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.

中文翻译：

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盐度和沉积物引起的分层对潮汐河口最大浊度的约束

盐度和悬浮泥沙浓度（SSC）的垂直密度梯度导致河口分层，这在湍流结构、水混合和泥沙输送中起着至关重要的作用。为了研究分层，特别是 SSC 引起的分层对维持河口最大浊度 (ETM) 的影响，我们于 2018 年 7 月对长江口南通道上游和中部 ETM 站点的沉积物动力学进行了原位测量。梯度理查森数被估计为可归因于盐度或/和 SSC 的分层的代理。我们发现盐分诱导的分层主要发生在地表和中层，而 SSC 诱导的分层主要发生在近底层。此外，在中央 ETM，当盐度诱导分层强于大潮时，小潮期间斜压作用增强。在小潮期间以最小速度洪水的早期阶段，盐度引起的分层抑制了沉积物的湍流和垂直扩散。此外，絮凝增强了水柱内的沉降过程。因此，在底部附近形成了高浓度的细粒沉积物，促进了 SSC 引起的分层，从而导致沉积物的不断积累和捕获。综上所述，“盐度和SSC诱导分层”过程之间的相互作用是长江口ETM时空变化的关键约束。