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Classical estuary circulation theory states that in the longitudinal direction of an estuary there exists a single circulation with landward, near-bottom, and seaward, near-surface flows; however, the situation becomes complicated with the presence of a river mouth bar. Here we conducted tidal-cycle observations in the South Passage of the Yangtze Estuary during both the wet and dry seasons of 2018 (July and December, respectively). The simultaneous current velocity, temperature, and salinity profile data were obtained from anchored boats and base tripods at three stations along the channel in the mouth bar area. The results reveal two distinct longitudinal residual circulation patterns: a classic circulation which is formed on the seaward slope of the mouth bar during the wet season, and a double circulation system which is composed of two circulation cells over the landward and seaward slopes of the mouth bar during the dry season. The Simpson number (Si), mixing parameter (M), and salinity data were used to quantify the mixing intensity, which shows that horizontal baroclinic pressure gradient is the dominant factor in the formation of the longitudinal residual circulation. Furthermore, the double circulation pattern during the dry season is related to the mouth bar bathymetry, which affects saltwater intrusion. The double circulations can significantly influence the spatial and temporal evolution of the stagnation point in the estuarine channel, which in turn modifies the distribution patterns of suspended sediment concentration in the maximum turbidity zone. Evidently, the changes in the mouth bar sandbar bathymetry in response to human activities and river basin hydrographic conditions indirectly affect the occurrence and scale of the double circulations. [ABSTRACT FROM AUTHOR]

The vertical density gradients of salinity and suspended sediment concentration (SSC) cause stratification in estuaries, which play a vital role in the turbulence structure, water mixing, and sediment transport. To investigate the effect of stratification, especially SSC-induced stratification, on maintaining the estuarine turbidity maximum (ETM), we conducted in situ measurements on sediment dynamics at the upper and central ETM sites in the South Passage of Changjiang Estuary in July 2018. The gradient Richardson number was estimated as a proxy for the stratification that is attributable to salinity or/and SSC. We found that salinity-induced stratification was observed mainly on the surface and in the middle layers, whereas SSC-induced stratification occurred mainly in the near-bottom layers. Furthermore, at the central ETM, the baroclinic effect was enhanced during the neap tide when the salinity-induced stratification was stronger than that during the spring tide. In the early phase of floods with minimum velocity during the neap tide, salinity-induced stratification suppressed the turbulence and vertical diffusion of sediments. Moreover, the flocculation enhanced the settling process within the water column. Consequently, high concentrations of fine-grained sediments formed near the bottom and promoted SSC-induced stratification, thereby leading to the continuous accumulation and trapping of sediments. In conclusion, the interactions among the "salinity- and SSC-induced stratification" processes served as crucial constraints of the temporal and spatial variations of the ETM in the Changjiang Estuary. [ABSTRACT FROM AUTHOR]

The vertical distribution of suspended sediment concentration (SSC) plays a key role in the residual suspended sediment transport in the estuarine environments. A field measurement was carried out in the turbidity maximum zone (TMZ) of the Changjiang Estuary in 2012 to study the characteristics and formation mechanisms of SSC profile in the partially stratified estuary. Based on the observed data, we found that the vertical distribution of SSCs is mainly determined by the density stratification caused by the interactions between the river runoff and the saltwater intrusion. The vertical profiles of SSC were categorized into three types: (I) two-layer structure profile, (II) exponential profile, and (III) linear profile. Type I mainly occurred in condition of strong stratification, whereas types II and III mainly occurred in weak stratification and well-mixed conditions, respectively. We developed two new empirical equations to simulate the vertical profiles, which can accurately describe the observed exponential and linear profiles and reasonably/accurately describe the observed two-layer structure profiles. The two new equations can be easily used in other well-mixed or partially stratified estuaries given the surface and bottom SSC data, which can be obtained from many ways, for instance, the buoy and bottom tripod survey system, the remote-sensed images, and numerical models. [ABSTRACT FROM AUTHOR]

The following are errors in the original article: Ya Ping Wang's first name was incorrect. It is correct as shown here. There is an error in Eq. 2. Following is the corrected equation. [ABSTRACT FROM AUTHOR]

Extreme flood events can significantly disturb hydro-sediment dynamics and affect turbidity maximum zone (TMZ) distributions, which are crucial to estuary evolution. This study used field data and eight Sentinel-2A/B MSI, GF-1/6 WFV, and Landsat-8 OLI satellite images to examine the behavior of the TMZ in the North Passage (NP) and South Passage (SP) of the Yangtze Estuary under the influence of the 2020 extreme flood event. Images covered average annual, usual flood, and extreme flood (river discharge above 70,000 m3/s) conditions between 2018 and 2020. The surface suspended sediment concentration (SSSC) of the Yangtze Estuary was retrieved and verified by the in-situ measured turbidity. Relative SSSC (RSSSC), the ratio of retrieved SSSC to region-averaged SSSC, was then calculated to determine the TMZ (RSSSC>1). The results show that the relation between the SSSC measured in the TMZ core of the SP and river discharge at Datong Gauging Station demonstrated an anticlockwise hysteresis, which can be divided into three stages. In the second stage, the observed SSSCs during the extreme flood were 0.18– 0.68 kg/m3, 0.10– 0.17 kg/m3 higher than before. SSSCs after the extreme flood were lower than those with similar discharge before it in this stage. The TMZ boundary location is correlated logarithmically with river discharge, and the upper boundary moves more seaward than the lower boundary. They moved 6– 20 and − 4– 11 km seaward in the NP and migrated 4– 13 and − 7– 9 km seaward in the SP during the extreme flood, respectively, compared with average annual and usual flood conditions. These findings further clarify TMZ characteristics in extreme floods and can be used as a reference for ecological and waterway projects considering the impacts of extreme flood events. • The surface SSC-discharge curve follows an anticlockwise hysteresis pattern. • The surface SSCs after the extreme floods are lower than before. • The turbidity maximum zone location is logarithmically related to river discharge. [ABSTRACT FROM AUTHOR]