Morphodynamics of world's river deltas are increasingly affected by human activities, which are of great ecological, economic and social implications. However, impacts of human interventions in deltaic regions are insufficiently
understood, especially superimposed upon diminishing sediment supplies. This study uses the heavily interfered Yangtze River delta as an example to address this issue. The morphodynamic impacts of the Deepwater Navigation Channel Project (DNCP) during 1997–2013 are investigated through process-based
modeling approach (Delft3D) and bathymetric data analysis. The DNCP was implemented in the mouth bar area of the Yangtze River delta including the twin dikes and 19 groynes with the total length of 132.0 km. Hydrodynamic simulations indicate that the training walls resulted in weaker tidal flow and longer slack period at the East Hengsha Shoal (EHS) and stronger tidal flow at the subaqueous delta. Thus, the EHS is characterized as a sediment accumulation zone after the completion of the training walls. Subsequently, morphological
modeling shows enhanced accretion at the EHS and enhanced erosion at the subaqueous delta when the training walls are taken into account. Numerical experiments further demonstrate that the above changes are mainly attributed to the seaward half of the northern training walls constructed in 2002–2005. This is probably the reason for the observed accretion peak of the EHS in 2002–2007 and the gradual increase in the erosion rate of the subaqueous delta after 2002. The schematized paths of sediment transport after the DNCP indicate that
sediment eroded from the subaqueous delta serves as an important source for accretion of the mouth bar area. It is suggested that siltation promoting projects within the mouth bar area increased shallow shoal accretion and aggravated erosion at the subaqueous delta. With the overall erosion of the Yangtze River delta due to river sediment reduction, large-scale estuarine engineering projects substantially increase the complicacy of its morphodynamic pattern, which merits close attention for sustainable delta management.

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.