Heyu Yu

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.

Plumes generated by deep-sea mining vehicles are multiphase flows in which hydrodynamic adjustment, entrainment, transport, settling, and particle state evolution act over competing time scales. How source configuration reorganizes these dynamics and their predictability remains unclear. We analyzed 22 stationary collection (SC), mobile collection (MC), and transit (TR) events using temporal analysis, ExtraTrees models, and recursive simulation. The modes represented a fixed collection source, collection combined with motion, and motion without active collection. Approximate velocity to concentration lags were 110 s for SC and MC and 170 s for TR, with broad uncertainty intervals. Particle irregularity decreased in five of six responsive MC events and increased in 10 of 11 TR events in the mapped records. Across withheld events, velocity carried appreciable predictive importance for concentration increments in the moving modes. ExtraTrees reduced velocity root mean square error (RMSE) by 11.1%–17.6% at 30–120 s, while derived flux RMSE decreased by 1.4%–17.5% at 10–120 s. Concentration and particle properties retained persistence as the final predictor, with particle scores influenced by sampling resolution. Recursive validation and initial state tests constrained the 95 scenarios to hypothetical comparisons. Together, the results show that the tested source configurations differ in measured temporal responses and fitted predictive dependence, while recursive scenario trends remain conditional on model validation.

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.

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.

Winter storms present a major erosional hazard to subaqueous deltas globally, but the associated dynamic mechanisms are still not well understood. Using high‐resolution in situ data collected over a 3‐week period in the abandoned Yellow River subaqueous delta, this study employs three‐dimensional quadrant analysis to examine turbulent structures and sediment response. The findings reveal a distinct cross‐stream asymmetry during winter storms, marked by strengthened positive ejections and negative sweeps—a pattern that is further enhanced when wind and currents are aligned. Storm‐induced changes in turbulent structure produce extended turbulent bursts that substantially facilitate sediment mobilization, while vertical sediment flux increases exponentially with the degree of turbulence asymmetry. Overall, this study demonstrates that storm‐driven modification of three‐dimensional turbulent structure is the primary mechanism for increased sediment export and morphodynamic evolution of subaqueous deltas.

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.