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Existing knowledge about groyne-induced effects is primarily based on riverine or coastal environments where salinity gradients are absent or limited. However, in estuaries, salinity gradients drive physical processes such as longitudinal and lateral residual flows. The effect of groynes is much more complex because they can modulate channel hydrodynamics and directly affect lateral salinity gradients. In this study, an idealized model is applied to investigate the effects of groyne layouts in estuarine environments, including effects on (1) channel hydrodynamics, (2) lateral water exchange, (3) Coriolis effects, and (4) saltwater intrusion. Model results show that the aspect ratio (the width of groyne fields to the length of groynes) of groyne fields plays an important role. Groynes also induce asymmetry of lateral flows, for example, increasing near-bottom shoal-to-channel flows during low water slack. The aspect ratio has opposite effects on horizontal and vertical components of water exchange. A large aspect ratio strengthens horizontal exchange and weakens density-driven currents. For a large-scale groyne field (several kilometers), Coriolis effects introduce a substantial difference in exchange mechanisms along the north and south banks. A medium range of aspect ratio (2.0–3.0) leads to the strongest saltwater intrusion during both neap and spring tides. Dikes and groynes are common engineering structures in waterways designed to increase along-channel flow velocities and prevent sediment deposition, thereby maintaining navigability. However, especially in estuarine environments, cross-channel flow velocities resulting from these structures may generate sediment transport toward the channel, in contrast to their original intention. Another important impact of these structures is on saltwater intrusion, which is crucial to freshwater resource management. Therefore, this research investigates the groyne-induced effects on cross-channel flows and the saltwater intrusion problem by comparing different layouts of groynes and their consequences. Two important findings may benefit practical applications. First, the interaction between saltwater and freshwater in estuaries enhances cross-channel flows during certain periods, thereby influencing sediment transport. Second, the maximal saltwater intrusion occurs for intermediate width-to-length ratios of the groynes, with lower saltwater intrusion for either very small or very wide groyne fields. [ABSTRACT FROM AUTHOR]

Lateral flows redistribute sediment and influence the morphodynamics of channel‐shoal systems. However, our understanding of lateral transport of suspended sediment during high and low water slack is still fairly limited, especially in engineered estuaries. Human interventions such as dike‐groyne structures influence lateral exchange mechanisms. The present study aims to unravel these mechanisms in a heavily engineered, turbid channel‐shoal system in the Changjiang Estuary, using a high‐resolution unstructured‐grid three‐dimensional model and in situ observations. Analysis of model results reveals two typical transport patterns during slack‐water conditions, that is, shoal‐to‐channel transport during low water slack and channel‐to‐shoal transport during high water slack. A momentum balance analysis is carried out to explain mechanisms driving the lateral transport of suspended sediment during high water slack, revealing the importance of lateral pressure gradients, Coriolis force, and the curvature‐induced term. Groyne fields play a crucial role in sediment transport, especially during low water slack. A model scenario in which one groyne is removed reveals that groyne fields strongly influence lateral sediment transport. The decomposition of the sediment transport flux reveals that the turbidity maximum is shaped by a balance between seaward advection by residual flows, and landward transport by tidal pumping and gravitational circulation. Within the turbidity maximum, sediment is laterally redistributed by lateral flows during slack‐water conditions, greatly influencing estuarine channel morphology. Plain Language Summary: Sediment deposition in navigation channels requires regular maintenance dredging. To minimize channel siltation, in‐depth knowledge of transport mechanisms and the effect of mitigation measures (such as groyne fields) is needed. These mechanisms include both longitudinal (along‐channel) and lateral (cross‐channel) processes. It is now well established that longitudinal processes play a key role in sediment convergence. However, during high and low water slack when the longitudinal velocity is small, lateral transport of suspended sediment becomes more important. These lateral transport mechanisms have a large influence on channel morphology. In this study, we investigate longitudinal and lateral transport of suspended sediment with a numerical model, revealing two typical lateral sediment transport patterns during high and low water slack which greatly influence the exchange of sediment between the channel and the shoals. The model also demonstrates the effect of groyne fields on lateral sediment transport. These findings expand our understanding of sediment transport in channel‐shoal systems in general, but in engineered systems in particular. Key Points: During stratified slack‐water conditions, the lateral pressure gradients drive sediment transport between the shoal and the main channelLongitudinal sediment convergence primarily results from advection, tidal pumping, and longitudinal vertical circulationLateral transport of suspended sediment and distribution patterns are influenced by groynes [ABSTRACT FROM AUTHOR]

Lateral flow significantly contributes to the near‐bottom mass transport of salinity in a channel‐shoal system. In this study, an integrated tripod system was deployed in the transition zone of a channel‐shoal system of the Changjiang Estuary (CE), China, to observe the near‐bottom physics with high temporal/spatial resolution, particularly focusing on the lateral‐flow‐induced mass transport. These in situ observations revealed a small‐scale salinity fluctuation around low water slack during moderate and spring tidal conditions. A simultaneous strong lateral current was also observed, which was responsible for this small‐scale fluctuation. A high‐resolution unstructured‐grid Finite‐Volume Community Ocean Model has been applied for the CE to better understand the mechanism of this lateral flow and its impact on salinity transport. The model results indicate that a significant southward near‐bed shoal‐to‐channel current is generated by the salinity‐driven baroclinic pressure gradient. This lateral current affects the salinity transport pattern and the residual current in the cross‐channel direction. Cross‐channel residual current shows a two‐layer structure in the vertical, especially in the intermediate tide when the lateral flow notably occurred. Both observation and model results indicate that near‐bottom residual transport of water moved consistently southward (shoal to channel). Mechanisms for this intratidal salinity variation and its implications can be extended to other estuaries with similar channel‐shoal features. Plain Language Summary: In channel‐shoal systems, lateral processes sometimes can be of great significance, for example, influencing salinity and sediment transport, even some biogeochemical matters that influence the water environment. Salinity and sediment transport will have a further effect on navigability, ecology, and so on, for example, sediment deposition issues, which significantly decrease the shipping capacity. Consequently, knowing the accurate information about the flow field and finding a criterion for pronounced lateral processes are important to better understand hydrodynamics and sediment behaviors in channel‐shoal systems. Here in this study, we conducted an in situ observation and used numerical simulation to detect and predict a strong lateral flow in the North Passage of the Changjiang Estuary. This shoal‐to‐channel lateral flow occurred around the end of ebb and caused an intratidal salinity variation at the observation site, close to the deep channel. Therefore, a link between pronounced lateral flows and typical salinity variation can be established. This link can also be found in other estuaries like Hudson River estuary. Findings presented here will help to easily distinguish pronounced lateral flows and better understand the effect of lateral flows. Key Points: Intratidal salinity variation (ISV) and a strong lateral flow in a channel‐shoal system were identified by an integrated tripod systemNumerical simulation reveals that lateral flows were primarily driven by the salinity‐induced pressure gradient forceLateral flows can be generated or enhanced by human interventions such as groyne fields [ABSTRACT FROM AUTHOR]

The concentrated benthic suspension (CBS) of mud, as a major contributor of sediment transport in the turbidity maximum of the estuary, is of great challenge to be correctly monitored through field measurements, and its formation mechanism is not well understood. A tripod system equipped with multiple instruments was deployed to measure the near‐bed hydrodynamics and sediments in the North Passage of the Changjiang Estuary, with the aim at determining the formation mechanisms of CBS. The measurements detected a significant dominance of high sediment concentration in the near‐bed 1‐m layer: ~20 g/L at the southern site and ~47 g/L at the northern site. Strong CBS occurred under weak tidal mixing condition and was directly relevant to the sediment‐induced suppression of turbulent kinetic energy and the enhanced water stratification due to saltwater intrusion and sediment suspension. During the weak‐mixing neap period, the typical thickness of CBS was about 0.2–0.3 m, with a life time of ~2.83 hr (suspended‐sediment concentration > 15.0 g/L). Enhanced water stratification reduced vertical mixing and confined the sediment entrainment from the near‐bed layer to the upper column. This enhancement was due to the suppression of turbulent kinetic energy as a result of the sediment accumulation in the near‐bottom column during the slack waterand also due to the appearance of a two‐layer salinity structure in the vertical as a result of saltwater intrusion near the bottom. These physical processes worked as a positive feedback loop during the formation of CBS and can be simulated with a process‐oriented, one‐dimensional vertical CBS model. Plain Language Summary: In the turbidity maximum, concentrated benthic suspension (CBS) frequently dominates the near‐bed sediment transport, which, however, is difficult to be correctly observed, especially in the stratified condition. And the formation mechanism of CBS is complex due to the interaction of sediment and hydrodynamics. In a stratified tide channel of the Changjiang Estuary, China, a comprehensive tripod system, integrated with many cutting‐edge instruments, is developed and deployed to measure the CBS and to determine the formation process. This system detected the existence of CBS in the lower near‐bed 1‐m layer, ~20 g/L at the southern site and ~47 g/L at the northern site, with the mean thickness of 20–30 cm. The corresponding salinity, tidal velocities, wave, and turbulent kinetic energy were also recorded. During the formation of CBS, tidal mixing is weak. And the salinity/sediment‐induced stratification greatly limits the vertical mixing and suppresses the turbulent kinetic energy production. The saltwater intrusion creates a two‐layer structure of salinity, leading to stratification and decreasing the mixing. These physical effects work like a positive feedback loop. A one‐dimensional vertical model had been developed to successfully simulate the formation process of CBS, indicating the stable CBS during weak‐tidal‐mixing neap cycle. Key Points: Concentrated benthic suspension (CBS) of mud in the turbidity maximum zone was observed by a comprehensive near‐bed tripod systemThe dynamics for the formation of CBS are determinedA one‐dimensional vertical model is developed to analyze the physical mechanisms responsible for the formation of CBS [ABSTRACT FROM AUTHOR]

Abstract: Based on the Delft3D-FLOW, a 2D/3D hydrodynamic and sediment transport model was set up for the region of Yangtze Estuary, China. After a series of model verifications, which showed that the model is capable to reproduce the hydrodynamic and sediment transport processes in the region, it is applied to study the storm surge problem and morphological evolution of the Jiuduansha Shoals. It is concluded that the effects of remote wind and waves should be considered to obtain accurate results for storm surges. For reproducing the morphological changes the transport of cohesive as well as the non-cohesive fraction of sediment needs to be taken into account. The model can be considered as a basic tool for managing and studying the Yangtze Estuary. [Copyright &y& Elsevier]

Examines a three-dimensional transport model for suspended sediment by waves and current. Variation of the averaged wave and turbulence situation; Equation of suspended sediment by waves and currents; Influence of waves on current field.