Morphodynamic Processes Of The Unsheltered Tidal Flats In The Changjiang Estuary

As a transition zone of land-ocean interaction,estuarine tidal flats are characterizedby high socioeconomic and ecological values because of their unique geographical environment.Nevertheless,in recent decades,global environmental problems have become increasingly severe,including the sharply decrease of fluvial sediment to the coast,frequently occurrence of exreme climate events and the remarkably accelerating sea level rise(SLR).These environmental pressures exacerbate the risk of globlal tidal flat degradation,including the Changjiang Estuary(CJE).CJE has undergone a significant reduction of riverine sediment because of the Three Gorges Dam operation in 2003.Additionally,the construction of Deep Navigation Project(DNP),Eastern Hengsha Shoal Reclamation Project,continuous SLR,and frequent typhoon storm surges in CJE also are crucial influences on the unsheltered tidal flats morphology,which would further affect the sustainable development of tidal flats resources in the CJE.Therefore,the study of unsheltered tidal flats morphodynamics in CJE is essential to reflect the coupling effect of land-ocean dynamics,and also has important practical value for tidal flats resource management and channel stability in CJE.In this study,three unsheltered tidal flats,Eastern Chongming(ECM),Eastern Hengsha(EHS),and Jiuduan Shoal(JDS),were selected to quantify the morphology changes of tidal flats,to detect the sedimentary characteristics and morphodynamic processes,and to diagnose the impact mechanisms between the tidal flat development and coupled natural and artificial forcings in the unsheltered tidal flats of the CJE based on long-time series of remote sensing images,hydrological records,sedimentological data and bathymetric map.The major results include:The ECM salt marshes and mudflats demonstrated a rapid deposition and expansion process with local erosion,while the submerged flats experienced a weak siltation.Affected by longshore currents and flood-ebb tide dynamics,there is an obvious coarse sand zone near the-5 m isobath in ECM tidal flats.The fine particles are concentrated on the northwest side of ECM tidal flat,while coarse particles are distributed on the southeast side because of the wave influence.Similarly,a remarkable deposition of fine sediment was detected in the north zone of the ECM tidal flats,while a weak deposition of coarse grains occurred in the southeast zone of ECM tidal flats.The deposition rates slowed down from north to south and transformed into erosion in the south.From 2002 to 2019,the area of salt marshes in the north zone suggested a maximum increasing rate of 2.64 km~2/yr,and the annual accumulation rates reached0.178 m;the margins of salt marshes and mudflats in the north and east zones showed seaward migration with the migration strength gradually enhanced from south to north;the salt marshes and the mudflats in the south zone presented erosion and landward recession.the area of submerged flats expanded at a rate of 2.5 km~2/yr,with the north zone and southeast zone as deposition center,the east zone as stable development area,and the south zone being the scouring area.The EHS tidal flats showed a rapid expansion pattern.Influenced by the northeast wave action,the blocking effect of the DNP,and the utilization of artificial dredged soil,the sedimentary pattern in EHS tidal flats shows the characteristic of finer sediment in the southwest area and coarser sediment in the northeast area.Meanwhile,during 2002-2020,the salt marshes and mudflats area of the EHS tidal flat exhibited a remarkable seaward expansion.The salt marshes and mudflats between spur dikes showed high expansion rate in the middle and low expansion rates in the east and west sides.Furthermore,the area of submerged flats in EHS exhibited slight change between 2002and 2020,but the area above-2 m expanded significantly.The-2 m isobath morphology transformed greatly from an initial elongated strip shape in 2002 to a stable long strip at pesent with the scouring channel in the middle of submerged flats migrating downstream continuously.The JDS salt marshes exhibited a significant expansion trend,the mudflats transformed from deposition to erosion,and the submerged flats underwent siltation and erosion fluctuation process.JDS sediments in the upper and lower tidal flats are finer,while the sediments in the central region are coarser due to the influences of south dike of the DNP and southeast wave.Moreover,in 2002-2020,the intertidal salt marshes area of the JDS expanded significantly,nearly tripling in size,with remarkable vertical deposition.The mudflats in the JDS transformed from deposition(2002-2014)to erosion(2014-2020),with an area decreaasing rate of 3.23 km~2/yr,a scouring rate of 0.11 m/yr.Meanwhile,the upper shoal of the JDS showed a landward expansion,while the lower shoal expanded seaward.Besides,the submerged flats area of the JDS remained stable,with its deposition/erosion process suggesting phase changes.Specifically,the submerged flats presented a total deposition thickness of 0.7m during 2002-2010,but an erosion thickness of about 0.4 m thereafter in 2010-2016,which is followed by a total deposition of 0.66 m subsequently between 2016 and 2020.The controlling factors of the unsheltered tidal flats’geomorphological change patterns have obvious differences.Although fluvial sediment is decreasing,the sediment from the sea and transporting from the longshore currents are the main sources supporting the continued development of the ECM tidal flats.The south zone of ECM tidal flat is directly influenced by the southeast wave action,while the north zone experiences weak hydrodynamic action.Moreover,the invasion of Spartina alterniflora and the reclamation projects have contributed to the differences in the spatial evolution pattern of the ECM tidal flats.EHS tidal flats formation is primarily dependent on the high-intensity artificial reclamation project,and dredged soil from the DNP provides a valuable sediment source for the continuous expansion of the EHS tidal flats.The construction of north dikes of the DNP provides a hydrodynamic environment that is conducive to sediment deposition and tidal flats expansion.Besides,although the decreasing fluvial sediment has slightly affected the salt marshes and mudflats deposition of the JDS,it has hindered the expansion of the submerged flats.Local suspended sediment supply ensures the development of the JDS tidal flats.In addition,the implementation of local projects and the invasion of Spartina alterniflora have,to some extent,interfered the natural evolution of the JDS.The evolutionary patterns of the three unsheltered tidal flats in the CJE and their response to SLR are different.Under the current discharge and sediment conditions,the mudflats in the north zone of the ECM is expected to reach its expansion limit in 2055,and then the salt marsh will completely occupy the mudflat area in 2100.The tidal flat in the east zone of the ECM will maintain a stable expansion trend for a long time because of the vast development space.In the south zone of the ECM,the salt marsh and mudflat will disappear in 2028 due to the coupled effects of tidal flats erosion and SLR.The EHS tidal flats are experiencing continuous deposition and are slightly affected by SLR.Nevertheless,the expansion status of tidal flats between spur dikes is subjected to the DNP.It is expected that the salt marshes will fully occupy the tidal flats space between spur dikes and then stop extending seaward by 2039.Under the coupled influences of SLR and limited space for tidal flat development,the JDS mudflats area will be completely occupied by salt marsh in 2033,with the tidal flats’slope becoming steeper.Besides,based on the current tidal flats development situation,the overall siltation rate of the three unsheltered tidal flats in the CJE is much higher than the rate of SLR,thus,the tidal flats deposition is temporarily unaffected by SLR,however,the risk of local tidal flats being vulnerable to erosion cannot be excluded.