Recent advancements have demonstrated that collectors based on the Coandă effect can effectively harvest polymetallic nodules from the seabed. However, the hydrodynamics of the flow around such collectors, particularly the mechanisms of ambient water entrainment, remain insufficiently explored. To address this gap, we performed three-dimensional numerical simulations to investigate the flow characteristics surrounding a Coandă effect-based collector, focusing on the effects of main jet velocity, secondary jet velocity, radius of curvature, and bottom clearance. The results show that increasing the main jet velocity enhances flow attachment and strengthens the pressure gradients beneath the collector, thereby increasing the entrainment of ambient water into the collection duct. Similarly, higher secondary jet velocities improve flow attachment and raise the collection duct flow rate but also lead to greater sideways water spillage. Furthermore, a larger radius of curvature reduces sideways spillage, consequently promoting greater ambient water entrainment beneath the collector. Likewise, increasing the bottom clearance enhances ambient water entrainment. Overall, these findings provide valuable insights for optimizing the operational parameters of Coandă effect-based collectors to maximize collection efficiency while minimizing water spillage.

Understanding the behavior of turbidity currents is crucial for the effective and sustainable management of natural and artificial hydraulic systems. This study employs a high-resolution numerical model based on the Large Eddy Simulation (LES) approach to investigate the effect of bed slope on the dynamics and depositional behavior of turbidity currents interacting with a triangular obstacle in a channel. Six bed slopes were studied ranging from 0% to 4.5%. Our analysis focused mostly on the quasi-steady-state flow conditions upstream of the obstacle. The results reveal that steeper slopes enhance sediment transport capacity, leading to reduced sediment deposition rates along the bed and thus a decline in the obstacle's sediment-retention efficiency. The increased transport capacity primarily results from higher flow velocities rather than increased sediment concentrations. Detailed analysis of velocity distributions upstream of the obstacle, under quasi-steady state, showed that the velocity profiles are distorted differently among bed slopes as a result of the interplay between flow inertia and the adverse pressure gradient induced by the obstacle. Recirculation zones are observed for the milder bed slopes (0–1.5%), whereas these zones disappear for steeper slopes (3–4.5%), indicating the dominance of inertial effects.

This study investigates the hydrodynamics of intrusive bottom-propagating gravity currents at channel confluences using large-eddy simulations, emphasizing interactions between buoyancy-driven flows and the ambient momentum of the main channel. The analysis systematically examines how the velocity ratio (β) between confluent flows regulates transport processes and turbulence dynamics during the transition from quasi-planar to fully three-dimensional gravity currents. Three distinct flow regimes are identified: buoyancy-dominated (β ≤ 0.47), transitional (0.47 < β ≤ 1.42), and momentum-dominated (β > 1.42) cases. In buoyancy-dominated cases, gravity currents form nearly axisymmetric, radially spreading fronts characterized by lobe–cleft patterns and coherent vortex rings. A distinct transition occurs beyond β = 1.42, where momentum dominance suppresses lateral spreading and promotes rapid downstream reorientation. Within this regime, vortical structures at the mixing interface evolve from laminar sheet-like layers to tube-shaped and arch-like vortices, eventually breaking down into smaller-scale turbulence. Streamwise-oriented vortices emerge as the dominant coherent structures within the mixing layer, resembling those found at natural river confluences, where momentum contrasts between converging flows sustain longitudinal vorticity. Despite geometric and dynamic differences between density-driven intrusions and homogeneous confluent flows, the interfacial vortical evolution exhibits notable similarity, suggesting that shear-induced instabilities primarily govern mixing dynamics.

Square piles are widely utilized in coastal engineering due to their economic efficiency and robustness in resisting large forces in the coastal environment. However, the removal of sediment particles due to the approaching flow around such structures, known as scour, raises concerns about the stability and safety of the structure. Therefore, this study investigates scour around square piles placed at 45 deg and 90 deg angles in wave–current flows. A newly developed sediment transport module within the open-source REEF3D framework is developed, incorporating a three-phase semicoupled approach with level-set method (LSM) for realistic representation of sediment bed and free surface interfaces. The developed model is first validated against experimental results of circular and square pile scour in different flow conditions, such as steady current, wave-only, and wave–current flows. Furthermore, the effect of the combined wave–current parameter (U_cw) and Keulegan–Carpenter (KC) number on the normalized equilibrium scour depth (S/D_w) is explored. This study provides new insights into how square pile orientation modifies bed topography and equilibrium scour depth in wave–current flows. Numerical results demonstrate that a higher S/D_w value was observed for larger U_cw and KC numbers for both piles. It is revealed that in wave-only and combined wave–current flows with low KC numbers (KC < 10), square piles oriented at 45 deg experience greater scour depths than those oriented at 90 deg. However, at a higher KC number (KC = 18), square piles oriented at 90 deg exhibit greater scour depths compared to those at 45 deg.

The cutter suction dredger (CSD) is one of the main vessels utilized in the dredging industry. The dynamic actions related to its rotating cutter head are the main trigger for sediment release and turbidity generation by this vessel. The ability to predict the evolution of this turbidity and suspended sediment concentrations is imperative for effective environmental management. This predictive capability allows ecologists to estimate potential damage, enabling environmental managers to propose appropriate mitigation measures. In this study, we conducted a qualitative numerical assessment of the characteristics of turbidity currents generated as a result of cutter suction dredging of densely-packed sand. To this end, we developed a one-dimensional physics-based model, providing the order of magnitude of sediment fluxes and concentration levels. In addition, a quantitative sensitivity analysis is performed to unravel the relative influence of key operational parameters on the generated turbidity currents. The results of this research reveal that breaching (dilative underwater slope failure) is a major source of sediment release by CSDs and should be incorporated in the source-term estimation. It is also found that the cut ratio is the most influential operational parameter on the generated turbidity.

Very limited research has been carried out to investigate sediment erosion caused by subaqueous inclined water jets, despite the fact that such water jets are used in subsea engineering (e.g., dredging, trenching, and deep sea mining). Therefore, we conducted a set of novel small-scale experiments to primarily study the effect of jetting inclination on cohesive sediment erosion. The experimental results reveal that vertical jetting results in the largest cavity depth (or ’erosion depth’), but not in the largest cavity size (sediment production). The erosion depth increases with the jetting angle reaching its maximum at 90° and then begins to decrease with further increase in the jetting angle. The results also indicate that the cavity width (or ’erosion width’) is not necessarily correlated with the impingement region but is instead associated with the erosion-effective jet width—the width of the jet where flow velocities are high enough to penetrate the bed. Analysis of the cavity size showed that the largest sediment production was achieved at a 65° jetting angle among the tested jetting angles (25°, 45°, 65°, 90°, 115°, 135°, and 155°). The erosion depth was found to be highly proportional to the impingement force exerted by the flow on the clay.

This study investigates the influence of multiple jet parameters on the flow field of translating impinging inclined water jets. We conducted full-scale stereoscopic particle image velocimetry and pressure measurements and three-dimensional computational fluid dynamics simulations for Reynolds numbers in the range of. Considering the complex mechanism of a translating impinging jet, a good concordance is observed between the experimental and numerical results. The translation-to-jet velocity ratio is identified as a critical parameter in determining whether the jet flow predominantly exhibits impinging characteristics or behaves as a jet in cross-flow. It is found that, for, jet impingement is minimal. The stand-off distance to nozzle diameter ratio determines the relative influence of the cross-flow on the jet flow. The effect of is similar to a stationary impinging jet, with the potential core extending up to, but entrainment is enhanced by the relative cross-flow. For an inclined jet, i.e. jet angle, the direction of the jet, either backward or forward, governs the deflection of the flow. Higher pressures are recorded for a backward directed jet compared with a forward directed jet for supplementary angles.

This study explores the scour phenomenon around a submerged square pile under the combined influence of waves and currents. To this end, a three-dimensional Computational Fluid Dynamics model was developed. The numerical model solves the Reynolds-averaged Navier-Stokes (RANS) equations with k-ω turbulence closure model. The Level-Set method is utilized to monitor free surface interface realistically within the computational model. The Exner formulation is used to compute the bed elevation variations. An extensive validation is conducted for square pile scour in steady current, wave only, and wave–current conditions. Subsequently, the validated numerical model is utilized to analyze the impact of the submergence ratio, wave-current parameter (U_cw), and Keulegan–Carpenter (KC) number on the normalized scour depth around the submerged square pile in combined wave-current flows. The numerical results show that an increase in submergence ratio leads to an increased normalized scour depth around submerged piles in wave-current flows. Furthermore, it was found that a larger U_cw results in a larger normalized scour depth around the submerged square pile. However, for larger KC values of 12 and 18, the effect of U_cw becomes negligible due to the suppression of lee-wake vortices by developed trailing vortices.

The chapter gives an overview of the sediment dispersion generated by the mining process. Within the field of dredging engineering, ample experience is available regarding equipment, turbidity generated by equipment, and sediment transport processes. High up the environmental impact mitigation hierarchy are avoidance and minimization. That is where engineering can provide (part of) the solution. It is our aim to predict and consider how we can improve the mining process and equipment. Within this context, our focus is on those processes that are likely to take place close to the seabed. On the one hand, our work focuses on the prediction and reduction of the amount of sediment that might get suspended. On the other hand, considering the conditions under which the suspended sediment might be released in the most optimal way to reduce dispersion, we have performed and analysed small-scale and full-scale laboratory experiments of a hydraulic collector design and various dynamic sedimentation experiments.

Turbidity currents have extensively been explored in quiescent environments. However, during several underwater activities (e.g., dredging and deep sea mining), generated turbidity currents could travel in opposite directions and interact with each other, which could largely influence their hydrodynamics and sediment transport capacity. Therefore, we carried out a set of dual-lock-exchange experiments to study the interaction of colliding turbidity currents. Our experimental results show that the interaction of identical currents results in the reflection of both currents with almost no mixing, forcing them to travel in the opposite direction of the pre-collision one. In contrast, when a turbidity current interacts with a lighter, less-energetic current, clear mixing is observed. Furthermore, it is revealed that the collision of turbidity currents reduces the suspended sediment transported by them, which is favorable from an environmental point of view, and slightly increases the vertical dispersion of particles. In the case of two identical counterflowing currents, a 35% reduction in mass flux, accompanied by a 6% increase in turbidity current thickness, was observed in our experiments.

Coandă-effect-based collection stands out as the foremost technology in polymetallic-nodule mining due to the absence of direct contact between the collector and the ocean floor. Yet, this collection method disturbs the ocean floor, and minimizing such disturbance is crucial from an environmental viewpoint. To this end, a solid understanding of the interplay between the collector and the sediment bed is required. Therefore, we carried out a series of small-scale experiments, where a collector drives over a subaqueous clayey bed. These experiments provide the very first quantitative data on cohesive sediment erosion caused by a moving Coandă-effect-based collector, as well as on turbidity currents generated behind the collector head. This paper discusses the observations and findings derived from these experiments. Our findings reveal a logarithmic relationship between erosion depth and the flow impinging force applied on the clayey bed. An increased flow rate in the collection duct results in a slower turbidity current generated behind the collector head. This study enhances the ability to forecast sediment erosion caused by Coandă-effect-based collectors, offering the possibility to optimize the collector operational conditions and minimize the magnitude of the resulting sediment plumes.

This article presents the current state-of-the-art understanding of underwater dilative slope failure (breaching). Experimental investigations are reviewed, providing critical insights into the underlying physics of breaching and pointing out knowledge gaps, which underscore the need for further research. Besides, field observations at several locations across the globe are outlined, highlighting the hazard of breaching and the need for effective coastal management strategies to mitigate the associated risks. Furthermore, existing methods for analyzing and predicting the slope failure evolution are discussed and reflected upon, including analytical approaches and numerical models, ranging from simplified 1D models to advanced 3D coupled flow-soil approaches. Lastly, open questions are posed and key future directions are identified to enhance our understanding of the breaching failure. Overall, this review paper provides a valuable resource for researchers and decision makers involved in slope stability and flow slide risk assessment.

To date, hydraulic collection is the most widely considered technology in polymetallic-nodule mining, since there is no direct contact between hydraulic collectors and ocean floor. To construct a hydraulic collector that results in the least sediment disturbance, it is critical to develop an insightful understanding of the interaction between the collector and sediment bed. To this end, we conducted a set of small-scale experiments in which several operational conditions were tested, delivering the first quantitative data for sediment erosion resulting from a hydraulic collector driving over a sand bed. This paper presents and discusses the experimental results and observations. It is found that the collector’s forward velocity is inversely proportional to the bed-sediment erosion depth, since the bed is exposed to the flow for a longer time when the collector drives slower and vice versa. In contrast, an increased jet velocity leads to a larger erosion depth. Furthermore, when the collector underside is nearer to the sediment bed, a larger sediment layer is exposed to the water flow, resulting in a larger erosion depth. Finally, the experimental results show that collector water jets strike the sediment bed under an inclined angle, destabilizing the upper sediment layer and consequently dragging sediment particles along toward the collection duct and behind the collector head. This study improves the predictability of sediment erosion created by Coandă-effect-based collectors, which is a crucial asset to optimize the collector design and decrease the extent of the associated sediment plumes.

This study utilizes three-dimensional simulations to investigate scour in combined wave–current flows around rectangular piles with various aspect ratios. The simulation model solves the Reynolds-averaged Navier–Stokes (RANS) equations using the k–ω turbulence model, and couples the Exner equation to compute bed elevation changes. The model also employs the level-set approach to realistically capture the free surface, and couples a hydrodynamic module with a morphological module to simulate the scour process. The morphological module employs a modified critical bed shear stress formula on a sloping bed and a sand-slide algorithm for erosion and deposition calculations in the sediment bed. To validate the numerical model, simulations are conducted in a truncated numerical wave tank with the Dirichlet boundary condition and active wave absorption method. After validation, the numerical model is used to investigate the effect of aspect ratio and the Keulegan–Carpenter (KC) number on scour depth in a combined wave–current environment. The study finds that the normalized scour depth is highest for a rectangular pile with an aspect ratio of 2:1 and lowest for an aspect ratio of 1:2. The maximum normalized scour depth (S/D) for aspect ratios of 2:1 are 0.151, 0.218, and 0.323 for KC numbers 3.9, 5.75, and 10, respectively, whereas the minimum normalized scour depth (S/D) for aspect ratios of 1:2 are 0.132, 0.172, and 0.279. Additionally, the research demonstrates that the normalized scour depth increases with an increase in the KC number for a fixed wave–current parameter (Ucw).

As a result of the dilation of soil matrix, dense submarine sand slopes can temporarily be steeper than the natural angle of repose. These slopes gradually fail by the detachment of individual grains and intermittent collapses of small coherent sand wedges. The key question is whether steep disturbances in a submarine slope grow in size (destabilizing breaching) or gradually diminish (stabilizing breaching) and thereby limit the overall slope failure and resulting damage. The ability to predict whether the breaching failure is stabilizing or destabilizing is also crucial for the assessment of safety of submarine infrastructure and hydraulic structures located along rivers, lakes, and coasts. Through a set of large-scale laboratory experiments, we investigate the validity of an existing criterion to determine the failure mode of breaching (i.e., stabilizing or destabilizing). Both modes were observed in these experiments, providing a unique set of data for analysis. It is concluded that the existing method has limited forecasting power. This was quantified using the mean absolute percentage error, which was found to be 92%. The reasons behind this large discrepancy are discussed. Given the complexity of the underlying geotechnical and hydraulic processes, more advanced methodologies are required.

We present an effective design of a hydraulic, polymetallic nodule collector, which fundamentally depends on the Coandă effect in harvesting nodules. The design was first developed based on 2D numerical simulations conducted using a computational fluid dynamics tool, ANSYS FLUENT. Following that, the design was tested in full-scale experiments, which provided insights into the collection efficiency of the collector and confirmed its functionality and effectiveness. The latter means, in the context of deep sea mining, high effective pick-up of nodules, with minimum sediment disturbance. Our observations indicate that our design hardly disturbs the tested sediment bed. The experimental results show that a higher jet velocity leads to a higher pick-up efficiency. Two forward velocities were tested and the higher forward velocity led to a lower pick-up efficiency. It is revealed that the available time for the nodules to respond to the pressure gradient under the collector is of great importance; if the available time is not sufficient, the nodules will not be picked-up even if the pressure gradient is adequate. The clearance under the rear cowl of the collection duct is found to play a major influential role in the collection process; a smaller bottom clearance results in a higher pick-up efficiency.

Owing to the absence of direct contact between hydraulic polymetallic-nodule collectors and seabed, hydraulic collection is deemed, from an environmental point of view, the most preferred technique in nodule mining. To design a hydraulic collector that results in minimum sediment disturbance, it is crucial to develop a solid understanding of the interaction between the collector and the sea bed. To this end, we performed a series of small-scale experiments where several operational conditions were tested, yielding the first quantitative data for sediment erosion resulting from the movement of a hydraulic collector over a sand bed. This paper presents and discusses the experimental results and observations. It is found that the collector’s forward velocity is inversely proportional to the bed-sediment erosion depth, since the bed is exposed to the flow for a longer time when the collector drives slower and vice versa. Contrarily, an increased jet velocity leads to a larger erosion depth. Furthermore, when the collector underside is nearer to the bed, a larger sediment layer is exposed to the water flow, resulting in a larger erosion depth. Finally, the experimental results show that a larger amount of water entrained into the collection duct results in a smaller erosion depth, implying that the flow velocities under the collector are lower in this case.

Polymetallic nodules are potato-sized rock accretions that form on vast areas of the abyssal plains of the global ocean. These nodules are rich in commercially precious metals, such as nickel, cobalt and copper, making them a target for potential future deep-sea exploitation. Generally, polymetallic nodules are partially buried in the seabed sediment, which is predominantly composed of clay. Among the existing mechanisms for mining polymetallic nodules (mechanical, hydraulic and hybrid), hydraulic collecting is deemed the most suitable technology in deep sea mining. This is primarily because hydraulic collecting hardly involves interaction with the seabed during the collection process (Agarwal et al., 2012); the collector generates a pressure gradient to harvest the nodules, thus substantially reducing the associated disturbance to the seabed.

Underwater slope failure is a common problem in the fields of geotechnical, dredging and hydraulic engineering, posing a major risk to submerged infrastructure and flood defences along coasts, rivers, and lakes. The term ‘flow slide’ refers to a specific, complex failure mechanism of underwater slopes, which occurs when a substantial amount of sediment moves downslope and eventually redeposits, forming a milder slope. A distinctive feature of flow slides is that the sediment running downslope is transported as a sediment-water mixture rather than as a sediment mass, and thus it behaves as a viscous fluid. Breaching is a particular type of flow slide, described as a slow (mm/s), gradual, retrogressive erosion of submerged slopes that are steeper than the soil internal friction angle. Breaching has remained unexplored until it was identified in the 1970s by the Dutch dredging industry as an important production mechanism for stationary suction dredgers. In that period, breaching was not known as a failure mechanism of underwater slopes outside of the field of dredging. In the Netherlands, breaching is now an important consideration in the safety assessments of dikes. Breaching flow slides are accompanied by the generation of turbidity currents, which can be described as buoyancy-driven underflows generated by the action of gravity on the density difference between the water-sediment mixture and the ambient water. These currents pose a serious threat to submarine structures placed at the seafloor, such as oil pipelines and communication cables. Breaching-generated turbidity currents run over and directly interact with the eroding, submarine slope surface (breach face), thereby enhancing further sediment erosion. The investigation and understanding of this interaction are critical to understand and predict the failure evolution during breaching. This is an important consideration for avoiding the risks of breaching during dredging and for the design of effective mitigation measures to protect hydraulic structures. In this dissertation, the evolution of the breaching failure and the associated turbidity currents are investigated through large-scale laboratory experiments and numerical modelling. This study begins by surveying the state-of-the-art knowledge of breaching flow slides, with an emphasis on the relevant fluid mechanics, providing a better insight into the physics and identifying the relevant knowledge gaps. Then, existing breaching erosion closure models were employed in combination with the three-equation model of Parker et al. (1986) and applied to a typical case of a breaching submarine slope. The sand erosion rate and hydrodynamic properties of the turbidity current were found to vary substantially between the erosion closure models, motivating further experimental studies on breaching flow slides, including detailed flow measurements, for validation purposes and improving the current understanding of the breaching phenomenon. At the Laboratory of Fluid Mechanics of Delft University of Technology, a set of unique large-scale experiments was conducted in which various non-vertical initial breach faces were tested, providing the first quantitative data for such initial conditions. Direct measurements of breaching-generated turbidity currents are thus provided, illustrating their spatial development and visualizing the structure of their velocity and sediment concentration. The analysis of the experimental results indicated that breaching-generated turbidity currents are self-accelerating; sediment entrainment and flow velocity enhance each other in a positive feedback loop. The turbidity currents accelerate downslope, and consequently the sand erosion rate increases downslope until a certain threshold, likely imposed by turbulence damping. This leads to the steepening of the breach face which induces the collapse of coherent sand wedges (surficial slides). These slides considerably enhance local sediment erosion and affect the hydrodynamics and thus increase the erosive capacity of the turbidity current. Even though breaching is a gravity-induced failure in the first place, the generated turbidity current seems to start dominating the failure just after its onset until the final deposition of the sediments. Owing to several difficulties encountered during the lab experiments, obtaining measurements of turbulence quantities of the flow was not possible. The lack of such measurements hampers the estimate of the flow-induced bed shear stress and hence the prediction of erosion during breaching. This motivated the use of an advanced 3D numerical model as a complementary tool to the experimental work, to gain additional insights into the behavior and structure of breaching-generated turbidity currents. Large eddy simulations of breaching-generated turbidity currents were conducted, providing deeper insights into their hydrodynamics and physical structure. Through these turbulence-resolving simulations, it was shown that the proposed numerical tool can reasonably reproduce several distinctive aspects of the flow, such as the vertical density distribution, and the spatial development down the breach face. A limitation of the model is that it underestimates the thickness of the current. The numerical results confirm the self-accelerating behavior of breaching-generated turbidity currents as indicated by the experimental results. Considering the challenging conditions of breaching, a new breaching erosion closure model was proposed and validated using the series of the laboratory experimental data obtained within this study. Good agreement is observed between experimental and numerically predicted erosion rates. Breaching-generated turbidity currents are found to exhibit a self‐similar behavior; velocity, concentration, Reynolds stress, and turbulent kinetic energy profiles take a self-similar shape. Based on a sensitivity analysis, sand erosion during breaching is found to be susceptible to the in situ porosity; the lower the in situ porosity, the higher the sand resistance to erosion. The experimental measurements acquired within this study may be utilized for the validation of existing and new numerical models used to simulate breaching flow slides. These models, based on the findings of this research, must be capable of reasonably reproducing the hydrodynamics and sediment transport of turbidity currents. The self-accelerating behavior of this current implies that it is quite dangerous, and that breaching could be a triggering mechanism for sustained turbidity currents in deep water. The knowledge gained from this dissertation may help towards the design of robust mitigation measures against breaching flow slides and towards the optimization of the sand production process during dredging while minimizing the associated risk for the surrounding environments. In addition, it may lead to a more accurate interpretation of the process responsible for the encountered submarine slope failures.

The ability to estimate the erosion rate along an underwater breach face is crucial to understand the evolution of breaching failure. To this end, breaching erosion models were developed and applied in numerical models. However, these erosion models have never been validated, owing to the scarcity of direct measurements of turbidity currents that accompany breaching. The aim of this study is to evaluate the existing breaching erosion models using direct measurements of recently performed laboratory experiments on breaching flow slides. We found out that the erosion model put forward by Mastbergen & Van Den Berg (2003) provides good agreement with the data and performs better than the one proposed by Van Rhee (2015). The latter tends to overestimate the erosion rate, particularly at steeper slopes.