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

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

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

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

This research focuses on a new system for nourishing the Dutch coast with sand, which utilizes semi-autonomous sailing barges and an electrical crawler to continuously replenish the beach. Yearly, the Dutch government grants contracts to nourish the Dutch coast with volumes around 10.000.000 m$^3$. The Dutch dune coast protects the low part of the Netherlands against inundation from the North sea. This sand is dredged by Trailer Suction Hopper Dredgers (TSHD) on a depth of 20 m and transported to the coast. This transport causes a CO2 emission of about 190 kilo tonne a year. As emissions are expected to grow with increasing sea levels the Dutch government looks for ways to improve the high-energy demanding strategy that is currently used. Sweco introduced the Slow Sailing, and believes in the energy reducing concept and therefore initiates this further research. The general idea of this concept is to reduce the energy demand and emissions associated with traditional sand nourishment practices by using multiple barges that sail slowly and continuously nourish the Dutch coast. The aim of this research is to concretize this concept, in which feasibility of the system compared with energy, emission and cost calculations are central. To achieve this goal, the Slow Sailing system is compared to the traditional trailer suction hopper dredger, currently used for sand nourishment in the Netherlands. The comparison is based on a scenario in the Northsea. This leads to the main research question: What is the gain in energy consumption, emissions and cost for the Slow Sailing concept compared to classical nourishment strategies? In order to achieve this goal a review of current sand nourishment strategies was conducted. Next, the OpenTNSim model was adapted to perform energy calculations and enable a comparison of energy demands and emissions between the new system and the traditional trailer suction hopper dredger. Finally, a cost evaluation was conducted to compare the cost per cubic meter of sand for a 1.000.000 cubic meter nourishment project at the Egmond aan Zee nearshore in the Netherlands.

Compared to the traditional trailer suction hopper dredger, the Slow Sailing system showed a significant factor 15 decrease in energy demand in the sailing component of the dredging cycle. The Slow Sailing system with barges of 1500 m$^3$ was found to be the most energy-efficient and cost-effective option, with an estimated project cost of 3.00 euro/m$^3$, while the TSHD with 4280 m$^3$ was found to be the cheapest option, with an estimated project cost of 2.55 euro/m$^3$. In terms of CO2 emissions, the Slow Sailing system emitted significantly less CO2 compared to the TSHD, with a difference of 1000 tonnes CO2+ emission in a one million cubic meter dredging project. Overall, the results suggest that the Slow Sailing system is a more energy-efficient and environmentally-friendly option for sand nourishment of the Dutch coast.

This thesis aims to investigate the effectiveness of the contained dredging system (CDS) in avoiding turbidity during dredging activities. Turbidity is a common issue that arises during dredging activities and causes the sediment to be suspended in the water column, which can have negative impacts on the environment. The CDS is a novel way of dredging that focuses on avoiding turbidity, and it has been developed by Hydrex, an Antwerp-based firm. The research aims to determine whether the CDS provides a solution to the environmental problems associated with turbidity at contaminated sites. The research methodology involves conducting a literature study, theoretical analysis, and real-life testing of the CDS. The main research question is ”To what extend does the CDS create turbidity?”, and a set of sub-questions is formulated to answer the main research question. Based on the analysis and findings presented in this thesis, it can be concluded that dredging activities can have a negative impact on the environment through the generation of turbidity. Traditional dredging techniques such as the clamshell, backhoe, trailing suction hopper dredger (TSHD), and cutter suction dredger (CSD) all create turbidity to varying degrees. However, methods such as silt curtains, sedimentation ponds, polymer flocculants, pH adjustment, vegetation buffers, and bio-engineering can be used to minimize the spread of sediment and reduce turbidity levels. Furthermore, new dredging techniques have been developed to minimize turbidity. The contained dredging system (CDS) was developed to minimize the negative impact of dredging on the environment by focusing on avoiding turbidity by dredging within an air chamber. However, the theoretical analysis of the CDS showed that it creates turbidity when the critical height of the dredged wall is exceeded. The CDS was designed to create no turbidity, but the theory and calculations presented in this thesis suggest that it does create turbidity. In conclusion, the CDS, although developed with good intentions, does not fulfill its requirement of not creating turbidity. The device creates turbidity due to the low critical height of the vertical wall, which will collapse after dredging. However, the CDS could still be used in non-sensitive areas or in very polluted areas where accurate dredging is necessary.

Cutter suction dredgers (CSD) are highly vulnerable to large wave loading due to their stiff, spud-based mooring. Many mitigations targeted at reducing the resulting stresses on the spud and other critical operating components have been proposed in recent years, most notably by adding a level of flexibility to the connection between the pontoon and the spud for rotational motions. Investigating the merits of the implementation of a flexible spud keeper however are hindered by the lack of accurate, CSD-specific models that are able to simulate real waves in the working area, as well as the multibody system of a CSD and its complex interactions with the sea floor. In this thesis, an ANSYS AQWA model is developed to conduct an operability analysis of a CSD in operation in offshore, coastal conditions. Three modelling objectives were established to provide a complete analysis of the effect of increased flexibility in a spud keeper: adding non-linear wave effects, improvement of the soil boundary conditions, and building a dynamic multibody system of a CSD in ANSYS AQWA.

A wave model is developed that expands upon standard Linear Wave Theory (LWT) through the use of Stokes 2^nd Order Wave Theory (S2). S2 was found to be valid for a larger range of water depths and wave parameters than other wave theories, mainly in shallow, coastal conditions, which is the main operating area of a CSD. These non-linearities cannot be implemented into a frequency domain (FD) model, therefore a time domain (TD) model is required. A combination of S2 waves in a JONSWAP spectrum is used to simulate the motion and force response in TD, as well as second-order forces from the interaction of wave groups with the CSD. The model output is then used as a means of verification of these assumptions and to show the effect of the wave shapes on the spud bending stress.
An altered spud model is proposed for shallow, wide spuds, using a rotational spring system instead of the commonly used clamped model. For the use-case of a CSD it shows a spud's tendency for rigid motion below the mudline, while also providing an upper operational limit for soil failure around the spud. The model output showed this upper limit to likely not be a important operational limit, but it can be used to investigate spud rotation below the mudline. Mooring effects from the cutter are absent in older models, but are added in the form of spring elements to better approach reality.
Finally, a number of CSDs with different levels of flexibility are tested for four operational limits. The model shows a clear shift in motion resonance towards lower frequencies when the stiffness is decreased, while operability analysis in TD presents a significant increase in operability when a CSD is free to move in pitch. This completely flexible connection provides a major reduction in spud bending stress, but is governed by the acceleration limit of the pontoon. Other reduced stiffness designs are shown to only provide limited benefits to the operability.

In the field of mining and dredging engineering, oftentimes slurries are involved in transport. Understanding the behaviour of these slurries in a flow is important as the energy and water consumption need to be determined. The slurries have an interesting rheology due to the presence of clay. The rheological parameters cause the flow to be non-Newtonian. Further, the presence of coarse solids (sand particles) in these slurries also influences the rheological parameters. Through Computational Fluid Dynamics (CFD) simulations, these aspects can be predicted.
Previous work in non-Newtonian CFD with coarse solids made use of a free surface flow through a rigid-lid approach (van Rhee, 2017). A shortcoming of this is that the flow needs to be uniform and the flowdepth needs to be known beforehand. In the mining and dredging fields, this is not the case and a different approach is required.
This report will focus on multiphase CFD simulations. One of the fluids will be a non-Newtonian model including sand particles, and the other fluid will just be air. The sand particles will be subjected to transport, segregation, and settling behaviour.
OpenFOAM is the weapon of choice, but as it stands it does not have a solver that’s capable of including sand particles. The interFoam solver is chosen as a starting point and it’s code is adjusted. A Bingham Plastic transport model is implemented, including sand particles. A sand transport equation is also implemented.
Using the adjusted interFoam solver, two sets of simulations are ran. A first set of simulations is performed using a 2D mesh for an open channel. The first simulation utilizes a Bingham Plastic fluid without any sand particles included. The resulting velocity profile is compared to the analytical solution and is found to agree well. The second open channel simulation includes sand particles. The results show a sand bed forming as well as the sand concentration to be roughly constant throughout the plug flow. This is compared to experimental work and concluded effects is captured well from a qualitative perspective.
The second set of simulations is performed using a 3D mesh for a pipe section. These results were not as satisfying as the results for the 2D open channel. Unfortunately, all 3D pipe simulations either showed the pipe to fully fill up with the Bingham Plastic fluid, or the solver crashed at some point. Many attempts have been made at this, and no conclusive reason has been pointed out as the cause for this.
It’s recommended to continue research in this direction with the main focus being the simulation stability.

The Port of Rotterdam experiences large amounts of siltation from the upstream rivers in its basins and canals every year. Furthermore, container vessels increase in size every decade and therefore require a larger navigational depth. These two factors cause the yearly amount of dredged material to increase. Currently, mainly trailing suction hopper dredgers are used to remove the large amounts of sediment, but these vessels come with high costs and operational time. A more time and cost-efficient dredging method is therefore desired.

A potential solution to the increasing problems of costs and operational time by dredgers is the technique of water injection dredging. To make this technique as efficient and effective as possible, this thesis has the objective to measure and analyze flow and sediment properties for different parameter settings of water injection dredging and find the optimal parameter settings of this dredging technique on mud from the Port of Rotterdam. Large-scale experiments have been conducted in the water-soil flume of Deltares, where a jetbar was trailed (or run) over a bed 27 meters long and 0.5 meters deep of port mud while injecting it multiple times with water.
A positive correlation is observed between the production rate and jet momentum. The production is related linearly with the jet momentum using the Vlasblom equation combined with a non-dimensionless empirical fitting parameter per run per traverse velocity.
What is remarkable is that the intrusion depth by the jets increases with ascending runs while the jet parameter settings stay the same. The data shows that the difference between the mass flux by the density current and the mass flux stirred up by the jets when the sediment concentration of an undisturbed bed is assumed, increases between runs when an increased intrusion depth between those runs is observed as well. This indicates that the volume penetrated by the jets contains a smaller amount of sediment than was initially assumed, thus is disturbed by the previous run and is therefore decreased in strength. This decrease in strength results in a larger intrusion depth during the run itself in comparison to the previous run.

The influence of the SOD of the jets is analysed by comparing runs with a SOD to runs without a SOD but with similar remaining jetting parameter settings. When a SOD is applied, the jet pressure applied to the bed is outside the flow development region and therefore the jet pressure decreases with distance from the jet nozzle. The mass of sediment stirred up by the jets, for a SOD of 300 mm, is lower in comparison to a SOD of 0 mm. The density current transports relatively more sediment, however, when a SOD of 300 mm is applied. So, if a large amount of sediment needs to be stirred up, and therefore a large intrusion depth is required, no SOD should be applied and when large horizontal transport by the density current is desired, a SOD outside the flow development region of the jets should be used.

Digital Soil Mapping (DSM) of soil types in geotechnical project areas is a top priority. These maps are often used in decision making and can have significant consequences related to costs and risks. Usually, these maps are generated by digital soil models that interpolate soil types at known locations. In practice, conventional spatial interpolation techniques are still often used for DSM of soil types, such as inverse distance weighting and kriging. However, conventional models are not well suited for predicting or interpolating soil types because of their inability to deal with categorical data properly. Besides, the design of the conventional models does not allow for incorporating the abundance of meaningful covariate information that is available nowadays. The flexibility of machine learning algorithms vanquish both problems and has become increasingly popular for DSM of soil properties in recent years. The results of machine learning techniques for DSM of soil properties are promising and generally outperform conventional models. However, few studies have used machine learning for DSM of soil types and is therefore still a relatively unknown field. Moreover, at the time of writing, there are no studies that use sequence models for DSM of soil properties or types. Hence, the author proposes to introduce a new method for DSM of soil types, namely a Long Short-Term Memory (LSTM) network. The intuition behind this introduction is that the spatial correlation can be captured in sequences and can improve soil type prediction.

Real project data from a cable burial project is used to evaluate and compare the performance of the conventional interpolation methods triangulation and kriging, the machine learning models random forest and XGBoost, and the newly proposed deep learning model LSTM. The project data consist of 757 vibro cores (VC), 718 cone penetration test (CPT), bathymetry data and sub-bottom profilers. The geotechnical data, i.e. VCs and CPTs, is received on separate PDF pages that require to be digitized first. This thesis describes a simple yet precise manner to extract this data from the PDFs. The VCs and CPTs are provided with a soil type interpretation and can be used directly for developing the models. The data is split into a training set to develop/train the models and a test set for evaluation. Ultimately, the best performing model is used to build a 3D stratigraphic soil model for the project area with associated prediction accuracies.

All state-of-the-art techniques outperform the conventional models and especially in predicting minority classes. The best performing model is random forest with an overall accuracy of 85.44\% and is comparable to the performance of XGBoost of 85.11\%. LSTM network achieved a slightly lower accuracy of 84.27\%. The results show that LSTM is suitable for DSM of soil types and has considerable potential for improvement as only a few possibilities of the model have been examined.

Subsea rock installation is a process in offshore engineering where rocks are placed on the seabed or subsea structures using a fallpipe. An example of a subsea structure could be a cable or pipeline that must be protected. In this research, the main scope is rock installation for scour protection of the foundation of offshore wind turbines. As the US government is planning to build 30 gigawatts of offshore wind capacity by 2030, many rock installation projects are planned. This is where GLDD wants to contribute by being part of a more sustainable future by building a subsea rock installation vessel. This vessel with a 20.000 ton rock capacity is placing rocks using a solid fallpipe. Accurate knowledge of how rocks are placed using a fallpipe is necessary to plan, manage and estimate the costs of a project. The main focus of this thesis project is to establish an optimal computer model for an inclined fallpipe for subsea rock installation. This model can be used to calculate rock velocity during operating the inclined fall pipe (IFP). Additionally, the particle concentration and the distribution of particles over the pipe’s cross-sectional area can be determined. The velocity of particles is essential for future models calculating where the rocks settle down after leaving the fallpipe. Based on the literature two models were made: Vertical fallpipe model 1 (VFM1) and Sliding bed model 1 (SBM1), these models are improved based on data and observations from lab research. These improvements lead to Vertical fallpipe model 2 (VFM2) and Sliding bed model 2 (SBM2). In the lab research, a scale model of a fallpipe is made. This is done by placing a transparent fallpipe in a 5x2.5x2 meter tank of water. This fallpipe is attached to a conveyor belt, making it possible to precisely control the amount of rocks per second placed in the fallpipe

Particle-driven gravity currents cause major geological problems. Turbidity currents are highly erosive and can be damaging to structures on the sea bottom such as telecommunication cables. Understanding the mechanisms of sediment transport and deposition is required to predict the erosive powers of turbidity currents (and of the distribution of turbidite deposits) which are fully dependent on the behavior of gravity currents. For this reason, the main question of this thesis was formulated: Which physical parameters of the gravity current are of importance for its behaviour?

The lock-exchange release experiment is a frequently used method to study gravity currents in a laboratory and was also used in this thesis. In order to answer the main question, the following parameters were investigated and their influence specifically on the 4 phases, the run-out length and the PSD: particle size, bed roughness and temperature. The influence of particles size was researched using mono-dispersed vs bi-dispersed experiments. In the bed roughness experiments, sandpaper was attached to the bottom and compared to smooth bed experiments. Finally, to investigate the influence of temperature on the gravity current, experiments with warm water were compared to experiments with colder water.

From these experiments, the most notable results are summarized below.
PSD : For all experiments applies that at low concentration the particles segregate over the run-out length of the gravity current. Smaller particles travelled further than the bigger particles with a higher settling velocity. This does not occur at higher concentrations and the PSD over the entire run-out length is similar.
Four Phases: In all experiments, the four phases could clearly be identified with one exception: the first phase in the rough bed experiments was difficult to distinguish.
Run out length: Some interesting findings were made that were in line with literature: adding fine particles to the mixture of the current cause the run-out length of the current to increase. However, it was also found that if the initial concentration is increased, this effect decreases.
Furthermore experiments showed that an increase in temperature can cause the current to travel less far when compared to experiments performed with water with lower temperature.

In the light of this research, the following recommendation are made:
Temperature should be taken into account for modelling gravity currents. Otherwise this can lead to an overestimation regarding the run-out length and an underestimation of the deposit density.
Furthermore, to get more insight in the effect of the particle sizes in the currents, it would be highly recommended to conduct more experiments with a greater difference between particle sizes. This would allow for a better assessment of the magnitude of the effect of hindered settling

On request of Royal Boskalis Westminster N.V. a comprehensive research was performed regarding emission free maintenance dredging in a harbour environment. The project site is the Maasmond at the port of Rotterdam. It covers an area of almost 10 km2 and, on average, a monthly volume of 400.000 cubic meters of sediment needs to be dredged. The operations are currently performed using trailing suction hopper dredgers (TSHD). Several new fully working emission free concept work methods were designed. These were assessed using a multi-criteria analysis, where emphasis was placed on energy reduction, reliability, interference, risk and safety. Given the scope of this research, costs are not decisive. General conclusions for the solutions contain the splitting of the total process. As the energy consumption of a conventional hopper is too high to operate on a battery cell, the work method is split into three different processes being:
(i) gathering, (ii) pumping and (iii) transportation. Two work methods scored best in this research, Sloped Water Injection Dredging (SWID) and the Fully Autonomous Submerged Dredger (FASD). SWID consists of Water Injection Dredging vessels, fixed structures and autonomous barges. FASD contains the design of a submerged dredging vessel. It can be concluded that a harbour environment is suitable to perform emissions free maintenance dredging with only small alterations to the current technology.

Siltation of fine (cohesive) sediment in navigation channels and harbour basins may cause serious problems for the shipping industry. Intensive maintenance dredging is required to ensure safe shipping traffic. Conventional dredging methods are highly expensive, and therefore port authorities seek for more efficient solutions to reduce costs. One of these solutions is water injection dredging (WID), which in general, proves to be cheaper than hopper dredging by leaving the sediment in place, thus, eliminating substantial costs for relocation of the dredged sediment.

WID is generally applied on soils, which show high concentrations of fines in their particle-size distribution. These kinds of soils, known as fluid mud, exhibit non-Newtonian behaviour. An extensive rheological analysis of fluid mud from the Caland Canal (Port of Rotterdam, The Netherlands), confirmed that fluid mud does indeed show non-Newtonian behaviour. Flow curves obtained by a rotational rheometer provided evidence that fluid mud can be classified as a visco-plastic fluid and furthermore exhibits thixotropical behaviour. The focus of this study is on gaining more insight into the influence of this non-Newtonian behaviour on fluid mud density currents created by WID using detailed numerical simulations.

The numerical simulations were carried out by a variable density 3D computational fluid dynamics (CFD) model. Visco-plastic behaviour was added through the Bingham-Papanastasiou model, in addition, two rheological models, relating the Bingham model parameters to the volumetric concentration of solids were used to capture the rheology of fluid mud. Numerical results were validated by new experimental measurements performed in the water-soil flume at Deltares. Comparison was made between a regular simulation, without rheology, and the two rheological models for different magnitudes of viscosity regularisation. The main difference between the two is the formation of a new bed layer due to the influence of yield stress. This is caused by the relatively high (apparent) viscosity, which is approximately two orders of magnitude larger, decelerating the density current. Apart from this, a sharper density gradient and a lower density current are developed as a consequence of the non-Newtonian rheology.

A 3D dynamic numerical model is established for modelling the excavation process for dredging purposes. The interaction between the solid and fluid phases is realized by a specially designed DEM-FVM coupling mechanism, where the fluid-particle interaction forces, the volume fraction information and the particle information are constantly updated and exchanged. Dry and underwater sand cutting simulations are conducted and validated against experimental results. Simulation results of cutting of cohesive soil in atmospheric condition match with the experimental data within acceptable error margin, while the underwater cutting simulations of cohesive soil have not been validated due to the lack of experimental data. Besides, the general applicability of using Discrete Element Modelling (DEM) to create rock samples, and the calibration of DEM rock samples have been investigated, which are essential for conducting atmospheric and underwater rock cutting simulations in the future.@en

Turbidity currents are common in the waters around the world. They can be caused by earthquakes, collapsing slopes or other geological disturbances. However, turbidity currents can also be caused by deep sea mining activities. Deep sea mining is done with special mining vehicles. There are three main operations within this mining vehicle: collecting nodules, separating the nodules from the mixture and discharging the mixture of sediment and water towards the environment. After impingement of the discharged flow with the seabed, it is expected that the flow will take the form of a turbidity current. Turbidity currents belong to a larger class of flows called gravity currents. Furthermore, turbidity currents are typically defined as dilute flows in which particles are dominantly supported by fluid turbulence. These currents have an interstitial fluid that is a liquid, generally water. The objective of this research is to increase knowledge of the behavior of these currents and to obtain experimental results that can be used for validation of CFD models. In this research the influence of the initial concentration on the dispersion, deposition and entrainment of the current is investigated. In addition, the spread of particles within the current is researched. Furthermore, different types of sediment are used with various particle size ranges and particle properties. The experimental methodology can be divided in two parts. The first part are experiments that involve a full-depth lock release of a fixed volume suspension of a sediment with different particle size ranges and particle properties into water. The initial concentration is varied and the obtained current is recorded with a high-speed camera. The second part consists of multiple calibration procedures. During these procedures, different concentrations of sediment are mixed with water and the resulting solution is recorded. This data is used to create a calibration function that in turn can be used to quantify the concentrations of sediment in the previous recorded currents. The experimental methodology differs from previous work due to this calibration procedure. Afterwards, video processing is used to perform an analysis of the current. The resulting turbidity currents go through three phases as observed in the experiments and previous work. At the start, there is an initial phase where the front is formed and the current accelerates. During this phase a limited amount of particles settle out of the current. Afterwards, the current transitions into a second phase when the velocity of the front starts to decrease due to inertial forces and due to the start of particle settling at the rear of the current. The last phase is when the backflowing bore created by the ambient fluid at the release of the gate reaches the front and decelerates the current until viscous forces start to dominate until the current vanishes. The experiments shows that currents transporting fine particles reach higher velocities and may travel for longer distances in comparison to currents composed of larger particles. Furthermore, the combination of fine and large particles has a substantial effect on the currents dynamics. The currents composed of a mixture of these two sediment types travel longer distances than currents with only large particles. The performed experiments also focus on varying the initial concentration and sediment type. These show that larger particles tent to entrain more with the ambient fluid in comparison to smaller particles and thus they have a larger vertical dispersion. Furthermore, currents composed of sediment with only fine particles will have a more rapid transition into a dense front and a less dense middle and rear pat of the current. Currents composed of fine sediment and a combined sediment with small and large particles have a clear division between a dense basal layer and a less dense top layer within the current. At last, deposition patterns are different per sediment type. Sediment composed of larger particles will have a larger deposition rate than sediment composed of smaller particles. Furthermore combining both sediment types will increase the deposition rate at the beginning of the current.

The research focuses on sedimentation in reservoirs. Subjects include settling distribution in a reservoir, dredging and flushing of sediment in reservoirs, impact of sedimentation on reservoir performance and financial effects of long term sedimentation in reservoirs.

Dredging is an energy-intensive operation and, due to the nature of the process, there are large and rapid fluctuations in the power requirement. With the signing of the Paris Agreement, implementation of IMO 2020 and expansion of ECAs, the external pressures for the reduction of different emissions(CO2, SO_x, PM, and/or NO_x) in dredging are rising. Additional motivating factors are the rise in the fuel expenses which form a major component of dredging project costs and the incentives from regulatory authorities to reduce the carbon intensity in dredging operations. Often, the achievement of one objective leads to deterioration of another, for example, the use of IMO-compliant fuel can increase the overall carbon emissions. In recent years, alternative fuels like LNG and biofuels have been explored. However, they suffer from their own set of issues and with the predicted trends, the usage of these alternative fuels would imply lower production and earnings, especially in large dredging projects. In this work, a marine power plant concept that has been rarely discussed in the context of dredging is explored and forwarded: a nuclear-based system. Fundamentally, such a power plant addresses the issues related to the emissions and essentially eliminates bunkering stops. This was the first study focused on nuclear-powered Trailing Suction Hopper Dredgers (TSHD), the most common type of dredging vessel. In this work, a system-level study was carried out to ascertain the retrofittability of a nuclear-based system on four existing TSHDs. The feasibility of retrofitting the nuclear-based system has been studied by comparison of mass and volume requirements of the nuclear power plant, with the mass and volume of the engine and fuel storage system of current dredging vessels. No re-design of the vessel was considered here.The ”inherently safe” High Temperature Gas-cooled Reactor (HTGR) with Nuclear Air-Brayton Cycle (NABC) was determined as the nuclear power system of choice. It appeared that for such a system,the TSHD sizes that are interesting for the deployment starts around 12000 m³ hopper capacities.The bigger the hopper capacities than this baseline, the better the nuclear system performed. It was found that despite the satisfaction of the mass and volume constraints, a redesign of the TSHD is required for the placement of the reactor and for the compliance with the nuclear related regulations.In addition to the nuclear power plant, the retrofitting of the TSHDs with Proton Exchange Membrane Fuel Cell (PEMFC) in combination with solid, compressed and liquid H₂ storage and batteries was considered. With the current commercially available offerings, PEMFC with liquid or 500 bar compressed H₂ storage were found to be suitable for maintenance dredging or capital dredging for a short duration(couple of days). However, it was established that the realisation of endurance level of current dredgers is not possible without a reduction of hopper capacities or factorial increase in energy density of storage. Further, the smaller TSHDs were found to be better suited to use PEMFC or battery-based systems.A part of this work also tried to answer the pertinent question of the third party liability insurance premiums for a nuclear-powered vessel and the regulations such a ship would be subjected to. Further, a preliminary business case was developed and the sustainability of the concept was evaluated. It was realised that the technological forces and trends like the development of Small Modular Reactors, deep-sea mining and autonomous ships, could favour the development of a fleet of nuclear-powered dredging vessels in the future. However, the regulations and the support for these vessels would be highly dependent on the flag country and operational location.

Dam reservoirs form a crucial part for human society storing water, controlling floods, providing hydropower, water for irrigation and drinking. Annually 1% of the worldwide dam reservoirs storage capacity is lost, caused by sedimentation. The inflow of sediment and reduction of flow velocity and turbulence in the reservoir pro- vides favourable conditions for settling. Several sediment transport mechanisms are responsible for this, one of these is the turbid density current. The turbid density currents settle as the reservoir becomes wider, and it is affected by forces along the top and the bottom of the current. Recently, focus on reservoir engineering has shifted from primarily structural dam design towards complete sediment management strategies. In order to improve the sustainability of dam reservoirs, many management techniques are developed that inhibit or mitigate sedimentation. However, the effectiveness of these techniques is not yet known. This thesis provides an additional concept for sediment management in dam reservoirs consisting of channelling of turbid den- sity currents in dam reservoirs. The channel provides controllable parameters. The aim is to study the effects of channelling turbid density currents.
The study starts with a literature review, to describe sedimentation, sediment transport, and turbid den- sity currents in dam reservoirs, including their analytical and numerical descriptions.
Two computational models study the concept: a steady-state model and a numerical model. The steady- state solution and is based upon an equation for open channel flows modified for turbid density currents. This model is used to investigate the effects of hydraulic radii and slope of the channel on the turbid density current — secondly, the dynamic numerical solution. An analytical description is provided using the one- dimensional shallow water equations, consisting of the continuity, momentum and particle conservation equations. The solution includes four sources: deposition, erosion, gravity and friction. It omits water en- trainment and bed deformation. The model is discretised using the Generalised Lax Friedrichs method. First validation and investigation of the quality of the source terms are done. Subsequently, the model, including the four source terms, is used to study the effect of slope, hydraulic radii, concentration and sediment size in the channel. Expanding the numerical study by a Water Injection Dredging case in which local velocity, concentration and height are increased along a certain length to study possible effects.
To conclude, channelling turbid density currents is a viable solution to improve sediment transport. The slope and depth of the channel have the most significant effects. The generalised Lax Friedrichs method provides a valid and straightforward discretisation method for the numerical model. Furthermore, the model provides an easy, quick and simple to use tool to make first estimations of the effects of channel dimensions.

Understanding the erosion process of sand will help improve dredging projects. Pickup functions are used to predict the amount of sand that can be eroded by a flow with certain parameters. The most common functions that are used have been validated to data in the low velocity regime (θ < 1). However, modern day dredging processes are in the high velocity erosion regime (θ > 1).An example of such a function is the widely used pickup function of Van Rijn (1984). It predicts the pickup of grains well at low flow velocities, but overestimates pickup at high flow velocities.Recently Bisschop (2018) has performed erosion experiments at high flow velocities. In 2015 Boskalis performed experiments in the same velocity regime during a preliminary study, however, a different setup was used. It was found that there is a large (factor 2) discrepancy between the two studies.The present study has performed experiments with a setup similar to that of Van Rijn (1984), however, the flow velocity was not limited to the low velocity regime. Erosion tests were done with bed shear stresses up to 60 Pa (θ<30). The setup in question uses a sediment lift to push a prepared sand bed up into the flow.The determination of the bed shear stress was questionable in preliminary study of Boskalis. Since it is a very important parameter in erosion predictions, it was a focus of the present study to determine the bed shear stress accurately. The bed shear stress was determined with three different methods which all agree within an acceptable range (up to 8%).The erosion velocity is determined by measuring the lift velocity, which is equal to the erosion velocity when the top of the sand bed is level with the floor of the setup. Whether the top of the sand bed is level or not has been determined with the use of a high speed camera.The produced data has been compared to existing data and pickup functions of other studies. From this comparison it is clear that no existing functions match the data of the present study.In the present study an analysis is done to find out why the new data does not match existing functions. Also, based on several existing functions, a new function is presented that has a good fit with the data of the present study.

A lot of uncertainties exist about the impact of deep-sea mining on the Benthic environment. One of these uncertainties concerns the sediment plumes created by the mining operation. The return slurry released above the sea floor creates a large plume of sediments. With numerical models the behavior of such plumes is modelled in order to minimize these uncertainties. Particle size distributions in these plume models are currently based on individual particle sizes. In situ, at the bottom of the ocean in an salt water environment, flocculation can occur. Flocculation can have effects on the particle size distribution, settling rates and floc structure. The objective of this research is to investigate the significance of flocculation on the settling behavior and if it should be taken into account in these plume models. Shear rate and particle concentration, as function of time, are the most governing processes that have effect on the flocculation behavior. The influence of these parameters on the flocculation process are investigated. Different methods are used in the search for a suitable technique to measure the in situ particle size distribution. Next to the particle size distribution the floc shapes, structure and settling rates are measured under the influence of the factors mentioned above. In the shear range of 6 s-1 and particle concentration of 0.7 to 40 g/l flocculation does occur and has a large impact on the settling behavior of the sediment. Flocculation at other shear rates and particle concentrations remain unknown. Figure \ref{fig:vol_dem_salt_floccu1} shows the time until 100 $\%$ of the mass is settled when the suspension is released at a height of 10 m. This graph shows the difference in settling time between a suspension in demi and salt water. A settling time difference of 85 days (25 days for salt water and 110 days for demi water) shows the importance of including flocculation behavior in plume models.