Salt intrusion poses a global threat to estuaries and deltas and is exacerbated by climate change through processes such as sea-level rise and prolonged droughts. This thesis aims to increase the understanding of salt intrusion dynamics in a complex delta system, particularly during drought conditions, using extensive field observations from the Rhine–Meuse Delta, including both moored and shipborne measurements.... ... Read more

Where rivers meet the sea, deltas form: branched, dynamic coastal ecosystems that have historically been attractive places for human settlement. Within these deltas, fresh riverine water and saline water meet in transitional zones, estuaries, and at the point where the river flows out into the sea, a river plume may be formed. To provide humans with services such as food, drinking water, and shelter, many deltas have been heavily modified. At the same time, these deltaic systems are typically highly vulnerable to such environmental change, so that the combination of human pressures and climate change can significantly amplify risks related to freshwater resources, ecosystem biodiversity, and coastal safety.

While climate-change-intensified flood risk has been extensively studied, risks to freshwater availability, such as salt intrusion, have received less attention. Salt intrusion is controlled by a complex interplay of river discharge, tides, wind, stratification, and human interventions. The combined effect of these processes on salt intrusion remains poorly understood, particularly in complex, multi-branch deltas or estuarine networks, such as the Rhine-Meuse Delta.

The overarching objective of this thesis is to gain more insight into salt intrusion dynamics in the coast-delta system of the Rhine-Meuse Delta. Understanding these dynamics is critical for both scientific and management purposes. This dissertation takes a modelling approach to addressing this objective. However, currently available realistic hydrodynamic models of the region often do not resolve all relevant scales, and analysis tools are unable to extract the underlying processes from the complex datasets they generate. Thus, this work is structured into two complementary parts: the first focuses on developing a suitable model and analysis tools, and the second applies these tools to investigate salt intrusion dynamics.

First, the scientific basis of this dissertation is established by reviewing the physical mechanisms governing salt intrusion in estuaries. The multi-scale nature of these processes is highlighted, from small-scale mixing to estuary-scale exchange flows. For salt intrusion in the Rhine-Meuse Delta, the most important processes are shown to occur on the scale of meters to the entire coast-delta system and on the intratidal to fortnightly timescale. Identifying these scales helps guide the choice of analysis methods. To extract the underlying processes from complex, realistic models spanning all these scales, this thesis analyses terms in the salinity variance analysis.

Identifying relevant scales for salt intrusion also informs hydrodynamic model selection. For this thesis, the realistic, three-dimensional, unstructured Delft3D Flexible Mesh Rhine-Meuse Delta model is chosen. The model is further developed to increase its predictive capability for an average-discharge year by adding a heat flux model and improving some of the boundary conditions. This new version of the RMD model is then validated for an average-discharge year using an extensive set of measurements in the region. This validation shows excellent reproduction of water levels, temperature, and good reproduction of salinity throughout the estuarine domain, thus ensuring that this model provides a reliable tool for investigating the mechanisms that control salt intrusion in the Rhine-Meuse Delta.

Next, the challenge of applying the chosen analysis methods to the RMD model is considered. Models with staggered unstructured grids, like the RMD model, are made up of polygons that can have any shape, as long as they adhere to certain orthogonality constraints. This, combined with the way datasets of models with staggered unstructured grids are structured, results in large, complex model output datasets. To circumvent computationally expensive calculations on these types of grids, output is often interpolated onto structured grids. However, the salinity variance analysis requires volume conservation and can thus not be applied to interpolated data. It is found that three practical implementation strategies are key to enabling complex computations on larger-than-memory unstructured datasets: unchunking alignment dimensions, ensuring perfect chunk alignment for element-wise operations, and strategically circumventing automatic alignment operations. A scientific software Python package, pySVA, is developed to apply the novel practical implementation strategy to model output datasets generated by the RMD model.

Using the tools developed in the first part of this thesis, the feedback between wind direction, river plume distribution, estuarine exchange flow, and salt intrusion is explored. Although river plumes are formed at the outflow of estuaries and the two are thus logically connected, it is unclear whether there is a dynamic two-way coupling between the estuary and the river plume — i.e., whether the river plume also influences the estuary. This is investigated by considering the influence of wind on the Rhine-Meuse Delta through the modification of the Rhine river plume. It is found that wind (direction and speed) modifies the plume's extent, stratification, and the strength and location of vertical mixing. Upwelling winds stretch and thin the plume, strengthen stratification, and influence the estuary by enhancing exchange flow through a shoreward bottom current. Downwelling winds thicken and narrow the plume, oppose estuarine circulation via altered cross-shore residual flow, and reduce exchange flow strength. Furthermore, downwelling winds can completely suppress cross-shore straining by prolonging the alongshore flood. Onshore winds attach the plume to the coast and suppress cross-shore straining, but through a different mechanism than under downwelling winds: by directly altering the cross-shore flow. Onshore winds decrease the cross-shore residual flow but increase the depth-averaged onshore current, resulting in relatively large exchange flows but low salt exchange efficiency. Thus, plume changes propagate into the estuary, altering the stratification, residual velocity profiles, and, ultimately, the strength and composition of the estuarine exchange flow. The net effect is that upwelling winds generally reduce salt intrusion, while downwelling and onshore winds increase it.

Finally, the effects of human intervention into the Rhine-Meuse Delta system are explored by considering the scenario of adding a second sea connection to the Rhine-Meuse Delta through permanently opening the Haringvliet floodgates. To assess this, two simulations with the RMD model are conducted: a reference scenario representing the average-discharge year and a scenario with permanently open floodgates and the same forcing as in the reference. It is found that the intervention changes the discharge distribution, alters tidal wave propagation and phasing, and reduces stratification in the northern branches of the estuary and the river plume. Placing the branches into the estuarine parameter space (Geyer & MacCready, 2014) shows that, as a result, almost all branches in the estuarine network undergo regime changes, and the salt intrusion length increases in all branches — an effect that cannot be explained by the commonly used steady-state salt budget. Additionally, comparing the two scenarios provides insight into the processes affecting salt intrusion in the Rhine-Meuse Delta in more general terms. Counterintuitively, analysis of the salinity variance analysis reveals that positive straining is negatively correlated to salt intrusion length, which challenges our current understanding of salt intrusion in salt wedge estuaries. The key to understanding this is to realise the importance of horizontal dissipation. Horizontal dissipation is found to be strongly correlated with straining, which is hypothesised to be related to baroclinic convergence and frontogenesis. Additionally, horizontal dissipation and the salt intrusion length are strongly negatively correlated, suggesting that horizontal dissipation is an important factor affecting salt intrusion in the Rhine-Meuse Delta.

Overall, this thesis demonstrates how coupled estuary–plume dynamics, advanced hydrodynamic modelling, and process-based analysis methods can be integrated to quantify and understand the physical drivers of salt intrusion, offering new insights into estuarine behaviour and informing sustainable management of complex deltaic systems. ... Read more

River plumes form when freshwater from rivers enters the salty ocean, creating buoyant water masses that strongly influence coastal circulation. By transporting freshwater, heat, nutrients, sediments and pollutants, they impact the ocean dynamics and ecosystems on local (10-100 km) and even (beyond) regional scales (>1000 km), depending on the size and dynamics of the plume. This thesis aims to improve our understanding of the Rhine River plume and its interactions with sea-level variations. The Rhine plume, located along the Dutch coast in the Southern North Sea, is highly dynamic system, influenced by tides and winds.
Chapter 2 investigates the variability of the wind-driven response of the Rhine River plume using numerical model simulations of a spring-neap cycle forced by idealized wind conditions. The difference in wind-driven response between spring and neap tide shows how the competition between straining and mixing, both induced by tides and winds, determines the structure and evolution of the Rhine River plume.
Chapter 3 examines the plume’s effect on sea-level variability along the Dutch coast by comparing barotropic and baroclinic model simulations. The Rhine plume induces a positive steric height anomaly, elevating the mean sea level along the coast and modulating the tidal signal near the river mouth. This highlights the need to include river plumes in sea-level studies.
In Chapters 4 and 5, an innovative method is developed for estimating sound speed profiles from multibeam echosounder measurements. The inversion method is based on minimizing the discrepancies between overlapping swaths and exploits empirical orthogonal functions to describe sound speed profiles using a limited number of unknowns. Since sound speed is influenced by depth, temperature, and salinity, this proof-of-concept provides a way to offer valuable insights into the vertical structure of the water column using routinely collected data.
Overall, this thesis advances our understanding of the Rhine River plume and its contribution to sea-level variability. In addition, the development of a proof-of-concept for retrieving sound speed profiles from multibeam echosounder measurements offers a promising approach to provide valuable information on stratification in river plumes. Together, these contributions support improved modelling and understanding of coastal oceans, particularly river plumes, which will become more and more important, especially in the face of climate change and its impact on coastal regions. ... Read more

The Hollandsche IJssel plays an important role in the freshwater provision of the province Zuid-Holland. Consequently, for Rijkswaterstaat it is key that salt intrusion is minimal in the Hollandsche IJssel. Recent studies noted that salt intrusion in the Hollandsche IJssel is limited due to a phase difference between tidal velocities in the main channel, the Nieuwe Maas, and the side channel, the Hollandsche IJssel. Earlier research investigated the impact of phase differences between branches and found it can lead to increased dispersion in the main channel, through a process known as tidal trapping. At the same time, this phase difference can prevent the saltiest water from entering the side channel, as was found at the Hollandsche IJssel. Because of this role, it is relevant to find out how this phase difference may be influenced by sea level rise, more extreme river discharges and particularly how it depends on the geometry of the main and the side channels. Especially the latter could help Rijkswaterstaat to minimize salt intrusion at locations relevant to freshwater intake, such as the Hollandsche IJssel.

The main objective of this thesis is to investigate how the geometry of the side and main channel influences the tidal phase difference between these two channels, and how this may impact the salt dispersion in the side channel. For this, an analytical model is developed describing harmonic wave propagation in multi-branch systems and this is used next to results from a 3D numerical model for the Rhine Meuse Delta (RMM3D). First, the influence of changes in geometry and forcing is systematically investigated for a network containing a single junction. This shows that the length and depth of the side channel are the most significant variables. The depth is one of the main variables impacting friction, which governs the type of wave which can form in the system. A decrease in friction allows a wave to transform into a standing wave pattern as the return wave becomes more important, while increased friction transforms it into a propagating wave. The length also controls the type of wave which can form as it determines the distance along which the friction can work. Additionally, the length also governs potential resonance in the side channel.

Next, the phase differences of the M2, M4 and M6 tide are determined for the junction with the Hollandsche IJssel in the Rhine Meuse Delta (RMD) based on the RMM3D model. The main tidal constituent regarding tidal trapping was found to be M2. However, this does not fully represent the time difference between flow reversal at the Hollandsche IJssel and the Nieuwe Maas, which was found to be around 75 minutes. Additionally, the phase difference at the Lek was investigated. For the M2 tide at the Hollandsche IJssel and Lek, a phase difference of 55⁰ and 31⁰ was found, respectively. These phase differences prevent salt intrusion in the respective side channels. The inflow of the side channels starts while the main channel still flows to the sea during the ebb. At this moment, the salt concentrations in the main channel have already returned to background levels... ... Read more

Due to climate change, low river discharges are expected more frequently. As a result, salt intrusion in the Rotterdam Waterway will be critical at least five times more frequently, negatively impacting freshwater availability. A possible new measure to reduce salt intrusion is providing additional mixing by internal wave generation, which reduces the estuarine circulation. The potential to reduce salt intrusion in the Rotterdam Waterway by internal wave generation is investigated. To determine this the irreversible mixing generated by internal waves is quantified and related to salt intrusion length. Numerical 2DV simulations of a decelerating flow over an undular bed were executed for an idealized section of the Rotterdam Waterway. The mixing was quantified by determining the background potential energy in a chosen control volume. This method can be applied if disturbances in the velocity or density profile do not reach the boundaries of the control volume. The mixing observed in the control volume is related to salt intrusion by the buoyancy frequency of the background state. The net salt flux imported by estuarine circulation was found to be proportional to the squared buoyancy frequency. Estuarine circulation is the main forcing mechanism of salt intrusion in the seaward part of the Rotterdam Waterway. For the small idealized section of the Rotterdam Waterway, internal waves generated over geometrical features are capable of reducing the salt intrusion caused by estuarine circulation by approximately 1.7% of the initial value, if bed waves are present over 27.5% of the control volume. ... Read more

In this study the contribution of the low-frequency residuals to the sea-level variability has been examined. This is done using 106-year old sea-level record obtained at Hoek van Holland. Computing the mean sea-level per season each year and the corresponding standard deviation one finds an increase in both these features of the sea-level record. The rise of the mean sea-level implies the effect of climate change. Moreover, it is found that the standard deviation in the sea-level, thus the intensity of variation, is the highest during fall and winter. This implies that the sea-level variability has a seasonal dependency. Furthermore, one finds an increase in the standard deviation on the long term. However, since a big part of the mean sea-level is influenced by tidal events, the increase in standard deviation is possibly linked to climate change in meteorological factors as has been found is in the study carried out by Gerkema and Duran-Matute(2017). With the use of the computational algorithms such as the Fast Fourier Transformation and the Wavelet Transformation one can extract the low-frequency residuals from the sea-level record. Inspired by the study of Gerkema and Duran-Matute(2017) a correlation between the wind speed and the low-frequency residuals have been found. Using the Wavelet Transformation it is found that the low-frequency residuals obtain the most energy during fall and winter, this empowers the finding that these low-frequency residuals are seasonal dependent. Moreover, when studying the low-frequency residuals closely it is found that the frequencies below 0.60 1 day obtain the most energy during fall and winter, especially the frequencies near 0.10 1 day . With these findings one can say that the contribution of the low-frequency signals, thus the low-frequency residuals, is correlated to meteorological events, such as the wind. Moreover, it is found that the variation in the low-frequency residuals is much smaller during spring and summer compared to the case during winter and fall. This seems not to be the case for the sea-level variability due to the tidal constituents and the high-frequency waves. Therefore, one may conclude that these low-frequency residuals contribute to a great extent in the standard deviation of the sea-level. ... Read more

In recent years, several logistic optimisation models have emerged as powerful tools to assess and optimise the planning, costs, and workability of marine operations. Nevertheless, these models often rely on two underlying assumptions: (1) the significant wave height and peak wave period adequately describe (directional) wave fields, and (2) these two parameters sufficiently describe the conditions causing weather downtime. However, little is known about how these underlying assumptions affect the reliability of logistic optimisation tools. This study, therefore, presents an alternative model that integrates response motions of vessels and turbine structures into a logistic optimisation tool to address and expose the implications that follow from using these assumptions. A case-study approach, on two recently realised offshore wind farm projects, showed that integrating response motions results in, up to 10%, less favourable workability conditions. Further analysis on the data showed that it is crucial to include the two-dimensional wave energy distribution to expose more complex sea states that induce weather downtime. Moreover, a failure analysis approach found that the conditions inducing downtime events are more accurately described by response motions instead of sea state parameters. Therefore, the findings of this study suggest that integrating response motions into logistic optimisation models improves the reliability of the model estimates. Besides, this study suggest that the industry’s approach potentially overestimates the true workability and, therefore, imposes unnecessary operational risks. Hence, the results of this study demonstrate the importance of integrating hydrodynamic engineering knowledge into the assessment and optimisation of project planning, costs, and workability. ... Read more

Key points:- Pre-closure salinity intrusion into Haringvliet-Hollands Diep is known to have reached Biesbosch National Park at flood tide. Post-Delta21 salinity intrusion is projected to be less extensive than pre-closure. Maximum intrusion is estimated to reach the westernmost Moerdijk ports mainly due to diminished tidal flow at the estuary mouth.- Freshwater intake in the Haringvliet is projected to be compromised for the majority of the year whereas freshwater intake in Hollands Diep is compromised during prolonged drought.- Salinity outwash from the Haringvliet on ebb tide is projected to be poor under both drought and normal conditions due to widespread salinity diffusion in lateral and vertical directions on flood tide. The Delta21 framework, in which this study is positioned, aims at increased flood safety and ecological restoration of the Haringvliet, a former estuary in the Dutch southwestern delta that has been closed as part of the Delta works. To achieve this, Delta21 proposes to reopen the Haringvliet freshwater basin to tidal effects in an attempt to restore its estuarine character. Implementation of Delta21 introduces the risk of compromising agricultural and industrial activities around the Haringvliet-Hollands Diep by reintroducing salinity and tidal movement in the basin. The central problem lies in quantifying the extent and stability of the expected periodic salt intrusion post-Delta21. This has been done by projecting a geographical bandwidth of salinity intrusion patterns depending on the severity of SLR (2020-2100) and low-to-mean Rhine-Meuse discharges (in 2100 CE) using a numerical model. Mitigation strategies are then recommended based on these numerical results. The OSR-HV model (owner: Port of Rotterdam) is used to run predictive scenarios of salinity intrusion for the lower reaches of the Rhine-Meuse basin. OSR-HV runs in TRIWAQ (Rijkswaterstaat), which is 3D numerical modelling software that employs coupled hydrodynamics and constituent transport thereby resolving salinity transport. An upper-end critical scenario consists of a Rhine (Lobith) discharge averaging 1000 m3/s for 31 days combined with 85cm SLR in 2100. This resulted in an intrusion pattern reaching the westernmost port of Moerdijk in Hollands Diep. Results show that the basin geometry, possibly nudged by Coriolis deflection, initially causes a preferential path of salinity intrusion along the southern bank of the Haringvliet. Lateral and vertical mixing is extensive in the western part of Haringvliet which is thought to be a combined effect of weakened tidal flow at the estuary mouth and robust and erratic geometry of the basin. Further up-estuary, the historical flood-ebb tidal channel structures is the main transporter of salinity. The Haringvliet shows distinctly different estuarine behaviour compared to the neighbouring Rotterdam Waterways where stratification is more stable, causing less up-estuary diffusion. The relative robustness of the Haringvliet and mild freshwater forcing give rise to extensive 3D mixing which subsequently limits the maximum horizontal excursion of salinity. Salinity intrusion into the Old Meuse is observed to aggravate upon opening of the Haringvliet sluices due to flow reversal in Spui which connects Haringvliet with the Rotterdam Waterways. This effect can even result in salt intrusion from the Rotterdam Waterways via Old Meuse and Spui back into the Haringvliet, causing a secondary spike in salinity on ebb tide there. Finally, height-limitation of the Haringvliet sluice gates is somewhat effective in countering horizontal excursion of salinity but runaway diffusion in the Haringvliet results in similar salinity profiles compared to full opening of the sluice gates. A shipping channel that crosses the current Haringvliet front delta is included in Delta21. This deep feature promotes advection of salt through the Haringvliet sluices. Subsequent diffusion patterns cause for poor washout of salinity from the basin upon ebb tide. Limiting the depth of this channel is recommended if salt intrusion is to be reduced. Full opening of the Haringvliet sluices furthermore causes approx. 0.50m lowering of Mean Low Water (MLW) near Moerdijk which affects busy shipping routes between Rotterdam-Moerdijk-Scheldt. Partial reduction of the conveying area of the Haringvliet sluices may be used to suppress the tidal wave penetration into Haringvliet-Hollands Diep. Significant gain in ecological value is likely post-Delta21 due to addition of approx. 1900 ha of intertidal areas and a 40 km salinity gradient (excl. front delta). Ample recommendations on further research have been made in this exploratory study. It is recommended to further study the effects of Delta21 interventions on macro hydrodynamics of the Dutch coastal shelf. The interconnected nature of the region requires a larger modelling domain to prevent the occurrence of non-physical effects obtained from the current numerical schematization. Furthermore, hydrological relations and bathymetry were generated from 2020 data. It is therefore recommended to precede the assessment of salinity intrusion into the Haringvliet with numerical projections on change to these environmental factors. Lastly, significant gain in accuracy may be obtained from applying spatially varying temperature and wind to the domain to better replicate baroclinic flows and turbulent mixing. ... Read more

The amount of suspended sediment spill released during dredging activities in the Fehmarnbelt strait is monitored along the excavated trench to prevent negative effects on local ecology. Stratification due to the interaction of saline water from the North Sea and fresh water from the Baltic Sea forces bi-directional flow of the layer above and below the density gradient, causing bi-directional spread of the dredging plume perpendicular to the trench as well. During such an event, monitoring on both sides of the trench is required. By mapping out the spatial and temporal behaviour of stratification, a prediction can be given on where and when measuring on both sides of the trench will be needed to include the baroclinic effect. Figures created with monitoring data show that the density profile is influenced by the salinity, rather than the temperature. Therefore, stratification predominantly depends on the in and outflow of water with respect to the Baltic Sea. The figures also show a larger density gradient during out than inflow. It can also be observed that where the water depth is restricted to 10 meters, wind and bottom friction mix the entire water column. Therefore, stratification occurs predominantly during outflow in sections deeper than 10 meters, indicating the need for monitoring the bi-directional plume spread during such circumstances. Whether stratification occurs during inflow in sections deeper than 10 meters likely depends on the duration and strength
of the wind forcing and the initial strength of the density gradient prior to the inflow event. Further analysis should be done to confirm this. Signs of Ekman transport, return flows and the deflection of the currents towards deeper water can also be observed in the measurement figures. Since these processes affect the plume spread direction, additional research can be done on the behaviour of the current direction in the Fehmarnbelt. ... Read more

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. ... Read more

Due to climate change and human interventions, saltwater intrusion is becoming a topic of increasing concern worldwide. Salt water intrudes into the Rotterdam Waterway (RWW) by an exchange flow, where the denser sea water propagates landwards at the bottom. The main competing mechanism for this stratified exchange flow is vertical mixing, which can be realised by internal wave induced shear instabilities or wave breaking. The goal of this study is to investigate whether internal waves generated over undular bottom topography in the RWW can generate additional vertical mixing. The underlying assumption is that a decrease in stratification decreases salt intrusion.   The approach to answer the main research question is a combination of an analytical and a numerical analysis. The analytical study is based on frictionless linear theory. Internal wave behaviour is further analysed with FinLab, a finite element model which includes the non-hydrostatic processes and effects of density differences. FinLab is evaluated for the application of this study by means of a validation case.  In the analytical study, linear theory is applied to obtain a relation between the bed wave parameters and average internal wave energy density E for internal waves generated over sinusoidal bottom topography in a linearly stratified fluid. The derived expression describes that the bottom topography amplitude h0 and bed wave number kT both have a positive quadratic relation with the energy. Additionally, kTkinfluences the resonance conditions.  To validate FinLab for internal wave breaking and mixing an experiment in a wave tank, according to an example from literature, is simulated. The validation case reveals a shortcoming in the turbulent mixing parameterization. However, on scales relevant for the RWW the effect of this will not have the same significance. The validation case offers a suggestion for a subgrid closure of diffusion, where density effects are taken into account.  Numerical simulations of a 2D channel stretch with sinusoidal bottom topography, a linearly stratified fluid and a linearly varying background velocity, show generation of resonant trapped internal waves for the first two resonant modes. These occurrences correspond to the highest values of kinetic energy as function of vertical velocity averaged over the bed wave domain. The vertical buoyancy flux b is downward directed during occurrences of internal waves and becomes upward directed for increasing background flow. Vertical mixing is associated with an increase in average potential energy Ep, which is 17% higher for the base case (containing bed waves) than for a similar case without bed waves. This increase is larger when bottom shear stress increases. Richardson numbers below 0.25, associated with shear instabilities and mixing, are only observed near the bed, mainly when internal waves are present. The effect of variations in bottom topography wavelength LT and amplitude h0 on internal wave energy can be explained by the analytical formulation. The effect of bed wave parameter changes on b and relative increase in Ep can be related to the effect of the changed amount of bed friction rather than the difference in wave energy.   The first resonant mode is the most energetic, however, the average energy density found for these waves is only 0.4% to 6.7% of the potential energy anomaly (PEA); the energy required to fully mix a stratified water column. In the simulations the only mechanism that could transfer internal wave energy to turbulent kinetic energy are shear instabilities near the bed. Over the full simulation, the net vertical buoyancy transport is of negligible magnitude, where Ep shows significant increases between 6% and 99% compared to similar cases without bed waves and is enhanced during the presence of internal waves.  The main discussion point is that the quantification of vertical mixing requires improvement, particularly to determine the importance of mixing by internal wave-induced shear instabilities and by bed shear. Mixing by local shear instabilities (of which the relevant scales cannot be resolved with the current grid resolution) does not have an adequate parameterization, because density effects are not included in the turbulence closure. The bed friction parameter, which greatly influences the behaviour of the system, has to be validated. Furthermore, cases where internal waves might break in practice (e.g. at banks) were not considered. Finally, the observed internal wave energy is of small magnitude, however field measurements by Pietrzak(1991) shows that turbulence production by internal waves was significant.    ... Read more

Phenomena like sea level rise, global warming and erosion together contribute to increasing flood risk in vulnerable coastal areas. As this flood risk increases, initiatives to mitigate the effects of climate change in the coastal zone are also increasingly sought. During recent years, the view that these measures should be circular has gained significant momentum. In line with this, the idea that sediment should not be treated as waste, but as a valuable product in a circular economy has been widely accepted. Within this theme, the sediment uses as resources in circular and in territorial economies (SURICATES) was created to develop and execute eco-innovative solutions for the reuse of sediments in Western Europe. As a part of SURICATES, over the course of a nine week pilot, a total of 500 tonnes of sediment was reallocated in a designated area in the Rotterdam Waterway, with the expectation that this sediment would be transported out of the Rhine-Meuse estuary into the North Sea. The SURICATES project is one of the first real large-scale efforts to reuse sediment for economic as well as ecosystem services. Effective implementation of the SURICATES project, however, requires a thorough understanding of the governing physical processes impacting the distribution of the deposited sediment locally. Furthermore, to assess the feasibility of similar future applications, efficient modelling of the pilot project is necessary. The work presented here aims to elucidate the hydrodynamic processes that are governing in the Rotterdam Waterway, within the framework of the SURICATES pilot project, and assess the reproducibility of these processes by (two) predictive models that are currently operational at the Port of Rotterdam. A special 6-hour monitoring survey was set up to measure salinities, velocities, temperature, and suspended particulate matter (SPM) along a transect crossing the reallocation location. In combination with a literature review, this dataset provides the basis for research into the predictive capabilities of two currently operational hydrodynamic models. Analysis of this dataset reveals the dominant terms in the momentum balance, the influence of Coriolis, the occurrence of internal waves, and the effect that all these mechanisms may have on the SPM distribution around the reallocation location. When the system dynamics are elucidated, the model performance of the two hydrodynamic models is assessed—both quantitatively and qualitatively. It is also investigated whether phase shifts are introduced in the models. It is found that the primary hydrodynamic processes in the Rotterdam Waterway are related to the barotropic tidal asymmetry imposed at the river mouth, the tidal excursion of the salt wedge, baroclinic exchange flow processes, and turbulence damping at the pycnocline. Turbulence damping at the pycnocline generally poses an upper limit to the (re)distribution of SPM over the water column, although field data suggests that this damping may not be sufficient to counteract diffusion processes locally. This effect occurs under certain forcing conditions and during low water slack. Furthermore, an internal Froude number analysis provides evidence for the possible generation of internal waves in one of the river’s bends. The evaluated models, however, are not capable of reproducing all of these hydrodynamic processes adequately. Although both models adequately reproduce water levels and the vertical velocity structure, they have difficulties predicting the pycnocline height. Additionally, it is found that both models introduce a small phase shift in the velocity and salinity prediction. The research presented here is a contribution to the understanding of the governing hydrodynamic processes in the Rotterdam Waterway, and the effect that (in)accurate modelling of these processes may have on future studies. Recommendations following from this research could improve future modelling practices. ... Read more

The port of Rotterdam is located within the Rhine-Meuse estuary where a substantial amount of fine sediment transport takes place. Therefore, the port of Rotterdam is subject to significant siltation, requiring maintenance dredging to guarantee a sufficient nautical depth of fairways and harbour basins. To optimise the dredging strategy in the port of Rotterdam, a pilot study has been carried out wherein sediment is reallocated in the Rotterdam Waterway, during ebb, instead of offshore in the North Sea. Between May and November 2019, 210,000 tons of sediment has been reallocated. This pilot study has been carried out in the context of the larger EU-Interreg Sediment Uses as Resources in Circular and Territorial Economies (SURICATES) project. The main goals are to re-use the sediment as a resource and to reduce the sailing time of the dredging vessels. Both ideas comply with the Building with Nature philosophy; a concept gaining popularity over recent years in The Netherlands focusing on, amongst others the optimisation of dredging strategies. It is expected that the reallocated sediment is mainly transported offshore, while at the same time some of the sediment will nourish the river banks of the Rotterdam Waterway enhancing its flood resilience. This thesis focuses on understanding the fine sediment behaviour of the SURICATES pilot project on two different scales. This is done by analysing measurements and model hindcasts. The measurement campaign is set-up by Deltares and the Port of Rotterdam. For the model hindcasts an operational hydrodynamic and sediment model is used. On the small scale this is done by focusing on the behaviour of a single disposal over a tidal cycle. The large scale focuses on the cumulative long term behaviour of all sediment reallocations. For the small scale two measurement surveys are analysed. In both surveys it is found that a sediment reallocation executed by bow coupling is subject to mixing up to halfway the water column. Subsequently, the sediment plume is advected around and below the pycnocline. Further measurements in the mid field are lacking, but it is hypothesised that the majority of the sediment settles during subsequent low water slack. For the other execution method, drawing the bottom doors, which is used to reallocate the majority of the sediment, useful measurements are absent. It is hypothesised that the majority of this reallocated sediment is confined in the salt wedge and therefore mainly transported upstream over time. To assess the long term behaviour of the cumulative behaviour of all sediment reallocations, a different measurement campaign is set-up. In this measurement campaign, bed samples are collected prior to and during the pilot study to determine the change of the bed composition. In this campaign an indication for increased sedimentation related to the pilot study is found for nearly all the sample locations. The short term model study is set-up to enhance the understanding of the short term behaviour of a sediment plume, to derive an accurate source term for the sediment disposals and to carry out a sensitivity analysis. This sensitivity analysis is executed to derive the influence of differences in disposal method, timing of disposal, and uncertainties in the model. It is found that the execution method has the largest influence on the critical sediment fluxes on the short term, followed by the timing of disposal. From the long term model hindcast, in which the entire pilot study is hindcasted, it is found that 27% of the total amount of reallocated sediment flows downstream from the location of disposal and 73% upstream. These estimations are in line with the hypothesis and long term measurement results, but a thorough calibration of the results is lacking. To conclude, in this thesis a pilot study utilising a different sediment reallocation strategy in the port of Rotterdam has been investigated. This study shows that majority of the sediment disposed, in the current set-up of the pilot study, is estimated to flow upstream. In the sensitivity analysis, it is predicted that this might be caused by the timing of disposal or method of execution. It is also found that the initial behaviour of the sediment plume and the long term measurement contain a large amount of uncertainties. As most important recommendation for future work an expansion of the current measurement survey is proposed with at least two fixed locations: one downstream and one upstream of the location of disposal. In this way sediment fluxes can be established, which can also be used to verify and calibrate the sediment model. ... Read more

TheAmazon-Orinoco river plume is a buoyant freshwater lens of 1.2 × 10⁶km², which has been traced over 2000 km from the Amazon river mouthinto the Caribbean Sea and along the Lesser Antilles. The river plume iswarmer than the surrounding open-ocean waters, with temperaturedifferences up to 1.5 ∘C caused by a stratification-induced barrier layerinhibiting vertical mixing and coloured matter increasing solar energyabsorption. Due to its magnitude, the river plume affects thehydrodynamics and the oceanic conditions in the Western Tropical NorthAtlantic (WTNA) substantially, but its variations on interannual time scales andthe corresponding relation to local sea-surface temperature (SST) are notwell understood. The Caribbean Sea is a region of high ecological value as itis home to extensive coral reefs, which are especially sensitive topersistent high SST. Therefore, this study investigates the interannualvariability of the Amazon-Orinoco river plume and its relationship to SSTsin the Caribbean Sea and the WTNA. It is hypothesised that fresh anomaliesof the river plume salinity pattern are indicative of a more extensivetransportation of the heat contained in the river plume. As a result, itis expected that interannual variations of dominant river plume pathwaysaffect the magnitude and location of anomalous SSTs. To test thishypothesis, model reanalysis fields of oceanic conditions from 1993 to 2017are used to conduct statistical analyses. In this context, the river plumevariability is determined using specific regions of freshwater influenceestablished using Empirical Orthogonal Function (EOF) analysis ofanomalous sea-surface salinity (SSS). Cross-correlations analysis relatingthese EOF modes of with atmospheric processes show that the interannualvariability of the river plume is dominated by wind-inducedadvective transport and -mixing. Strong winds along the Brazilian shelfare related locally increased SSS, while a weak southward component makesfor extensive spreading of the low-salinity plume waters. Additionally, weshow that high river discharge affects SSS east of the Lesser Antillesafter a lag of three months. Through its modulation of these atmosphericprocesses, there is a strong indication that the El Niño-SouthernOscillation affects SSS variability in the main along-shelf northwestplume pathway, with low SSS 1–9 months after a La Niña event. DecreasedSSS are found in phases 2 and 3 of the Madden-Julian Oscillation, whileincreased SSS was observed in phases 6 and 7. However, the evidence forthis relation is weak and should be investigated in further research.
The results show that, opposed to the hypothesis,a more extensive river plume is not associated with higher SSTs in theCaribbean Sea. However, strong correlations are found between river plumesurface area and SSTs at a lag of 1 year. Based upon results of previousstudies, we argue that the river plume has the ability to pre-heat themixed layer in the WTNA leading to extreme temperatures in the followingyear. It is wise to conduct a Lagrangian parcel back-tracking experimentto verify this mechanism. ... Read more

The oceanic transport of heat and salt from the equator northward is one of the main reasons for the mild climate of Europe. This transport occurs in the upper layer of the ocean. In the north, strong cooling occurs due to the large difference in temperature between the ocean surface and the atmosphere. The cooled watermass has a higher density and therefore sinks and returns toward the south at depth. This so-called AtlanticMeridional Overturning Circulation is driven in part by the wind and in part by the difference in temperature and salinity between the equator and the poles. Polar climate change will result in warmer and fresher oceans whichwill likelyweaken this global overturning circulation. Especially processes that concern the transformation from the light (warm) watermasses to dense (cold) watermasses are sensitive to changes in buoyancy forcing. This thesis focuses on an area where a large part of this transformation from light to dense watermasses takes place; the Nordic Seas. The Nordic Seas are located between Greenland and Norway and consist of several sub-basins, like the Lofoten Basin, the Greenland Basin and the Norwegian Basin. The main aim of this thesis is to better understand the dynamical processes involved in the watermass transformation in the Nordic Seas. ... Read more

The Rhine Region of Fresh Water Influence (ROFI) is a shallow frictional river plume in front of the Dutch coast. Each tidal cycle a new tidal plume front with fresh-water is released. Recently, internal gravity waves have been observed in this plume. Using a Froude number analysis, support for the internal wave generation mechanism by a tidal plume front is found. It is shown, that on averaged neap tides the density stratification is large. This results in a larger area where the internal waves can be released from the tidal plume fronts compared to spring tides. As the Rhine ROFI is located in shallow water, it is investigated what the effect of bathymetry variation is on internal waves. This is studied by extending the standard KdV model derivation to account for a variable bed. This resulted in a small correction on the propagation speed inside the KdV equation. Observations of internal waves have been used to validate the KdV model. By scaling analysis of the observed waves it is obtained that the relative wave height and relative depth balance for most of the observed events. For these events the wave period and velocity amplitude of the KdV model are well matched with the measurements. This showed that the KdV model may be used for a first estimate of internal waves in the Rhine ROFI. By developing a TGE fitting procedure, it was possible to obtain the pycnocline depth and the direction of wave propagation with only limited data available. Therefore, the parameters have been varied and the error between the TGE solution and the velocity potential obtained from the velocity measurements has been minimized. ... Read more

This study aims to get an insight into the particle trajectories that microplastics follow, after having been released in the North Sea.

For the computations daily-mean values of the surface currents are used, retrieved from the Mercator global ocean model. 2D particles trajectories are simulated for a year, with a 3rd party Python toolbox for Lagrangian simulation of particles: OceanParcels. Particles released from any location in the North Sea eventually get trapped in the Norwegian Coastal Current (NCC). From here they are being further advected to the North, at different moments in time for the particles released at different locations. The coastal processes in the NCC are mainly linked to wind and stratification, hence variations in ow patterns near the coast are linked to the seasons. When these ow pattern include large scale eddies, the particles follow a meandering and erratic path. Floating plastic particles released in the North Sea will flow northwards along the coast of Norway. Eventually those particles will end up in the Arctic region or get
trapped in the Norwegian fjords, independently of the location of release. However, the time scale of the northward advection depends both on where the particle has been released and the environmental conditions.
... Read more

Coastal bays in the Caribbean accommodate different marine ecosystems, including seagrass meadows and coral reefs, which provide important ecosystem services. However, these marine ecosystems are endangered. Seagrass ecosystems have a key role in coastal bays, but despite their alarming rates of loss, they receive little attention compared to other marine ecosystems. This study aims to provide a better understanding of the role of seagrass ecosystems in coastal bays by assessing their impact on the morphodynamic behavior. In order to achieve this, an existing hydrodynamic model has been extended to include morphodynamics. Two different bays have been investigated: Baie Orientale and Baie de l’Embouchure (St. Martin). These bays represent a partially exposed and fully sheltered bay, respectively. Both regular wave conditions, as well as extreme storm conditions, have been investigated to understand the consequences of seagrass loss on coastal erosion for different environmental climates. The coastal erosion has been found to be more prominent in the exposed region, whereas the more sheltered areas are more resilient. The erosion takes place in shallow waters but remains limited under regular swell conditions. The increased wave energy, during the extreme storm event, increases the erosion rates considerably. However, the regular swell conditions are normative in shaping the morphological development of these coastal bays when longer timescales are considered. The seagrass counters erosion most effectively in the foreshore and much less in deeper regions. Removal of seagrass in the foreshore (between 1-3m) halves the sediment stabilization. In contrast, the sediment stabilization increases if the meadows are able to spread especially towards the shore. In addition, the wave energy and waveform have a significant impact on the sediment stabilization. The stabilization typically decreases for higher and shorter (storm) waves, whereas it increases for smaller and longer (swell) waves. The long-distance interactions between seagrass meadows and coral reefs have led to their mutual coexistence, which is also found to be beneficial for the erosion control of the coastal bays. The dissipation of wave energy on top of the reefs fosters the sediment stabilization by seagrass. Moreover, the impact of the reefs on sediment stabilization also increases in the presence of seagrass meadows due to the additional drag exerted by the seagrass. The seagrass meadows are typically more effective under swell waves, whereas the reefs are more dominant in the dissipation of storm waves. Seagrass meadows and coral reefs form thus a synergy, in which the stabilization of sediments provided by the individual ecosystems is facilitated, reinforced, and complemented by the proximate presence of the other ecosystem. Finally, the role of seagrass on sediment stabilization has been explored under climate change. The coastal erosion increases for the considered sea-level rise scenarios. The impact of the seagrass on sediment stabilization decreases when the bays, seagrass and reef ecosystems are not able to keep up with sea-level rise. The synergy between seagrass and reefs emphasizes that the responses of both ecosystems are of importance for the future of tropical sheltered bays and should therefore not be managed in isolation. ... Read more

This thesis introduces a new algorithm for optimising shipping routes within a dredging project. Highly dynamic and time-dependent hydrodynamic features influence shipping routes. Due to the complex interactions between the horizontal tide, vertical tide, stratifying forces, wind-driven forces, and limited water-depth, shipping routes were previously only optimised for large scale routes (order of 1000 km). This study presents an algorithm that can optimise shipping routes that are influenced by these small scales (order of 10 km) hydrodynamic features. This algorithm uses graph theory to solve for the time-dependent fastest path between start and destination. Graph theory searches for the optimal path through a set of nodes that are connected with edges. This study uses the time-dependent shortest path algorithm which accounts for the FIFO-criteria (Waiting criteria) and can solve the non-convex nature of the problem

The input of this algorithm is a hydrodynamic model. These models are Computational Fluid Dynamic (CFD) models that calculate currents and water levels in a specific domain. The domain is discretised into cells and nodes to calculate these hydrodynamic features. This study uses the nodes of this hydrodynamic model as the vertices of the graph. However, for some cases, the hydrodynamic model has too many nodes for the shortest path algorithm. This study presents a method for reducing the number of nodes without reducing the spatial resolution. The nodes are reduced based on a combination of the vorticity and the magnitude of the flow.

This algorithm is implemented in a python software package named Hydrodynamic Algorithm for Logistic enhancement Module (HALEM). HALEM can determine the optimal shipping route for a given hydrodynamic model. Defining different cost functions results in different optimisation purposes. This thesis presents cost functions for the fastest route, shortest route, cheapest route and least polluting route. This software is then implemented in the OpenCLSim software so that this combination of software can optimise routes of entire projects. A case study simulates a beach-nourishment at Schouwen Westkop Noord to demonstrate the practical use of HALEM and OpenCLSim. For this project, 425,500 m3 sand should be dredged offshore and pumped onto the beach. Due to the narrow gullies and tidal changes in hydrodynamic features, the routes were hard to predict. The simulation with HALEM and OpenCLSim shows an increase in the production with 21 % compared to the simulation with just OpenCLSim.
... Read more