Formation of Concentrated Benthic Suspension in a Time‐Dependent Salt Wedge 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: 20g/L at the southern site and 47g/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 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 0.2-0.3m, with a life time of 2.83hr (suspended-sediment concentration>15.0g/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 turbulent kinetic energy as a result of the sediment accumulation in the near-bottom column during the slack waterand 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. Plain Language Summary In the turbidity maximum, concentrated benthic suspension (CBS) frequently dominates the near-bed sediment transport, which, however, is difficult to be correctly observed, especially in the stratified condition. And the formation mechanism of CBS is complex due to the interaction of sediment and hydrodynamics. In a stratified tide channel of the Changjiang Estuary, China, a comprehensive tripod system, integrated with many cutting-edge instruments, is developed and deployed to measure the CBS and to determine the formation process. This system detected the existence of CBS in the lower near-bed 1-m layer, 20g/L at the southern site and 47g/L at the northern site, with the mean thickness of 20-30cm. The corresponding salinity, tidal velocities, wave, and turbulent kinetic energy were also recorded. During the formation of CBS, tidal mixing is weak. And the salinity/sediment-induced stratification greatly limits the vertical mixing and suppresses the turbulent kinetic energy production. The saltwater intrusion creates a two-layer structure of salinity, leading to stratification and decreasing the mixing. These physical effects work like a positive feedback loop. A one-dimensional vertical model had been developed to successfully simulate the formation process of CBS, indicating the stable CBS during weak-tidal-mixing neap cycle.