| Yangtze River as the world’s third largest river, the runoff of Datong Station had no significant change in trend. But with the constant interference of the drainage basin sediment yield and sediment transport processes of human activities on the Yangtze river sediment amount stepped descending.1950to2002average sediment concentration for0.479kg/m3, since the mid-1980s, the sediment concentration was significantly reduced from1986to2002years, the average sediment concentration is0.267kg/m3, In June2003years, Three Gorges Project after the impoundment to the2010annual average sediment concentration only0.176kg/m3, water and sediment of the Yangtze River estuary this is bound to make the appropriate response. Based on this issue, the paper firstly based on analyzes the characteristics of runoff and sediment transportation of the Yangtze River in nearly10years, select the rainy year in2003, average year in2007,2011dry year flood and dry seasones multi-station the synchronous continuous25-hour field measurement data, and this corresponds to the rainy year in2003, average year in2007and dry year in2011just typical watershed hydrological year. In order to focus on the spatial and temporal distribution of estuarine sediment, sediment deposition and resuspension in the Yangtze runoff and sediment boundary conditions, study contrast the different river channel sediment movement process, to deepen people’s understanding of the basin to sand sharp drop in conditions of estuarine sediment concentration distribution and the process of change in this regard.The results show that:In the recent, the hydrodynamic conditions in the Yangtze estuary was basically stable, the main channel to the stratosphere net conveyance and water to the sea, usually, the trend of the characteristics of that is the flood season>dry Season,spring>neap,ebb> flood. Flow distribution remained north branch> south branch, the north channel> south channel, North Passage> South Passage but in special dry month happen South Passage> North Passage; Vertical,north tributary speed slot for the north-south flow rate of the smaller area is mainly distributed in the south branch to south channel. Vertical velocity gradient in the past10basic rendering ebb> high tide, the tide> neap tide, north branch, north-south trough the Turbidity Maximum velocity gradient generally larger than other river channel. Impact runoff conditions change on the Yangtze estuary each slot ebb forces, specific performance of the ebb tide of the flood season> amount of high tide tide, ebb advantages flow significantly, the tide ebb preferential flow in more than61%, the advantages of a small tidal flow at68%above; Dry season estuarine areas ebb still dominant, ebb flow advantages percentage decrease, it is natural, dry season fluctuation tide amount of difference between the two is small compared to the flood season, especially at North Passage and South Passage in the dry season flow minimum2007happened high tide in the amount of dominant phenomenon.As we all know, in recent decades, especially the past10years sharp drop in the amount of sand basin to60%of the conditions, the estuary of the river sediment concentration distribution has been made with varying degrees of response, the suspended sediment concentration in south branch appears to reduce, in south and north channels decrease a littel, the suspended sediment concentration changed of north and south passages suspended sediment concentration changed little. Entire estuarine sediment concentration distribution law10years ago watershed runoff and sediment was a period of quasi-natural state is essentially the same. Hanging sand concentration in the groove of north and south passages and the north branch of the turbidity maximum development of the river channel, the southern branch of the lowest concentration of suspended sediment, followed by north and south channels.At the same time, in the flood season, the North Branch and South passage in a tidal cycle appear three sand peak, appear later in the fall anxious moment ebb moment, up anxious moment. North Passage of the river within a tidal cycle two sand peak. Dry season the North Branch of the rivers in the north and south passages within a tidal cycle there are two sand peak.Estuarine areas of suspended sediment particle size was small, suspended sediment mainly clayey silt. Vertical point of view, the median diameter of suspended sediment experienced "small-large-small change, North and South Port river channel suspended sand median diameter, the particles most coarse suspended sediment particles, followed by the south branch of the north-south trough main tank smaller. Sand and silt particles are involved in the southern branch of the main components of the North Port water and bottom sediment exchange water segment of the north and south channels involved in the exchange by the end of sand to clay with silt.Based on information measured continuously for a long time, fitting suitable for different river channel of the Yangtze River estuary sediment transport capacity formula, with good correlation to reflect recent riverbed sediment transport capacity.The main channel in the stratosphere mainly for sediment transport, the Tl sediment transport direction is always to the sea, there is flood season> dry season, tide> neap. In flood season, the T1value of qinglonggang in north branch and South channel was largest, located in north passage T1value of the minimum stratospheric sediment transport capacity is reduced, resulting north passage obvious siltation with the north passage of the river bed constantly frequent changing of erosion and deposition;dry season north branch,the north and south-south tank T1, is conducive to watershed sediment transport seaward. T2net transport direction to the land-based, its absolute value is only less than T1, the general tide of neap tide. In addition, the effect of the influx of pump, which neap tide of flood season tide pump effect is stronger than the dry season, indicating that the exchange process between the water and the riverbed and seasonal variation, related to changes in the size of the tide. It is worth noting that, in the turbidity maximum zone area, South Passage in the flood season tide pump effect is most obvious, followed by the north branch, North Passage tide pump sediment load minimum.In addition, the exchange of research results show that the South Passage and Nanhui Shoal Water and Sediment advection term sediment flux in the total net amount of sediment in the the beach trough water and sediment exchange, the largest contribution to the role of the tide pump sediment input in the South Passage areashift also plays an important role, especially in shallow areas, the role of tidal pump more obvious contribution of net sediment load increases. During the spring tide, the South Passage and Nanhui Shoal exchange in trough water sand, beach, between the flood season, during the flood spring tide, net sediment transport is seaward at deep stations NCH2、NBH4and NBH7, and landward at shallow station NBH6, which forms the sediment transport pattern of input in shallow area and output in deep that leads to slowly silting-up at nanhui beach front, Besides, Formation of transverse circulation supplement sediment further south into the groove main channel, making South Passage of the main tank sediment concentration as a whole can still be maintained at a high level. |
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