The Study Of The The Temporal And Spatial Variation, Vertical Profiles And Long-term Variation Trend Of Suspended Sediment Concentration In The Yangtze Estuary

Suspended sediment concentration (SSC) is one of the important properties of the water in estuaries. It also has important geomorphological significance on ecology, geomorphology and engineering. Influenced by tide, wind, wave, Terrain and human activities, the SSC is complex. Although many domestic and foreign scholars carried out a lot of work on the study of SSC in the estuary, but the knowledge about the changes of SSC in time and space remains to be studied in the further.The Yangtze River is the third longest river in the world, and also is the fourth largest river in sediment discharge and fifth largest river inwater discharge. Affected by reservoir construction in the Yangtze River (especially the Three Gorges Dam), water diversion and other human activities, recently there has been a dramatic decline in sediment discharge. In addition, there were a series of major projects implemented in the Yangtze River Estuary, which has changed the river boundary conditions in a certain extent. In this context, the overall level and distribution pattern of SSC in the Yangtze River Estuary will be how to change, is a worth exploring scientific proposition. It not only has important practical significance in the management of Yangtze River estuary, but also is helpful to enrich basic theories of estuarine science knowledge.Based on the simultaneous observations at ten measuring points during spring and neap (both 26 hours) in July 2012 and July 2013, The spatial and vertical variation, spring and neap variation, inter-annual variation of SSC was analyzed. By comparison about SSC with 1982,2003 and 2005 in the same season, tide and stations, we discussed the interdecadal variation of SSC in the Yangtze River Estuary. The variation trend of SSC in the next decades in the Yangtze River was also analysed. The main results and conclusions are as follows:1) Space pattern of SSC in the Yangtze River estuary. SSC in Xuliujin, south branch, the upper of north channel, the next section of north channel, the offshore section of north channel, south channel, north passage, the offshore section of north passage, south passage, the offshore section of south passage respectively is 0.15g/l, 0.19g/1,0.18g/1,0.40g/l,0.14g/10,0.21g/1,0.55g/1,0.19g/l,.0.68g/1,0.30g/l. (1) The SSC had little changed along inner estuary (about 0.2g/l), while it sharply increased by several times in the mouth-bar region, then a downward trend to the level of inner estuary was identified in outer estuary. Accordingly the mouth bar region became a turbidity maximum zone. (2) The horizontal difference of SSC during spring tide was significantly greater than that during neap tide. (3) The horizontal difference of SSC in surface water was obviously weaker than that in the near bottom water. (4) The location of turbidity maximum zone corresponded well with the variations of minimum water depth, maximum tidal range, salinity front and the mutation from sand to mud.2) The vertical variation of SSC in the Yangtze River Estuary.Although instantaneous SSC profile in evey hour is complex and changeable, there is a significant regression relationship (correlation coefficient R2> 0.99) between SSC and water depth at each average profile of spring and neap tide. The bottom SSC bottom in average profile is 5.3 times as the surface SSC. The vertical difference of SSC in the turbidity maximum zone (The SSC in the bottom in average profile is 7.2 times as the SSC in the surface) was significantly higher than the mouth channel (about 2.9 times) and offshore area (about 4.9 times).3) The variation trend of SSC in the Yangtze River Estuary recently. Compared with ten years ago, the average SSC decreased by 30% in ten stations of Yangtze River Estuary in flood season (July) of recent two years. which is similar to the decline on sediment discharge (33%) and SSC (31%) in Datong station in the same period. The average SSC in outer estuary respectively is 0.18g/l,0.13g/l,0.22 g/1 in 1982,2003 and 2005,2012 and 2013. It needs to be studied deeply because the variation trend is not obvious. In a word, because of the decreasing of sediment discharge and SSC, the SSC in the Yangtze River Estuary has spatial differences obviously.4)The prospect of variation trend of SSC in the coming decades in Yangtze River Estuary. In the next few decades, due to the impact of human activities such as the dam builted newly, the suspended sediment flux to the sea and SSC in the Yangtze River Estuary will be reduced further. Because of the surface sediment coarsening and the increasing of downward extrusion degree of sediment (the enhancement of performance about resistance to erosion), the erosion of the Yangtze River Delta is gradually weakened, and the resuspension sediment is also reduced. As a result, the overall level of SSC in the Yangtze Estuary in the next several decades is likely to fall further. However, the spatial variation of SSC and short-term changes (such as seasonal, tidal, inter-annual variation, storm event) will be very obvious.