The Observation And Simulation Of The Hydrological And Sedimentary Dynamic At A Tide-dominated Estuary

As the main dynamic factor of the tide-dominated estuaries,tide plays an important role in the hydrodynamic process and geomorphic evolution of the tide-dominated estuaries.As a typical tide-dominated estuary,the evolution of hydrodynamics and geomorphology in the north branch of the Yangtze River estuary has become more complicated due to climate change,the reclamation and ecological restoration projects in this area.Under the new background,the study of the hydrodynamic and sedimentary process of the North Branch of the Yangtze River estuary is of great guiding and practical significance for the geomorphological evolution and engineering application of the North Branch in the future.Based on the comprehensive data collected by the tripod observation system placed on the tidal flat and in the river channel of the North Branch estuary in April2018,in addition with the fixed-station observations were executed during spring and neap tides,the hydrodynamic and sedimentary process of the tidal flat and main channel of the North Branch estuary was studied.Meanwhile by applying The Unstructured Grid Finite Volume Community Ocean Model(FVCOM)that established by the colleagues in my research group,the hydrodynamic process of the North Branch estuary was simulated,besides the influence of wind on the hydrodynamic and sedimentary process in the tidal dominated estuary was discussed.The results are depicted as follows:(1)Apparent tidal characteristics were found in hydrodynamic process and sediment transportation on the tidal flat located at south of North branch of Changjiang estuary,the velocity during spring is far more large than it during neap,and there is also an asymmetry exist in tide.The unit width net sediment transport of the tidal flat during spring and mid-tide are 1.61×10~4 kg/m and 8.28×10~3 kg/m respectively,while it is1.30×10~3 kg/m during neap.The net sediment transport direction towards to the inlet during spring tide and neap tide,during middle tide,on the contrary,to the sea.Sediment transport is mainly driven by advection and tidal pumping.According to the model results,it was found that wind plays a vital role in the dynamic process of hydrology and sediment in the offshore shallow water area when the tide is weak.During the mid-tide,the study area experienced a strong south wind period,the average wind speed during this period reached 7.55m/s.The strong south wind strengthened the ebb tide on the tidal flat and south channel of Guyuansha,caused the direction of residual flow field at this area towards sea,this explained the seaward transportation of sediment on the tidal flat during mid-tide.In addition,the hourly net sediment flux of the tidal flat and south channel of Guyuansha tends to transport to each other during the middle tide period and neap illustrated the tendency of the process of sediment exchange.(2)The hydrodynamic process is different in two main waterways of Guyuansha,besides wind plays an important role in the hydrodynamic process.There are obvious asymmetries in duration and velocity of ebb and flood in two main waterways of Guyuansha.The ebb tide duration of both two main branches were longer than that of the flood tide.Under the influence of south wind,the ebb flow velocity of the north waterway increased and gradually exceed the velocity of flood flow after spring.While the flood flow velocity of the south waterway was always larger than that of the ebb.Model result of residual and velocity field showed that the flow in south waterway has a tendency to run across the western shoal of Guyuansha during spring,besides the model results revealed that ebb of the investigation area was enhanced by south wind during mid-tide while north wind strengthened flood in spring and neap.(3)The difference of sediment transportation between two main waterways of Guyuansha is obvious.From the aspect of sediment transport,the unit-width sediment transport of two channels during the spring tide both are greater than that of neap,the unit-width net sediment transport of the south waterway was larger than that in the north waterway.The characters of lateral flow at the bottom of the north and south waterways of Guyuansha are distinct,which indicated the difference in the dynamic process of water and sediment in different channels.In north waterway,since the lateral flow is more remarkable during ebb,it turned out that the lateral flow carries suspended sediment to the northern shoal of Guyuansha,while in south waterway,on the contrary,the strong lateral flow caused the sediment deposited at the austral shoal of Guyuansha.These trends of sediment transport exposited an expand potential to both north and south side of the eastern shoal of Guyuansha.