Cross-shelf sediment transport in the Yangtze Delta frontal zone: Insights from field observations

The Yangtze River, which has the largest runoff in China, discharges massive amounts of sediment into the East China Sea every year, which has an important influence on the biogeochemical processes of estuaries and adjacent seas. However, how much Yangtze River sediment is transported to the East China Sea shelf and the dynamic mechanisms of cross-shelf sediment transport remain unclear, especially given the lack of sediment dynamics studies based on field observations. In this study, we performed 25 h field observations of cross-shelf dynamics in the frontal zone of the Yangtze River (122.74E, 31.04N, water depth: 28 m) in the summer and winter of 2011. The observations show that the strong tidal current with the maximum current speed of 0.8 m/s causes bottom shear stress up to 0.4 Pa, which is the main force for seabed sediment erosion and suspension, while wind waves play a negligible role. Significant stratification appears at the observation site with a maximum buoyancy frequency of 0.01 s- 2 in winter and 0.2 s- 2 in summer, which inhibits the high turbidity water within 10 m from the seabed. The cross-shelf sediment transport at the mooring site is mainly driven by the baroclinic force in the middle and bottom layers, and is influenced by wind in the surface layer. During the observations, the sediment from the Yangtze Delta was transported to the southeast in winter and to the northwest in summer, with sediment transport fluxes of 2.8 and 2.3 g/s/m2, respectively.