Alteration Of Estuarine Circulation Under The Influence Of Morphological Evolution

An estuarine delta is densely populated and highly influenced by human activities,and is sensitive to the land-ocean interaction.Mass transport in estuaries is a key process in the land-ocean interaction and is highly dependent on estuarine hydrodynamics.Under the combined effects of river runoff,tide,wave and interaction between fresh and salty water,the hydrodynamics in an estuary is complex.In brached estuaries,the dynamics becomes more complicated due to the transverse variability in bathymetry.The traditional two-dimensional estuarine circulation structure has been well explained in previous studies.In recent years,the understanding of lateral circulation and its important role in estuarine dynamics has developed.However,the unique channel-shoal water exchange in branched estuary and its influence on hydrodynamics is rarely known.In this study,the three-dimensional dynamics structure in a large-scale branched estuary,Changjiang Estuary,is investigated.The study will provide new knowledge in estuarine dynamics and benefits to understanding of sediment transport,pollutant diffusion and morphological evolution.The study was carried out by ultilizing a three-dimensional numerical model.The model was developed for Changjiang Estuary and the marginal seas,and was calibrated with measured tidal elevation,tidal current and salinity.The results show that the model is capable to reproduce the hydrodynamics of the estuary.Based on numerical model,the transverse structure of estuarine circulation was presented and its association with vertical mixing and channel morphology was explored.The role of lateral circulation in driving estuarine circulation was explained by analyzing the momentum balance.By changing the model bathymetry,we exhibited the the impact of reducation of channel-shoal exchange on estuarine dynamics.The model results indicate that the change in hydrodynamics caused by large infrastructure extended to the subaqueous delta instead of limiting in the inner estuary,resulting in local morphological adjustment.The main results and conclusions are presented as follow:1.The transverse structure of estuarine circulation was exhibited,and its temporal variation and driving mechanism was explored.Previous studies of estuarine circulation focus on the along-channel direction,neglecting the three-dimensional structure.The stratification has a large impact on the vertical momentum exchange,which results in different transverse structures of estuarine circulation.The transverse structure of estuarine circulation in the North Channel featured a vertically-sheared structure with inflow near the surface and outflow at the surface.While in the North Passage and South Passage,the inflow extended to the upper layer of the water column under relatively strong vertical mixing.Spatially,the outflow was constrained on the southern part of the channel,which generated a laterally-sheared structure of exchange flow.The deepening and narrowing of the estuary in 2010 modified the structure of exchange flow significantly.The enhancement of stratification led a decrease in vertical mixing.Thus,the transverse structure of estuarine circulation remained as vertically-sheared in the North Channel.Because the wide and shallow bathymetry of the South Passage,the structure of extuarine circulation was still laterally-sheared.In the North Passage,the inflow was constrained near the bottom due to weak mixing.The structure of exchange flow shifted to vertically-sheared.2.The channel-shaol water exchange actes as the main driving force for lateral flows in branched estuaries.The lateral flow played a significant role in stratification.Unlike single-channel estuary,the amplitude of lateral flows is controlled by cross-shoal flow.During the late flood,the barotropic force set by the cross-shoal flow drove northward lateral flow in the channels.While during ebb tide,the differential advection became the dominant driving force for lateral flow due to the disappearance of cross-shoal flow.Thus,the lateral flow featured a divergent structure,with water flowing from the deep channels to the shoals.By investigating the evolution of vertical salinity gradient,we found that the stratification was dominated by tidal straining,advection and vertical mixing.The along-channel straining reduced the stratification during flood tide.While during ebb tide,the vertical shear of tidal current strained the isopycnal,which enhanced the stratification.Similar to along-channel straining,the interaction between lateral flows and salinity gradient generated lateral straining.The impact of lateral straining was apparent near flood slack,leading to rapid stratification in the early stage of ebb tide.The lateral straining was more important than along-channel straining due to increase in lateral flow and lateral salinity gradient.The cross-shoal flow was largely reduced due to accretion on shoal and local engineering works in 2010.As a result,the magnitude of lateral flows declined significantly.The amplitude of lateral flows in North Channel,North Passage and South Passage decreased by 28%,35%and 27%during,and decreased by 22%,29%and 26%during neap tide.The impact of lateral straining was reduced due to reduction in lateral flows.As as result,the stratification was dominated by along-channel process.3.The non-linear advective acceleration is an important driving mechanism for estuarine circulation.The amplitude of non-linear advective acceleration can reach 0.2×10~-44 m/s~2,0.4×10~-44 m/s~2 and 0.2×10~-44 m/s~2 in North Channel,North Passage and South Passage,respectively,which has the same magnitude of pressure gradient,suggesting the advective acceleration is important in estuarine momentum.During flood tide,the lateral circulation advectes the low-speed water in shallow shoals to the main channel,deaccelerating the flood current in the southern part of the channel.Meanwhile,the high-speed water was transported to the shoal in the northern part of the channel,accelerating the flood current.While during ebb tide,the non-linear advective acceleration has the opposite effects.On subtidal timescales,the non-linear advective acceleration enhance the landward inflow on the northern side and seaward outflow in the southern part of the channel,which acts as an extraordinary driving mechanism for estuarine circulation.4.On local scale,the spatial distribution of accretion/erosion was controlled by the change in hydrodynamics induced by local engineering works.Previous studies of subaqueous delta erosion focus on the reduction of riverine sediment flux,which neglected the impacts of local engineering works.The annual sediment flux from the river into the estuary between 1986 and 1997 was 348×10~6 t/year and declined to217×10~6 t/year between 1997 and 2010.The Changjiang Estuary exhibited a continuous accretion between 1986 and 2010,whereas the accretion rate decreased from 16.7 mm/year to 9.1 mm/year during that time span.Spatially,the tidal flats accreted whereas the subaqueous delta switched from deposition between 1986 and1997 to erosion between 1997 and 2010.Two large erosion zones were aligned along the 10-m isobaths over the south-north direction in the submerged delta front.We used two indicators,tidal energy dissipation and erosion rate,to quantify the change in hydrodynamics and found that the erosion of the subaqueous delta in recent decades can readily be explained by the alteration of the hydrodynamics.Under the influence of engineering works,the erosion rate increased significantly in seaward of North Passange and South Passage(between 5-10 m isobaths).Similarly,the erosion rate in the seaward region(between 5-10 m isobaths)of North Channel increased by 30%.On the shoals,the erosion rate decreased due to weak dynamics condition.The change in hydrodynamics agreed well with the spatial distribution of accretion/erosion,suggesting that local engineering works was also an important factor in controlling morphological evolution.