Tidal hydrodynamics and fine-grained sediment transport on the radial sand ridge system in the southern Yellow Sea

Abstract

The large radial sand ridge (RSR) system located in the southern Yellow Sea off the Jiangsu coast is characterized by a radial current field and high suspended sediment concentrations (SSCs). Significant rectilinear currents are present over the northern sand ridge area, whereas rotary currents prevail over the southern area, the transition between the two being dominated by a locally generated trapped wave. Observations undertaken in 2006 and 2007 indicate a seasonal pattern in the SSC and associated dynamic characteristics. The results of a calibrated hydrodynamic model show that the M2 tide dominates the RSR area, the maximum tidal amplitude in the core region close to land being approximately 2m. An amphidromic point is located some distance north of the RSR off the abandoned Yellow River mouth (121.686° E, 34.6° N). Specifically, the Eulerian and Lagrangian residual currents flow southward in both summer and winter, whereas the Stokes' drift is directed southward in the north and northward in the south. Moreover, in deep water the Lagrangian residual current is mainly determined by the Eulerian residual current, whereas it is strongly influenced by the Stokes' drift in shallow water. The seasonal variation in the residual current in the southern Yellow Sea is a good indicator for the location of the Yellow Sea Warm Current (YSWC) in both summer and winter. Measurements show that a SSC peak appears off the abandoned Yellow River delta in response to the strong currents and plentiful supply of fine-grained sediment from coastal erosion. Another SSC peak lying close to the North Branch of the Changjiang River mouth is related to the abundant suspended sediment supplied by the river. Seasonal variations in SSC are observed in the study area, maximum average values and widest diffusion from land occurring in winter, this being contrasted by minimum mean values in summer. Sensitivity experiments demonstrate that the main factor influencing the distribution of SSC is the tide, while river discharge, wind, and wave effects are small and very localized. To explain the seasonal variation in SSC, the role of hydrodynamics, initial conditions, and temperature are tested in the model. The results show that the hydrodynamics play a significant role, whereas the initial conditions only have a minor regional influence. Temperature, on the other hand, is also shown to greatly influence the seasonal variation in SSC because of substantial changes in the viscosity of the water and the erosion shear stress. The modeled results are used to calculate sediment fluxes, which are shown to be consistent with the Lagrangian residual current pattern, while wind waves can significantly change the fluxes in winter. Bed elevations change substantially on a monthly time scale. In general, the deep channels are undergoing erosion, while the shallow ridges and coastal tidal flats are accreting, the net sediment transport being directed towards the coastal tidal flat and the Changjiang (Yangtze) River subaqueous delta.