Abstract:

The spatial and temporal characteristics of current, salinity and the concentration of suspended sediments were systematically investigated using long time series of hydrological observation data of the South and North Branches in the neap tide and spring tide during dry and wet seasons in 2009 and 2010 in the Yangtze Estuary. The relationship between the South and the North Branches was studied in this context. The salt water and sediment intrusion were discussed, especially the sediment intrusion from the North Branch into the South Branch which would influence the concentration of suspended sediments in the South Branch. The results indicate that the changes of salinity and suspended sediment concentration (SSC) in the North Branch were significant. Normally, the salinity and SSC in the spring tide were higher than those in the neap tide. The salinity and SSC during the dry season were higher than those during the wet season while the salinity and SSC in the normal flow year were higher than those in the high flow year. The salinity and concentration of suspended sediments of the South Branch were about 025‰ and 02 kg/m3, respectively, which were only an order of magnitude smaller than those of the North Branch due to the runoff and it presented more complex change that the salinity and suspended sediment concentration will increase significantly in a short time during the ebb tide. The North Branch is the only bifurcation channel that dominated by tide in the Yangtze Estuary. The sediment transports backwards from the North Branch to the South Branch as the saltwater flows backwards. The concentration of suspended sediments of South Branch will increase about 005 kg/m3 in the spring tide during the wet season due to sediment intrusion. Especially in the spring tide during the dry season, the concentration of suspended sediments in the South Branch will be maintained about 025 kg/m3 for a long time and the maximum value can be up to 062 kg/m3 because of sediment intrusion. Recently, the slender sediment intrusion can be observed in the spring tide during the wet season. If the runoff is reduced excessively, the impact of the sediment intrusion from the North Branch into the South Branch will increase. Because of sediment intrusion, the median grain size and grain gradation of the surface water suspended sediment in the northern part of the South Branch (Chongxi Station) is quite different from that in the southern part of the South Branch (Xuliujing Station). The clay content at Chongxi Station is lower than that at Xuliujing Station while the silt content is higher. Sometimes the surface water suspended sediment at Chongxi Station would contain a little sand which may be taken from the North Branch