The goal of achieving net-zero emissions by 2050 requires innovative ways to reduce energy consumption in all sectors. This thesis presents an in-depth analysis on the effect of coastal currents on energy consumption of sailing dredging vessels and the potential to make use of these currents to minimize energy consumption in dredging projects. To model the highly dynamic and time-dependent currents of the North Sea, an analytical solution for the tidal-induced currents and residual density-driven currents was used to create a synthetic 3D flow field, respectively derived by Prandle and Heaps. To quantify the energy consumption of dredging vessels sailing through a 3D flow field, a python-based model was developed. This model utilizes the Holtrop and Mennen method to calculate the sailing speed corresponding to a desired engine power, with a modification to account for current-induced drag resistance on the rudder caused by currents perpendicular to the sailing direction. The model was validated with sensory vessel data of Van Oord containing information of a dredging project in the North Sea, in combination with measured data for the currents at the location of the project. The validated model was used to explore a dredging strategy for a sand nourishment project that minimizes energy consumption by waiting for favourable marine currents before sailing. A sand nourishment project was chosen because the primary cross-shore movement of the vessel interacts with the bi-weekly occurrence of alternating cross-shore currents. Two cases were simulated, a hypothetical case in which the vessel does not consume energy when waiting, and a realistic case in which the vessel's energy consumption continues due to utilities such a lighting and heating of on-board accommodations. The hypothetical case showed a reduction of energy consumption of 3\% when sailing with the cross-shore currents as opposed to neglecting these currents, which is at the lower boundary of the energy reduction due to voyage optimization predicted by IMO (1\% - 10\%). However, the energy reduction was outweighed by energy consumption during waiting in the realistic case, making this strategy unsuitable for conventional dredging vessels. The model developed in this thesis can be of significant value for both dredging companies as well as researchers. These stakeholders can use the model to plan and optimize dredging activities, reduce environmental impact, and identify areas for innovation and improvement in the dredging industry.

As the environment is changing temperatures are changing, becoming more extreme. This is expected to affect the oceans and its transport, specifically the Atlantic Meridional Overturning Circulation (AMOC). The Labrador Sea is a part of the AMOC, where overturning in depth and density space occurs, due to deep convection. Deep convection is the process of seawater losing its heat to the atmosphere, due to atmospheric cooling during the winter. This causes the seawater to become colder and denser, and it therefore sinks towards the bottom of the basin. Deep convection is previously studied extensively as it is a unique and important process of the global ocean circulation system. The key process that causes the AMOC water to overturn, is due to buoyant eddies shedding from the boundary current into the interior. The buoyant eddies exchange their buoyant boundary current water with the dense interior water, causing the boundary current (and in extension the AMOC water) to cool down.

Previous studies have shown that the properties of the boundary current water are strongly dependent on the eddy exchange, and therefore on the surface heat loss. However, it is not known how consecutive strong winters impact the dynamics of the Labrador Sea on various timescales, which will therefore be the focus of this thesis. Data for this research will be obtained by using an idealised model configuration of the Labrador Sea, where the hydrostatic primitive equations of motion are solved by the MIT general circulation model (MITgcm). Different types of scenarios are defined to analyse different effects on the dynamics. These scenarios are analysed by looking into how the mean basin temperature changes, how the eddy kinetic energy (EKE) and mixed layer depth (MLD) develop, and how the properties through a transect of the basin change. The effects of these interactions are then studied by looking at how the transport of water throughout the boundary current, per density class and per vertical layer change.

The thesis mainly shows that the mixed layer depth in the interior increases during a strong winter. As a result, the eddy kinetic energy increases significantly in the boundary current, as the horizontal density gradient increases, thus causing an increase in boundary current velocity in the downstream direction. Additionally, more and denser interior water accumulates, depending on how many consecutive strong winters occur. This deep convected water in the interior partly remains near the bottom of the basin. In the next winter, it is mixed again due to deep convection, consequently a positive feedback loop occurs. Meaning, that the number of consecutive winters positively impacts the interactions in the basin, as the horizontal density gradient increases, and thus the velocity and eddy kinetic energy increase as well, in respect to the previous winter. The effect of the strong winters persists in the years afterwards, as the interior remains relatively cold. Additionally, a part of the accumulated convected interior water resides too deep in the basin to be exchanged by the eddy exchange and therefore flows near the bottom out of the basin, due to a pressure difference. The flow near the bottom is a negative feedback loop, as the volume of dense convected water decreases and can therefore not be further cooled during consecutive strong winters. Finally, the properties and the transport of the boundary current water are directly related to the interior water and eddy exchange. As the MLD in the interior and eddies in the BC are still relatively large in the years after the additional surface heat loss, the export of boundary current water therefore also remains affected. In conclusion, the effect of wintertime surface heat loss on the Labrador Sea Water in the short term has the most influence on the MLD and EKE, however the influence of the MLD and EKE remains and therefore in the long term affects the export through the BC. These conclusions can help to better interpret the limited available measurements of the Labrador Sea Water.

Remotely Operated Vehicles (ROVs) are positioned relative to an underwater acoustic positioning system: Ultra-Short BaseLine (USBL). The accuracy is affected by refraction artifacts, caused by the variations of the sound velocity in the water column. Accurate positioning would require continuous measuring of the sound velocity profile (SVP) which is unpractical and preferably to be minimized. It requires alternative or inversion methods to obtain SVP information. In this thesis, two inversion methods are introduced: HISOM (Hull In Situ Ocean Model) and OMBES (Overlapping MultiBeam EchoSounder).

HISOM analysis whether the SVP can be approximated via other sources: 1) a constant profile based on the in situ surface (ISS) sound velocity, or 2) a profile derived from ocean model data in combination with the ISS sound velocity. These simplifications inevitably introduce a refraction error but for fallpipe ROVs operating at small incident angles (e.g. θ<6°) this error may be within acceptable margins (e.g. threshold of 0.2 m). The applicability is assessed by estimating the horizontal refraction error with a ray-tracing technique using daily-mean SVPs, derived from freely-available ocean model data. A spatiotemporal quantification for the North Sea yielded maps of sea areas where SVP measurements are necessary, and areas where constant surface SVPs suffice. The latter are the shallow parts of the North Sea (<80 m), where the error is always smaller than 0.2 m. For deeper locations, the gradients of the SVP cannot be neglected. Then, the model-based SVPs can be used with or without the use of ISS sound velocity data. Comparing a collection of observed SVPs in the North Sea revealed that these daily-mean model-based SVPs are accurate enough for the positioning of fallpipe ROVs at least up to 370 m depth.

OMBES uses the synchronous overlap in depth measurements between two dual-head multibeam echosounders (MBES). In previous studies, the overlap is obtained by sailing two adjacent tracks with one MBES on a ship. Here, we propose to use two MBESs deployed on the same ship, thereby reducing the uncertainty of the ship’s motions that affect the quality of the depth measurements. The mean sound velocity can be inverted by mathematically minimizing the depth differences in the overlap. This technique completely minimizes the refraction error with frequent updates of the inverted mean sound velocity on the flight. Subsequently, the mean sound velocity can be used to locate ROVs that operate close to the seafloor in shallow water (<80 m), even for large incident angles (e.g. θ~65°). Simulations showed that the best performance of the inversion technique is established when maximizing the distance between the multibeam heads, and by inward-tilting one or two heads. Practically, it means the deployment of MBESs on either side of the ship, rather than one pair of MBESs at mid-ship.

With increasing access to reliable ocean data, HISOM methods can potentially be run on ocean forecast model data to assess where and when refraction artifacts become dominant for fallpipe ROV positioning. This automatic assessment tool supports the SVP measuring strategy in subsea rock installation projects. OMBES can potentially be used as monitoring tool by comparing near real-time updates of the inverted mean sound velocity with the measured SVP. This data-driven decision tool can assist when to take an additional SVP. OMBES also improves the accuracy of multibeam bathymetric surveys by automatic collection of SVPs.

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.

In times of rapid urbanization, engineering interventions in coastal systems have become more common. It is important to understand the interplay of these engineered solutions with their natural environment to minimize hazardous side effects to our ecology, economy and coastal safety. In a numerical study by Rijnsburger (2021), a flow recirculation with a diameter of 10 to 20 km - similar to an eddy - was identified in the North Sea. It remained unclear what causes the recirculation, but its location in the Rhine region of freshwater influence and near the artificial Maasvlakte headland, suggest that the recirculation could both be baroclinic or barotropic. This numerical model study is set out to understand what processes drive the recirculation. Identifying these processes is important to understand what is required to properly model the hydrodynamics along the Dutch coast, and accordingly, the spreading of sediment, pollutants and phytoplankton.

A realistic 3D hydrodynamic model was used to assess the recirculation during a validated spring-neap cycle. Our analysis shows that the recirculation has a maximum diameter of roughly 15 km during neap tide and 5 km during spring tide. The recirculation occurs in the buoyant top layer of the water column and grows offshore from its onset north of the Maasmond around HW+3 until it is overtaken by ebb tidal velocities around HW+5. This study shows that the onshore advection of negative (clockwise) vorticity water results in most water flowing back to the river mouth into an expanding clockwise recirculation, the vorticity and strength of which are strongly influenced by buoyant river outflow.
Our analysis shows that the headland is not a prerequisite for its onset. Nevertheless, two mechanisms are identified of how the headland influences the recirculation. First, during spring tide, the headland is found to ‘shelter’ the recirculation from strong ambient currents in the midfield which would otherwise hamper the recirculation. Secondly, the geometry of the river mouth influences the vorticity input in the North Sea associated with the buoyant river outflow.

To further investigate what processes influenced by the river outflow could contribute to the recirculation, a scale analysis and particle tracking study were deployed. Although, the study is not yet conclusive on what drives the onshore advection of the spinning water that flows into the recirculation, several potential processes have been identified.

For future studies that want to model the spreading of freshwater, pollutants or suspended sediment near the river mouth of the Rhine-Meuse system, this study underlines the importance of using a three-dimensional numerical model that accounts for density differences and the influence of the studied recirculation. The latter is especially important when the boundaries of the numerical domain are located close to the river mouth, due to which the influence of the recirculation is not resolved unless explicitly accounted for in the boundary conditions. This study therefore contributes to the improvement of future modelling studies of the Rhine ROFI and to our understanding of the processes that govern the hydrodynamics along the Dutch coast.

The Labrador Sea is one of the deep convection sites in the world's oceans and the water masses formed here are an important component of the Atlantic Meridional Overturning Circulation (AMOC). To study this linkage, one study in particular used an idealized model of the Labrador Sea where the density variations consists only of temperature variations. In this study, it is questioned whether the assumption of neglecting salinity is appropriate, by analysing the pathways of water mass and water mass transformation in the Labrador Sea.
This is investigated by using that same idealized model (here called the reference run) and comparing this to a model where salinity variations are added whilst keeping density variations the same (Sconstant) to produce a similar circulation pattern. Furthermore, a model configuration is created which investigates if a seasonal cycle in salinity impacts the circulation pattern of the Labrador Sea (Sseasonal). The pathways of water masses in these model configurations are analyzed by Lagrangian particle tracking from A to B.
It was found that with the same initial density variations the maximum surface eddy kinetic energy (EKE) increases by 41 % when salinity is incorporated in the model. An increase in EKE is often associated with more water mass leaving the boundary current (BC) due to an increase in instabilities. Surprisingly, the opposite was found: 7.02 and 8.22 Sv are transported through the BC for the reference run and Sconstant, respectively. Furthermore it was found that most of the water mass leaves and re-enters the BC near the maximum EKE for each model configuration. An increase was found in maximum overturning in density space from an Eulerian perspective: from 3.9 to 4.8 Sv for the reference run and Sconstant, respectively, where about 10 % so called density compensation occurred for Sconstant. No significant annual changes are found when adding a seasonal cycle to the model. For all model configurations a large discrepancy exists between Eulerian and Lagrangian calculations in downwelling. This discrepancy is due to Lagrangian particles that reside in the models at the end of their simulation duration Thus the overturning in the Labrador Sea is significantly influenced by particles that have a long residence time (longer than 4 years in these model simulations). Between 22 and 25 % of the Lagrangian volume transport does not reach the outflow of each model simulation.
There are also properties that salinity did not influence: no significant changes were found between the model configurations for the overturning in depth space, the annual MLD and barotropic streamfunction. In conclusion adding salinity to the idealized model showed only minor changes in the pathways of water mass and water mass transformation: the order of magnitude of all analyzed properties stays the same. Density compensation however is neglected when no salinity variations are added in the model. This means that for a highly idealized model of the Labrador Sea, salinity variations can be neglected, when density variations due to salinity variations are represented by temperature variations.

KEY POINTS (I) A biophysical model framework (BMF) for corals is developed in which four environmental factors are included: (1) light; (2) hydrodynamics; (3) temperature; and (4) acidity. (II) The full feedback loop between corals and their environment forms the core of this model framework, where the morphological development is new and closes the feedback loop. (III) The developed BMF predicts the coral response to environmental input via (mainly) process-based relations within the accuracy of climate projections. (IV)- The BMF supports both the deep reef refugia hypothesis and the turbid reef refugia hypothesis. (V) The BMF contributes to the development of protection and recovery programs and is not site-specific. (VI) The BMF is developed for long-term predictions - in the order of decades to centuries - but runs on daily averages and is therefore applicable for assessing the response of corals on shorter time-scales; such as months to years. SUMMARY The increasing pressure on Earth’s ecosystems due to climate change becomes more and more evident. These pressures are especially visible at coral reefs. Therefore, a good understanding of the biophysical mechanisms controlling these ecosystems is needed, so that accurate predictions of their survival can be made. Such an understanding is also needed to develop efficient recovery and protection programs vital to the maintenance of these ecosystems. Because the research on marine ecosystems is relatively young and the phenomenon of coral bleaching is yet to be fully understood, there is no comprehensive framework in which the complex interactions between corals and their environment are combined. In this study, a biophysical model is developed in which four environmental factors are included in a feedback loop with the coral’s biology: (1) light; (2) hydrodynamics; (3) temperature; and (4) acidity. Literature from multiple disciplines is combined to find the interdependencies between the corals and their environment. These relations include coral growth, coral bleaching, storm damage, and recruitment/recolonization of corals. For the connection with the hydrodynamics, a coupling is made between the biological model developed here and Delft3D-FM. The composed biophysical model is a big leap forward in understanding the world of coral reefs, as it is the first construction of a model framework including four environmental factors in which the hydrodynamics are included in the feedback loop. Furthermore, it creates the ability to assess recovery and protection programs based on the four aforementioned environmental factors; e.g. the susceptibility of coral bleaching can be reduced by increasing the attenuation of light through the water column. Because more environmental factors have a role to play in the coral dynamics, the framework is constructed such that these can be added relatively easily.

The Atlantic Meridional Overturning Circulation (AMOC) is a key component in the Earth System. Given its important role in the climate system, variability in the AMOC strength is expected to have great impact on the global climate. The current observational timeseries are not long enough to make climate projections for the end of the century or even longer. Therefore, coupled climate models play an important role in the making of end of century climate projections on the AMOC strength. A known issue with the current generation of global coupled climate models is that the grid resolution is generally too coarse to resolve smaller scale processes such as mesoscale eddies. Observations and modelling studies suggest that mesoscale eddies play an important role in the exchange of water between convection regions and downwelling regions. When such processes are absent or parametrized incorrectly, it can have an influence on the climate projections based on these model simulations. This study analysed the AMOC characteristics in two different simulations of the Community Earth System Model; a reference simulation (referred to as piControl) and a simulation in which the atmospheric CO2 concentrations have been increased to four times the initial concentration (referred to as 1pctCO2). First, the AMOC characteristics in the piControl simulation are analysed using both a Eulerian and a Lagrangian approach. The Eulerian analysis shows that deep mixed layers, an indicator for convection, are present in the subpolar North Atlantic. Compared to observations and higher-resolution ocean-only models are these located closer to the West-Greenland coast. Strong vertical velocities are found over the continental slopes, especially over the steep continental slopes around Greenland. Second, the Lagrangian analysis showed the consequences of the coarse grid in the model. Only a single pathway around the subpolar gyre was observed. This implies that particles will experience convection while crossing the interior of the Labrador Sea, but only will experience downwelling when their individual pathway comes close enough to the continental slopes around Greenland. Furthermore, there is only limited exchange of particles with the regions north and south of the subpolar gyre. In the export of deep waters to the subtropical gyre, does it seem that the particles are being blocked by the North Atlantic Current. Third, the changes in the AMOC characteristics in the 1pctCO2 simulation are compared to the piControl simulation. In the 1pctCO2 simulation have deep mixed layers disappeared from the subpolar North Atlantic and convection in this region has shut down. A new fresh(er) surface layer in the Labrador Sea has intensified the stratification and prohibits the formation of deepmixed layers. Instead of the deep mixed layers in the subpolar North Atlantic have new deep mixed layers emerged between 30N and 40N. In general are velocities reduced in magnitude in the 1pctCO2 simulation, but the stronger vertical velocities can still be found over the continental slopes. In conclusion, the results show that the CESM model can reproduce the two components of the AMOC reasonably well, but the connection in the formof mesoscale eddies is missing. This illustrates that a overturning streamfunction simplifies the complexity of the overturning process. The overturning streamfunction is a measure for the overturning strength, but it does not take into account how and if the convection process and the exchange between convection and downwelling are represented.

The Weddell Polynya, a large hole in the Antarctic sea ice, reappeared in 2017. The polynya forms due to deep convection, which is caused by static instability of the water column. Observations and model studies show periodic heat accumulation in the subsurface layer prior to a polynya. This heat accumulation could be caused by internal ocean dynamics: the Southern Ocean Mode. Periodic subsurface heat and salt accumulation could be the major driver in causing periodic deep convection, which is in contrast with earlier studies. These studies focus on surface processes, and see the polynya as an irregular event. In this study a simple convective model is used to look into this contrast. Model simulations excluding and including periodic subsurface heat and salt fluxes have been performed. Multiple polynya events were only simulated in the model set up including subsurface fluxes. The dominant frequency for polynya events in these simulations equals the frequency of the subsurface heat and salt accumulation. This frequency is still visible in runs with white noise added to the freshwater flux, showing the importance and dominance of the subsurface forcing. In combination with earlier studies, this study suggests that periodic subsurface processes are most dominant and govern the initial formation and periodicity of the Weddell Polynya.

Thermohaline staircases are characterised by stepped vertical temperature, salinity and density profiles, which are formed and maintained by the double diffusion of heat and salt. Because double diffusion is the primary mixing agent in regions with staircases, it is the topic of extensive studies. Previous studies, however, are mainly theoretical and modelling orientated and observational evidence is needed to verify the results. In our study we use an extensive dataset with 460 vertical profiles of temperature and salinity. We found that staircases in the Caribbean Sea are related through temperature and salinity, indicating that
staircases in the Caribbean Sea are constant in time and space. Individual steps, however, differ and were characterised in four types: well-developed steps, transitional layers, inversions and absence of steps. A case study of a strong anticyclonic eddy gave the indication that steps are influenced by short term processes. The eddy induces lateral gradients and hereby positions the water masses in the interior and exterior of the eddy such that thermohaline
intrusions are initiated. The apparent preconditioning by the eddy, leading to thermohaline intrusions, allows us to speculate that the eddy is a catalyst in double diffusive diapycnal buoyancy transport.

Regular maintainance dredging is a large expense to the Port of Rotterdam, therefore the installation of sediment traps is considered. By increasing the bathymetry locally, an increase of local accumulation is expected and a decrease deeper in the harbour basin. This thesis describes the functioning of sediment traps in a stratified tidally energetic estuary, where sediment is supplied by the river and sea. A numerical 2DV representation is set up for the Botlek Harbour with the hydrostatic Delft3D software. A calculated salinity time-series by Operationeel Stromingsmodel Rotterdam (OSR) is combined with a measured water level time series for the same time period to describe the hydrodynamic boundary conditions. Simulations are done with a variable and constant Suspended Particulate Matter (SPM) time series boundary conditions, that were generated based on the measurements of De Nijs (2012). Flow expansion caused by the sediment trap reduces the flow velocity, but increases the turbulent kinetic energy locally. A reduction of bed shear stress is observed in the trap, except near the edges where an increase is observed. Density currents caused by salinity differences govern the vertical flow velocity distributions, while tidal filling causes the net exchange. The depth of the trap plays a significant role in the internal flow characteristics. The sediment trap changes the properties of the internal flow, which may change the hydraulic state of the flow, i.e. from supercritical to subcritical, resulting in large instabilities and even an internal hydraulic jump. Shallow sediment traps result in less frequent internal hydraulic jump and weaker jumps compared to deeper sediment traps. To investigate the dominant mechanism for the trapping of sediment in the sediment trap, a distinction is made between an erosion and a fluid mud scenario. The erosion scenario shows the largest agreement with the survey data, i.e. maintenance dredging data and Echosounder multibeam surveys, but contribute only marginally to the trapping of sediment. The fluid mud scenario yields the largest contribution to the trapping of sediment. The decreased amounts of accumulated sediment in the basin for substantial to lots of fluid mud behaviour may vary between 10% and 14%, depending on how this fluid mud behaviour is modelled. Various shapes have been tested on erosion and fluid mud scenarios. Both for erosion and fluid mud a more extreme choice of parameters might yield also more extreme results, this is however not considered realistic. The creation of an overdepth has shown to result in a marginal improvement in the capturing of sediment in an environment where erosion is important. A regular sediment trap decreases accumulation in the harbour basins by 2%. A trap twice as shallow increases this amount to 4%, but deepening the trap further may even enhance accumulation in the basins. A shorter or longer trap did not improve the situation. Installation of a sill did however result in a decrease of 6% compared to the situation without trap. It is concluded that this effect can be largely contributed to the internal flow properties and the presence of internal hydraulic jumps. The presence of an overdepth results in a significant improvement in the capturing of fluid mud flow. For the trapping of fluid mud flow, an overdepth results in 6% less sediment in the harbour basins no matter the depth or shape of the trap. The length of the trap did influence the accumulation in the basins. A trap twice as short shows an increase of 4 % of accumulation in the basins compared to a regular trap. A trap twice as long or installation of a sill results in similar accumulation in the harbour basins as a regular trap. For fluid mud flows, any type of overdepth decreases accumulation of sediment considerably. The amount of overdepth or the shape does not influence this. The length of the trap should be sufficient. A trap that is too short results in an increase of accumulation in the basins. For environments where erosion is important, only shallow sediment traps have proven to reduce accumulation in the basins. Traps that are too deep actually increase accumulation in the basins. A sill has proven to be the best measure for both mechanisms. If overdepth is desirable for navigation a shallow sediment trap is advised. This leads to a reduction of accumulation in the harbour basins for both erosion and fluid mud scenarios.

This thesis investigates the important physical processes related to underwater sound propagation due to offshore pile driving with the first commercial and numerical model called the Underwater Acoustic Simulator (UAS). To make adequately use of noise mitigation measures during offshore wind projects and to protect marine life the prediction of sound levels in water is important. The aim of this thesis is to assess if the UAS is suitable to give insight in the physics contributing to sound propagation and if the UAS is suitable to quantify sound levels in water for the application in future wind park projects. This is done by comparing the predicted sound levels with the UAS with the measured sound levels obtained during the construction of the Gemini wind park. The results obtained with the UAS identify the parameters related to the seabed properties as the most important for governing the attenuation of the noise levels as function of distance and frequency. In the UAS the seabed is modelled as a fluid. Especially frequencies < 800 Hz are sensitive to the choice of geo-acoustic parameters. These frequencies show a trend of underestimation of the sound levels compared to the acoustic data. An increase of the predicted sound levels is observed for increasing the coarseness of the sediment of the upper seabed layer and decreasing this layer thickness. The UAS parameters can be tuned to obtain sound levels that agree with the measured sound levels in the Gemini area. However, these parameter values do not represent the properties of a realistic seabed. Additionally, no evident choice of values exist to arrive at sound levels that agree with the acoustic data. It is concluded that the choice of values compensates for the exclusion of the shear effect in the fluid approximation of the seabed in the UAS. The exclusion of the shear effect in the representation of the seabed in the UAS is regarded as essential. Tsouvalas (2015) shows that the propagation of sound energy at low frequencies is governed by the shear of the soil. These lower frequencies propagate in the deeper parts of the seabed where shear effects play a role. Also, sound energy in the form of Scholte waves propagates at the water-seabed interface, thereby contributing to the sound levels in water. The reason the sound levels propagating at low frequencies is sought in two aspects. In a fluid seabed the geo-acoustic parameters are adjusted and account for the damping associated with shear, while the propagation of sound energy at these wave forms is not modelled. Another possible explanation is the trapping of the low frequency pressure waves in the seabed. Although the UAS is able to predict sound levels in the Gemini area that agree well with the measured sound levels, this is not due to the correct implementation of the physic regarding the seabed. As a consequence, the model is tweaked to arrive at the measured. The UAS is therefore not considered suitable for application in future wind park projects.

Large-scale changes in ocean conditions caused by installing and operating Ocean Thermal Energy Conversion (OTEC) have not been studied in depth. Conversely, the effect on an OTEC plant by oceanographic features are not researched in depth either. The aim of this research is to describe the natural patterns and variability of the ocean currents around Curaçao, an island in the Caribbean Sea and a potential OTEC deployment location. This research is carried out in order to assess possible implications for the OTEC industry. Ten years of data from the Mercator Ocean Model with a spatial resolution of 1/12˚ and temporal resolution of one day was analyzed. The model is forced by wind data from ECMWF. A strong current jet is found to dominate the flow from east to west in the Caribbean Sea. The jet is identified as the Caribbean Current and it is forced by currents in the north equatorial Atlantic Ocean and large-scale wind patterns. The Caribbean Low Level Jet, an intensification of the Trade Winds over the Caribbean Sea, is strongest in winter and weakest in fall. Consequently, the current jet is found to have a peak from December to March and a trough in October and November. The largest surface velocities of the order of 1 m/s are found along the coast of Venezuela, where wind-driven upwelling enhances surface flow to the west. Along the Venezuelan coast, subsurface currents to the east, in the opposite direction to the surface currents, are also found. The period from April to September is characterized by the meandering of the jet and the formation of large (diameter > 200 km) anticyclonic eddies that cause large local surface velocities. These eddies contribute to the great variability observed in the Caribbean Sea. The origin of these eddies has not been clearly identified. Due to upwelling, no OTEC system should be deployed more than 50 km south of Curaçao to avoid cold surface water decreasing the system's performance. Hydrodynamic forces due to the calculated expected maximum velocity of 1.8 m/s, induce stresses in the cold-water pipe that do not exceed the yield stress.