen. Moreover, only mussel covered cylinders are compared to a smooth cylinder for a scale of 1:6. During the experiments, the flow velocity, force, and surface elevation are measured, and the drag and inertia coefficients are obtained with the help of the Morison equation. The drag coefficient shows a clear relationship with the Reynolds numbers for steady flow. Although the drag coefficients display the same qualitative behavior as in other studies in the literature, the drag coefficients of the smooth cylinder are more than 40 % larger than in the literature. This is probably due to large variations in the offset of the velocity meters (EMS) during the experiments. The large mussels show higher drag coefficients than the smooth cylinder. For the wave experiments, the drag coefficients show good correspondence to other studies in the literature, but the inertia coefficients are a factor two larger than expected for smooth cylinders. Treating the peak and trough of the cnoidal wave as individual sinusoidal waves shows similar results and suggest that cnoidal waves can be represented by KC numbers, as it is done for linear waves. The best fit to the literature values was found by solving the Morison equation with the measured force with the depth-integrated velocity squared and acceleration from the Fenton theory. This suggests that the results of the wave experiments have been uncertain due to inaccuracies in the EMS recordings. This thesis presents a method to schematize mussels for laboratory experiments and provides a qualitative comparison of the forces acting on poles with and without mussels

The added drag on a buoyant body traveling in a solid-body rotation flow has been rigorously studied over the last century with G.I. Taylor being the first one to describe it in 1922, a hundred years ago. A recent publication by Duinmeijer (2021) investigated the capacity of free-surface vortices to transport solids as a practical application to avoid solids accumulation in wastewater pump sumps. In that research, the assumption was made that a Taylor column exists upstream and downstream of the solids body, aiding the downward motion in the vortex core. This has however never been experimentally confirmed for free-surface vortex flows. The goal of this research is to experimentally check whether or not the Taylor-column induced drag force is the main mechanism for the downward motion of buoyant particles in a free surface vortex core. An experimental set-up consisting of a 600 x 1000mm (diameter x height) Perspex tank is used, in which controlled vortices can be generated. A novel combination of Laser Doppler Velocimetry (LDV) and Particle Tracking Velocimetry (PTV) is deployed simultaneously to obtain synchronised data of both the flow velocities and particle motion. The LDV device measures the tangential and axial flow components in a small measurement volume. Many point-measurements along a horizontal line through the vortex enable us to create the tangential and axial velocity profiles. A deconvolution process is applied on the LDV data to account for the spatial averaging effect of the LDV measurement volume and the wandering of the vortex core. The PTV system is used to determine the particle location over time, from which the velocity is obtained. For simplicity, only one type of particle - a buoyant sphere of 25 mm in diameter - is considered in this study. To obtain repeatability of the experiments, a particle dropping device is introduced to insert the sphere in the vortex core without entrapping air. A PID control system is added to the set-up to keep the discharge through the system constant. The first part of this research focusses on the performance of the measurement system, where the measurements of the flow characteristics of the free-surface vortex in the absence of a buoyant particle are compared to the results obtained by Duinmeijer (2020), repeating one of the experiments from that thesis without a buoyant particle. The tangential velocities measured with LDV, coincide well with the tangential velocity profiles found during PIV measurements by Duinmeijer. The axial velocity profiles however do not agree. Duinmeijer measured maximum axial velocities around the vortex core radius where in this thesis an axial velocity profile with two maxima is found; one in the vortex centre and one at 60-70% of the vortex core radius. Several additional experiments were performed to rule out possible causes for this difference, from which it is concluded that the axial velocity is non-zero in the vortex centre. The second part of the thesis focusses on the characteristics of the flow in the vortex core in the presence of a buoyant sphere. With the synchronized deployment of the LDV and PTV-system, the flow mechanics and the motion of the buoyant sphere can be simultaneously characterized. Such that the influence of the sphere's presence on the tangential and axial velocity components is quantified. New findings were obtained from these measurements, with the most striking one being a significant difference that is observed in the tangential velocities of the flow just above and below the buoyant sphere. Although this effect is described in theory e.g. by Moore and Saffman and Maxworthy (1970; 1968), it has not been measured before in any lab experiment before and therefore it was not expected to be observed so clearly during the experiments. The clear tangential velocity discrepancy experimentally confirms the presence of the Taylor column above and below the sphere.
The last part of the thesis is dedicated to quantifying the Taylor column induced drag force, derived from the tangential velocity difference of the flow above and below the particle. This difference in tangential velocities induces a pressure difference over the particle leading to a downward pointing force. The magnitude of this force is shown to be 75±17% of the total drag on the particle, making the Taylor column induced drag the main downward transport mechanism for the buoyant sphere in the free-surface vortex core given the conditions used in this experimental set-up.
It is recommended to investigate further the effect of the vortex core radius/particle characteristic length ratio on the Taylor column induced drag. The axial free-surface vortex flow is not radially uniform, with a region of high axial flow around the core radius. This axial flow being pushed into a Ekman layer on the sphere surface is generating the pressure difference over the particle and thus the downward force. It is therefore suspected that vortex core/particle size ratio strongly influences the Taylor column induced drag magnitude and thus be researched further.