Suspended Sediment Transport Processing And Associated Mechanism In The South Passage Of The Changjiang Estuary During The Dry Season

Estuary is the most intense and sensitive area of land-sea interaction.Rivers will carry a large amount of water and sediment into the ocean through the estuary.The area is affected by multiple dynamic factors such as runoff,tidal current and waves,and the hydrodynamic conditions are complex.In recent years,with the construction of the Three Gorges Project and other projects,the sediments in the Yangtze River estuary area have dropped sharply.Large-scale silt-promoting reclamation projects have changed the shape of the river channel and affected the water and sediment transport mechanism of the estuary channel.The South Passage is located in the area of the turbidity maximum zone of the Chingjiang Estuary.The material migration process in this area is relatively strong.Therefore,this article takes the South Passage of the Yangtze River Estuary as the research object.In December 2018,three marine systems were carried out using a variety of marine instruments.The station's 13 tidal cycles of neap-spring tides were continuously observed at fixed points,and data such as water depth,velocity,flow direction,temperature,salinity,and suspended sediment concentration were obtained;the flux mechanism decomposition method was used to quantitatively analyze the effects of different physical processes on suspended sediment transport.The contribution of transportation volume,discusses the dynamic characteristics and sediment transport mechanism of different sections of the South Passage during the dry season.The main phenomena and conclusions drawn in this article are summarized as follows.In this paper,the South Passage of the Chingjiang Estuary was used as the research object.In December 2018,a shipboard system of various marine instruments was used to conduct continuous observations of the spring tide-neap tide at 3 stations and13 tidal cycles,and obtained water depth,velocity,flow direction,Data such as temperature,salinity and suspended sediment concentration;the flux mechanism decomposition method is used to quantitatively analyze the contribution of different physical processes to the suspended sediment transport volume,and to explore the dynamic characteristics of different areas in the upper,middle and lower sections of the South Channel during the dry season.Sediment transport mechanism and its main influencing factors,analyze and summarize the differences in suspended sediment transport mechanism and dynamic characteristics between the beach trough exchange zone and the main trough of the south channel.The main phenomena and conclusions drawn in this article are summarized as follows.The South Passage is dominated by semi-diurnal tidal waves,and the nature of the tide is irregular semi-diurnal tidal waves.The amplitude of the M2 tidal component is dominant at each station.The upper tidal wave is dominated by standing waves,and the middle and lower sections are dominated by forward waves.From the upper section to the lower section,the degree of tidal wave deformation gradually weakens.During the observation period,the measured maximum wind speed reached 10.8m/s.The southwest wind will inhibit the intrusion of salt water and promote the vertical mixing of the water body,keeping the salinity of the water body at a low level.The northwest wind and the northeast wind are the opposite.The tidal flow velocity gradually decreases from the upper section to the lower section of the South Passage.The average vertical tidal velocity is 0.85m/s,0.77m/s and 0.72m/s,respectively.The upper and middle section is more affected by the change of tidal range than the lower section.The measured concentration of suspended sediment in the upper section is relatively high,the average value of the vertical tidal cycle is between 0.22?1.22kg/m~3,the concentration of suspended sediment in the middle and lower water bodies is relatively low,and the average vertical line of the tidal cycle remains around 0.3kg/m~3.There is an obvious positive linear relationship between suspended sediment concentration and flow velocity at each station.Affected by factors such as sediment particle size,there are significant spatial differences in the correlation between suspended sediment concentration and flow velocity.The upper station is significantly stronger than the lower station.The intrusion and stratification of salt water will change the vertical structure of the water velocity and the concentration of suspended sediment,and indirectly affect the transportation of suspended sediment.During the observation period,the Euler residual currents at all stations are uniformly facing the sea,and the Stokes residual currents are uniformly facing the land.Under the influence of tidal waves,the intensity of the Stokes residual currents in the middle and lower sections is greater than that in the upper sections.The suspended sediment transport mechanism and dynamic characteristics of the South Passage have obvious spatial variation characteristics.For the station in the upper section of the South Passage,it is strongly affected by the deformation of runoff and tidal waves,and the asymmetry of the ebb and flow duration,flow velocity and suspended sediment concentration is significant.During the observation period,there is a net transport of sand to the land and its intensity is greater;the lower station is affected by tidal currents.Impact,the net transport of sand to the sea.The longitudinal advection and tidal pumping effect of the two stations in the upper and lower sections are the key factors to control the size and direction of the net sediment transport in each tidal cycle.In the middle section of the South Passage,the exchange of water and sand in the beach and channel is strong,and the strong lateral flow controls the intensity and direction of the net transport of suspended sediment.Such a sand transport mode means that more sediments are transported from the tidal flat to the main trough,which may cause the erosion of the upper shoal.During the dry season,there is a spatial transport pattern of net sand transport from the middle and upper parts to the land and from the lower part to the sea in the South Passage.Advection and tidal pumping are the key factors that affect and control the net sediment transport.Their intensity and contribution have obvious tidal cycle and spatial changes.The vertical circulation sediment transport is very weak and contributes to the net sediment transport.The asymmetric phenomenon of ebb and flow of sediment transport in the South Passage is obvious.The water depth,velocity,salinity,suspended sediment concentration and duration all have certain ebb and flow asymmetry.The asymmetry intensity and direction of ebb and flow of these factors jointly control each station.The intensity,direction,and temporal and spatial changes of sediment transport in different horizons.In the upper and lower sections,the asymmetry of the flow velocity is weak,and the asymmetry of the suspended sediment concentration and the ebb and flow of the duration directly affect the net sediment transport,while at the middle section,the flow velocity and the concentration of the suspended sediment are always asymmetric dominated by the flooding tide.In contrast,the diachronic asymmetry is offset and the throw controls the net sand transport to the land.