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Saltwater intrusion is a common feature in the Changjiang Estuary affected by river discharges and tidal flows. It leads to a two‐layer flow structure during the flood‐to‐ebb tidal transient period: the seaward tidal flow in the upper water column and onshore intruded salt flow in the lower column, even though the lower column water usually experiences an ebb flow eventually. Our recent measurements with a tripod deployed in a tidal channel of the North Branch in the East China Sea challenged this feature. We detected that the two‐layer flow structure disappeared in the ebb tide period during the neap cycle due to intense saltwater intrusion. A constant onshore flood‐like flow predominated the entire water column. The physical mechanism for the flow reversal was examined using the Finite Volume Community Ocean Model (FVCOM). The FVCOM was robust to capture the observed flow reversal in the tidal channel during the neap tidal cycle. The momentum balance analysis results suggest that the flow reversal occurred when the saltwater intrusion‐induced onshore baroclinic pressure gradient force and baroclinic tidal rectification overwhelmed the seaward barotropic pressure gradient force. A parameter‐driven criterion was derived theoretically to determine the potential occurrence of a stable ebb flow reversal in the tidal channel. Plain Language Summary: Many estuaries experience saltwater intrusion, bringing saltwater from offshore regions to nearshore and river channels. In the Changjiang Estuary, it could create a two‐layer flow structure during the flood‐to‐ebb transient period, a seaward fresher ebb tidal flow in the upper water column, and an onshore intruded salt flow in the lower water column. Could the increased saltwater intrusion destroy this two‐layer flow structure and form an onshore flow throughout the water column in the ebb‐tidal period during a neap tidal cycle? If it does, what is the critical driving mechanism? To answer these questions, we deployed a tripod system integrated with state‐of‐the‐art marine instruments in a tidal channel of North Branch in the Changjiang Estuary, an area where saltwater intrusion frequently occurs. The observational data captured a flow‐overtured feature, showing a stable flood‐like intruded flow throughout the entire water column in the ebb‐tidal period during a neap tidal cycle. We applied the Finite‐Volume Community Ocean Model to examine the physical driving mechanism for flow‐overturning. The results suggest that the flow‐overturning could occur when the along‐channel saltwater intrusion‐induced baroclinic pressure gradient force and stratified tidal rectification overwhelmed the along‐channel tide‐induced barotropic pressure gradient force. Key Points: The bottom tripod system identified an overturned tidal flow in the North Branch channelA numerical model revealed the baroclinic gradient force from saltwater intrusion dominated the overturnThe balance criteria were proposed to estimate competition between these two dynamics [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]

Fluid mud (FM) is a unique sedimentary feature in high‐turbidity estuaries, where it can make a rapid contribution to morphodynamics. Insufficient field measurements and fixed‐point monitoring lead to deficient understandings of the formation, transport, and breakdown of the FM under extreme weather conditions. A field survey was conducted in the Changjiang Estuary during the period of turbidity maximum, just after Typhoon Haikui. The measurements captured the formation of the FM beneath the suspended layers, particularly around the lower reach of the North Passage. The thickness of the observed FM gradually decreased landward along the channel, with the maximum value reaching ~0.9 m. The major features of the observed storm‐induced FM were simulated using the Finite‐Volume Community Ocean Model. The results indicated that the initial appearance of the FM was the result of a typhoon‐intensified, salinity‐induced stratification in the outlet region. The subsequent landward propagation of the FM was driven by the combined effects of the FM‐induced mud surface pressure gradient force and saltwater intrusion near the bottom. Weak mixing during the subsequent neap tidal period sustained the FM as it rapidly extended into the middle region of the North Passage. This produced a large velocity shear at the interface of the FM and upper suspension layer, increasing the entrainment from the FM to the upper suspension layer. As a result of the increased tidal mixing, the FM weakened and then finally broke down in the subsequent spring tidal period. Plain Language Summary: The environment along the large river to estuary continuum is generally turbid and frequently produces highly concentrated benthic sediment suspensions, that is, fluid mud (FM). The FM is a sediment feature, with a concentration mostly in the range of 10 to >100 g/L. It is difficult to track the FM's movement and breakdown. The response of the FM to extreme atmospheric and oceanic conditions is therefore not well understood. In this study, a comprehensive field campaign identified the large‐scale formation of the FM after a severe tropical storm in the Changjiang Estuary. The life cycle of the FM in this estuary cannot, however, be determined through the field observation alone. A two‐layer FM model was developed to achieve this goal. The experiments suggested that the key physical factor was the stratification resulting from the typhoon‐enhanced saltwater intrusion. This led to the formation of the FM in the near‐bottom layer. After the FM formed, it extended onshore along the channel under the influence of an ambient saltwater intrusion. The FM was constrained in the benthic layer as a consequence of weak mixing from the saltwater intrusion. The breakdown of the FM was governed by enhanced tidal mixing in the subsequent spring tide. Key Points: A strong fluid mud (FM) formed during the passage of a tropical stormA storm‐increased stratification initially triggered a massive formation of the FMThe FM was sustained and transported by saltwater intrusion, and it subsequently broke down during the strong‐mixing spring tide [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]

Coastal ecosystems are strongly influenced by terrestrial inputs of freshwater, sediments, and nutrients, particularly in a megariver estuary of the Changjiang River. A remarkable increase in nutrient loading from the Changjiang River to the shelf has been observed over the period from 1999 to 2016 and turned the region into a high eutrophication condition. The Finite‐Volume Community Ocean Model and the European Regional Seas Ecosystem Model were coupled to assess the impact of the nutrient loading on the interannual variability of nutrients and phytoplankton. The model was first validated via observational data, and then dynamical analysis were conducted. Singular vector decomposition analysis indicated that the rapid change of local ecosystem was highly correlated with the change in river nutrient contributions. The Changjiang estuarine ecosystem was phosphate limited. The phosphate exhibited local variation, while the abundant nitrate from the river was diluted by the low‐nitrate oceanic water. The suspended sediment was significantly correlated with phytoplankton but not with nutrients. The ratio of diatom biomass to dinoflagellate biomass respected a rapid response to strong oscillations in the river nutrient input. High diatom primary production occurred near the sediment front, whereas the dinoflagellate bloom extended significantly offshore. The spring diatom and dinoflagellate blooms had major peaks in the empirical orthogonal function Modes 1 and 2, and the autumn bloom is characterized by secondary peaks from Mode 2 in the autumn. Plain Language Summary: There was an increase in the nutrient input into the Changjiang Estuary with an increased use of agricultural fertilizer as food demand rises globally. Further, there will be additional changes to river and estuary fluxes due to global anthropogenic activities in the future. This study presented the results of a novel coupled physical‐biogeochemical model which was designed to examine the seasonal and interannual variability of nutrients and phytoplankton dynamics in the Changjiang Estuary over the period of 1999–2016. The variations of nutrient source from the Changjiang Estuary in the last 18 years had been identified. The model demonstrated that this estuary ecosystem had a rapid response to the changes in riverine nutrients. The total nutrient concentrations, as well as their fluxes of the system, had similar variation patterns with river discharge, indicating that the river was the principal source of nutrients for the nearshore and offshore regions in that area. The phytoplankton population in terms of the ratio between diatoms and dinoflagellates responded quickly to changes in river nutrient fluxes. Key Points: Physical and biogeochemical models have been coupled to study pelagic planktonic ecosystem variability in a river estuary systemConsequences of changing nutrient fluxes over the last 18 years into the estuary on nutrient biogeochemistry and plankton were studiedPelagic ecosystem responded rapidly to river discharge and nutrient flux but exhibited different patterns for nutrients and phytoplankton [ABSTRACT FROM AUTHOR]