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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]

Estuarine fronts have significant effects on estuarine circulation, water quality, and productivity. However, there are limited studies on the joint ecological effects of sediment and plume fronts caused by tidal mixing and a low-salinity plume in and off the Changjiang River Estuary (CRE). Based on observational data during the summers of 1988–2016, we analyzed the spatial correlations between environmental factors and chlorophyll-a (Chl-a) concentrations along the two frontal boundaries. The water mass that is shoreward of the sediment front was characterized by the highest nutrients and total suspended matter (TSM); extensive light limitation caused by TSM consequently led to the lowest Chl-a. The water mass between the sediment and plume fronts displayed the highest Chl-a, which benefited from increased light availability due to rapid deposition of TSM across the sediment front and higher nutrients contributed by the plume and coastal upwelling. Shelf water mass beyond the plume front showed the lowest TSM and nutrients; the distinct reduction of nutrient supply resulted in a relatively low Chl-a. The spatial pattern of summer phytoplankton biomass tended to be a result of the balance of light and nutrients constructed by the two fronts and coastal upwelling in and off the CRE. [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]

A high-resolution numerical model system is essential to resolve multi-scale coastal ocean dynamics. So a multi-scale unstructured grid-based finite-volume coastal ocean model (FVCOM) system has been established for the East China Sea and Changjiang Estuary (ECS-CE) with the aim at resolving coastal ocean dynamics and understanding different physical processes. The modeling system consists of a three-domain-nested weather research and forecasting model, FVCOM model with the inclusion of FVCOM surface wave model in order to understand the wave-current interactions. The ECS-CE system contains three different scale models: a shelf-scale model for the East China Sea, an estuarine-scale model for the Changjiang Estuary and adjacent region, and a fine-scale model for the deep waterway regions. These three FVCOM-based models guarantee the conservation of mass and momentum transferring from outer domain to inner domain using the one-way common-grid nesting procedure. The model system has been validated using data from various observation data, including surface wind, tides, currents, salinity, and wave to accurately reveal the multi-scale dynamics of the East China Sea and Changjiang Estuary. This modeling system has been demonstrated via application to the seasonal variations of Changjiang diluted water and the bottom saltwater intrusion in the North Passage, and it shows strong potential for estuarine and coastal ocean dynamics and operational forecasting. [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]

• Two surveys were conducted to estimate flood impacts on environments in estuary. • Salinity/nitrate persisted, but phosphate/chlorophyll recovered rapidly after flood. • Dam improves the dissolved oxygen condition, but the flood can enhance hypoxia. Land-ocean interaction plays an essential role in the transport fate of terrestrial matters in the coastal and shelf regions. Flood discharge from a mega river, containing massive water, sediment, and nutrient loads, could result in substantial and complex impacts on the physical and biogeochemical dynamics of coastal systems. In this study, field campaigns were conducted in a region from the Changjiang River Estuary to the East China Sea (ECS) before and after a significant flood. The impacts of the flood on physical and biogeochemical environments were assessed. The results revealed that the fluvial flood enhanced the offshore expansion of the low-salinity river plume and associated sediment/nutrient fronts. However, the area of elevated chlorophyll-a at the river mouth did not expand noticeably. A numerical model was applied to quantify the contribution of the Three Gorges Dam (TGD) to the spatial intensity and temporal duration of fluvial flood effects on estuary–shelf continuum. The results predicted a maximum of 2° latitudinal offshore displacement of the shelf water. Salinity and nitrate exhibited conservative expansions, with a longer relaxation time (~2 months) than chlorophyll-a and phosphate. After the TGD-regulated flow event ceased, salinity and nitrate effects persisted, but phosphate and chlorophyll-a recovered rapidly. The flood decreased the dissolved oxygen (DO) concentration around the river mouth and the offshore region, but not in the nearshore transient area. In contrast, the non-TGD regulation increased the regional DO concentration, which reduced the hypoxia risk. The TGD has become a crucial anthropogenic driver of environmental changes in the Changjiang Estuary-ECS continuum. [ABSTRACT FROM AUTHOR]

High-turbidity water is a common feature in the estuary and inner shelf. Sediment suspension functions as a modulator that directly influences the interactions among nutrients, phytoplankton and other related ecosystem variables. A physical-biological coupling model system was applied to examine the impact of sediment front on interactions among on suspended sediment, vertical mixing, nutrients and phytoplankton over the inner shelf off the high-turbidity, phosphate-limited Changjiang Estuary. The physical model was the Finite-Volume Community Ocean Model (FVCOM) and the biological model was the European Regional Seas Ecosystem Model (ERSEM). Results revealed that in the nearshore region the growth of phytoplankton over the spring-summer seasons was limited by suspended sediments and intensified vertical mixing during the autumn-winter seasons extended the sediment-induced suppression extended offshore to restrict the phytoplankton growth over the shelf. Nutrients were diluted by spreading of freshwater discharge and significantly decreased off the suspended sediment front due to the depletion by the offshore phytoplankton growth. The simulation results showed that although the diatom phytoplankton dominated the Chlorophyll a (Chl- a) concentration, the non-diatom group had a more contribution to the biomass. The relatively high phytoplankton biomass was found over the offshore deep underwater valley area as results of remote advection by the Taiwan Warm Current and weak turbulent mixing. • Physical-biogeochemical model was developed to study sediment-ecosystem interaction. • Model revealed high nutrients within the front and dense Chl a out of the front. • Chl a was mainly from diatom, however non-diatom group contributed more on biomass. [ABSTRACT FROM AUTHOR]