Infrastructure is at risk to climate uncertainty due to a combination of long life spans, complexity of the systems it is embedded in, and the high investment costs often necessary. Current infrastructure planning approaches lose efficacy under deep uncertainty, necessitating new approaches that function better under conditions where the future cannot be predicted. The approaches that attempt to deal with this are also called Decision Making under Deep Uncertainty (DMDU). Bangladesh, the Netherlands, and New Zealand have all already adopted the use of these DMDU approaches in their delta protection guidance. One popular DMDU technique is Robust Decision Making (RDM). RDM can be seen as a computational extension of scenario planning, where proposed plans are tested against every potential combination of uncertainties. The Deep South Challenge (DSC), a New Zealand based research institute is looking into using RDM on a regional scale to discover vulnerabilities and identify robust strategies based on them. One of the test cases is in Helensville, where a Wastewater Treatment Plant (WTP) serving a small community is located in the middle of a floodplain. The WTP discharges its effluent into a strongly tidally influenced river, which drains the entire watershed and flows past large tidal flats into a dynamic estuarine environment. In order to identify potential vulnerabilities in the system, robust decision making uses a vulnerability analysis. This consists of a scenario discovery and global sensitivity analysis which sample through every combination of uncertainties to characterize the vulnerabilities of the system. In order to facilitate this, usually simple conceptual models are used due to the high number of runs necessary. However, these types of models can oversimplify complex physical processes and topography. These complicating factors are all present at the current case site selected by the DSC. This research investigates whether the added computational demand of a complex model is worth it compared to a simple conceptual model. To do this, two models are selected and forced for the same event. They are then compared on predicted system behavior, identified vulnerabilities, and potential policy advice. From a larger selection, the FLORES and SFINCS models were chosen. FLORES uses a simple hydrological balance to calculate the water level in the subbasins for each timestep. SFINCS is a reduced physics solver which only uses the Local Inertial Equations (LIE). Both models are forced by a compound rainfall and stormtide event for a storm with a 24 hour duration, for which they were calibrated and validated using the results of previous modeling efforts in the region. After the calibration and validation, a sensitivity analysis and scenario discovery were run for both models. The results of the sensitivity analysis show similar model behavior between FLORES and SFINCS. The upstream part of the model domain is only sensitive to rainfall, while the downstream part is mostly sensitive to storm surge and mean sea level, and to a lesser degree to tidal amplitude. This downstream part includes the wastewater treatment plant. Compared to SFINCS, the outcomes for FLORES on average overestimated water levels at the WTP. This is most likely due to the lack of flood attenuation taken into account by FLORES compared to SFINCS. The scenario discovery showed similar results for each model’s box describing 73% of the outcomes where failure occurs. Both models had the same three thresholds: storm surge, mean sea level, and tidal amplitude. The main difference between the boxes was the storm surge threshold being 21 centimeters lower for FLORES compared to SFINCS, indicating FLORES overestimates the water level at the WTP. The results of the scenario discovery also showed a linear relationship between these three factors. From this relationship, it is possible to see that, keeping all else similar, and with the same high tidal amplitude and storm surge, for SFINCS the plant only starts flooding when a mean sea level of at least 0.4 meters is reached, while for FLORES this is 0.25 meters. Using a RCP4.5 emissions scenario, a mean sea level of 0.25 meters will be reached in 20-30 years, and a mean sea level of 0.4 meters in 50 years. The proposed policy options for both SFINCS and FLORES would be to mitigate storm surge as long as possible, since the water level at the WTP is most sensitive to this factor. Once this is no longer possible, the WTP should be relocated. The results of SFINCS indicate this relocation is necessary later than for FLORES. These results show that while the behaviors exhibited by both models is relatively similar, the small differences in accuracy affect which are most likely due to the lack of flood attenuation taken into account for FLORES lead to a different timing of proposed adaptations. This leads to reason that while a conceptual model such as FLORES works well to identify important factors within the system, a more accurate model such as SFINCS can be more helpful once timing of adaptation becomes important. Further recommendations are to repeat this research for more models, further calibrate and validate the models, and to include scenario discovery methods that better deal with the found linear relation.

The centre piece of Galveston Bay’s 34 billion dollar flood risk reduction plan is a 3.6 km long storm surge barrier built across the Bay’s main tidal inlet. The barrier, which consists of a series of vertical lift gates and two large floating sector gates, will close when a hurricane approaches in order to reduce the volume of hurricane driven storm surge entering the Bay. However, the rotating wind fields of passing hurricanes can blow offshore directed winds over the Bay which can generate a "reverse head" condition, where water levels on the bay side of the closed surge barrier exceed water levels at the open coast. The resulting reverse load threatens failure of the barrier’s two floating sector gates, which can be pulled from their supporting ball joint sockets. This Master Thesis demonstrates why reverse loading is an important load that must be adequately accounted for in the design of the surge barrier. A model is set up to determine reverse loads generated by a specified hurricane. The model accounts for reverse loading due to reverse head and wave action in the Bay and is comprised of; a parametric hurricane model by Holland (1980), a coupled hydrodynamic flow-wave model by Xu et al. (2023), and load formulations for the reverse heads and wave conditions calculated by the hydrodynamic model. Deterministic application of the model shows that reverse heads/loads are a common occurrence and can be generated by hurricanes landing both West and East of Galveston. Furthermore, hurricanes approaching landfall from oblique Eastern directions can generate a high reverse load before the arrival of a high coastal surge, which threaten more severe consequences as in addition to floating sector gate failure, the following coastal surge can enter Galveston Bay and increase flood risk. Probabilistic application of the model is used to estimate the exceedence probabilities of reverse head/loading magnitudes, for an assumed operation procedure where a decision is made to keep the surge barrier either permanently closed or permanently open depending on assumed surge forecast uncertainties

Under growing pressures of climate change, it is vital for our society to implement cost-effective reinforcements in a flood defence system. This thesis proposes a method for optimising reinforcements in a flood defence system with multiple lines of defence under sea level rise.

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...

Tidal energy has a large potential to contribute to achieving the sustainability goals in the Netherlands, due to the long coastline and many estuaries. The investment costs for the implementation of offshore tidal energy are however, still a drawback. A possibility to lower the investment costs for tidal energy is to implement tidal energy extraction into existing coastal infrastructure. An example of this is the current pilot project in the Eastern Scheldt barrier where five Tocardo turbines are attached to the barrier's geometry. The flow passing the turbines in the barrier is contracted by the barrier's geometry and the weir of the foundation of the barrier. Deltares executed an environmental impact assessment for the pilot project with a measurement campaign and a computationally expensive but accurate blade resolved numerical model. In this thesis, a cheaper computational model is added to the these assessment tools. To do so, a turbine parameterization is introduced in the finite element model FinLab. The relatively cheaper computational model should make it possible to execute more extensive sets of simulations of the pilot project in the future. FinLab solves the non-hydrostatic three-dimensional incompressible Navies-Stokes equations in an unstructured mesh with an optional moving free-surface. The used meshes are refined in certain areas of complex flow, such as the turbine parameterization, near the bed, the recirculation zone of the weir and the wake of the turbine. An extra source term is added to the Navier-Stokes equations on a chosen number of turbine integration points, to implement the turbine into the mesh. The placement of these integration points in a two-dimensional mesh is rather simple, the integration points are evenly distributed over the diameter of the turbine. In a three-dimensional mesh, a Fibonacci series is used to implement an actuator disc (AD) into the model domain. A benefit of the Fibonacci circle is that each turbine integration point represents a similar area. The force in the turbine integration points is determined by three methods: an actuator disc with a uniform thrust distribution (uniform AD), a rotational averaged actuator disc blade element momentum method (AD-BEM) and a non-rational averaged actuator line blade element momentum method (AL-BEM). In the uniform AD method, a measured thrust force is divided equally over all turbine integration points. In the BEM methods axial and tangential forces are determined for each turbine integration point based on the local flow speed and experimental lift and drag coefficients. With these methods, it is possible to estimate the generated power and thrust by the turbine. A tip-correction is added to the AD-BEM method to achieve accurate results of the simulations. Furthermore, in both the AD-BEM and AL-BEM methods a small actuator disc is applied in the nacelle region to reproduce the nacelle's blockage of the flow. The uniform AD, AD-BEM and AL-BEM parameterizations are validated with measurement data of flume experiments of a single turbine, flume experiments with different turbine-weir geometries and flume experiments with multiple turbines. The combination of these last two sets of experiments does very well represent the situation in the Eastern Scheldt barrier. As stated above, the AD-BEM and AL-BEM methods are able to estimate the turbine performance. The results of the AD-BEM simulations show a relative error of the turbine's generated power of within 20% in comparison with the flume experiments. The accuracy of the thrust fluctuates more. The AD-BEM method shows to be able to accurately estimate the location of optimum power harvesting in a combined turbine-weir geometry. Both the power and thrust accuracy depend on the mesh resolution and the turbine's rotational speed. Accurate results can be obtained by fine-tuning these settings. The AL-BEM method should be able to produce at least similar similar accuracy to the AD-BEM method when it comes to power and thrust. The AL-BEM method in this thesis is however, not yet able to do so. The results of the simulations with the three methods are also compared with time-averaged velocity measurements of the flume experiments. In the near wake, the uniform AD method does not imply any wake rotation. The rotation of the wake is represented accurately in a time-averaged sense by the AD-BEM method. The AL-BEM method adds transient features such as the downstream trailing tip-vortices to the flow. The velocity shear and turbulent eddies in the near wake as caused by the different methods influence the TKE in the near wake and the recovery of the far wake. The uniform AD under-estimates the wake recovery. The extra velocity shear in the AD-BEM method improves the accuracy but still under-estimates the recovery rate. Resolving the transient features of the near wake in the AL-BEM method further improves the accuracy of the wake recovery. In the simulations with the weir, the geometry seems to be dominant and the wake recovery rate is more accurate than in the simulations without the weir. The AD-BEM method is applied to the Eastern Scheldt field case. Five turbines are introduced in a mesh which represents Roompot 7 to 9 in a simplified manner. Due to the simplification of the mesh, the resistance of the barrier on the flow appeared to be too low. Therefore, the simulations are executed with an upstream discharge boundary. This discharge boundary makes the model less useful to estimate the environmental impact of the turbines in the barrier and at the moment not yet an alternative for the blade resolved model by Deltares when it comes to estimating these effects. With the discharge boundary, the AD-BEM model predicts the average thrust over the five turbines with a relative error of 3% and the average power with a relative error of 12%. This is only slightly less accurate than the results by the blade resolved model by Deltares, while the computational costs of the Eastern Scheldt field case simulations are only a fraction of the earlier executed blade resolved simulations by Deltares.

Salt intrusion poses a threat to the fresh water supply function of inland waterways. How far inland salt intrusion reaches is dependent on a balance of buoyancy forcing, water depth, discharge of the canal and the amount of mixing. Wind, bottom roughness, and sailing ships contribute to mixing. Currently, little is known about the contribution aof the latter to mixing. It is important to better understand the role of ship traffic in order to make more accurate models and take the right measures to protect the fresh water supply function of inland waterways. The flow field in an unstratified canal has been well studied, however, little is known about the effects that ships have in a stratified canal. Ships move water when sailing through a waterway. This induces three water movements: primary and secondary waves and the propeller jet flow. These processes can contribute to mixing by shear instabilities and internal wave breaking.A sailing ship in a stratified canal has been modelled using a moving grid approach in the 3D non-hydrostatic finite element model FinLab to study these effects in detail. The propeller has been neglected to simplify the model. A parameter study has been performed to observe the influence of several parameters (such as the canal blockage, internal Froude number, and canal bank slope) on the flow pattern and the amount of mixing. The amount of mixing is found to be in the order of magnitude of 1 percent for one vessel over 600m canal length a representative range of parameters of canal geometry, density profile, vessel draught and vessel speed. The actual effect will be larger since the internal wave field is still present at the outflow boundary and since the propeller has been omitted from the model set-up. Processes around the vessel and the internal waves contribute about equally to the amount of mixing in the model domain. The internal waves are estimated to have a contribution of about twice as large if the modelled domain was longer. As the interface comes up behind the vessel, it is likely that the propeller jet can have a large impact on the density field directly behind the vessel. The vessel speed, density profile, relative density difference, layer distribution in a two-layer flow, canal blockage, draught to top layer height ratio, and the slope of canal banks are important parameters in the amount of mixing generated. Due to the cumulative effect of the ship traffic, mixing by ship traffic is estimated to be of large importance on the density distribution in a canal. More research is needed to include this effect in large-scale numerical models and find a good parameterisation. How mixing by ship traffic could finally be implemented will be dependent on the numerical model that is used, and the amount of detail that is needed.

River bars are large-scale bedforms formed by the local deposition of sediments, the length of which scales with the channel width and the height with the water depth. They create suitable habitats for aquatic fauna and riparian vegetation, making them useful in river restoration projects. Understanding the dynamics of river bar development is required for a proper design of the restoration project and for developing a sustainable management scheme. The channel half-width-to-depth ratio is the key controlling parameter for the formation of alternate bars. Existing stability analyses define a resonance point and critical width-to-depth ratio, marking the transition between the bar regimes. Seasonal variations in the river discharge and the propagation of flood waves make the half-width-to-depth ratio a time-dependent parameter, varying over periods of days to months. This research aims to create a better understanding of the development of alternate bars in varying discharge conditions. For that, insight into the timescale of development of the bars is necessary to link this with the timescales associated with natural discharge variability. A straight river channel is modelled in Delft3D, with non-erodible banks to which a fixed perturbation is added, to trigger the formation of free bars and to which hybrid bars become fixed. In the first set of simulations, the upstream boundary condition is a constant discharge, with magnitudes between 50 and 1250 m$^3$/s, corresponding to a half-width-to-depth ratio between 77 and 8. The different types of alternate bars and the time to develop the bars are analysed from this constant-discharge analysis. The damping length and wavelength of the hybrid bars showed good agreement with the values determined from the theory of Struiksma et al. (1985). Free bars became suppressed when a hybrid bar pattern developed, showing that hybrid and free bars do not coexist locally. The resonance point is clearly obtained and assigned to a half-width-to-depth ratio of 37-38. Contrary to existing stability analyses, the critical half-width-to-depth ratio is marked by a gradual transition at half-width-to-depth ratios from 11 to 20 instead of a single value. In this transition, free bars develop with intervals of a few months. A clear difference in the development of hybrid bars and free bars results in a shorter timescale of development for free bars than for hybrid bars, which is in line with laboratory experiments (Fujita and Muramoto 1985; Crosato et al 2010). Also, in the mathematical derivations for describing the two types of bars, the distinction between an initial response (free bars, Colombini, Seminara, and Tubino (1987)) and a final stable bed (hybrid bars, Struiksma et al. (1985)) is considered. The numerical results show that free bars develop fastly when the bed becomes unstable, in the case of a relatively flat bed. The associated timescale shows little variation over the different simulations and stays around 20 days, which is in the same order of magnitude as the timescale related to seasonal variation. A pattern of hybrid bars develops in downstream direction, starting at the fixed perturbation. Based on the definition of the damping length by Struiksma et al. (1985), a theoretical expression is defined, which gives a good fit to the time at which a pattern of hybrid bars has developed. The transition of alternate bars around resonant conditions is modelled using a steadily increasing and decreasing discharge. The distinction between the timescales of hybrid and free bars results in a different response of the bars to a varying discharge. The first response of the bed in the transition from subresonant to superresonant conditions is the adaptation of free bars towards the new bar regime. A pattern of hybrid bars follows, developing in downstream direction. The reversed transition is governed by a migration of the hybrid bar pattern, making room for the damped bar pattern. The difference in the transitions between rising and falling discharge in combination with a bed adaptation after a temporal in- or decreased discharge led to a type of hysteresis in the transition around the riverbed's resonance point, which was not earlier detected. Hysteresis is ascribed to the increased bed stability when a hybrid bar pattern has developed, suppressing the formation of free bars in the transition from superresonant to subresonant conditions and to the time-lags associated with lower discharge conditions. This thesis assesses the timescales of alternate bars and the transition around the resonance point. The numerical model is able to assess both free and hybrid bars and therefore gives valuable information in the difference of their development. The difference is linked to the (linear) theories describing the bars and applied for answering the research question, showing the development of free bars should be largely influenced by discharge variation due to seasonal variation. The varying discharge simulations provided insights into the response and transition of alternate bars around resonance conditions, leading to a type of hysteresis which was not detected earlier. Recommendations are made towards increasing the river geometry complexity and discharge variation and the assessment of the transition around critical conditions to verify analyses around critical conditions.

For both developed and developing nations, coastal zones form an attractive location for urban settlements. With the expected increase in the earth’s population, coastal areas will experience a further increase of inhabitants. Floating city development could therefore be an interesting alternative for land-based urban expansion on land [41]. Expanding an urban settlement towards the ocean however, will make it more susceptible to extreme forces such as a tsunami waves. By generating more knowledge on how a floating structure interacts with a tsunami wave, it can show the potential value that a floating city can bring. If a floating city limits the effect of (extreme) events such as tsunamis, it can protect the coastal zone which is located near a floating city. To provide an answer to this question, this research will focus on how a floating structure can reduce the transmission of a tsunami. With the construction of an analytical model, representing the floating structure and the tsunami wave as simplistic as possible, the system can be understood more quickly. If the problem is solvable by generic programming language, this would mean that it can solve a larger range in the spectrum of the problem. The conceptual model features two options: one where the platform has no freedom of movement, the other where the platformcan move vertically. They both assume that hydrostatic pressure holds during wave propagation and a linearization of the momentum equation describing the water particle interaction. For each option, the transmitted wave height is determined based on varying the floating structure dimensions. This gives an indication on which parameters are of influence in the transmission of the wave. First, the conceptualmodel is analysed by changing the platformdraft and the length for both the motionless and the vertically moving platform. Both options are influenced most by the length of the structure. The situation with the motionless platformshows this effect earlier, by a higher wave attenuation percentage for the same platform length. Whether the draft also has an influence is strongly dependent on the value representing the length of the platform. The difference between the two platform movement options shows that the effect of changing the allowed movement of the platformis significant. Next to the reduction in transmission, the conceptual model shows signs of resonance. The moving platform option in the model is formed by a second order differential equation. Fitting this equation, resonance is evident and therefore visible for certain combinations of the platformdimensions. In addition damping is present, ensuring that there are some parts where, despite the natural frequency pointing there, no resonance occurs. The amount of damping is strongly linked to the platformlength, with a higher level of damping for a longer platformlength. Finally, the results from the conceptual model are compared to the outcome in SWASH. This numerically based model has the possibility to simulate a tsunami wave in its development towards the coast and also features a buoyancy function for structures. This comparison serves as a validation of the conceptual model. In general, the conceptual model always results in a less reduced wave attenuation percentage and can be said to be more conservative. Due to the assumptions of leaving out the non-hydrostatic pressure and a lower level of detail, a maximum of 5% deviation in both model results occured. This however, matches with the fact that it is a less detailed model and adds to the reasoning that the conceptual model provides what it is meant for. Next to the effect of the structure itself, the positioning of the structure is also of large importance. Wave height and intensity of the wave will vary due to the surrounding local coastal features. Next to that, the local water depth is determinant in compressing the wave, therefore decreasing the wave length when the water depth decreases. The maximum wave attenuation that can be achieved according to both models is 10%, considering platform dimensions and location variations. The conceptual model appears to work for which it is intended: modelling the resulting transmission between a floating structure and a linear tsunami. It is expected that modelling programs can be expanded and/or improved, so that more realistic floating structures can be modelled. However, it will remain difficult to accurately model a tsunami as it is complex in behaviour. Yet, this research brought the field one step closer to evaluating the transmission of tsunami waves when interacting with a floating structure of certain dimensions.

Large-scale measures within the river programmes such as Room for the River, Natura2000 and Water Framework Directive have increased the biodiversity in Dutch rivers and have achieved a more natural landscape. However, recent river programmes have shown conflicts between safety against flooding and riverine nature rehabilitation with maintaining navigational water depths. Therefore, an integrated approach is called out upon to improve the river management within these programmes. Conventional groynes are typically used to improve functions within the river programmes, whose primary function is to maintain navigational water depth. Groynes are transverse structures in which large turbulent structures are observed around the groynes. Consequently, significant bed shear stresses are developed, leading to significant local scour. Investigations for improving alternative groyne configurations are often studied for optimizing the groyne structure. The flexible groyne is considered an optimization of the conventional groyne structure. The flexible groyne consists of steep slopes and has a permeable characteristic. Although a fair amount of research has already been carried out on various groyne configurations, detailed flow characteristics around and through permeable, sloped groynes are limited. Hence, this research aims to numerically quantify the effects of permeability and head steepness of groynes. A literature study is executed to investigate the critical processes required for capturing within the numerical model to answer the research question. From the literature study, it appears that the most important processes are the non-hydrostatic effects, adapting large turbulent structures and implementing the porous zone using a non-linear function. Multiple software packages have been analyzed. Fluent is chosen, which is analyzed to capture the required processes adequately.
A new numerical model has been set up for investigating the research question by simulating flow around groynes. The model is validated against three experimental studies for various characteristics. The important mean flow characteristics have been validated within an acceptable range. Still, it appears that the numerical model tends to underestimate the mean streamwise flow velocities, overestimate the Reynolds shear stresses and shift the peak values of the Reynolds shear stresses downstream. Four configurations are identified for the simulations of varying head steepness and porosity. It appears that the increase of the porosity reduces the large turbulent structures and bed shear stresses close to the groyne and shifts the peak values of the Reynolds shear stresses, and bed shear stresses further downstream. The porosity reduces the maximum Reynolds shear stresses and bed shear stresses compared with the Reynolds shear stresses, and bed shear stresses for an impermeable groyne. These reductions are because of the momentum exchange between the free flow region and the flow through the porous structure, which reduces the mean flow velocity in the free flow region. For decreasing steepness, the large turbulent structures and bed shear stresses are observed close to the groyne due to increasing deflection. The flow appears to follow the geometry of the sloped groyne more smoothly. This research is seen as a first approach for modelling a porous, sloped groyne. Further improving and analyzing numerical modelling for porous, head sloped groynes are advised to increase the model's accuracy. Furthermore, more simulations for varying the head steepness and porosity is suggested to improve the relations between the increase of porosity and head steepness for the flow characteristics. In addition, including sediment transport models within the model is expected to increase the understanding of the hydrodynamics and morphology around these specific groynes.

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.

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.

Shallow jet flows are often coupled to high flow velocities and complex flow patterns. To properly ensure the stability of the barrier between both water bodies surrounding the jet, a scour analysis is often a necessity. Such analyses are inevitably coupled to the hydrodynamic processes occurring in jet flows, either in the form of loads on potential bed protections or equilibrium scour hole depths. Numerical models could assist in the design of bed protections by supplying accurate hydrodynamic input parameters for the design formulae or by predicting the hydrodynamic processes at play. Due to the limited computational power available, simplifications must be made within the numerical model to ensure reasonable computational costs. One of these simplifications is to resolve the flow in depth-averaged sense to create a 2DH model. To allow this simplification to be made, the performance of a 2DH model compared to a fully 3D model needs to be researched. By comparing simulations of a controlled environment and of a prototype scenario, insight in the predictive capabilities of the numerical models for possible other projects can be gained. This work aims to research the performance of 2DH and 3D numerical models regarding the reproduction of the hydrodynamic processes in shallow jets. First, it was investigated which hydrodynamic processes thrive at a prototype shallow jet by analysing measurement data available at Waterdunen. Afterwards, we systematically determine the performance of both a 2DH and 3D numerical model by setting up FINEL models both for a laboratory experiment of a shallow jet and for the shallow jet at Waterdunen. Insight in the predictive capabilities of 2DH and 3D numerical models for shallow jets can contribute to future design problems surrounding shallow jets. Information regarding the shallow jet at the seaside of the culvert at Waterdunen was obtained by Svasek Hydraulics, commissioned by the public government Waterschap Scheldestromen and the province of Zeeland, during a measurement campaign split into six individual measurement scenarios. Immediately at outflow from the culvert, the jet is subject to a 1:6 slope in streamwise direction. The geometry of the seaside channel is asymmetric, with 1:4 lateral slopes at either side of the channel. Furthermore, morphological changes as a result of the jet flow introduce an asymmetric topography in the channel. Visual observations during the measurement campaign indicated the flow can be characterized by an horizontal contraction over the longitudinal slope and an asymmetric flow profile depending on which casings were active within the culvert. The analysis of the measurement data shows the jet concentrates on the eastern side of the channel during all six scenarios. As a result, the recirculation zones on either side of the jet can be characterized as a small, non-dominant recirculation zone on the east side of the jet and a large, dominant recirculation zone on the west side of the jet. The streamlines show a continuous horizontal contraction until the flow hits the eastern side of the channel. Over the longitudinal slope, the flow seems to remain attached to the bottom rather than separating. However, due to a relatively large clearance of the ADCP measurements with respect to the bottom, this cannot be proven directly. It was decided to simulate the shallow jet of experiment 2.4.1 of van de Zande (2018) to investigate the performance of a 2DH and 3D numerical model for a laboratory simulation. In this experiment, the behaviour of an asymmetric shallow jet over a longitudinal slope at the point of the horizontal expansion was investigated. It was shown that both the 2DH and 3D numerical model were capable of reproducing the most dominant hydrodynamic processes within the jet. However, the accuracy with which the processes were reproduced differed significantly. The 2DH model reproduced too much curvature of the flow towards the lower wall of the flume compared to both the 3D model and the measurement data. As a result, the high flow velocities in the jet center were located further down in the channel. The flow velocities in the jet center were modelled with a maximum error of 12.4% in the 2DH model and 3.02% in the 3D model. The lower accuracy of the 2DH model is caused by the comparison between depth-averaged velocities and surface PIV velocities. Due to the additional curvature the recirculating velocities in the non-dominant recirculation zone were modelled too high by the 2DH model, with an error of up to 56.6% relative to the jet center velocity. In the 3D model, this error decreased to 18.9%. However, it was remarked the recirculating flow velocities in the PIV data were subject to clumping of tracer particles. The horizontal streamline contraction was simulated in the 2DH model, but can only be directly related to the changes in water depth. In both the 3D model and the measurements, this horizontal streamline contraction was shown to continue further downstream of the slope. It was concluded the horizontal streamline contraction is directly related to the vertical velocity profile, which was observed to differ from the standard log-profile both on the slope and downstream thereof. The 3D model was capable of reproducing the vertical velocity profile with a maximum relative error of 10%. In the numerical simulations at Waterdunen, both the 2DH and 3D models were shown to behave similarly. Both the 2DH and 3D numerical models were capable of reproducing the eastern concentration of the flow. However, the curvature of the jet further downstream was modelled more accurate in the 3D model. The better reproduction of the flow curvature in the 2DH model compared to the laboratory simulations is the result of the asymmetric inflow boundary, which allows the 2DH model to more easily resolve the flow curvature. Both models underestimate the flow velocities throughout the entire DOI. It was concluded this was caused by errors in the model input. Similar to the laboratory simulations, the 2DH model reproduced the horizontal streamline contraction over the slope but failed to reproduce the contraction further downstream. The 3D model was capable of reproducing the continuing streamline contraction. Furthermore, the 3D model reproduced vertical flow attachment over the slope, which complies well with the measurement data. It was concluded that a 3D numerical model can better reproduce the flow curvature in case of asymmetrical flows. Furthermore, the recirculating flow is better reproduced by a 3D model compared to a 2DH model. Finally, a 3D model is capable of accurately reproducing the vertical velocity profile, which supplies additional information of accurate flow velocities at the bed. However, on a prototype scale the differences between a 2DH and 3D numerical model are small. In channels with symmetrical boundaries, it is suggested to use a 3D numerical model in case the flow is subject to the Coanda-like effect. However, if the inflow into the channel is already asymmetrical, a 2DH numerical model can be considered to save computational effort. Nevertheless, when employing a 2DH model one should be wary of geometric and topographic asymmetries, as the effect of both factors on the flow symmetry is still unclear.

In the proximity of the Eastern Scheldt storm surge barrier (ES-SSB), one of the largest hydraulic structures of the Dutch Delta Works, scour holes have developed that potentially threaten the stability of the structure. Although, these scour holes were anticipated in the design phase recent bathymetry measurements show that they are growing deeper than initially predicted. Uncertainties in the predictions of the equilibrium dimensions partially arise from the presence, thickness and erodibility of clay layers, but first and foremost from a lack of understanding of the flow conditions in the vicinity of the scour holes. High flow velocities are still observed near the bed in the scour holes, that can be associated with high bed shear stresses. This implies that an equilibrium state has not yet been reached and that the scour holes still become deeper. The presence of these high flow velocities is not fully understood, making predictions on the development of the scour holes and mitigation strategies difficult. As a result, the risk of geotechnical instabilities increases, which may potentially endanger the stability of the ES-SSB.

The objective of this master thesis is to obtain more fundamental understanding of the flow patterns and turbulence structure in the vicinity of tidal barriers, such as the Eastern Scheldt storm surge barrier, and how these may contribute to ongoing scour. In this thesis, we experimentally investigate the development of a shallow jet that experiences a streamwise increase in depth, with and without the effect of grid turbulence. The aim of this thesis is to obtain additional insights into the fundamental processes in scour holes using experimental results, that can contribute to the improvement of equations and models that estimate scour near hydraulic structures.

Two polluted Ghanaian lagoons are investigated and possible engineering solutions are suggested and researched.
The first lagoon is the Sakumo lagoon, located between Accra and Tema. This lagoon is connected to the sea through a small culvert, which enables a limited amount of water exchange. Since a few years, fishermen have been unable to catch fish in this lagoon, because of invasive plants restraining them from entering the basin. The siltation rate is high due to increasing friction because of these plants and limited sediment outflow through the small culvert. Furthermore, the water quality in the lagoon is poor. An overland flow model and a mixing model are used to evaluate the effects of a change in layout of the lagoon mouth. From this, it follows that a larger connection to the sea is beneficial to the water quality while still maintaining flood safety. However, decisions on the redesign of this lagoon mouth should be made with close regard to stakeholder interests.
The second lagoon is the Klottey lagoon, located in the city centre of Accra. The surroundings of this lagoon are planned to become an area of tourism. Neighbouring the lagoon, a new fishing harbour is planned. The water quality in this lagoon is poor and its water flows along the shore of Accra. The water quality in and near the lagoon is investigated with the development plans of the area in mind. Furthermore, the shoreline response as a result of these interventions is assessed.

Tropical cyclones (TCs) have tremendous impact on coastal communities in terms of damage due to flooding and high wind velocities, as shown by the recent hurricane season of 2017. Coastal flooding due to TCs can be contributed to different types of forcing (e.g. high offshore water levels, rainfall, etc.), or multiple at the same time (i.e. compound flooding). While the impact of TCs increases, there is the need for better predictions and early warning systems (EWSs). Probabilistic forecasting by modelling different ensembles is wanted to account for uncertainties, which requires fast models. Current modelling options are static models (bathtub approach), which are fast but too simple. Furthermore there are advanced process-based models (like Delft3D, XBeach) which are accurate but too computationally expensive. The solution is the use of a semi-advanced model which solves all relevant processes, but does that in a computationally efficient way. The semi-advanced SFINCS model is developed to solve all necessary processes with computationally efficiency in mind, but is still in its development phase.

This research assesses how compound flooding due to TCs can be modelled in an accurate and computationally efficient way. Besides assessing the relevant physical processes, the implementation in a semi-advanced model is tested. In multiple tests it is shown that for conceptual situations a semi-advanced model can give accurate results within certain limits. Also it is shown in what conditions the advection term of the momentum balance needs to be solved and that a swash zone model approach with an indirect random forcing can give realistic results in 1D runup tests.

A case study of the compound flooding at Jacksonville, Florida, during Hurricane Irma (2017) shows that using a semi-advanced model, a similar accuracy compared to Delft3D can be achieved while being two orders of magnitude faster. Furthermore, a first test with wave-driven flooding in a real case study at Hernani, the Philippines, during Typhoon Haiyan (2013) shows that wave-driven processes have to be explicitly included to model all types of compound flooding. The approach to include these processes with the semi-advanced SFINCS model gives reasonable results compared to XBeach, although they can be further improved with more research. The resulting computational efficiency seems to get in the right range as needed for EWSs.

There is a growing interest in tidal energy, thanks to its constant availability in comparison to other renewable energy sources. Also in the Netherlands, tidal turbines have been installed. This has been done in the storm surge barrier that is positioned in the inlet of the Eastern Scheldt. These turbines have an impact on sedimentation. Therefore, the objective of this study is to analyse the effect of tidal turbines on the sediment transport in the Eastern Scheldt. Within this research, there is a distinct focus on how the tidal situation plays a role in this. Literature research has been carried out to gain knowledge that is needed to fulfill this objective. From this literature research it could be concluded that the effect of tidal turbines on sediment transport is much greater when the tide is (horizontally) asymmetric. It means that the maximum flow velocity in ebb-direction does not equal the maximum flow velocity in flood-direction. This results in a net sediment transport in one direction. It can be concluded that the greater the tidal asymmetry, the greater the net sediment transport. Now, it was time to qualify the aspects of the tide where the sediment transport depends on. This has been done by analytical elaborations of the relation between sediment transport and flow velocity. From this, it could be concluded that the sediment transport is depending on the wave height amplitude, discharge amplitude and the phase difference between the wave height and the discharge. The obtained relation was implemented into a Matlab model that computes the wave height and the discharge due to arbitrary tidal waves. The model has been extended to a network of channels that represents the Eastern Scheldt. With the extended model it was possible to simulate a net sediment transport, due to tidal asymmetry. The storm surge barrier and the tidal turbines could be implemented in this model, represented by a higher resistance. This resulted in a lower wave height- and discharge amplitude and with that also the net sediment tranport decreased. Since the net sediment transport is directed seawards, a decrease meant less erosion of the channels. But from the literature research it was concluded that the construction of the storm surge barrier and of the tidal turbines resulted in more erosion of the channels. An explanation for this difference in results may be that in the model, only sediment transport due to tidal asymmetry is taken into account. Other forms of transport are not considered. Also, many assumptions have been made in the model and there is still room for improvement to represent the reality more accurate with this model, which could be subject to further research.

Dams are an essential part of the Dutch flood defence works to protect the hinterland from flooding by the sea. Sand closures are the construction of closure dams with sand as the main building material. The rising and falling tide generates a current through the closure gap, which increases in strength as the gap becomes smaller. This current is quite strong and transports a significant amount of sand from the gap to areas where the current is weaker. The sand that is transported away from the closure gap is called the sand loss. The changing tide and constantly moving sand grains result in a dynamic environment. This makes it difficult to estimate the total amount of sand that is necessary to close the dam. It is important to be able to assess the amount of sand loss accurately, because it could make the difference between needing an extra dredging vessel or not. The costs of these dredging vessels are about 1 million euro's a week; therefore, having an unnecessary vessel present due to very conservative sand loss predictions is costly. Laboratory experiments and a numerical model were used to investigate the morphology during bedload transport under a lateral expansion. A streamwise bar formed behind the tip of the lateral expansion for both the laboratory experiments and the numerical simulations. However, it was not possible to replicate the bedforms that occurred during the laboratory experiments with the numerical model. These bedforms are deemed to be important for the morphology during bedload transport. The numerical model was used to expand the research to suspended sediment transport by decreasing the grain size of the sediment. In the obtained morphological patterns, four areas are distinguished where the morphology was found to depend on the flow velocity and the degree of turbulence in the flow. It is expected that a variation of these patterns can be found during a sand closure, although these findings should be validated with a laboratory experiment or in situ measurements. Therefore, it is recommended to set up a laboratory experiment to investigate the morphology under a mixing layer for suspended sediment transport. Based on the results of the laboratory experiments and the numerical model, it is advised to look into the possibility to use the sand deposited as a streamwise bar during a sand closure as an additional borrow pit to aid the closure process, since it would lead to less sailing time for the dredging vessels. For the first stage of a sand closure, it is advised to use a two-dimensional model that incorporates a bedload transport formula to model the sediment transport. It is important to use a parametrization of the bedforms that occur during bedload transport. For the final stage of a sand closure, which is the most critical phase due to high flow velocities, high degree of turbulence and time restriction, it is advised to use a three-dimensional model with a pick-up equation that takes the influence of turbulence on the grains into account, such as the pick-up formula of Okayasu et al.

Additional thesis - The largest structure of the Dutch Delta Works, that protect the Netherlands against high water levels at sea, is the Eastern Scheldt storm surge barrier. The Eastern Scheldt barrier is constructed as an
open structure that can separate the Eastern Scheldt estuary from the North Sea. In case that water
levels of +3.00 m NAP are expected, the steel gates of the barrier close and the hinterland is protected
against floods. Besides the protective function of the barrier, the location also shows potential for tidal
energy extraction.

Valdivia is a Chilean city located near an estuary system, 800 km south of Santiago. The navigation capacity of the Valdivia river mainly determines the present state and future possibilities for the welfare of the city. Since sedimentation problems arose in the river, the capacity for navigation became limited. The ministry of Valdivia therefore desires a solution for this problem in order to create possibilities for future growth. However, it is not known to what extent solutions for the sedimentation problem will actually contribute to an increase of welfare. By identifying the problem for the Ministry of Public Works of Chile, the boundaries and goal of the project could be set. In this research an attempt is made to answer the following research question: How can the surplus of sediment in the Valdivia river be remedied and to what extent will this contribute to the economic and social values of the city of Valdivia? In order to know how to remedy the surplus of sediment, a qualitative analysis of probable causes of the sedimentation was necessary. The main subjects which were researched by means of data and literature are tidal influence, river discharges, sediment composition and salt intrusion. With this knowledge it was concluded that the directions and magnitudes of flow lines in the river system can give good indications on what locations sedimentation can occur. The interaction between tidal currents and river discharges are the main drivers behind these flow characteristics. To verify the theoretical analysis, a basic Delft3D model was set up containing only tidal movement and river discharges. The pattern of the flow lines which was obtained from the Delft3D model supports the possibility that sedimentation occurs on the current identified sedimentation locations. Although other hydrological and morphological processes were found to possibly influence sedimentation rates in the river, qualitative data for studying these processes were missing. The current Delft3D model is therefore a good result regarding the available data and can be seen as a part of preliminary research in order to support further studies. Because the exact causes of the sedimentation were not identified in this research, it was not possible to come up with suitable solutions and to research how these solutions could affect the system. However, in order to gain insight in the effects of a river system without sedimentation problems, a fictitious scenario was studied by means of a social cost benefit analysis. The goal of the social costs and benefits analysis was to give insight in what factors should be taken into account when considering a project plan which solves the sedimentation problem. A dredging design was made in order to make the Valdivia river navigable for larger cargo vessels to increase the trading capacity of Valdivia. This dredging design comprises the deepening of the Valdivia river to a minimum water depth of 10 meters. Considering investment and maintenance costs of the dredging activities of this magnitude, it is concluded that the costs are not profitable compared to the social and economic effects of a higher traffic intensity on the river. With the social costs and benefits analysis all effects of solving the sedimentation problem are inventoried. When the Delft3D model can identify what the remedy of the sedimentation could be, the impact on the economic and social values of Valdivia can be determined. This research was not sufficient to answer the research question. Both the Delft3D model and the SCBA model were lacking accurate and reliable data. To expand this research, it is necessary to collect the data of which an overview has been made. Therefore, this research can serve as a preliminary study for further research. It is recommended that after data collection, solutions will be identified which can solve the sedimentation problem. These solutions need to be compared with a scenario analysis in order to decide on which is the most suitable solution for the sedimentation problem. Besides this, it is advised to research whether it is actually necessary to expand the river for transport when looking at all actors involved.

In the Rhine-Meuse delta many scour holes are present. These scour holes form a potential risk on the surrounding bridges, levees, weirs, dikes and tunnels. In general, scour holes can develop when there is a local change in hydrodynamic conditions or erosive capacity. Extensive research has been done in the past on scour holes close to hydraulic structures in a homogeneous subsoil, however, a large amount of scour holes are not related to hydraulic structures. Additionally, in deltas, the subsoil mostly consists of alternating layers of soil with different erosive capacities. This experimental study focused on a simplified situation of scour holes in heterogeneous subsoil with a three-dimensional character. The bed of the flume consisted of a concrete layer with a hole in the centre in which sand was exposed to the flow. Water depth, flow velocity and the size of the geometry were systematically varied to investigate their influence on scour hole development. Additionally, general observations were performed on the flow structures and scour development in and around the scour hole. It was observed that the scour development is different compared to the results of earlier investigations as a result of the non-erodible top layer. The process of undermining is a limiting factor for sediment transport out of the scour hole. The water depth showed a significantly smaller influence on the scour depth compared to existing empirical formulas. Additionally, the size of the geometry seems to be the most important varied parameter in this study. Relative to the size of the geometry, the scour depth remained approximately equal with different experimental conditions. Compared to two-dimensional experiments, the scour holes became deeper, which suggests importance of three-dimensional flow for scour hole development.