The Study On Sediment Transport In The Changjiang Estuary And 3D Sediment Numerical Modeling

The sediment transport is the most complex problem in estuary. The physical model, numerical model and field observation are the most important methods to research sediment transport, which have developed enormously in recent years. The physical model is still a main means for research estuarine engineering, which can be used to research some mechanisms that can't be described by numerical expression, such as bed load transport, stabilization of hydraulic engineering and et al. The numerical model has been developed to be a more and more important method to research sediment problems. These mechanisms of sediment advection, diffusion, deposition and resuspension are three dimension processes, so three dimension sediment numerical model is need to research those mechanisms. The field observation is the most direct and convincing method for research on sediment transport, which can be used to reveal some new mechanisms of sediment transport. In this paper, these three methods were used to research sediment transport in Changjiang Estuary.There are many interlaced shoals and channel and the river pattern is very complex in Changjiang Estuary. The bed load transport play a key role in the general process of erosion in shoal head and deposition in shoal tail. Based on the in-situ samples collected by sampling in field and flow flume experiment, the appropriate model sediments were selected to simulate the bed load sediments in a validated physical model. The path line and deposition of sediment from several shoal head and main channel in South Branch, North channel and South channel in Changjiang Estuary were investigated in physical model.To investigate the dispersal pattern and the fate of dredged material disposed in pre-selected disposal site, a field tracer experiment of simulation of dredged material dispersion was conducted in North Passage of Changjiang Estuary during 2005 flood season. Three tons dredged material and 2.792kg Sodium hexachloroiridate (â…£) hexahydrate (SHH) were used to make tracer sediment. Three round sampling were carried out in pre-selected sections on the third, fourth and fifth day after the release of dredged materials. All samples were analyzed by neutron activation analysis and results showed disposed dredged material mainly dispersed and deposited between navigation channel and south dike in the North Passage, whose path line were nearly parallel to navigation channel. Only few dredged material entered or came across navigation channel and the ratio of back silting in navigation channel was about 5%. Disposed dredged material dispersed southeasterly beyond two dike heads.Four bottom-mounted instrument-equipped tripods were deployed at two sections spanning the region characterized by severe sedimentation rates in the Deepwater Navigation Channel (DNC) along the North Passage of Changjiang Estuary in order to observe currents, near-bed suspended sediment, and salinity. Seaward residual currents predominated in the up-estuary section. In contrast, a classical two-layered estuarine circulation pattern occurred in the down-estuary section. Flow moved seaward in the upper layer and a heavier inflow, driven by the salinity gradient, moved landward in the lower layer. The near-bed residual currents in the up-estuary section and the down-estuary section acted in opposing directions, which implies that the region is a convergence zone of near-bed residual currents that trap sediment at the bottom. The maximum salinity gradient at the maximum flood current indicates the presence of a strong front that induces sediment trapping and associated near-bottom convergence of sediment.Observation on south dike and shoal showed huge net sediment transport flux from Jiuduansha shoal to North Passage expect slight inverse flux from S3 to S4 groins during the south dike was submerged. The net sediment transport flux from Jiuduansha shoal to North Passage was dominant. The high SSC flow from Jiuduansha shoal can't transport directly to navigation channel in North Passage.A three dimensional sediment transport model was setup based on ECOM-si model. A TVD scheme was adopted to calculate the sand deposition process and a convenient flocculation formula was reduced to express the effect of salinity and SSC on sediment setting velocityso sediment deposition process can be simulated more accurately. The effect of wave on turbulent and bottom shear shtree were adopted to include wave. A bottom-mounted instrument-equipped tripods was deployed to observe near bed SSC, current and bed level to calculate several key parameter for sediment model, such as critical initial shear, critical deposition shear, erosion rate and deposition rate. Based on those work, the sediment model can be validated well, which can be used to study sediment transport in Chanjiang Estuary.The sediment transport process was investigated by the validated sediment model. The study results show that the sediment Eular transport flux in main channel is seaward but that on shoal is landward. The sediment Stocks transport flux decrease the seaward Eular transport flux in main channel but increase the landward sediment Eular transport flux on shoal. The sediment Lagrange transport flux pattern is similar to Eular transport pattern. The tidal pumping flux is landward at up-estuary of mouth bar but seaward at down-estuary of mouth bar. The tidal pumping flux is a divergence pattern over mouth bar. The vertical shear sediment transport flux is intensive over mouth bar and is much less than that of Lagrange flux and tidal pumping flux at other area.