Sediment Transport In Tide-dominated Channels ——A Case Study Of The North Branch In The Yangtze Estuary

Estuaries are located in the transition zone between rivers and oceans.They are densely populated,providing important resources for navigation,ports,production,entertainment and other activities,as well as habitats for plant and animal.They have important social,economic and ecological values.The development of estuarine geomorphology affects ecological and economic development.It is very important to understand the evolution of estuarine geomorphology for future management and prediction.The North Branch is one of the branches of the Yangtze Estuary.Impact by both human and nature,it shows a tendency of strong siltation,which brings challenges to channel management.It is of great significance to understand the historical geomorphic evolution characteristics of the North branch and explore the mechanism of hydrodynamics and sediment transport in the North Branch for scientific research and management decision.This thesis focuses on the historical hydr-morphodynamic changes of the North Branch,using bathymetry data from 1977 to 2019 and satellite data from 1976 to 2020 to quantitatively analyze the coastline and geomorphic changes of the North Branch,and discusses the characteristics of the historical geomorphic evolution process.By means of numerical simulation to analyze the main factors affecting the change of shoreline and landform,which provides information for further management and protection of the North Branch.The main conclusions are as follows:(1)The development of shoreline,siltation of riverbed and width narrow are the significant characteristics of the North Branch in recent 40 years.The average annual migration rates of the northern and southern shorelines were 60 m/yr and 15 m/yr,respectively.The change of the shorelines increased the convergence of the channel width which amplified the tidal effect in the channel.During the 42 years,the channel volume of the North Branch decreased continuously,and the total sedimentation reached 1.754 billion m~3.The proportion of channel volume above 2 m isobath line increased,while the proportion of channel volume below 5 m isobath line decreased significantly.The contribution rate of shoreline narrowing and siltation to the shrinkage of the North Branch was 27% and 73%,respectively.(2)Tidal asymmetry is the main cause of landward net sediment transport in tidedominated estuaries.Diversion ratio,channel geomorphology and tidal flat development significantly affect the net sediment transport.The sedimentation of the North Branch channel is the comprehensive result of the changes in hydrodynamic and sediment pattern and human reclamation.The decrease of the diversion ratio and the reclamation of the channel lead to the change of channel morphology,which leads to the enhancement of tidal dynamics and the positive feedback effect of tidal dominance and sediment import.The long-term positive feedback of dynamic landform of tidal flat reclamation should be considered in the management of North Branch.(3)Human activities have a significant impact on the hydrodynamic dynamics of the North Branch channel.If the tidal flats in the lower part of North Branch was reclaimed,it will reduce the tidal prism,change the tidal asymmetry characteristics of the channel,and promote the deposition of sediment in the channel.The reclamation of the lower reaches resulted in the change of the channel plane shape,decreasing the flood and ebb velocity of the channel rapidly,and make the asymmetric to flood dominant in the reclamation area,which provided a favorable environment for the sediment deposition of the North Branch channel.If dredging take place in the inlet section,the diversion ratio of the North Branch will be increased,and the ebb velocity in the upper reaches of the North Branch channel will increase,while the flood velocity will be decreased.The pattern of upper ebb dominant and lower flood dominant may increase the sedimentation of North Branch,which is not help to do river management and maintenance.In this paper,satellite data and numerical simulation methods was used to analyzed the main features of the historical geomorphic evolution of the North Branch and explored the human impact on the geomorphic evolution of the North Branch.The main factors affecting the siltation of the North Branch channel was explored,and some references can provide for the future management of the North Branch channel.Although this paper only discussed the sediment transport mechanism of the North Branch from the perspective of tide and runoff,and has gained some understanding,it also has the deficiency of not considering the influence of wind field and salinity on the geomorphological evolution of the North Branch.