The Study On Flow And Sediment Characteristics In The Turbidity Maximum Of Changjiang Estuary

The Turbidity Maximum is a special region of estuary, which influenced by river flux, tidal dynamic and estuarine circulation. It’s a sedimentation dynamic process of sediment that drastically affects aggregation and transport of the fine sediment, further have an impact to the development and evolution of the estuary grooves. Due to the high suspended sediment concentration and high turbidity, making estuarine turbidity maximum affect the transport of heavy metals and organic matter, biochemical reactions and biological activities. As the Hengsha Shoal, deep water channel project in North passage and Jiuduan Shoal are located in the turbidity maximum, it’s necessary to research about the water-sediment process, shoal and mouth bar characteristics such as formation, development and evolution, for government’s renovation and construction of the Yangtze estuary.Combined with the hydrological and sediment data of dry and flood season in1982, dry season in2011and flood season in2013at the measurement points which in the turbidity maximum for the following tasks:(1) According to the multi-point continuous measured data, analyze spatial and temporal characteristics of the current of the Changjiang estuary turbidity maximum;(2) Based on the analysis of the hydrodynamics of turbidity maximum zone and adjacent areas studying the characteristics of suspended sediment, including suspended sediment concentration (SSC) distribution, particle size characteristics, planar distribution, vertical distribution, and characteristics change over time;(3) Comparing with the hydrological and sediment data between1982and2011,2013, analyzing of the variation of current and suspended sediment characteristics and change between30years;(4) By calculating the unit width sediment flux and making an analysis of that of each measuring point, analyzing the sediment transport of turbidity maximum.Through analysis, the conclusions are as following: (1) From the area out of the estuary to turbidity maximum zone, tidal current transformed from rotating flow for reciprocating flow. The current velocity of stations in flood and dry season were almost same, but the current direction angle in flood season is larger than in the dry season, this is because the runoff of Changjiang into the estuary is forced to the south under the action of Coriolis force, makes the flood tide velocity direction to the northwest and the ebb tide velocity direction to the southeast. The velocity of spring tide is larger than that of neap tide. In the North Channel and North passage, from the estuary to the outside lengthways, velocity decreases substantially, and outside the Nanhui Shoal, velocity increase gradually. On the transverse comparison, the velocity of stations in the core region of the turbidity maximum decreased gradually from north to south, and increases gradually at the offshore areas, but the maximum velocity was still located in the core region of turbidity maximum.(2) The vertical current gradient of stations at the core region of turbidity maximum is the biggest. Among the three water ways located in the core region the current gradient at North Channel is bigger than that at North Passage, and gradient at South Passage is the minimum. But in offshore areas the situation is completely opposite. At the offshore area at Nanhui Shoal, current gradient of the three stations are approximately equal to0.04s-1.In a spring-neap tidal cycle, the maximum current gradient appears at rapids moments, and the one at maximum ebb is bigger than that at maximum flood.(3) The suspended sediment concentration of stations at core region of turbidity maximum is much higher than at other areas, and gradually reduced to the edge, offshore. The stronger current power of spring tide than neap tide makes the spring tidal suspended sediment concentration higher. Because of open areas in the Changjiang estuary and bigger wind force at dry seasons, more bed material will suspend, and suspended sediment concentration is higher than at flood seasons. Suspended sediment concentration at various stations affected by current dynamic strength over time there will be a cyclical change, due to the lag of sediment resuspension, maximum suspended sediment concentration usually appear after1to2h of rapids moments. With stronger resuspension ability in North Channel at spring tide of dry season and South Passage at spring tide of flood season, the suspended sediment concentration is high overall depth at low water. Among the three channels at the core region of turbidity in the study area, the suspended sediment concentration in North Passage is the largest commonly, South Passage comes secondly, and North Channel is minimal.(4) The median particle diameter at surface layer in the core region and edge of the turbidity maximum were closed, but at the bottom, the median particle diameter in the turbidity maximum core region is larger than the edge areas. Ordinary at spring tide, the current power is stronger and sand can be found in the flow, so the median particle diameter is bigger than at neap tide. There were more sand at North Passage than the North Channel at the similar hydrodynamic condition.(5) The flow discharge of Changjiang is unchanged, with the sediment discharge has decreased about60%in the last30years. The velocity of stations located at turbidity maximum show little change. But the SSC of stations between1982and2011,2013decreased obviously, the SSC in2011,2013decreases about50%to the stations in1982with the same tide situation.(6) By calculating suspended sediment flux of the stations at turbidity maximum, found that the fluxes at stations located in the North Channel and the North Passage transport sediments toward east and southeast respectively, and the fluxes at stations near Nanhuizui tidal flat transport sediment toward north or south. Away from the Changjiang Estuary, the suspended sediment moves toward southeast. Suspended sediment flux is affected by the magnitude of suspended sediment concentration and current velocity, and typically spring tide suspended sediment flux is greater than neap tide. Flood and dry seasons suspended sediment flux contrast features is not obvious at the edge of turbidity maximum, as at the core region of turbidity maximum the suspended sediment flux in flood seasons is bigger than dry seasons. Suspended sediment flux at the core region of the estuary turbidity maximum should be larger than at other areas. Among the three channels (North Channel, North Passage and South Passage), the maximum suspended sediment flux occurs at North Passage. By making an analysis of suspended sediment flux, the dominant mechanism of net transport of suspended is that of non-tidal steady advection transport, mass Stokes drift, tidal trapping and net vertical circulation. They are the main factor in formation turbidity maximum.By analyzing of the water and sediment characteristic of turbidity maximum of Changjiang Estuary, found that at the core region the hydrodynamic force is the biggest, and the current gradient is the biggest, too. The great water power and bottom current gradient make the sediment at bed suspend sharply, and the median diameter of suspended sediment is big. The great hole vertical current gradient can keep the suspended sediment suspended. The dominant mechanism of net transport of suspended is that of non-tidal steady advection transport, mass Stokes drift, tidal trapping and net vertical circulation. They are the main factor in formation turbidity maximum. The biggest water power and high SSC at core region result in great suspended sediment flux.