| Freshwater from rivers and saltwater from the oceans meet at estuaries,resulting in density flow due to differences in density.High density sea water intrudes into land from the lower layer of the water body,and low density river water flows from the upper layer of the water body to the sea,thus forming a vertical stratification of the water body.Saltwater wedge is a typical high stratification state.The existence,movement and dynamic characteristics of saltwater wedge greatly affect sediment suspension,transport and deposition in estuary area.Exploring its internal dynamic mechanism will help us to deepen our understanding of the characteristics of sediment transport in the estuary area and to analyze the fundamental causes of environmental changes in the estuary area.In this paper,the dynamic process of sediment movement in the estuary under the condition of brine wedge is studied through physical experiments.Firstly,the acoustic inversion analysis of suspended sediment concentration is carried out by using high precision instrument profile velocimeter(ADVP)under the condition of static water in laboratory and dynamic water in flume.Then a physical experiment system of rectangular flume with upstream runoff as muddy water and downstream saltwater intrusion was established.The inverted instrument ADVP and point velocimeter(ADV)were used to synchronously measure the velocity and sound intensity(AMP)values.The experimental conditions under different water depth,saltwater flow rate,muddy water flow rate and sampling frequency were designed to analyze the variation of suspended sediment concentration during saltwater wedge intrusion and calculate turbulence.Dynamic parameters explain the cause of suspended sediment concentration change,and obtain the law of suspended sediment concentration change in brine environment.The main research contents and results are as follows:(1)In acoustic inversion analysis,the acoustic intensity signal(AMP)measured by the instrument is positively correlated with the sediment concentration of muddy cement,and an empirical formula for inversion of sediment concentration by AMP value under hydrostatic conditions is proposed.Through verification,the formula has good adaptability in the case ofsmall sediment concentration(0.3g/L).(2)Based on the flume experiment,it is concluded that the saltwater wedge intrusion will lead to the increase of the concentration of muddy cement sand near the salt-freshwater junction,but the influence range has a certain limit,which is about 1 cm near the interface,and can last for a period of time.At the same time,for small sediment concentration,the measurement results of sound intensity signal by ADVP instrument are better than those by ADV instrument.(3)Through the calculation and analysis of turbulence characteristic values,it is concluded that the main dynamic action of sediment concentration increase at the salt-fresh water interface of brine wedge is vertical velocity shear.It produces large-scale vortices,which promote turbulence upward transport,so the particle concentration increases significantly near the brine wedge interface due to turbulence.Although the upper sediment gradually settled,it remained suspended due to the small-scale turbulence of the interface when it approached the saltwater wedge interface,which weakened the sedimentation.Thus,the concentration of turbidity sediment near the saltwater-freshwater interface was observed to increase.At the same time,the emergence of high shear does not necessarily mean the increase of turbulent dissipation.The large-scale eddies generated by high shear are beneficial to particle suspension,but the small-scale eddies do not increase or develop significantly.It can be seen that the reason why the sediment concentration increases for a period of time and then returns to the size of saltwater is the turbulence of the small-scale eddies of the turbidity flow itself. |
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