Breaching Flow Slides and the Associated Turbidity Currents: Large-Scale Experiments and 3D Numerical Modelling

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.