Suspended Sediment Transport In Stratified Waters

The formation of stratification in estuarine waters plays an important role in controlling estuarine dynamics.Changjiang is known for its significant river discharge and large sediment load.The Changjiang Estuary exhibits periodic stratification throughout a fortnightly tidal cycle.Stratification prohibits the vertical diffusion of suspended sediments.An estuarine turbidity maximum is formed at the landward limit of the salt wedge,which leads to the deposition of the suspended sediment.Therefore,understanding the onset and breakdown of stratification as well as the transport of suspended sediment in stratified waters is of great importance both in scientific means and in practical means.In the hope of making a little contribution to the estuarine science community,these questions are studied using a numerical model combined with observed data.Before numerical simulations are implemented,the numerical model is improved so as to better reproduce the transport of salinity and sediment in the Changjiang Estuary,with the presence of energetic turbulence and engineering constructions.The alternating direction implicit method is adopted to calculate the water level.Secondly,the vertical diffusion module of suspended sediment is adapted to ensure that the downward settling and upward diffusion of sediment occurs at the same timestep in the model.The last improvement to the model is a dike-groyne algorithm,which helps to simulate the current structures around dikes and mass transports over these constructions.Three boats were deployed along the North Passage of the Changjiang Estuary over a complete spring-neap tidal cycle in the dry season,measuring the currents,salinity,and suspended sediment.This observation reveals that the movement of salt wedge has an important impact on the transport of suspended sediment.Pronounced stratification was observed during the late flood tide and the following early ebb tide,along with the advancing and retreating salt wedge,whereas strong mixing occurred during the late ebb when the effect of the salt wedge faded.Therefore,the SSC in the flood-ebb tidal cycle tended to be asymmetric,which could evoke the tidal pumping transport of sediments,especially in the lower layers.The numerical model results of the Changjiang Estuary show good agreement to the observations.At the timescale of a spring-neap tidal cycle,the residual transport of suspended sediment in the North Passage is seaward,with the magnitude much smaller than the instantaneous value.This implies that the Changjiang Estuary acts more like a trap rather than a conduit to the suspended sediment.As indicated by the results from flux decomposing analysis,the seaward Lagrange transport is balanced by the landward tidal oscillatory transport and vertical shear transport.Influenced by the estuarine circulation,the two-layer structure of residual sediment transport is observed during all the four stages in a spring-neap tidal cycle,with the upper layer flowing seaward and lower layer flowing landward.During the neap tide,evident landward bottom transport of suspended sediment is detected,indicating the the effect of strong baroclinic pressure gradient.The landward bottom transport of suspended sediment ends at the curvature of the main channel,which could induce severe sediment siltation.A new settling term is formed by combining the settling velocity of sediment and the buoyancy term caused by turbulence.This new settling term represents the relative vertical velocity of sediment particle with respect to the water.When tidally averaged,the new settling term is downward in the region of the estuarine turbidity maximum on both flood tide and ebb tide,suggesting the importance of suppression of turbulence by stratification on the formation of the estuarine turbidity maximum.With the freshwater input from the river and saltwater input from the ocean,an estuary produces intermediate salinity water and exports the brackish water back into the ocean.This mixing process fundamentally depends on the horizontal salinity gradient between the riverine freshwater and the oceanic saltwater,which is a fundamental characteristic of estuaries.This horizontal salinity gradient drives estuarine circulation,which provides the mean shear that strains the horizontal salinity gradient,giving rise to stratification.Vertical stratification is another essential estuarine variable,and it has significant influence on various estuarine processes by affecting vertical mixing.This paper applies a vertical salinity variance methodology for studying the roles of straining and mixing in controlling stratification,applying the approach to the temporal and spatial variation of stratification in the Changjiang estuary.At the beginning,three kinds of salinity variances are introduced: the total variance,the horizontal variance,and the vertical variance,with the former being made up of the latter two.Then it is demonstrated that the vertical salinity variance can be used to quantify stratification.The vertical variance equation indicates that stratification is produced by the straining induced by the vertically differential advection of the horizontal salinity gradient,which represents the local conversion of horizontal variance to vertical variance.While straining provides exchange between horizontal and vertical variance,the dissipation of variance is accomplished only in the vertical dimension via the mixing of stratified fluid.In the Changjiang Estuary,the spring-neap variations of stratification are the result of intensified straining during neap tides and intensified dissipation during spring tides.Throughout the spring–neap tidal cycle,straining is almost always larger than dissipation,indicating a net excess of production of vertical variance relative to dissipation.This excess is balanced on average by advection,which exports vertical variance out of the estuarine region into the plume.It is worth noting that,though advection is not as significant as straining or dissipation when volume averaged in the estuary,it plays an important role in the exchange of salinity variance between the ocean and the estuary: it brings total variance in the estuary and then brings vertical variance out.Two different mechanisms are observed in generating peak stratification regions in the Changjiang estuary.One is the great excess of straining over dissipation,which is balanced by the divergence of stratification.The other one is the convergence of the transport of stratification,which is balanced by dissipation and negative straining.