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.@en

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.@en

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).@en

The interest in polymetallic nodule mining has considerably increased in the last few decades. This has been largely driven by population growth and the need to move towards a green future, which requires strategic raw materials. Deep-Sea Mining (DSM) is a potential source of such key materials. While harvesting the ore from the deep sea by a Polymetallic Nodule Mining Tool (PNMT), some bed sediment is unavoidably collected. Within the PNMT, the ore is separated from the sediment, and the remaining sediment–water mixture is discharged behind the PNMT, forming an environmental concern. This paper begins with surveying the state-of-the-art knowledge of the evolution of the discharge from a PNMT, in which the discharge characteristics and generation of turbidity currents are discussed. Moreover, the existing water entrainment theories and coefficients are analyzed. It is shown how plumes and jets can be classified using the flux balance approach. Following that, the models of Lee et al. (2013) and Parker et al. (1986) are combined and utilized to study the evolution of both the generated sediment plume and the subsequent turbidity current. The results showed that a smaller sediment flux at the impingement point, where the plume is transformed into a turbidity current, results in a shorter run-out distance of the turbidity current, consequently being more favorable from an environmental point of view. @en

This paper starts with surveying the state-of-the-art knowledge of breaching flow slides, with an emphasis on the relevant fluid mechanics. The governing physical processes of breaching flow slides are explained. The paper highlights the important roles of the associated turbidity current and the frequent surficial slides in increasing the erosion rate of sediment. It also identifies the weaknesses of the current breaching erosion models. Then, the three-equation model of Parker et al. is utilised to describe the coupled processes of breaching and turbidity currents. For comparison’s sake, the existing breaching erosion models are considered: Breusers, Mastbergen and Van Den Berg, and Van Rhee. The sand erosion rate and hydrodynamics of the current vary substantially between the erosion models. Crucially, these erosion models do not account for the surficial slides, nor have they been validated due to the scarcity of data on the associated turbidity current. This paper motivates further experimental studies, including detailed flow measurements, to develop an advanced erosion model. This will improve the fidelity of numerical simulations.@en

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.@en

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.@en

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.@en

Recent studies have revealed that breaching, rather than liquefaction, is the dominant failure process in underwater slopes of fine sand and the main driver of observed flow slides in nature. As a result, breaching is getting more attention from hydraulic and geotechnical researchers. Measurements of breaching-generated turbidity currents are substantial for understanding the interaction between the turbidity current and the slope surface, as well as for the validation of numerical models. However, these measurements are scarce in the literature. To this end, laboratory experiments are planned to be carried out in the water lab of Delft University of Technology, the Netherlands. This paper describes the special experimental setup that will be employed to obtain the required data.@en

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.@en

Breaching flow slides result in a turbidity current running over and directly interacting with the eroding, submarine slope surface, thereby promoting further sediment erosion. The investigation and understanding of this current are crucial, as it is the main parameter influencing the failure evolution and fate of sediment during the breaching phenomenon. In contrast to previous numerical studies dealing with this specific type of turbidity currents, we present a 3D numerical model that simulates the flow structure and hydrodynamics of breaching-generated turbidity currents. The turbulent behavior in the model is captured by large eddy simulation (LES). We present a set of numerical simulations that reproduce particular, previously published experimental results. Through these simulations, we show the validity, applicability, and advantage of the proposed numerical model for the investigation of the flow characteristics. The principal characteristics of the turbidity current are reproduced well, apart from the layer thickness. We also propose a breaching erosion model and validate it using the same series of experimental data. Quite good agreement is observed between the experimental data and the computed erosion rates. The numerical results confirm that breaching-generated turbidity currents are self-accelerating and indicate that they evolve in a self-similar manner.@en

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.@en

Breaching flow slides are accompanied by the generation of turbidity currents. Measurements of these currents are critical for understanding the interaction between the turbidity current and the slope surface, as well as for the validation of numerical models. However, there are insufficient data available detailing the velocity distribution or sediment concentration in these currents. This paper presents experimental results of unique large‐scale physical model experiments on breaching flow slides conducted at the water lab of Delft University of Technology, The Netherlands. The model tests were carried out in a tank with a subaqueous sandy slope steeper than the internal friction angle for that sand. We performed a series of novel experiments in which various non‐vertical initial slope angles were tested, providing the first quantitative data for such initial conditions. Measurements of flow thickness, velocities, and sediment concentrations are obtained, providing the spatial and temporal evolution of the turbidity currents and the resulting underwater slope morphology. The experimental results reveal that the breaching‐generated turbidity currents are self‐accelerating, and that they dominate the problem of breaching flow slides. We evaluated the theoretical expression for the erosion velocity in the case of pure beaching ‐ where the turbidity current has no influence on the erosion ‐ and it overestimated the observed erosion. The analysis of the failure evolution showed that the sand erosion rate increases due to the acceleration of the turbidity currents downslope, until a certain threshold, leading to the steepening of the breach face and thus the occurrence of surficial slides.@en

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.@en