The Sediment Transport And Its Mechanism In Changjiang Estuary And Its Adjacent Waters

Estuary and coast are the tie between land and ocean,also is the key research areas of the Land Ocean Interaction in the Coastal Zone(LOICZ)program.Sediment transport is the key link of geomorphological evolution and pollutant transportation.In this thesis,we explore the key process of sediment transport and its mechanism in Changjiang estuary and its adjacent areas,which has great significant to scientific research and application.The hydrodynamic module used in this thesis is based on Ecom-si.A sediment module was coupled with the hydrodynamic model to simulate sediment transport.The model domain covered the entire Yellow an East China Seas(YECS)and the Bohai Sea.The model was driven by wind,tide,wave and open boundary.The sediment module takes into account the settlement,resuspension and deposition process.The sediment parameter such as settling velocity,critical shear stress for erosion and critical shear stress for deposition was determined.The sea surface wind and wave parameter was provided by Weather Research and Forecasting(WRF)model and Simulating Waves Nearshore(SWAN)model.A distinct sediment plume exists over the Yangtze Bank in the YECS in winter,but it disappears in summer.The formation mechanism for the sediment plume is controversial nowadays.We used 3-D sediment transport model to explore the formation for the sediment plume over the Yangtze Bank.Numerical experiments showed that the bottom sediment could be suspended by the bottom shear stress and diffuse to the surface layer by vertical mixing in winter;however,the upward diffusion is restricted by the strong stratification in summer.The sediment plume is generated locally due to bottom sediment resuspension primarily via tide-induced bottom shear stress rather than by cross-shelf sediment advection over the Yangtze Bank.However,the coastal sediment is the main source of bed material in the Yangtze Bank by crossshelf transport for the long time scales.Numerical experiments were conducted to investigate the dynamics of sediment transport in Subei waters.Control experiments showed that sediment transport northward in Subei waters during winter and then transport offshore in the vicinity of the Old Yellow River Delta.The dynamics of this sediment transport pattern is tideinduced residual current.Most of the offshore sediment from Subei waters transport southward,the transport mechanism is tidepump transport.The rest of offshore sediment from Subei waters transport southeastward which is mainly dirven by tideinduced residual current and wind-driven residual current.Sediment transport northward with conbine effect of tide and wind.The northwestward wind-driven residual current confine the sediment from transporting off the coast.Sediment transport espectialy the sediment exchange between Changjiang estuary and Subei waters and its mechanism was explored by numerical simulation.Most of the river sediment load deposited in the mouth bar area,part of it transport southeastward and northward.The sediment transport from Changjiang estuary to Subei waters in shallow wates(depth less than 20m),while sediment in Subei waters transport to Changjiang estuary in waters where depth greater than 20 m.Stokes transport is the main mechanism of the northward transportation from Changjiang estuary to Subei waters,while Euler transport is significant only during neap tide.Tidal pump which is related to asymmetric sediment transport is the main mechanism of sediment transport from Subei waters to Changjiang estuary.