Yang Lu

Wave‐induced seabed liquefaction can enhance coastal sediment suspension, but the wave‐cycle‐scale influence of interfacial‐wave motion remains poorly understood. Wave‐flume experiments were conducted on a silty bed under two regular‐wave conditions to compare near‐bed hydrodynamics and sediment suspension between a non‐liquefied condition and a liquefied condition. Under the non‐liquefied condition, suspended sediment concentration (SSC) exhibited a double‐peaked intra‐wave pattern and generally varied with turbulent kinetic energy and Reynolds shear stress, suggesting predominantly shear‐related suspension. After liquefaction, sustained interfacial waves developed at the mudline, with a root‐mean‐square amplitude of approximately 1.4 cm. Around the onset of sustained interfacial‐wave motion, near‐bed SSC increased from approximately 1.3 to 2.6 g L ⁻¹ within 4–5 wave cycles, the intra‐wave SSC distribution shifted to a single‐peaked pattern, and its relationship with Reynolds shear stress weakened. During rapid mudline descent, upward turbulent sediment transport was strongest and accounted for approximately 93% of the positive cycle‐integrated turbulent sediment flux. Under the experimental conditions examined, these observations suggest that interfacial‐wave motion may provide an additional phase‐dependent mechanism modulating sediment suspension over a liquefied seabed, offering a physical basis for improving the representation of sediment transport over liquefied beds.

This study employed field observations to investigate the evolution of deep‐sea sand ripples under the influence of internal waves. An observational system was deployed at a depth of 720 m in Dongsha, where cameras captured four distinct types of sand ripples: linear, curved, transitional, and covered ripples. A joint analysis of the sand ripple images and the observed hydrodynamic conditions revealed that internal waves facilitate the transformation between different sand ripples. Additionally, analysis of seabed echo intensity indicated that internal waves can cause the resuspension of seabed sediments, resulting in the emergence of covered sand ripples. Furthermore, under the same bottom flow velocity conditions, different types of sand ripples as bedforms result in varying concentrations of suspended sediments. Specifically, the sediment resuspension caused by transitional sand ripples is greater than that caused by curved sand ripples, which in turn is greater than that caused by linear sand ripples. These direct observations not only confirm the evolution of sand ripples under the influence of internal waves but also provide new insights into the mechanisms of sediment resuspension in the study area.

Large deformation runout is a key factor in assessing the hazards posed by underwater landslides. However, conventional kinematic analyses often neglect both the progressive degradation of slope materials and the hydrodynamic response accompanying the interaction between the moving mass and the overlying water. Taking the well-documented St. Niklausen underwater landslide as a representative case, this study employs a coupled Eulerian–Lagrangian (CEL) model to investigate the earthquake-triggered initiation, large deformation movement, and hydrodynamic response of the landslide. A Python 2.7.15-based stress mapping method is developed to establish an accurate initial geostatic stress field for the irregular slope profile. The numerical model reproduces the principal stages of landslide initiation, runout, and deposition. The results reveal a progressive retrogressive failure mechanism in which successive sliding masses interact through a high-strength compression zone. The rear sliding mass continuously transfers compressive work to the frontal mass, thereby maintaining its downslope movement and indirectly promoting basal erosion to a maximum depth of approximately 6.2 m. In addition, rapid landslide motion generates pronounced vortical flow in the overlying water. These flow structures reflect the hydrodynamic response induced by landslide motion, although their net influence on basal resistance and final runout cannot be isolated from the present coupled simulation. These findings clarify the internal mechanical evolution of underwater landslide movement and characterize the accompanying hydrodynamic response, providing a methodological basis for assessing landslide mobility and related underwater hazards.

The collapse of immersed granular columns serves as a proxy for submarine landslides, yet natural sediments are rarely the ideal spheres typically assumed in models. To elucidate the poorly understood influence of particle geometry on immersed granular collapse dynamics, this study employs a coupled computational fluid dynamics and discrete element method framework explicitly accounting for non-spherical characteristics. Using the aspect ratio to parameterize particle shape, we analyze the collapse and transport processes of granular columns composed of particles with varying aspect ratios in a fluid environment by digging into the evolution of collapse morphology, particle contact characteristics, energy budgets, and excess pore-fluid pressure responses. This study reveals that increasing particle aspect ratio (i.e., more elongated particles) enhances geometric interlocking effects among particles. This enhanced interlocking constrains both translational and rotational movements, thereby reducing the kinetic energy and energy dissipation of the immersed granular column. Consequently, a greater proportion of energy is retained as potential energy within the deposit, ultimately manifesting as decreased front propagation velocity and reduced final runout distance. Furthermore, variations in the particle aspect ratio influence the dilatancy–contractancy behavior of the granular column, resulting in diminished excess pore-fluid pressure fluctuations with increasing particle aspect ratio. The study also demonstrates that the final runout distance is proportionally correlated with the front kinetic energy, and this relationship remains independent of the particle aspect ratio. These findings advance our understanding of submarine landslide dynamics and provide a theoretical foundation for risk assessment and mitigation of marine geohazards.

Wave‐supported gravity flows are a key process driving near‐bed sediment transport in deltaic systems. However, previous observations have focused primarily on active river deltas with abundant sediment supply. The sediment sources and dynamics of wave‐supported gravity flows in abandoned deltas with low sediment availability remain poorly understood. In this study, in situ observations were conducted on the abandoned Yellow River Delta of northern Jiangsu Province, China, and four wave‐supported gravity flow events were observed within 48 hr during a storm event. The sediment source mechanisms in the study area include local resuspension, wave‐induced liquefaction, and tidal advection, and these different sources influence the dynamic characteristics of the flows. Wave‐supported gravity flows developed solely by local resuspension from bed shear stress exhibited lower suspended‐sediment concentrations and shorter durations. In contrast, when the sediment source mechanisms of wave‐supported gravity flows involved wave‐induced liquefaction or tidal advection, the wave‐supported gravity flows achieved suspended sediment concentrations 7 to 14 times higher. Despite large variations in suspended sediment concentration, flow velocities among the events were not significantly different, ranging from 3.6 to 5.8 cm/s, a result attributed to increased bottom drag and viscosity in the high‐concentration fluid mud layers. Furthermore, all observed events were short‐lived (1–2 hr) and displayed a distinct tidal periodicity. These findings suggest that in fine‐grained sediment limited abandoned deltas, tidal advection is the dominant mechanism triggering frequent, short‐lived wave‐supported gravity flows, effectively compensating for the lack of local sediment supply.

The evolution of turbidity currents covers multiple physical processes, such as fluid entrainment, self-acceleration, and sediment deposition, which are associated with sediment particle behaviors and yet not well understood. This study uses a fully coupled computational fluid dynamics and discrete element method model to investigate the particle-scale dynamics of turbidity currents and their responses to different bedforms. Results show that the turbidity currents controlled by viscous drag exhibit distinct flow features, including changes in morphology, velocity evolution, and other fluid/particle kinematic indicators, depending on their initial particle volume concentration. An increase in initial particle volume concentration enhances the fluid/particle motions accompanied by generating interfacial Kelvin–Helmholtz waves. The fluid/particle indicators, with the exception of the energy loss for particle–bed collisions, have strong relevance with particle concentrations, which can be described by linear or power-law functions. Furthermore, specific bedforms play unique roles in the propagation process and deposition pattern of turbidity currents. Slope beds enhance the motion, suspension, and collision of sediment particles, and cause wave-shaped sediment deposits along the slope particularly in the high-concentration case. By comparison, weakening of particle migration on obstructed and wavy beds is accentuated by blocking effects, mainly resulting from the convex bed morphology. However, the continuously convex and concave features diminish the blocking effect of wavy beds by intensifying particle motions along the lee sides of wave-shaped bumps. The particle-scale dynamics of turbidity currents is linked to the relative sizes of the underlying bedforms, which should be noted and further studied in our future work.

Two relatively comprehensive and long‐term observation campaigns of bottom sediment dynamics have been conducted at the subaqueous Yellow River delta in the winters of 2016 and 2018. High suspended sediment concentration (SSC) up to 10 g/L were observed in 2016 and active events of wave‐supported fluid mud (WSFM) were found to exist at the abandoned river delta during storms in 2018. A set of calibrated optical backscatter sensors measured the near‐bed SSC >30 g/L with a maximum WSFM thickness >60 cm. WSFMs emerging at the abandoned river delta were classified into three types: single‐peak, multi‐peak, and hybrid according to the time‐varying characteristics of WSFMs. The dominant factors triggering the WSFM generation and dissipation are storm‐wave actions and tidal effects, with limited contribution from current‐induced turbulence. In this study, several lines of evidence were put forward to prove the presence of seabed liquefaction to a depth of 10–16 cm, and thus we can infer that another significant factor enhancing the WSFM thickness scale may be wave‐induced liquefaction. Observations showed obvious signs of WSFM motion, or in other words, wave‐supported gravity flow (WSGF) moved downslope. The downslope velocity (around 1.9 cm/s) was estimated by solving a buoyancy‐friction force balance. This study observed frequent WSFM events at the abandoned Yellow River delta for the first time and demonstrates the need to attach more concerns about its roles in sediment transport and offshore engineering in the future.