Longitudinal Residual Circulation Patterns Of The South Passage Of Changjiang Estuary Influenced By Mouth Bar Morphology

The estuary is where the river flows into the sea;it possesses multiple resources associated with shipping,fisheries,and fresh water,presenting an important zone that supports human activity and life.In the study of nearshore and estuary areas,riverbed morphology is an important influencing factor,continuously interacting with hydrodynamics with complicated feedbacks.As a result of the interaction between the river and the sea,estuary circulation usually has a two-layer structure with the surface seaward and the bottom landward in a vertical profile,which plays an important role in net sediment transport and geomorphological evolution of the estuary.A number of factors influence the longitudinal residual circulation.In addition to runoff and tides,the longitudinally inclined topography can also affect the formation of the two-layer structure circulation.Although the characteristics of local circulation and its formation mechanism in terms of the coupling of runoff and tide have been studied,quantitative analysis of the longitudinal topographic effects is rare.As a sedimentary landform developed in the estuary,the mouth bar has an upward convex shape in the longitudinal direction,which is different from the ideal,gentle terrain adopted in the classical circulation theory.Therefore,the analysis of the longitudinal residual circulation in relation to the mouth bar helps improve our understanding of the circulation patterns,which have important theoretical significance for the study on sediment transport,landform morphology and ecological environment evolution.In the present study,field surveys were carried out in the mouth bar area of the South Passage of the Yangtze Estuary,during July 13?20 and December 18?25,2018.The simultaneous current velocity,salinity profile data were obtained from anchored boats and base tripods at three stations along the channel in the mouth bar area,and the topographic data were obtained from multi-beam measurements.In addition,the ERA-Interim wind speed data during the field observation period and the salinity data of the navigation observations over the East China Sea from August 31 to September 1,2019,were collated.Based on the analyses of the data sets obtained,the main conclusions are as follows:(1)The South Passage of the Yangtze Estuary has a typical mouth bar,which is high in the middle with a steep slope,and the outer part has a gentle slope.The longitudinal changes in topography are more significant than the lateral ones.Outside the South Passage the bed slope is around 0.0004.During the observation period,the wind direction was relatively stable,with the wind speed being less than 10 m/s,and the influence of wind on the longitudinal residual circulation was relatively weak.(2)The direction of flood and ebb tidal currents in the South Passage was basically parallel to the direction of the river channel,and the lateral flow was insignificant.The ebb tide duration is longer than that of the flood tide.The velocity gradually weakened from land to sea during the ebb,but it was opposite during the flood tide.Thus,there was a remarked tidal asymmetry at the landward slope of the mouth bar.The salinity data showed that the saline water intrusion was limited to the seaward slope of the mouth bar during the wet season and stratification occurred on both the seaward and landward slopes during the dry season,with the central part of the mouth bar being associated with a well-mixed water column.The salinity data of the sea area outside the mouth indicated the occurrence of upwelling,with its influence reaching the seaward slope of the mouth bar.According to the low-pass filtering result with a filtering window of 34 h,a single circulation pattern was formed at the seaward slope of the mouth bar in the flood season,but there was a double circulation pattern in the dry season,which was composed of the circulations of the landward and seaward slopes of the mouth bar.(3)Combining the Simpson number,the M value and salinity distribution that characterize the estuarine mixing,the longitudinal residual circulation in the mouth bar area of the South Passage was related mainly to the baroclinic pressure,with the tidal straining during the neap tide playing a secondary role.In the dry season,the formation of the double circulation pattern was due to the influence of the mouth bar topography on the saline water intrusion.Besides,the outer slope circulation being possibly affected by the upwelling outside the mouth.Based on the Delft3D-Flow model,together with the establishment of an ideal,semi-closed ideal estuary,a group of 7 sets of numerical experiments were undertaken to reveal the circulation patterns in relation to the variations in mouth bar topography(length L and height ht),runoff and tide.The major findings are as follows.(1)The presence of the mouth bar is a necessary prerequisite for the formation of the double circulations,but if L is too large then the dual circulation mode may disappear,and if it is too small then the landward slope circulation will be weakened.In the situations that the double circulations are present,the ht value is negatively correlated with the longitudinal extent of the landward slope circulation.(2)It is relatively easy for the double circulations to form during the dry season when the runoff is weak;further,the weak runoff favours the formation of a larger longitudinal range of the landward slope circulation.Because of the weaker mixing,the double circulations mainly occur in the neap tide.(3)The mouth bar is a necessary prerequisite for the formation of double stagnation points in the estuary area.The position variations of the stagnation points have a similar relation to L to that associated with the longitudinal residual circulation;however,with an increase of ht,the location of the stagnation points may change.For the situation of the single circulation pattern,the location of the stagnation point in the neap tide will move to upstream,compared to the one during the spring tide,due to the effect of mixing.The double circulation structure in the mouth bar area can significantly affect the temporal and spatial changes of the stagnation point and the suspended sediment concentration in the maximum turbidity zone,and can be a dynamic mechanism for seasonal erosion and accretion changes at the bed.There is a continuous interaction or feedback relationship between the hydrodynamic forces and the riverbed morphology.Moreover,Furthermore,human activities in the drainage basin and over the estuarine waters may cause the morphological changes of the mouth bar,which in turn may affect the behavior of the double circulations.The results of the present study add new insights to the theory of the longitudinal residual circulation,and propose further scientific issues for further research:(a)the interactive processes between the mouth bar and the longitudinal residual circulation,especially the influence of the latter on the former;(b)the response of the estuarine system composed of mouth bar and the longitudinal residual circulation to future climate change and human activities;and(c)the systematic comparison of the mechanisms responsible for the double circulation formation in different estuaries.