The Lateral Circulation And Its Regime Shift In A Multiple-channel Estuary

The estuary is a key area where land-to-sea interaction and human activities are highly aggregated.Water and sediment transport in the estuary is not only driven by natural processes such as runoff and tide but also deeply affected by human activities.Previous studies have focused on the structure of longitudinal water and sediment transport in the main channel and only a few studies focused on lateral exchange.Along with engineering constructions in the estuary delta in recent decades,especially for the multi-channel system,researchers began to find that the lateral transport process between the main channel and the shoals would significantly affect the water and sediment transport in the entire estuary,thus affecting our complete understanding of the estuary dynamic field.It is not difficult to find that human activities such as reclamation,navigation channel deepening,and reservoir building have significantly changed the geomorphology of the estuary,which has modified in the longitudinal and lateral circulation.Recognizing this change will improve our scientific understanding and help us to analyze the impact of human activities.Taking the Changjiang Estuary as an example,the circulation structure of each main channel has its characteristics and is affected by geomorphology and human activities.Within this context,it is important to study the changes in material exchange under the influence of human projects,especially the lateral exchange among channels and shoals.Furthermore,it is enlightening to find out the structure of the circulation and mechanism controlling the material transport,which will extend our knowledge of the water transport process,sediment transport,pollutant transport,and landform evolution.This study is based on a combination of measured data analysis and numerical model analysis.It aims to study the water exchange pattern of the Changjiang Estuary and its response characteristics to human engineering such as Deep Waterway Projects(DWP)and reclamation.Besides,it will explore the key process of lateral water transport in a multiple-channel system.This study focuses on the influence and mechanism of the lateral exchange on the main channel circulation structure and the lateral circulation pattern in the Changjiang Estuary.By establishing an ideal model with cross-sectional morphology as the variable,the structural change trend of the lateral circulation in the Changjiang Estuary is predicted.With the analysis of the observation data,the response of estuary sediment distribution under the background of regime shift in lateral circulation will be discussed.The main findings are summarized as follows:1.The function of lateral water exchange in the estuarine water transport has been clarified.The lateral water transport is significantly affected by human activities.This study has shown that the lateral transport and longitudinal water transport are equally important.Residual lateral transport flows from south to north during the wet season,and the residual flux is of the order of 10~4 m~3/s.During the dry season,from north to south,the residual flux is the order of 10~3 m~3/s.Geomorphology changes caused by reclamation,dike building have resulted in a reduction in the residual lateral fluxes among the North Branch,the North Channel,the North Passage and the South Passage during the wet season by 26%,33%,and 50%respectively.Among them,the growth of the estuary tidal flats and the surrounding reclamation not only block the lateral water exchange but also enhance the stratification in the North Branch and the North Channel.Furthermore,the water exchange rate among the Changjiang Estuary and the adjacent areas has reduced.The water left the Changjiang Estuary in 5 days has decreased by10%,comparing to the situation before human activities.2.Propose the lateral circulation pattern in the Changjiang Estuary.In a natural state,it is‘overtopping in the surface layer and rotating in the bottom layer'.After reclamation,it is shifted into‘weakened laterally transport and rotating dominance'.When the dike is inundated,the pattern is‘rotating southside and overtopping northside'in the North Passage.The lateral circulation patterns are similar in the North Channel,the North Passage,and the South Passage.At the high slack,water moves from south to north in the surface layer,while in the middle layer and the bottom layer,water rotates clockwise.At the low slack,water moves from north to south,controlled by the cross-shoal lateral exchange.The reclamation weakens the lateral exchange in the North Branch,the North Passage,and the South Passage,reducing the maximum lateral velocity by 42%,43%,and 28%respectively.At the same time,the range of the clockwise rotating in the middle layer and the bottom layer extends,and the intensity weakens.After the building of the dike,the overtopping flow across the Jiuduan Shoal brings high salinity in the surface layer during the flood tide,generating a strong clockwise circulation in the south shoal area in the North Passage,overturning the vertical density gradient.The overtopping flow enhances the vertical mixing in the south shoal area in the North Passage,lasting for 1-2 hours.3.It is found that the DWP and geomorphological changes are the main reasons for the shift of the lateral circulation pattern in the North Passage.The lateral exchange of water is weakened and the stratification is strengthened,which further suppresses the vertical diffusion of suspended sediment.By comparing the hydrodynamic structure of the estuary before and after the DWP,and the characteristics of the suspended sediment distribution,it is found that the deepening of the channel caused by the DWP led to the shift of the lateral circulation pattern.The lateral flow moves from south to north in the natural state,while it decreases and rotates in the lower layer after the DWP.The vertical structure is hit from a well-mixed state into a strong stratification state,which can be reflected by a one-time increase of the potential energy anomaly.The enhanced stratification suppresses the vertical diffusion of the hyper-concentration of suspended sediment in the near-bottom layer and increases the vertical gradient of the suspended sediment concentration.4.Attempt to use the depth-to-width ratio to study the relationship between the cross-sectional shape before and after the channel deepening,and its influence of stratification.In the narrow-and-deep channel,the lateral flow velocity is small,the water body controlled by the vertical flow mechanism,and the stratification is strong.The traditional theory bases on the width-to-depth ratio to describe the shape of the cross-section,taking root of width and dividing by the averaged depth of the cross-section.In the estuary area,due to the large width of the river,the average depth will cover the changes caused by the channel deepening.In this study,based on the actual situation in the estuary area,the depth-to-width ratio is selected to describe the shape of the cross-section.The depth-to-width ratio is the maximum depth divided by the average river width to emphasize the change of water depth.The North Channel,the North Passage,and the South Passage are classified by this method.After inspection,the depth-to-width ratio can better describe the cross-sectional shape changes brought by the deepening of the channel.It is found that,in a narrow-and-deep channel,the vertical structure always maintains a relatively stratified state and will no longer change.By decomposing the physical mechanism that controls the stratification-mixing process,it is found that the effect of vertical flow is the dominant control factor for the strong stratification in the narrow-and-deep channel,and the proportion of each mechanism is always greater than 20%.In general,under the influence of human activities,the water exchange pattern and water exchange rate in the Changjiang Estuary shows a clear regime shift.Horizontally,the human projects blocked the lateral water exchange and changed the lateral and longitudinal exchanges from the natural state of the same magnitude to the longitudinal exchange dominates.Vertically,the lateral circulation structure in the channel changed from the natural state with north-south strong exchange pattern to a low-velocity pattern dominated by rotating flow with an enhanced stratification.The research and prediction of the regime shift of the lateral circulation of the multiple-channel estuary make up for the shortcomings of the theory in the multiple-channel estuary,which can deepen the understanding of the hydro-geomorphological process of the estuary under strong human intervention,and provide important scientific and technological support for the management and protection of the estuary.