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

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.