Distribution and diffusion of suspended sediment in the offshore area of Changjiang estuary,China

Abstract

The Changjiang estuary located in the coastal area of East China is the largest estuary of the country.The offshore area is the main place where sediments spread and pile up in the Changjiang estuary,and is also the remarkable area of land-ocean interaction.This paper takes the offshore area of the Changjiang estuary as a study area,utilizes the abundant in-situ data to study the characteristic of suspended sediment under the different water conditions and calculates the vertical diffusion coefficients. The difference of suspended sediment concentration is very remarkable in this area,the transient suspended sediment concentration is low,ranging from less than 0.1kg/m~(3) to over 8 kg/m~(3),the different runoff,tidal current,salinity and topography determine the complexity of the spatio-temporal distribution of the sediment concentration.The study result shows that the main features of the suspended sediment are in an asymmetric distribution which presents high concentration in the west and south and low in the east and north.This pattern of the suspended sediment concentration distribution proves that the suspended sediment spreading course is obviously influenced by the runoff and ocean dynamics,the topography of relative function and restriction in the study area. The difference in longitudinal distribution is very remarkable in the offshore area of the Changjiang estuary,the sediment concentration reduces gradually from the west to the east,and it is obviously higher in the spring tide than in the neap tide.Salinity is greater than the density of the silt-contained water,as the diluted water from the upper reaches float above the high-density salt water,and fine sediment deposits in the mouth of the salt wedging. Generally,the suspended sediment concentration increases gradually from surface to bed,because of asymmetric distribution and fluctuation of tidal current velocity,the vertical instantaneous velocity is always changing.So it is very difficult to divide various kinds of vertical distribution.The vertical diffusion coefficient is relatively huge in the west and middle water area,and is irregularly distributed in the vertical mixing,so the suspended sediment concentrations increase from the surface to the bottom equably in the area.Along with the strengthening of the degree of the vertical mixing,the suspended sediment concentrations also increase remarkably.As a result,two high suspended sediment concentration zones come into being in the south and the north of the offshore area of the Changjiang estuary.