Formation of Concentrated Benthic Suspension in a Stratified Tidal Estuary

The concentrated benthic suspension (CBS) of mud, as a major contributor of sediment transport in the turbidity maximum of the estuary, is of great challenge to be correctly monitored through field measurements, and its formation mechanism is not well understood. A tripod system equipped with multiple instruments was deployed to measure the near-bed hydrodynamics and sediments in the North Passage of the Changjiang Estuary, with the aim at determining the formation mechanisms of CBS. The measurements detected a significant dominance of high sediment concentration in the near-bed 1-m layer: ~20 g/L at the southern site and ~47 g/L at the northern site. Strong CBS occurred under weak tidal mixing condition, and was directly relevant to the sediment-induced suppression of turbulent kinetic energy (TKE) and the enhanced water stratification due to saltwater intrusion and sediment suspension. During the weak-mixing neap period, the typical thickness of CBS was about 0.2-0.3 m, with a life time of ~2.83 hours (SSC> 15.0 g/L). Enhanced water stratification reduced vertical mixing and confined the sediment entrainment from the near-bed layer to the upper column. This enhancement was due to the suppression of TKE as a result of the sediment accumulation in the near-bottom column during the slack water, and also due to the appearance of a two-layer salinity structure in the vertical as a result of saltwater intrusion near the bottom. These physical processes worked as a positive feedback loop during the formation of CBS, and can be simulated with a process-oriented, one-dimensional vertical CBS model.