Sediment Mixing And Exchange Processes In The Yangtze Estuary

Based on the data of grain size distributions (GSDs) of suspended and surface sediments, current velocities, suspended sediment concentrations (SSC) and salinities collected synchronously during 2003 to 2007 in the Yangtze Estuary and adjacent area, the temporal and spatial characteristics of GSDs were carried out to study the sediment exchange processes under the estuarine mixing conditions, as well as to explore the fate of suspended sediments in the Yangtze estuary-shelf systems. This study was to analyze the GSDs of the suspended and surface sediment in the earlier 21^th century, and to study the characteristics of sediment dynamics and the sediment exchange. The grain size spectral analysis provided by this study can be used for the study of sediment dynamics in other tidal environments. The analyses of mixing and exchange processes based on the GSDs can enrich the methods and theories of the sediment dynamics and sediment exchange, as the reference for the relative research in other estuaries.The spatial distribution of the sediment exchange ratios demonstrated that small amounts of suspended sediment were deposited onto the seabed of the upper estuary (exchange ratio < 0.1), because the fine-grained suspended sediments in this region were transported to the mouth bar area and subaqueous delta by the ebb-dominated tide flow. The sediment exchange ratio in the outer estuary also showed very low values (< 0.1) due to the oceanic currents offshore that prevented the diffusion of riverine sediments further seaward. However, the intensive sediment exchange occurred in the inner estuary due to the sediment mixing which controlled by bidirectional tidal flows. In addition, a high sediment exchange ratio occurred in the muddy area (>0.9) at the river mouth, which implies that this is the depocenter of the Yangtze mud. Relative higher sediment exchange ratio occurred in the mouth bar area (0.4～0.6), which indicates that the proportion of the suspended sediment deposit in the mouth bar area is not the maximum in the estuarine system. The sediment in the mouth bar areas with high sedimentation rate is come from the sediment transport from the muddy area with flood tidal flow.The analyses of grain-size spectra were conducted in an attempt to quantify the sediment mixing and exchange processes in the Yangtze Estuary and the adjacent region based on the principle of mass balance. This study makes great progress in the sediment dynamics and analysis methods. Based on the average GSDs of the suspended sediments, approximately 47% of the sediments from the Yangtze River accumulate in the mouth bar area and subaqueous delta, while the rest are transferred to the Hangzhou Bay and transported southward along the coasts. This study provides another analysis method to examine the suspended sediment transport and its fate through the analysis of GSDs.The temporal-spatial characteristics of suspended sediment grain-size distribution and its response on the conditions of sediment transport decrease from the Yangtze River are also carried out in this study. A clear fine-coarse-fine pattern of suspended sediment median diameter (D₅₀) is shown from the upper estuary to continental shelf. The coarsest suspended sediments are present in the mouth bar area, and the coarser are present in the upper estuary, while the finest are present in the continental shelf. The increase of D₅₀ in the mouth bar area is resulted in the sediment resuspension and sediment exchange between channel and shoal. Clayey silt is the dominant sediment type in the study area. The contents of clay, silt and sand are 30%, 65% and 5%, respectively.The D₅₀ of suspended sediment in neap tide is decreased by 24% compared with that in spring tide, due to the coarse silt and sand components are difficult to suspend into the water during the neap tide. The SSC at the upper estuary in the year of 2007 is 66.5% lower than that in 2003, and the D₅₀ shows no significant variation trends. However, The D₅₀ at the mouth bar area in 2007 is 20% lower than that of 2003, and the SSC is only 10% lower than that in 2003. The D₅₀ of the Yangtze Estuary in the year of 2007 was decreased by 11% in the comparison with that in 2003, while the SSC was decreased by 22% in the same period. Under the situations of reduction of sediment loads from the river basin, the high SSC was maintained in the mouth bar area due to the inner sediment supply from the estuarine systems: sediment resuspension and sediment exchange between channel and shoal. The responses of SSC in the upper estuary and D₅₀ in the mouth bar area on the reduction of sediment loads from the river basin are more sensitive than the other region of the Yangtze Estuary.Symmetric sediment mixing characteristics are present in the suspended sediment of Yangtze Estuary, and the fine-grained and coarse-grained sediments display a simple and perfect linear mixing, which are primarily deposited as flocs. However, asymmetric sediment mixing characteristics are present in the surface sediment. Specifically, the fine-grained sediments display a simple linear mixing of the cohesive and cohesionless fractions, which are primarily deposited as flocs. The coarse-grained sediments display a selective deposition of the cohesionless sediment fraction, which is primarily deposited as single grains. Furthermore, high rates of flocs deposition were recorded in the outer estuary between 122°E and 123°E along the coast. The fine-grained sediment belt along the south coasts beside the Hangzhou Bay and Zhoushan Islands indicates the deposition and southward pathway of suspended sediments from the Yangtze Estuary in the form of flocs.Conversion of the dimensionless exchange ratio, p, to the exchange rates of the fine-grained sediments was achieved by considering the bulk sediment accumulation rates. The sediment from the Yangtze Estuary is deposited in the muddy area firstly, then, which is transported southward into the Hangzhou Bay and south coasts with the tidal flow. In fact, the muddy area is the "transfer station" of Yangtze sediment southward transport.These results are confirmed by the analyses of the remote-sensing image, current velocities and SSC in the outer of the Yangtze Estuary. Estuarine sediment from the North Channel, North and South Passages are transported to the southeast and converged at the outer of Nanhui Headland. The ebb flow and residual current are converged at the outer of Nanhui Headland too. Moreover, the SSC in this region is higher than other regions of the outer mouth bar area. On the decadal to centennial time scale associated with our results, a clearly defined depocenter of Yangtze mud with the exchange rates of greater than 2-3 cm/yr is present in the south of the estuary. This depocenter, which extends to the south of the estuary, is located between 122°E and 123°E longitude, and around 31°N latitude. Our study of sediment exchange rates provides a clear picture of suspended sediment transport pathways in the Yangtze Estuary and adjacent region, and allows us to trace the fate of suspended sediment supplied by the Yangtze River.The sediment mixing processes are conducted by the analyses of the synchronous current velocities, SSC and salinity. The gradient of D₅₀ showed a small-great-small pattern in the Yangtze Estuary, which were consistent with those of velocity, SSC and salinity gradients. The great gradients are present in the mouth bar areas. The increase of silt component conducted by the resuspension, which was controlled by the shear flow, contributed to the increase of SSC and D₅₀.Turbulence is largely suppressed by the sediment. The proper intensity of shear flow redounds to maintain the high SSC in the estuary. Stratification of SSC reduces the turbulence intensity when the shear flow is low. The SSC gradient increases with the increase of turbulence, as a result, the stratification of SSC suppresses turbulence production until the Richardson number (Ri) is near to the constant 0.25. If the critical level of Ri is exceeded to 0.25, however, turbulence will be damped and sediment settling will exceed further resuspension until Ri returns to the critical value of 0.25.The recent GSDs of suspended sediment and the sediment mixing processes in the Yangtze Estuary and adjacent area are conducted in this study, moreover, the transport pathway and the fate of the Yangtze sediment in the continental shelf of East China Sea are carried out as well as.