Existing knowledge about groyne-induced effects is primarily based on riverine or coastal environments where salinity gradients are absent or limited. However, in estuaries, salinity gradients drive physical processes such as longitudinal and lateral residual flows. The effect of groynes is much more complex because they can modulate channel hydrodynamics and directly affect lateral salinity gradients. In this study, an idealized model is applied to investigate the effects of groyne layouts in estuarine environments, including effects on (1) channel hydrodynamics, (2) lateral water exchange, (3) Coriolis effects, and (4) saltwater intrusion. Model results show that the aspect ratio (the width of groyne fields to the length of groynes) of groyne fields plays an important role. Groynes also induce asymmetry of lateral flows, for example, increasing near-bottom shoal-to-channel flows during low water slack. The aspect ratio has opposite effects on horizontal and vertical components of water exchange. A large aspect ratio strengthens horizontal exchange and weakens density-driven currents. For a large-scale groyne field (several kilometers), Coriolis effects introduce a substantial difference in exchange mechanisms along the north and south banks. A medium range of aspect ratio (2.0-3.0) leads to the strongest saltwater intrusion during both neap and spring tides.@en

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.@en

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.@en