Variation Characteristics And Mechanism Of Turbidity Maximum Zone Of The Yangtze River Estuary Under The Condition Of Declining Fluvial Sediment Discharging

The turbidity maximum zone of the estuary is generally located at the interface of salt and fresh water in the estuary,and the suspended sediment concentration(SSC)is significantly higher than that in the upper and lower reaches of the estuary.The maximum turbidity zone of the estuary is affected by multiple factors,such as the source of sediment,the dynamic process of the estuary,and the interaction of salt and fresh water,and has a very complex movement law.The study of the change process,characteristics and influence mechanism of the SSC in the maximum turbidity zone will affect the development of estuarine landforms,delta evolution is of great significance.In the past 40 years,the continuous reduction of sediment in the river basin has had a profound impact on SSC of the turbidity maximum zone of the estuary.The research is based on the measured data obtained by the research group in September 2019 and July 2020 to carry out the comprehensive hydrological survey of the turbidity maximum zone in the flood season of the Yangtze River Estuary.The stations can be compared with historical data(1982,1984,1985,1996,2003,2004,2013)to analyze the variation characteristics of runoff,the maximum turbidity zone and the isobath 10 m outside the mouth,and to study the continuous flow of runoff sediment.The decline process,influencing factors and dynamic mechanism of SSC in the maximum turbidity zone of the estuary under the background of reduction,the preliminary results are as follows:(1)The continuous decrease of incoming sediment of the river basin is the main reason for the decrease in sediment concentration in turbidity maximum zone of the Yangtze River EstuaryThe continuous reduction of sediment in the river basin is the main reason for the decrease of SSC in the turbidity maximum zone of the estuary.On the one hand,the reduction of sediment from the basin leads to low sediment runoff entering the estuary area,reducing the source of sediment in the turbidity maximum zone.On the other hand,the continuous reduction of sediment from the basin causes the estuary delta to experience sedimentation slowdown,equilibrium and local erosion processes,and the estuary delta is resuspended.The source of sediment is reduced,and the supply of sediment in the turbidity maximum zone of the estuary is affected.There is a high correlation between the decrease of the SSC in the runoff and the decrease in the turbidity maximum zone.From 1982 to 2019,the average SSC in the turbidity maximum zone decreased from 1.5 kg/m3 to 0.44 kg/m3,and the average SSC in the runoff decreased from 0.54 kg/m3 to 0.11 kg/m3.But there is a certain lag in the decline of SSC in the turbidity maximum zone,roughly around 2004 as the division,the decline rate of runoff SSC is characterized by "fast before and slow"(1982 from 2004 to 2004,the average SSC of runoff decreased by 0.36 kg/m3,and from 2004 to 2019,the average SSC decreased by only 0.07 kg/m3),while the decline rate of the SSC of the turbidity maximum zone was roughly around 1996 as the division,which the decline rate It has the characteristics of “slow before and fast”(from 1982 to 1996,the average SSC in the turbidity maximum zone decreased by only 0.1 kg/m3,and from 1996 to 2019,it decreased by 0.96 kg/m3).The effect of surface SSC is more direct.(2)Sediment resuspension delays the decreasing trend of SSC in the maximum turbidity zone of the estuaryIn the past 40 years,the sediment in the runoff has continued to gradually decrease.In 1982,the SSC in the runoff was 0.54 kg/m3,which was higher than 0.42 kg/m3 outside the mouth during the same period.By 2019,the amount of sediment will drop to 0.25 kg/m3,and the decline in runoff SSC is higher than that outside the mouth.The calculation results show that the sediment outside the mouth tends to be transported into the mouth,and the resuspension of the sediment outside the mouth can provide sediment supplement and delay the decreasing trend of SSC in the turbidity maximum zone of the estuary.(3)The longitudinal circulation of the estuary is an important reason for the high sediment concentration at the bottom of the stable maximum turbidity zone.Due to the difference in the density of salt and fresh water,the estuary water body forms a vertical circulation system in which the surface fresh water moves to the sea and the bottom sea water moves to the land.The longitudinal circulation of the estuary is an important dynamic process for the transportation of resuspended sediment from the outside of the mouth to the turbidity maximum zone.The resuspended sediment from the outside of the mouth is transported into the mouth under the action of the longitudinal circulation of the estuary,which is the important reason for the high SSC at the bottom of the turbidity zone.There are few sources of runoff and sediment,and the surface and bottom SSC of the water body in the turbidity maximum zone shows a downward trend.The SSC in the bottom water body is recharged by the resuspended sediment outside the mouth.