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Regime shifts in the Changjiang (Yangtze River) Estuary: The role of concentrated benthic suspensions.
- Source :
-
Marine Geology . Mar2021, Vol. 433, pN.PAG-N.PAG. 1p. - Publication Year :
- 2021
-
Abstract
- 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. • A regime shift was observed in the Changjiang Estuary, with increasing SSC near the bed and a decrease in mid-lower layers. • The Changjiang Estuary exhibits a mobile pool of concentrated benthic suspensions (CBS) after the deepening. • The friction/drag coefficient is constant and minimum in the presence of CBS, with a drag reduction of 60‐–80%. • Slack-water and lateral flow asymmetries cause sediment trapping in the middle and south flank of channel, respectively. [ABSTRACT FROM AUTHOR]
Details
- Language :
- English
- ISSN :
- 00253227
- Volume :
- 433
- Database :
- Academic Search Index
- Journal :
- Marine Geology
- Publication Type :
- Academic Journal
- Accession number :
- 148930099
- Full Text :
- https://doi.org/10.1016/j.margeo.2020.106403