As offshore wind energy expands, the environmental impact of monopile foundations on ocean stratification has become a growing concern. Thermal stratification occurs when layers of seawater with different temperatures form due to solar heating, creating a stable density gradient that limits vertical mixing. However, monopile-induced turbulence can disrupt this layering, a process known as destratification, which may have significant ecological consequences.

This study investigates a novel perforated monopile design as a potential solution to mitigate these effects. Using Computational Fluid Dynamics (CFD) simulations in OpenFOAM, the research compares the hydrodynamic behavior of regular and perforated monopiles, focusing on turbulence intensity, wake formation, and destratification rates.

The results indicate that perforated monopiles reduce turbulence intensity and modify wake dynamics compared to regular monopiles, by allowing partial water flow through the structure and reducing recirculation zones. However, their impact on destratification rates remains minor. While perforations alter hydrodynamic behavior, they do not significantly mitigate stratification breakdown.

These findings suggest that while perforated monopiles influence flow dynamics, their effectiveness in reducing destratification is not significant enough to definitely recommend these structural modifications. Future research should explore different perforation patterns and other alternatives develop more sustainable offshore wind turbine foundations.

Over 30% of the world's coastline consists of permafrost and large sections of these coasts are subject to erosion. In the Arctic, unlike with temperate low-latitude coastlines, thermal processes affect erosion mechanisms. There is a lack of long-term predictive capability of morphodynamics for Arctic coastlines, which results in a knowledge gap regarding the coastal processes inducing permafrost erosion and how these processes will change under the effects of climate change. Previously developed parametric morphodynamic permafrost models lack generalizability because they either do not include all erosion mechanisms or were calibrated for specific eroding coastlines. Comprehensive morphodynamic models that include permafrost dynamics are too computationally expensive to perform long-term analysis and are thus applied to storm time scales only.

This study implements a novel method that integrates thermodynamics, hydrodynamics, and morphodynamics to predict the morphodynamic evolution of a permafrost-affected coastline. We developed and validated a process-based numerical model for Barter Island (North Slope, Alaska). This model showed skill in predicting the ground temperature distribution and the erosion of a permafrost bluff. Sensitivity analyses indicated that the environmental drivers affected by climate change (i.e., air and sea temperatures, water level) are expected to accelerate the erosion of permafrost-affected coastlines under the effects of climate change, confirming the findings of previous work. Rising temperatures will compound with diminishing sea ice to widen the annual window during which erosion can occur, which will increase the number of storm events that lead to erosion. Lower bluffs composed of finer sands are especially vulnerable.

The low computation costs mean that the model can be used to predict coastal erosion for larger regions, potentially benefiting strategic coastal management and policy-making.
Additionally, the developed model will improve global climate models. It can facilitate the mapping of permafrost degradation and organic carbon release, with the release of organic carbon through permafrost erosion being one of the greatest unknown drivers of global warming. Though further calibration is required, the developed model can be used as a tool to research the quantitative effects of climate change on the erosion of Arctic coastlines and gain a deeper understanding of how climate change affects the processes that ultimately lead to the erosion of permafrost bluffs.

In Dutch polders, numerous structures like bridges, weirs, culverts, and pumping stations have
been constructed over centuries to manage water levels. These structures play a crucial role in
maintaining water levels within predefined targets. The flat topography of the Dutch landscape
combined with the collective impact of head losses, induced by these structures may result in
flooding of polders during high runoff scenarios. Over time, culverts and bridges may
underperform due to alterations in the water system, increased pressure from climate change,
evolved design rules, insufficient maintenance, and shifts in land use.
A challenge is the potential hydraulic underperformance of structures and the need for their
premature replacement, which is costly. Waiting until the end of their technical lifespan may
contribute to floods. Therefore this thesis focuses on improving existing structures to mitigate
the need for replacement, specifically by streamlining inlet and outlet openings to reduce
energy losses. This leads to the research question of this thesis: “How can the head loss over
existing (too tight) culverts be minimised by adding an inlet or outlet profile and does this lead
to a substantial enhancement in the performance of these culverts, providing a practical option
to postpone the replacement of underperforming culverts?”
To answer this question, the problem is explored by looking into the fundamentals of energy
losses, including entrance losses, friction losses, and exit losses. This gives an understanding of
the conditions under which these losses manifest. However, these basic calculations have
inherent limitations due to their reliance on predefined coefficients. This renders them
inadequate for evaluating the effects of introducing new profiles onto an existing structure.
To overcome this, a flume experiment has been performed to verify whether it is possible to
measure water level differences for various profiles at the culvert entrance and exit. With a 3D
Computational Fluid Dynamics (CFD) model (OpenFOAM), flows around different culverts
are simulated. The results of the CFD model are compared to the flume experiment, after
which the CFD model is used to simulate a variety of scenarios, with different profiles, culvert
dimensions, velocities, and water depths.
As such, this thesis addresses challenges and uncertainties in quantifying head losses in culvert
structures through experimental methods and CFD modelling. Experimental setups struggle
with controlling all flow-influencing parameters, while CFD modelling offers flexibility but
requires careful consideration of uncertainties and limitations. The discussion emphasizes the
complexities of comparing experimental and model results, highlighting trade-offs and
uncertainties in each approach.
The conclusion answers the central research question, confirming that specific profiles added to
culverts can significantly reduce entrance losses up to 65%, thereby lowering headwaters for a
constant discharge.
The recommendations section outlines possibilities for further research, including optimizing
profile dimensions and conducting sensitivity analyses of influential parameters. Practical
recommendations involve aligning large-diameter concrete culverts with the socket end in the
flow direction and integrating groove or rounded profiles during construction for cost-effective
inlet loss reduction.

Geotextiles are permeable fabrics used in various civil engineering and construction applications for the protection against erosion and scour. Geotextiles have predominantly been manufactured using synthetic materials due to their durability and precise design pore sizes. However, the environmental concerns associated with synthetic geotextiles have led to the exploration of sustainable alternatives, including natural fibers and biodegradable thermoplastic polymers.

This study addresses the problem of designing stable geotextile filters. The problem statement resolves around the assumption that geotextiles of natural fibers are considered "open" filters. Existing design criteria for open filters do not directly account for load conditions or the use of fibers in the fabric. Instead, they rely on the relationship of the geometrical properties of the base and filter material. To address this gap, a new approach is proposed based on hydraulic pressure gradients. This approach considers the actual gradient and critical gradient at the geotextile interface. To achieve this, the geotextile will be evaluated within the context of an open channel with uniform flow conditions.

The main research objective is to formulate a design method for geotextiles based on the principles of an open filter structures under a single granular filter layer. A comprehensive literature review has been conducted to investigate the hydrodynamic processes that define the actual gradient at the filter/base interface where the geotextile is positioned. The framework of linear wave theory proved to be a valuable tool for investigating and modeling these hydrodynamic processes. To define the actual gradient, the following hypothesis is proposed:

The actual gradient can be defined by adding the average gradient to a turbulent component derived from measured turbulent wall pressure spectra at the top of the filter layer. Based on spectral approach and linear damping, it is hypothesized that these turbulent components undergo damping within the filter layer, resulting in a diminished turbulent component at the filter/base interface.

From the literature review, it can be inferred that the dimensionless turbulent rough wall pressure spectra developed by Blake (1970) offer the most appropriate representation for the relevant hydraulic conditions (rough horizontal bed, uniform open channel flow). In a turbulent wall pressure spectrum, outer scaling variables and inner variables relate to different length scales within the flow, representing large-scale and small-scale fluctuations, respectively. In this hypothesis, it is assumed that the low-frequency region is scaled by the average velocity and water depth, while the scaling variables for the high-frequency region are presented in terms of the shear velocity and nominal diameter of the filter material. By combining both spectra, converting the pressure spectrum to the gradient spectrum, and transforming the spectral domain to the spatial record, the fluctuating gradient can be determined.

The hypothesis was verified using measured data from experiments conducted by Van de Sande (2012) and Wolters & Van Gent (2012). By incorporating Van de Sande's measured dataset into the scaling parameters of the dimensionless pressure spectra and determining the stability parameter for open filters, it was concluded that qualitatively the results align with Van de Sande's findings, indicating stability or instability of the filter layer. However, in some test cases, the hypothesis suggested instability while the experiments indicated stability.

This discrepancy may be due to the fact that while the stability parameter may indicate an unstable filter, it does not necessarily correlate with significant base material transport through the filter layer. In a quantitative analysis, the amount of dimensionless transport of base material was compared to the stability parameter. According to the data presented in figure 4.12, it can be observed that the stability parameter exhibits a value that is three times larger then the stability parameter defined by Schiereck (2012), while not inducing any significant transport of base material. This finding aligns with previous experimental investigations, such as the study conducted by Wolters & Van Gent (2012), which similarly demonstrated that the stability parameter could reach values that are two to three times larger, while still not resulting in notable base material transport.

A direct quantitative comparison was conducted between the measured dataset of Wolters & Van Gent (2012) and the calculated gradients obtained from the hypothesis. The results revealed that the calculated gradients exhibited a comparable magnitude, ranging from 0.25 smaller to 1.50 larger in comparison to the measured gradients reported by Wolters & Van Gent (2012). This indicates a reasonably close agreement between the calculated and measured gradients.

In conclusion, the research objective in this study was to formulate a design method for open geotextiles under a single granular filter layer. To achieve this, it was necessary to take some steps back and define the actual gradients, as well as identifying a suitable method for calculating these gradients. The actual gradient is calculated with the sum of the average gradient and the fluctuating gradient. The fluctuating gradient can be characterized by the turbulent wall pressures, which are derived from the pressure spectrum measured at the top of the filter layer, as described in Blake (1970). Assuming linear damping of the pressures within the filter layer, the actual gradient can be computed at the interface of the geotextile. Consequently, the assessment of stability entails a comparison between this calculated actual gradient and the critical gradient measured by Lemmens (1996).

With a clearer understanding of how to calculate the actual gradients at the geotextile interface, the same approach employed in the hypothesis is applied to address the design of a stable open geotextile under a single granular filter layer. This has been demonstrated through a case study, where the thickness of the filter layer was determined to create a stable filter structure.

Several recommendations can be made to improve the understanding and design methodologies in relation to the hypothesis. Firstly, there is a need for a better understanding of the spectral density of wall pressure fluctuations in open channel flow. This understanding will enable more accurate estimations of fluctuating gradients. Secondly, further research is necessary to incorporate the distribution of pressure at the filter/base interface for non-linear damping. Thirdly, it is advised to conduct tests using modern natural geotextile materials to refine the critical gradient determination and explore alternative approaches, such as considering critical filter velocities. Lastly, it is recommended to extend the design method to encompass wave conditions. Verifying the effectiveness of the design approach under wave conditions through comparison with measured data will ensure its applicability and reliability in practical scenarios.

While further investigations are needed, it still offers a fresh perspective on evaluating the stability of open filter structures. If successfully refined, the design method could become a valuable tool in geotextile design and potentially find applications in other contexts involving "open" structures.

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.

The influence of surface roughness and shape on the submerged floating tunnel is investigated in wave and current conditions. The hydraulic forces caused by varying tunnel shapes and roughness designs are measured and analyzed.

The ramifications of plastic pollution on the environment are becoming increasingly serious in various forms throughout the world. In this context, rivers are the most important suppliers of plastics entering the marine environment. However, rivers that contain high loads of plastic waste also directly harm the livelihoods of people living near these rivers. An example is the flooding of urban areas in Indonesia due to clogging of the hydraulic and drainage systems, which is caused by the blockage of local hydraulic structures by plastic debris.
At this moment, there is a lack of knowledge on this accumulation process and its underlying dynamics, since observational and experimental research is lacking. Numerical modeling has proven to be a great tool for expanding experimental research. However, no suitable numerical method has been identified yet to model the plastic accumulation process, since traditional mesh-based CFD numerical methods are expected to be not a viable option, due to their inability to model the individual interaction between plastic particles, critical during this process. A possible solution could be the SPH-DEM method, which is a two-way coupled numerical approach that simulates fluid and debris as discrete particles and elements.
The objective of this report was to find out if SPH-DEM could be a suitable numerical method to model the dynamic processes of the plastic debris accumulation against hydraulic structures. To accomplish this, the first goal was to realistically model a turbulent open-channel flow and the buoyancy of individual plastic debris, which would be validated by experimental research. The second goal was to investigate which are the most important (numerical) parameters affecting the mentioned plastic debris accumulation.
In this report, experimental research was carried out in the form of buoyancy tests and flume tests, and numerical research was carried out in the form of the design of numerical simulations. In the buoyancy tests, the rising velocities of four plastic fragments that differed in size and density were measured, which were released multiple times in a graduated cylinder filled with water. In the flume tests, first the water elevation was measured along the the flume, after which the passing ratio’s and carpet lengths were measured for the four different released fragments for three different gate configurations. Two types of numerical models were designed that represented both types of experimental tests, for which several design choices had to be made to compensate for several physical phenomena, which can’t be directly represented in the model design.
The numerical buoyancy test was validated with the rising velocities obtained from the experimental equivalent. It was discovered, that for relatively low resolution modeled fragments, the rising velocity is heavily influenced by numerical diffusion. The smoothing length was identified as an important numerical parameter, which can compensate this effect. Furthermore, it was discovered that the degree of numerical diffusion is dependent on the depth of the fragment in the water. The numerical flume test was validated with the water elevation obtained from the experimental equivalent. For uniform flows, by adjusting the boundary viscosity coefficient, smooth turbulent velocity profiles could be simulated throughout the flume corresponding to theoretical values. However, no single value of was found in which the velocity profiles of the uniform flow and the validated water elevation of the gradually varied flows were both in agreement with their theoretical values. After validation, fragments were added to numerical flume model. Per fragment type and gate configuration,
four different numerical scenarios were executed, where each scenario was defined by a combination of a certain density ½s and restitution coefficient e. Finally, the best corresponding scenarios were used to simulate mixed fragments released in the flow. It was found that the gate opening height , density and restitution of the plastic fragments have the largest influence on the passing ratio’s , carpet length , carpet shape and carpet stability. Furthermore, it was confirmed that individual fragment interactions play a crucial role in the accumulation process. However, the model is mainly limited by its low resolution and the absence of suitable turbulence models. This means that many forms of fragment behavior seen in the experimental research such of buoyancy, trajectory and individual interactions, which are heavily influenced by turbulence, cannot be sufficiently represented in the numerical model. However, it is shown that by adjusting the density the buoyancy behavior can be partly replicated and by adjusting the restitution coefficient the turbulent individual interactions can be partly replicated. In conclusion it can be stated that SPH-DEM is an interesting option to model the dynamic processes of the accumulation of plastic debris against a sluice gate; however, further improvements in computational power and turbulence models are needed to be more widely applied.

The coastal zone is becoming more vulnerable due to growing concentrations of human population, settlements, and socio-economic activities. The Rhine-Meuse Delta is low-lying land, which makes it vulnerable to flooding. The Maeslant barrier is a storm surge barrier which should prevent flooding in the low-lying land behind the barrier. Various climate models have predicted that sea level rise will occur. The predicted sea level rise is more than what was anticipated during the design stage of the Maeslant barrier. Due to the increasing water levels, the closing frequency of the barrier will increase. This has a negative effect on the reliability of the barrier, as the components are used more often. The Maeslant barrier also needs to discharge river water during a storm closure when the water level on the rear side of the barrier becomes too high due to river inflow. This discharging process is a complex process which contributes greatly to the failure probability of certain components.

Delta21 is a spatial plan to redevelop a part of the Dutch delta to mitigate the effects of climate change which are sea level rise and increased river discharges. Delta21 claims to improve the safety of the entire Rhine-Meuse delta till a sea level rise of 1.1 m. Delta21 pleads for a central approach to focus on improving pump capacity in the main water systems instead of raising and strengthening all the dikes. The main goal of Delta21 is to reduce the flood risk in the downstream area. Due to the large pump capacity available, Delta21 can replace the discharging function from the Maeslant barrier. This simplifies the closure operation of the Maeslant barrier to only retain seawater during storms on the North Sea. The effect of the simplified closure operation has consequences on the reliability of the Maeslant barrier.

Therefore, the main objective of this master thesis is to find out how the failure probability of closure of the Maeslant barrier with the simplified closure operation changes in the Delta21 configuration.

To fulfil the objective, the first step was to find out how the Maeslant barrier itself works and how the discharging procedure impacts the failure probability of closure. Then, a hydraulic system analysis was done to understand how Delta21 impacts the current configuration of the Rhine-Meuse delta. In the next step, the new situation with Delta21 was schematized into a simple hydraulic model which has been verified and validated.
By making the variables in this model probabilistic, a Monte Carlo simulation was done to find out what the probability of a negative head difference larger than 1.5 m was. The simulation was done for the situation without and with 0.6 m sea level rise.
The next step was the failure probability analysis. In the qualitative analysis of the failure probability analysis, different failure mechanisms and failure scenarios are identified for the Maeslant barrier in the current situation, and for the Maeslant barrier in the Delta21 configuration. In the quantitative analysis, the probabilities for the defined failure mechanisms and failure scenarios are calculated according to the upper bound approximation, for the current situation and with Delta21. The probability which was calculated with the Monte Carlo simulation is also used here. Ultimately, a fault tree can be composed on the basis of the qualitative and quantitative analysis for the Maeslant barrier in the current situation and the Delta21 configuration.

The failure probability of closure has been reduced by a maximum of approximately 10%. This is because many systems and operations were simplified because of the simplified closure operation.
Additionally, the failure probability analysis also provided insight in how the probability of flooding changed for Rotterdam with the Maeslant barrier in the Delta21 configuration. While the failure probability of closure was reduced by a maximum of 10% with Delta21, the probability of flooding for Rotterdam has increased by 53% when using the full pump capacity from Delta21. This increase is not significant since the probability of flooding remains very small. Due to the removal of the discharging function with Delta21, the probability of several flooding scenarios increased drastically which resulted in the increase in the probability of flooding for Rotterdam.
With a sea level rise of 0.6 m, the probability of flooding increased approximately 500% for the current situation (without Delta21). When comparing Delta21 with no sea level rise to Delta21 with 0.6 m sea level rise, the increase was also present with approximately 375%. This is less than 500%, which indicates that Delta21 is effective to mitigate the effects of sea level rise compared to the situation without Delta21.

Dredging is performed for various reasons, including port development, coastal defense and land reclamation. The economic and social importance of such projects is widely acknowledged, but there are growing concerns about the possible environmental impact of the dredge plumes that form as a temporary by-product of these activities. Suspended sediment blocks sunlight, and sediment that settles on the seabed buries coral and other organisms. These effects accumulate over the duration of a dredging project, and should be minimized.

An important step towards this goal is predicting plume formation and development. Here, we focus on an overflow plume from a trailer suction hopper dredger (TSHD). Although this subject has been studied for years, much is still unknown, due to the complex interaction of the processes involved. Because of this complexity, it is not uncommon in the industry to use rules of thumb, derived from empirical studies.

One such rule of thumb, based on an influential publication by Becker et al. (2015), relates to the fraction of the overflow plume that is still (or again) in suspension once the developing plume moves from the dynamic near-field to the passive far-field. This source term fraction (STF) is important, because the large-scale area models that model plume development in the far-field require information about the characteristics of the plume entering the far-field zone. Becker et al. (2015) presented a realistic range of 0-20% for the source term fraction. This range is based on a combination of measurements and expert opinions. Although Becker et al. (2015) recommend characterization monitoring at the start of a dredging project to determine whether the 0-20% range needs adjusting, this is not always done in practice, due to time pressure, costs, complexity or lack of necessity.

Building on the work by Becker et al., this thesis aims to support a more informed implementation of TSHD source terms on the computational grid of far-field models. In order to explore the behavior of SHD dredge plumes, numerical simulations were carried out using computational fluid dynamics model TUDflow3D. In these simulations, the values of three model input parameters – the water depth, the overflow density and the crossflow velocity – were varied to gain insight into the behavior of the plume under the conditions studied. For each of the three parameters, three representative values were chosen on the basis of expert advice, and all combinations were tested, leading to 27 numerical simulations.

The results of the numerical simulations show that the amount of sediment in the plume has reduced to less than 20% of the sediment in the overflow plume at 11-42 minutes of plume development, corresponding to a distance of 0-1895 m from the dredging location (500-3800 m from the moving vessel), depending on the specific conditions. It is important to note that (only) three conditions were varied
in this study. These conditions were all steady state (e.g. no tidal flows). In real-life situations, many more (combinations of) conditions and hence a wider range of results are possible. Conditions where the plume was found to travel relatively long and far into the far-field, are a high ambient current, a low water depth and low overflow densities. The effect of these factors corresponds with existing studies.

The contribution of this work lies in the broad characterization of the developing plume under these various conditions: not only the magnitude of the plume is investigated, but also its vertical and lateral
distribution. Specifically, the contribution of the bottom 6-13 % of the water column to the total plume flux is highlighted. This is particularly of interest, as this zone is typically not included in ADCP
measurements. This increases insight in the processes underlying plume development and supports the appropriate determination and implementation of the far-field source term, including a more informed
use of the 0-20% source term fraction range proposed by Becker et al. (2015).

Tailings storage facilities (TSF) are engineered structures that retain mixed waste material (known as mine tailings material) from mining processes in liquid or slurry form. One of the issues regarding TSF management is the lack of site investigation and sample analyses, which cause the uncertainty of geotechnical properties of mine tailings materials. For a proper operation of tailings storage facilities, the owner should study the material involved and understand the physical and chemical properties associated with it. In case of mine tailings dam failure, the resulting flood wave supposes an environmental, social and economical disaster for society.

Mine tailings dam break studies use numerical models to predict the flooding area and assess the possible damaged area. Historically, these studies were carried out according to Newtonian modelling principles, but the presence of solids within the fluid suggests that the resulting flood wave of a TSF failure should be treated as a non-Newtonian fluid. Absence of laboratory data regarding the geotechnical properties of mine tailings materials make difficult the prediction of such flood wave, since the composition of the mixture is unknown. Therefore, the aim of this research is to study the flow behaviour of mine tailings materials in case of failure of tailings storage facilities. Understanding the flow behaviour of the non-Newtonian fluid is essential to analyse the possible failure event for an existing structure, in order to plan and organise emergency procedures that anticipate and mitigate downstream damages.

Currently multiple projects investigate the effect of lowering the crest of groynes in the Dutch rivers with the aim to lower the backwater effect of groynes under high water levels. There is a major uncertainty however in the modelling of groynes, and therefore in the prediction of the effect of groyne lowering. Currently submerged groynes are modelled subgrid as weirs. The flow processes differ however between groynes and weirs as discharge over a groyne is not purely defined by the contraction and expansion over the groyne, but also by lateral shear and interaction with the surrounding main channel and floodplain. The result is a lower resistance of the cross section than expected from weir modelling. For this reason a numerical investigation was started, comparing the modelling of groynes as different subgrid weirs in a 2D model and the modelling of groynes included in the bed topography in a 3D (non-)hydrostatic model. There were large differences between all different methods of groyne modelling. Furthermore the available experimental data did not suffice to explain the differences.
For that reason a new physical model has been set up in the Delft University of Technology, at the hydraulic laboratory of the Faculty of Civil Engineering and Geoscience. The experiment includes a 1:30 scale model of a representative transect of the Waal river. The model is 5 m wide and 30 m long and includes 6 groynes and 5 groyne fields. It further includes a part of the main channel at one side and a part of the floodplain at the other side of the groyne fields. Gravel is fixed to the bed to ensure hydraulically rough flow conditions. In the experiment many measurements were done. The first aim of the measurements was to obtain a good spread of points to validate numerical models on. The second aim was to gain insight in the different flow processes in order to describe the differences between groyne and weir flow and quantify the resistance groynes have on a flow.
An important observation was the existence of a region of low flow, at the tip of the groyne. The observation indicates a complex three-dimensional flow which effectively redistributes discharge over the transverse. The contribution of this flow to the two-dimensional momentum balance is then directly as a secondary circulation and indirectly by alternating the distribution of discharge. Complex flow furthermore invalidates the hydrostatic pressure assumption. The observed flow needs further investigation with either more accurate measurement devices or in a 3D non-hydrostatic model. Another observation includes the contraction and expansion of flow over the groyne crest. The expansion of flow over the main body of the groyne seems comparable to the expansion of flow over a weir, which can be predicted as Carnot head losses. The observation of expansion losses is however clouded by other flow processes, such as bed shear stress, the lateral flow around the groyne and possible three-dimensional processes induced by the groyne tip, so that the observed head loss over the groyne is only partially explained by weir-like expansion losses.
The comparability between groyne flow and weir flow does not seem to hold at the groyne tip, where no separation of flow is observed. Measurements were performed 10 cm away from the transition between the groyne tip and main body of the groyne. This means that at full scale there is a region of 3 m on either side of this geometrical transition where the transition lies between weir-like overflow and non-weir-like overflow.
With the here performed simulations it is possible to adapt weir head loss formulas on the observed head losses over groyne. It should be possible to validate a three-dimensional non-hydrostatic model, which can further quantify the effect of three-dimensional flow and differentiate the different contributions to groyne head loss. Based on these models proper tuning parameters can be chosen to represent the complex flow in a two-dimensional model and separately model the weir-like expansion.

The Eastern Scheldt storm surge barrier (ES-SSB) is the largest hydraulic structure in the Netherlands. Its semi-open inlets allow for North Sea waters to enter and leave the Eastern Scheldt estuary with each tidal cycle, and can be closed during extreme storm events. The flow through the barrier is strongly contracted, and complex flow patterns emerge. Among characteristic flow features are the shallow jet and shallow mixing layer, generated as a result of large transverse shear stresses with horizontal lengths scales greatly exceeding the water depth. Large mean velocities in combination with the developing lateral non-uniformity of the flow between slack water and maximum flood gives rise to higher bed shear stresses. A bed protection up to a distance of about 600 m from the barrier is applied to stabilize the bed against increased hydraulic loading. Scour hole development adjacent to the applied bed protection was anticipated for, but expected equilibrium depths have not yet been reached. Reaching local depths of 60 m with respect to the water surface, these scour holes may on the long term be a threat to the stability of the barrier. Broekema (2020) concluded that during flow contraction, separation of flow near the bed of the scour hole is suppressed and high flow velocities in streamwise direction are found near the bed. Cyclic variations in lateral non-uniformity affect turbulence intensities and subsequent mixing of mass and momentum. Scour growth is therefore enhanced in two ways: i) velocities in the main flow remain high due to horizontal flow convergence, and ii) lateral velocity gradients are associated with larger turbulence intensities that are likely leading to larger bed shear stresses...

Two current assumptions of the Zeesluisformulering (ZSF) regarding a homogeneous lock density and instantaneous shipping are tested against refined descriptions according to literature: a stratified lock density and shipping according the continuity approach. These are implemented into a conceptual model constructed in Python, after which optimizations are quantified by means of a sensitivity analysis and a case study. It was found that by using the new assumption set, a maximum of approximately 10% less salt intrusion is obtained as a result of the influence of the return current on the lock exchange.

Two control structures in the form of compound weirs were built near Pannerden and in the Hondsbroeksche Pleij. These compound weirs consist of multiple adjacent gates with individually configurable weir heights. To make optimal use of the flexibility of the structures, research must be done into how different compound weir configurations affect the stage-discharge relationship. Both perfect and imperfect flow are researched.
This thesis has the following research question: “How is the flow over a compound weir affected by the configuration of the individual weirs?”
The research on imperfect weirs was caried out by constructing two analytical models based on a combination of momentum- and energy conservation, and a combination of Carnot losses and energy conservation. This was done for two cases, one case in which the equations were solved explicitly by using the average weir height, and the second case in which the equations were solved by solving a system of equations (one for each gate). The research on perfect weirs was caried out by making use of the Rehbock formulation, again for both the individual weir heights and the average weir height. To validate the models and to research interaction between adjacent gates, experiments were carried out in a 3-meter-wide flume with a scale model of the control structure near Pannerden.
In the experiments it was found that in the case of perfect weir flow, the discharge coefficients hardly change for different configurations with the same average weir height. The recorded maximum change was 1.9%. The best performing model for perfect flow was the model based on the average weir height. In the case of imperfect flow, it was found that the discharge coefficient can vary up to 13%, so the configuration influences the discharge coefficient significantly. No unambiguous results were found on how certain configurations affect the discharge coefficient. The models for imperfect flow performed well for an average weir height of 10 cm, but not for an average weir height of 7.5 cm. The least performing model was the model based on individual gates with the momentum equation.
With PIV (Particle Image Velocimetry) it was found that lateral flow starts at a greater distance from the weir if the lateral travelling distance is bigger. Furthermore, the streamlines hit the weir at an increased angle if the transition between high and low weir heights was sudden instead of gradual.
Further research should focus on adding more forms of energy head loss to the analytical model. Focus should be on one specific average weir height and more data should be collected on fewer configurations to get more insight into the behaviour caused by specific properties of a configuration.

One of the preferred features of plastic is its high durability. This merit is its main disadvantage in the natural environment. When poorly disposed of, plastic waste can enter rivers from surface waters and subsequently end up in the ocean. Here, plastic spreads out over the entire ocean and ultimately ends in ocean gyres. Plastic material in the ocean causes environmental damage. Marine animals can get entangled in plastic debris, plastic accumulation occurs when consuming seafood and the material spreads toxins in the seawater. It has also caused economic damage: marine plastic accumulation led to €5 to 18 billion to key economic sectors for 87 coastal countries in 2020 (Deloitte, 2021). Attempts have been made to capture buoyant plastics from the ocean with interception devices, with mixed success.

 

The placement of an interception device in a river is thought to be a better approach to the problem. A standard interception design is that of a floating boom. Plastic debris is transported by the river flow and these objects guide buoyant plastics into a collection tray. Currently, these interception designs are still in their infancy. A difficulty in the optimisation of these interception devices lies in the limited knowledge of the trajectories of plastic waste. Computational fluid dynamics (CFD) modelling can help to create a better understanding of these aspects. This tool can provide a rapid assessment of different flow conditions with different types of plastic debris modelled. Further knowledge into this field can ultimately help into better retention of plastics in rivers.

 

Plastic modelling for buoyant particles has been performed in the marine environment (Van Utenhove, 2019) and in the river (Van Welsenes, 2019) with a Lagrangian study. However, both works did not perform a validation study. The combination of a CFD model and the results of physical model tests can be valuable in removing uncertainties related to plastic trajectories and the modelling of these.  One physical model test performed on plastics is the release of plastic particles in a flume (Zaat, 2020) Small films were released in a 2DV physical model, representing plastic bags. It was found that these particles followed an inverted Rouse profile for spherical particles times a shape factor.

The study by Zaat (2020) was replicated in the CFD modelling environment of OpenFOAM in this thesis. Subsequently, the retention of plastic particles was investigated. Three hydrodynamic conditions were investigated, with uniform inlet flow velocities of 0.10, 0.55 and 0.90 m/s. The $k-\epsilon$ turbulence model was used. An Euler-Euler approach was applied with a discrete and continuous phase. This modelling approach is computationally less expensive compared to a Euler-Lagrangian framework and can be easily established for different cases. The drag models Gibilaro, Wen-Yu and Syamlal-O'Brien were tested to investigate which concentration profile best fits an inverted Rouse profile for rising particles. The interception device was modelled as a square obstruction of 10\% of the water depth and the model was assumed as a rigid-lid. The retention of particles by the system was explored for neutrally buoyant particles, light material and high-density polyethylene (HDPE) and polypropylene (PP) films.

 

This study found relative differences in near-surface flow and the analytical approach of 3 to 4%. The Gibilaro model was best applicable and model coefficients for the virtual mass, lift and turbulent dispersion of 0.5, 1.6 and 1.0 were found. The concentration profiles followed the inverted Rouse profile and that for films closely for the medium and high flow cases, but not for the low flow case. A lower retention of plastics was observed for increasing flow velocities. In the extreme case of low flow and light particles a build-up of particles near-surface can be observed and runs against the streamwise direction.

Several limitations were present in this study. Only two parameters of the plastic particles were adjustable, which were the particle diameter and density. In reality, plastic debris has different shapes will experience drag differently, especially the friction drag. The rigid-lid model is valid only if the velocity head is smaller than that of 10% of the water depth, which is not the case for the high flow velocity case.

 

The results show that on the basis of a rise velocity and a density a representative diameter can be determined from Stokes’ law. The Euler-Euler modelling approach provides an accurate assessment and gives a proper representation of the particles for the Rouse distributions. This is done without handling the particles individually and thus results can be found more quickly. Both for HDPE and PP inverted Rouse profiles were found that followed the theoretical profile with an underestimation of 10\%-20\% near-surface. The retention was influenced mainly by the size of the wake. It is evident that the interception system can work for buoyant particles. The object can be placed during low flow conditions for the highest efficiency. For high discharge in rivers, more particles will be in suspension and thus further away from the water surface, which makes this design less preferred during high flow conditions.

 

Recommendations for further research are to perform more physical model tests, implement a non-spherical drag model applicable for plastic material and to apply a three-phase model. This removes uncertainties related to the model and to the current knowledge in particle dynamics. The addition of physical model tests would benefit the current knowledge the greatest. It is only possible to expand the complexity of the CFD model with a larger availability of these physical model tests.

This experimental study is about the bed-load transport over inlet sills of longitudinal training walls. The focus is on the flow structures around the inlet: the shape, the processes and the effect on sediment transport. Groynes have been replaced by longitudinal training walls in an 11-km long pilot project to optimize training of the river Waal in the Netherlands. These training walls have many advantages, making it worth exploring the performance of these new river training structures further. The inlet sill affects the discharge and sediment transport distribution and hence the morphological stability of the system. However, currently operational morphodynamic models cannot reliably compute the sediment flux from the main channel towards the sheltered side channel behind the longitudinal training wall. Therefore, the research aim was to develop a description of the sediment transport over the inlet sill of these longitudinal walls for implementation in two-dimensional depth-averaged river models. Since knowledge on the flow structures and validation data lacked, a series of flume experiments with the inlet area has been performed. This study shows that the blocking effect due to the sill that functions as obstacle is dominant at the upstream side of the inlet and helical flow due to curvature at the downstream side of the inlet are the dominant three-dimensional flow structures at the upstream side of the sill. Further research is needed for the identification of the shape of the flow structure due to blocking.

Currently, there is no infrastructure between the two Indonesian islands Flores and Adonara. The islands are separated by the Larantuka Strait, which has a width varying between 600 and 1000 meters. The local government would like a bridge between the two islands. However, as the water in the 18 meter deep strait is heavily subjected to tidal forces - creating a tidal amplitude of about 1.5 m and tidal flow velocities ranging up to 4.5 m/s - structural design is challenging. As traditional bridges were deemed too expensive, a new type of bridge was introduced: the `Tidal Bridge'. The pendulum founded floating bridge, which is proposed to span the deepest 400 meters of the cross-section, is designed with tidal turbines attached to the bottom of the structure. The energy production mitigates the financial burden that the 225 million US dollar Palmerah Tidal Bridge will bring. At the time of writing, a pre-feasibility study has been performed by Antea, proposing initial structural dimensions. BAM took over the design process, which lead to questions regarding the dynamic stability of the design. The objective of this report is to answer the following two research questions: 1. How can the dynamic response due to two-dimensional forcing of a Tidal Bridge be determined? 2. What design choices can further optimise the dynamic behaviour of a Tidal Bridge? A numeric tool has been created to predict the dynamics of the proposed design as function of an input of wind, waves, and current. Based on (experimental) literature, hydrodynamic coefficients determining the fluid-structure interaction were determined. The complex shape of the floaters did not allow appropriate validation of the accompanying added mass and radiation damping coefficients. Comparison with a model constructed in Ansys Aqwa showed values of similar magnitude, but precise magnitudes could not be determined. In order to find these important coefficients, a set of experiments has been performed to determine the added mass (moment of inertia) and radiation damping (moment of inertia) for heave and roll motion. The experiments showed that the added mass equations for heave were well defined, while the added mass moment of inertia equations for roll motion deviated up to 250 per cent. The acquired data was used to find better relations between the added mass (moment of inertia) and the floater dimensions. Please note, the empirical relations are based on limited data and with little mathematical background. Hence, the relations should be used with care. The radiation damping coefficients that were also extracted from the experiments showed no clear trend, but did confirm that the hand-calculations were of correct magnitude. Using the constructed model, forcing characteristics of the Tidal Bridges are investigated. In these computations, the Palmerah Tidal Bridge dimensions are used as a case study. It was noted that the extreme non-linearity of different elements of the problem (changing pendulum angle, hydrodynamic pressure field, and particle velocity/acceleration field) do not allow for linear approximation. While a linear mass-spring system predicts that the natural period is about 6.7 seconds, the maximum dynamic amplitude is found for wave periods of 9 seconds. This coincides with the largest expected waves for the Palmerah Tidal Bridge location. Reducing the natural period of the design is recommended. Additionally, research was done into the contribution of the various types of forcing, where it was found that traffic weight has negligible effect on the dynamics. Wind forces add only a few percent to the pendulum forces, but do have a significant contribution to the displacements. Furthermore, research into the impact of an approaching wave field showed that accelerations during first impact may overshoot the maximum steady state acceleration by more than 200 per cent. A parametric study on the dynamics of Tidal Bridges was performed, which did research into the the forcing combinations that lead to most amplification of the dynamics. It shows that different loading combinations are governing for accelerations in the three different degrees of freedom present in a two-dimensional system. In here, difference was found for the dynamic behaviour induced by waves from the two different wave directions, leading to two sets of three forcing combinations. This data was used to investigate the effect of three design parameters: the angle of the pendulum, the hinge location of the pendulum and the depth of the strait. Moreover, a sensitivity study is performed on a set of parameters defining the Tidal Bridge. It shows that the mass of the segment, the added mass, the floater length, the design turbine force, and the pendulum angle are most important if it comes to design optimisation. Based on the parametric study and the forcing characteristics, conclusions and recommendations are made for improvements of Tidal Bridge designs in general and more specifically: the Palmerah Tidal Bridge.

Siltation affects navigation in harbors and waterways by reducing the water depth. To regain depth and safeguard navigation, dredging is required. Water Injection Dredging (WID) is one of the available hydrodynamic dredging techniques, in which pumps on a vessel inject water at high flow velocities and low pressure into the bed and fluidize the bed material into a mixture of water and suspended sediment. The fluidized material travels as a turbidity current to deeper areas, driven by natural processes. Sediment transport is then less controlled and the final destination of the dredged material is more difficult to estimate than for other dredging techniques. The passage of a WID-induced turbidity current produces an increase in suspended sediment concentration (SSC) in the water column that increases turbidity. Turbidity increase may have a negative environmental impact. Therefore, to evaluate and mitigate the environmental impact of WID, the increase in SSC should be estimated. Additionally, from an operational perspective, the sedimentation footprint of the turbidity current should be determined. Deltares developed with Boskalis a numerical tool named “Lagrangian 1DV model” (Deltares, 2019). The Lagrangian 1DV model can calculate the thickness and density of the WID-induced turbidity current at a distance from the dredger, as well as the deposition rate. It provides the information to estimate the increase in SSC in the water column and the sedimentation footprint. The tool was defined to be a rapid assessment tool, as it is necessary at the early stages of WID projects when there is usually a limitation of time and available data. Such a tool is useful to analyze different dredging strategies and different solutions to mitigate environmental impact.
As its name indicates, it follows a Lagrangian 1DV (1-Dimensional Vertical) approach. This approach follows the turbidity current in space as it moves away from the dredger. It is an innovative approach that requires less computational effort than a 3D or a 2DV (2-Dimensions in the Vertical plane) approach.
Three main steps were followed by Deltares (2019) for conceptual and mathematical modelling: - Step 1: assume that the turbidity current is a 2DV process, thus neglecting lateral gradients - Step 2: schematize the 2DV process as a Lagrangian 1DV approach. - Step 3: select the 1DV Point Model (Uittenbogaard and Winterwerp, 1997) as the basis to solve velocity and concentration profiles over the vertical. In order to schematize the 2DV process as a Lagrangian 1DV approach (Step 2), two main points were assumed:
- Uniformity of the 1DV Point Model (in the Eulerian frame of reference) was assumed as stationarity in the Lagrangian frame of reference - The Lagrangian velocity was defined as a concentration-weighted velocity over the water column, expressed as the ratio between the mass flux and sediment load.
The validity of these assumptions to estimate sediment transport was analyzed in this report. For that purpose, the Lagrangian 1DV model (Lagrangian 1DV approach) was compared to a 2DV computation in Delft3D-FLOW (called Delft3D model) in four sets of numerical experiments. The validity of the Lagrangian 1DV approach for uniform flow was confirmed with the first set of numerical experiments. Experiment Set 2 showed that the Lagrangian 1DV approach is as valid as the Eulerian 2DV approach to represent sediment transport in non-uniform flow when density differences are negligible. Differences between the Lagrangian 1DV model and the Delft3D model in velocity, concentration and, consequently, in the concentration-weighted velocity (calculated in the Delft3D model as the ratio between mass flux and sediment load), arose in non-uniform flow for the cases in which density differences became significant. The results suggest that the origin of the differences between the Lagrangian 1DV model and the Delft3D model in the concentration-weighted velocity relies on the expression of the advective transport term of the sediment transport equation.
In Experiments Set 3 and Set 4, the numerical models were based on laboratory experiments on turbidity currents by Parker et al. (1987) and van Kessel and Kranenburg (1996), respectively. The baroclinic pressure gradient was higher for these sets experiments. Thus, differences in the baroclinic pressure gradient between the models produced important differences in velocity.
In the case of Experiments Set 3, based on Run 13 of Parker et al. (1987), the concentration-weighted velocity was lower for the Lagrangian 1DV model along the analyzed reach. Experiments Set 4, based on Run 2 of van Kessel and Kranenburg (1996), presented the opposite behavior: a higher value for the concentration-weighted velocity for the Lagrangian 1DV model. A possible explanation for these results was found, based on the variation of the sediment load along the channel in the Delft3D model and the magnitude of the baroclinic pressure gradient. In order to analyze possible implications of the observed differences when the model parameters are in the range of the WID application, the variation of the concentration-weighted velocity was calculated in the Lagrangian 1DV model for a typical case of WID. The trend of the variation along the reach was similar to the case of Experiments Set 3. Then, the results suggest that the estimated concentration-weighted velocity in the Lagrangian 1DV model may be lower than the result for the 2DV approach. As a consequence, the travelling distance of the turbidity current may be shorter. More specific cases can be addressed by comparing the Lagrangian 1DV model and the Delft3D model, following the methodology presented in this report.

For a project in the province of Zeeland, The Netherlands, a culvert was installed trough a dike to restore the tidal flow. Shortly after commissioning of the facility a scour hole at the end of the bottom protection developed. Although the scour hole was anticipated in the design, the scour hole was developing more rapidly than predicted. Furthermore, the hydrodynamics of the tidal jet flow from the culvert were different than what was expected in the design. It is expected that the uncertainties in predicting the scour hole process lies probably the most in a lack of understanding of the flow conditions in the vicinity of scour holes.

The scour hole depths can be predicted by the Breusers empirical relations. The development of the scour process entirely depends on the flow velocity and the relative turbulence intensity at the transition of the fixed to the erodible bed. The empirical equations do not account for all the hydrodynamic processes in a scour hole. Furthermore, the scour studies are predominantly considering a two-dimensional vertical situation in which lateral uniform flow is assumed. However, in practice lateral non-uniform flow is the rule rather than the exception.

The objective of this master thesis is to research the scour development under laterally nonuniform flow and to obtain more fundamental understanding of the scour process. Physical experiments were conducted in order to investigate the influence of lateral non-uniformities in the flow field on the scour process. In the scale experiments, the lateral velocity differences originated from two parallel streams with different streamwise velocities.

The scour pattern under laterally nonuniform flow is different than the scour pattern under laterally uniform flow. The scour hole in laterally nonuniform flow is deeper and it develops faster compared to the scour hole development in laterally uniform flow under the same hydraulic conditions. Furthermore, the scour pattern under lateral non-uniform flow shows spatial variations in lateral direction. The larger maximum scour depths are not observed in the zone with high flow velocities, but in the mixing layers.

A horizontal contraction of the flow at the scour hole is observed for lateral non-uniform incoming flow, whereas the presence of the scour hole does not affect the horizontal structure of the flow for lateral uniform incoming flow. The vertical structure of the flow showed relatively high near-bed velocities in the scour hole indicating flow attachment to the bed for laterally nonuniform flow. In addition, the vertical structure of the flow appears to become more depth uniform in the scour hole. The vertical structure of the flow is important of the sediment transport in the scour hole. For vertically attached flow, it was indicated that sediment could not be transported from the scour hole in the direction of the bed protection.

Design criteria for the stability of rock filters on river beds (i.e. rock bed protection) are extensively researched and successfully applied in practice. The most common stability criteria are the Izbash and Shields criteria. These methods define a critical flow (Izbash) or parameter (Shields). Rijkswaterstaat (RWS) wants to explore a more sustainable bed protection by using logs. A pilot project is started where logs will be used as bed protection. It is yet unclear if the design and construction approaches for rock bed protections can be used for log bed protections. The most dominant aspect is that logs are cylindrical objects, while rocks are spherical. This means that the design criteria for rock filters might not be directly applicable to log filters. This research aims to verify if the Izbash and Shields criteria for rock can be used for logs to create functional and safe designs. To achieve this, two experiments are performed at the TU Delft faculty of Civil Engineering & Geosciences. The first experiment, done in a water filled tank, explores the settling behaviour of logs for multiple drop methods. Insight is gained in the settling velocities, horizontal spread and magnus effect (force exerted on a rotating object, e.g. the curve of a football spinning through the air) of logs settling in a water column. Results from these experiments are used in the second experiment. This experiment is done in a 14.3m long flow flume where a log filter is constructed. The log filter is constructed using the drop method that was preferred from the first experiment. The roughness, stability and porous flow of a log filter are investigated. The results are compared with what is known for rock filters. Tree branches were used as model logs. This was done to be able to correctly scale the results to prototype scale. Primary reason for this was the effect of bark on the roughness of a cylinder. This is difficult to replicate on model scale. Using branches of trees that will be used Saturating the model logs however was more time consuming than initially expected. Attempts were made to accelerate the process but they were futile. One method, using a diaphragm vacuum pump, could not be applied due to lack of resources. For any future research on the topic of tree branches as model logs it is highly recommended to use a vacuum pump to ensure that the maximum density is reached. From the first experiment it was concluded that large quantities of logs can still be used to create functional log filters. This is a positive result as this will reduce construction time on prototype scale. The method used was a funnel. This method was applied in the flume to create the log filters. By measuring the velocity profile for multiple discharges the roughness of a log filter was measured. This also resulted in an equation of the shear velocity as a function of the discharge. By increasing the discharge step by step, several mobility stages of a log filter were found. This resulted in a dataset that could be directly compared with the Izbash and Shields equations for rock. Higher critical Shields parameters were observed than for rock of the same diameter. The behaviour of a log filter differed from a rock filter in the transition from one stage to another (stable to mobile to transport to failure) did not occur slowly, but almost instantaneous. This behaviour is unwanted because it is difficult to monitor in what stage a log filter is if no changes can be observed between stages. Thus, although applying the equations for rock filters to log filters are conservative, the behaviour of log filters are more sudden and prone to escalation close to their critical thresholds. Final conclusion is that more research is required to better understand the significance of variables for the settling behaviour and stability. These are water depth, log diameter independent of density and vice versa, log orientation and log length to diameter ratio. This can be done by doing more experiments in a similar fashion where only one variable is changed at a time. For the application of the design criteria for rock to a log filter with regards to the pilot project, it is recommended to be conservative. Based on these experiments it is safe to assume that when the most unfavourable scenario is used (e.g. low log density, small log diameter, high flow velocities near the bed) a sufficient design is made, especially if the top layer(s) of the filter are placed parallel to the flow direction. Backfilling of the log filter did not increase the stability significantly in this research and is only be beneficial for scour protection.