Constraints of salinity- and sediment-induced stratification on the turbidity maximum in a tidal estuary

Abstract

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.

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Acknowledgments

We thank Jieping Tang, Lifen Zhang, Sheng Xu, Yaqing Zhao, Hui Sheng, Dezhi Chen, and Tingfei Lan for their assistance in the field measurements and laboratory works. We sincerely thank the two reviewers and the editor for their valuable comments and suggestions, which are very useful and helpful to make great improvements for the manuscript.

Funding

Financial support for the study was provided by the National Natural Science Foundation of China (41625021 and 41876092) and the Innovation Program of Shanghai Municipal Education Commission (2019-01-07-00-05-E00027).

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Correspondence to Ya Ping Wang.

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Lu, T., Wu, H., Zhang, F. et al. Constraints of salinity- and sediment-induced stratification on the turbidity maximum in a tidal estuary. Geo-Mar Lett 40, 765–779 (2020). https://doi.org/10.1007/s00367-020-00670-8

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Keywords

  • Suspended sediment concentration
  • Salinity
  • Stratification
  • Flocculation
  • Changjiang
  • Estuarine turbidity maximum (ETM)