Characteristics And Numerical Simulation Of Water And Sediment Transport In The Tide-dominated Estuary

The tide-dominated estuary's main-controlled dynamic factor is tide,geomorphological evolution processes is influenced significantly by the water and sediment transport.The North Branch mouth is a typical tide-dominated estuary.Perfecting for the research of water and sediment transport in this area can help us understand the whole Yangtze Estuary and provide the key indicators for predicting the geomorphological evolution processes,which is of theoretical and practical significance.A comprehensive survey including the bottom tripod system survey,the fix station survey and the cross-section survey is designed and implemented in the North Branch estuary mouth in April,2018.Moreover,a high resolution numerical FVCOM model is set up and used to analysis the characteristic and mechanism of water and sediment transport in the North Branch estuary mouth.The results suggest:1.The waterways in the North Branch Estuary have different net water transport directions.The net tidal current transport in the section of Santiao Port is landward;Guyuansha north waterway's net transport of current has seaward direction.In the south waterway,the net transport of current is landward,which is caused by the propagation direction of lateral tide and the orientation of Guyuansha south waterway.Guyuansha waterways are asynchronized in ebb and flood phase.When the north waterway begins to ebb,the south waterway is still in the flood phase.Tidal current from the Guyuansha south waterway would cross the shallow shoal and reach the north waterway,so the ebb flow in the north waterway is increased.2.The diversion process in the North Branch tide-dominated estuary is very complicated,the diversion ratio will change in the flood and ebb phase during the spring-neap tide cycle.The flood-tide and ebb-tide diversion ratio of the Guyuansha north waterway are 29.7% and 47.2% during the spring tide period.Most of tidal currents spread up from the south waterway,the spatial distribution of flood tide flow is significantly uneven in the these two waterways of Guyuansha during the spring tide.The flood-tide and ebb-tide diversion ratio of the Guyuansha north waterway are 41.6% and 43.1% during the neap tide period.Therefore the spatial distribution difference of ebb tide flow is reduced because of the weakened tidal dynamic.The cross-sectional area of Guyuansha south waterway is much bigger than that in the north waterway,causing the flow diversion ratio of north waterway is smaller than 50%.3.The waterways in the North Branch Estuary have different net sediment transport directions.During the spring tide,the direction of net sediment transport and current transport is consistent.The net sediment transport in the section of Santiao Port is landward;Guyuansha north waterway's net transport of sediment has seaward direction.In the south waterway,the net transport of sediment is landward.During the neap tide,the net transport of sediment in the center channel of the Guyuansha north waterway is landward.Drastic saltwater intrusion occurs during the neap tide.The sediment will be restricted in the bottom layer because of the water stratification and the tidal currents at middle and bottom layers flow upstream and has higher velocity because of the baroclinic gradient force induced by the salt intrusion.Therefore,the net sediment transport is landward in the center channel of the Guyuansha north waterway.