Hydrodynamic Characteristics And Suspended Sediment Transportation In The Tubidity Maximum Zone Of Yalu River Estuary

Turbidity maximum(TM) zone of estuaries is suffered from complex dynamic conditions, and affected by many processes, such as tide, estuarine circulation, runoff et al.. Well understanding the sediment dynamics are not only beneficial to study the hydrodynamic and suspended sediment transport in the TM, but also helpful to the coastal engineering and management of local resources exploitation. In this study, several observations in different position of the Estuary of Yalu River were carried out. During the August,2009, synchronization data including current, salinity, temperature and suspended sediment concentration (SSC) at both upstream and downstream of TM zone (Y02, Y03) were collected during both spring and neap tides. A mooring observation within long-term period (Y01) was observed from19th Jun. to2nd Aug,2010, and the same haydrodynamic parameters were obtained in station Y02and Y03. In addition, a cross-section observation from the upstream to the entrance of Yalu River was explored on2nd Aug,2010. Subsequently, the distribution of velocity, SSC, salinity and temperature were analyzed. Using method of flux decomposition of suspended sediment transportation, the mechanism of sediment transport was discussed. Finally, the TM forming mechanisms of the Yalu River Estuary were discussed.The analysis results of these observations showed that the Yalu River estuary, with a maximum tidal range of6m, was characterized by reversing current, and the ebb duration was longer than the flood duration. The maximum depth-averaged velocity were observed3-4hours before and2-3hours after the flood slack, with a range from0.9m s-1to1.2m s-1. Contrasting to the depth-averaged velocity, the flood current which was much greater than that the ebb current were recorded in the near sea bed current. The max difference between the bottom velocity between ebb and flood reach0.5m s-1, which was quite different with the depth-averaged velocity induced by the runoff.Lagrangian, ulerian and stokes residual current were caculted. The results showed that, lagrangian residual current was seaward, except during the neap tide of Y02. Ulerian residual current was seaward and Stokes residual current was landward for all the stations. All the three kinds of residual current were much larger during the spring tides rather than that during the neap tides. Y01was a long-term observation station. The lagrangian residual current, ulerian residual current and the stokes residual current was5.5cm s-1(seaward),15.2cm s-1(seaward), and9.7cm s-1(landward), respectively.The SSC distribution during the flood was different from that during the ebb. The maximum flood velocity were always connected with the uniformly vertical distribution of suspended sediment concentration, and the corresponding maximum SSC was up to1000mg L-1. Contrastly, high SSC were only observed in the bottom layer, in terms of high quantity of1600mg L-1, when maximum ebb velocity occurred; however, The SSC of upper water body was quite small, which was always smaller than400mg L-1. This phenomenon may be caused by the bottom suspended sediment diffusion blocked by stratification. During the flood phase, the saltwater wedge could reach station Y01, which accelerate the diffusion of bottom suspended sediment. In contrast, during the ebbs, there was no saltwater wedge, and the diffusion was slow down, causing a high value of SSC only in the bottom water body.The result of flux decomposition of sediment transport showed that, the items of Eulerian Residual current (seaward), stokes residual current (landward) and tidal trapping played important roles on the suspended sediment transport in the Yalu River Estuary. In the Y01, the suspended sediment transport was dominated by Advection transport. However, tidal pumping show a high linear correlation with the suspended sediment transportation (R2=0.82), implying that the variation of suspended transport was determined by that of tidal pumping.The bottom boundary observation showed that, SSC of100cm was much bigger during ebb than that during flood. The maximum SSC was always smaller than1000mg L-1during the flood phase, and larger than2000mg L-l during the ebb phase, which may be related with the difference of the resuspension time between the ebb and flood phase. Calculation result showed that resuspension time during the ebb was more than two times of resuspension time during the flood.During the observation of Y01, flood discharge of upstream reservoir increased the quantity of runoff. Although the resuspension of bottom sediment was not significantly influenced, the bottom suspended sediment diffusion was strongly restained, especially during the flood phase. Therefore, the decreased diffusion resulted to the lowerening of the total SSC. Furthermore, the variation ranges of instantaneous item of flux decomposition was also reduced, and the landward sediment transport was correspondly decreased..Strong resuspension of bottom sediment is the most important factor that maintain TM of Yalu River. The resuspension in TM not only increases the SSC of water body, but also supply rich sediment source which transported landwards and seawards, respectively.