Regime shifts in the Changjiang (Yangtze River) Estuary

Regime shifts in the Changjiang (Yangtze River) Estuary: The role of concentrated benthic suspensions

Lin, J. (TU Delft Coastal Engineering; East China Normal University)
van Prooijen, Bram (TU Delft Environmental Fluid Mechanics)
Guo, Leicheng (East China Normal University)
Zhu, C. (TU Delft Coastal Engineering; East China Normal University)
He, Qing (East China Normal University)
Wang, Zhengbing (TU Delft Coastal Engineering; East China Normal University; Deltares)

2021

Channel deepening often triggers positive feedback between tidal deformation, sediment import and drag reduction, which leads to the regime shift in estuaries from low-turbid to hyper-turbid state. In this study, a transition in profiles of suspended sediment concentration (SSC) is hypothesised by including a positive feedback loop of vertical mixing and settling. Such a hypothesis is validated by the historical observations in the North Passage of Changjiang (Yangtze River) Estuary, with decreasing SSC in mid-lower layers and increasing SSC near the bed after the deepening. A mobile pool of concentrated benthic suspensions (CBS) develops in the North Passage, with a tidally averaged length of ~20 km and a mean thickness of ~4 m. The width of the CBS pool is limited (<1 km) as the CBS is concentrated in the Deepwater Navigational Channel. The movements of the CBS pool, combined with tidal asymmetry (e.g., slack-water asymmetry and lateral flow asymmetry), results in sediment trapping in the middle reaches and on the south flank of the channel. Observations by a bottom tripod system show the response of friction/drag coefficient to sediment concentration: (1) nearly linear decrease within low SSC (<10 kg/m³); (2) constant and minimum coefficient (with drag reduction up to 60–80%) in the presence of CBS (10–80 kg/m³). An empirical relationship was derived, which can be used to predict the friction coefficient and the magnitude of drag reduction for sediment transport studies, particularly for modelling regime shifts in estuaries.

Channel deepening
Concentrated benthic suspensions
Drag reduction
Estuarine turbidity maximum
Sediment trapping
Turbulence damping

http://resolver.tudelft.nl/uuid:8c46bc53-d826-4d04-80fd-ea2a722736fb

https://doi.org/10.1016/j.margeo.2020.106403

2021-07-02

0025-3227

Marine Geology, 433

Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.

Institutional Repository

journal article

© 2021 J. Lin, Bram van Prooijen, Leicheng Guo, C. Zhu, Qing He, Zhengbing Wang