Multi-time-scale Morphodynamics Of The Changjiang Estuarine Marginal Shoal

Estuarine tidal flats,developed at the terminal end of rivers,possess extreme economic and social values in providing reserve land for urban expansion and mitigating disasters along coasts.However,the estuarine tidal flats worldwide are facing crisis of degradation under the couplings of global warming and intensive human activities.Thus,it is extremely urgent and important to understand the development mode and evolution mechanisms of the tidal flats,predict their variation trend and put forward adaptive policies.Meanwhile,morphodynamic processes of the tidal flats,which directly reflect the river-sea interactions,therefore serve as important contents in deducing and discriminating dynamic mechanisms between land and sea and predicting evolutionary tendency of estuaries,and are of exceedingly scientific significance.The manifestation and dominating factors in morphodynamic development of tidal flats vary dramatically in different time scales.In a given scale,it is hard to ascertain the morphodynamics of estuarine tidal flats under the coupled natural and artificial forcing.Previous researches mainly focus on morphodynamic processes in macroscopic or microscopic scale,but rarely refer to morphodynamic development and corresponding mechanisms of estuarine tidal flats in multi-time scales.Therefore,in this study,based on the collected historical hydrological,sedimentological and bathymetric data and conducted high-frequency successive surveys,the Nanhui Shoal(NHS),Changjiang Estuay,is selected to diagnose linkage mechanisms between morphodynamics of estuarine tidal flats and potentially impacting factors in multi-time-scales by means of cluster analysis,wavelet analysis,empirical orthogonal fuction and Gao?Collins model.Major results include:(1)Daily sedimentary dynamic processes of the intertidal flat controlled by the interactions of tidal regime and wind weather The middle intertidal flat at the south edge of the NHS keeps a suspended sediment transport mode of 'westward transport in the along-shore direction and seaward transport in the cross-shore direction'during the spring and medium tide,with the intensity of the along-shore transport being 3 times of the cross-shore transport.However,the suspended sediment transport mode tends to reverse during the neap timed,when the intensity of transport decreases dramatically.In this process,the offshore wind could aggravate the seaward transport,while the onshore wind tends to induce landward transport.The intertidal flat exhibits significant cross-shore variations and spatial zonation.A landward zone,on which sand acts as the major component of the sediments,a transitional zone,on which sand content decreases abruptly,and a seaward zone,on which sediments are dominated by silt,could be further divided seaward.The zoning mode of the sediments changes slightly during the daily survey(2014.12-2015.1),while migration of the zonation is detected under the impacts of wind,with a seaward progradation during the strong wind and a landward retreat during the calm weather.Moreover,the sediments in the landward and seaward region are sensitive to the alterations of tidal regime,whose compositions show fluctuations of 2-4 and 7 days.In a spring tidal cycle,the upper and middle intertidal flat suffers erosion and the lower intertidal flat experiences siltation,which furtherly result in erosion and deposition in longer scale.The daily geomorphic changes of the intertidal flat are usually accompanied with staged erosion or deposition and could transform from a 'significant deposition from spring tide to medium tide'status to a 'mild siltation or dramatic erosion from medium tide to next spring tide'status,which is recognized as status transition of 'dramatic siltation towards mild siltation(erosion)'.Besides,the erosion and deposition intensity under strong wind is 5 times of those during normal weather.(2)Monthly evolution of the intertidal flat controlled by development and extinction of tidal creek Tidal creek is a basic geomorphic unit on the intertidal flat of NHS,whose development and extinction in a great extent control the morphodynamic evolution of the intertidal flat.Bathymetric data during 12.2014-6.2016 show that the monthly geomorphic processes of the intertidal flat could be divided to 3 stages:When the tidal creek is developed,width of zonation changes slightly while the lower intertidal zone experiences dramatic siltation.With the extinction of the tidal creek,the middle intertidal zone suffers severe erosion and the lower intertidal zone migrates landward significantly.Subsequently,the intertidal flat presents an adjustment status with relatively insignificant erosion and deposition.Finally,morphology of the intertidal flat transforms from a 'double sand-body systems,single tidal creek' to a'single sand-body system' structure with the profile growing steeper and exhibiting an'L' shape.Meanwhile,the sedimentary zonation of the intertidal flat alters dramatically,among which the landward region retreats by 62.5 m,the transitional region narrows by 75 m and the seaward region migrates landward by 137.5 m.The sand content in sediments of the landward region decreases slightly during 12.2014-6.2016,with the maximum value being observed in 1.2015,7.2015 and 2.2016,while the silt content in sediments of the seaward region remains stable.The sedimentary processes of the intertidal flat are highly coupled to the geomorphic changes,shown by the consistency between the alteration of the sedimentary zonation and the geomorphic processes,which indicate narrowing of the upper and middle intertidal zone and landward migration of the lower intertidal zone,and fining of sediments in the seaward region with siltation of tidal flat.(3)Annual morphodynaics of the NHS dominated by engineering of catchment and in estuary The northern and southern NHS,divided by the Dazhi River,exhibit different evolution modes under the intensive human interferences of catchment and in estuary.Thereinto,the northern NHS changes from a 'tidal channel-tidal ridge'structure to a structure with 'even flat at top and bottom,and steep slope in the middle'owing to continuous siltation in the tidal channel.Meanwhile,the northern NHS exhibits staged changes,namely,'mild siltation with a complete tidal channel during 1998-2002','gradual filling of the tidal channel during 2003-2008' and 'large-scale siltation during 2009-2013'.Finally,the northern NHS undergoes an increase by 1.5×108 m3 in volume.The southern NHS experiences continuous siltation,accompanied with development,extinction and rebirth of the supratidal and intertidal flat during 1986-2013.Specifically,the southern NHS manifests 'dramatic siltation and westward extension(around the Nanhui Spit)of the supratidal and intertidal flat during 1986-1999','seaward migration of the coastline and extinction of the supratidal and intertidal flat during 1999-2003 due to reclamation'and 'rebirth and northward extension of the supratidal and intertidal flat during 2003-2013'.Siltation of the northern and southern NHS indicates that the Three Gorges Dam induced fluvial sediment decline plays a limited role in erosion and siltation of the NHS at present.The annual geomorphic evolution of the NHS is likely to be controlled by the siltation promotion and reclamation projects and is sensitive to the progress of the works.(4)Decadal evolution of the NHS dominated by natural processes and human activities alternatively The NHS experiences dramatic changes in the planar geometry before 1989,with the cusp transforming from a triangle to an arc shape.Heave forming and disappearing are detected successively at the seaward edge of the northern NHS owing to the development and separation of the Jiangya Shoal.In this period,fluctuant siltation and erosion occurs on the NHS,with the volume changing slightly and the area above-5 m increasing by 36 km2.After 1989,the spatial mode of the tidal dynamics and the channel-shoal suspended sediment transport remains stable,while the surface sediments on NHS fines gradually.The NHS experiences a significant siltation in region west of the tidal ridge(Meimao Shoal)with the volume increasing by 3.5×108 m3,and slight changes in the planar geometry despite of a slight retreat at seaward edge of the north section and a mild progradation of the cusp with the area above-5 m staying stable.The estuarine regime adjustment of the southern Changjiang Estuary in response to the new configuration of'3 ordered bifurcation and 4 outlets to sea',is the key factor dominating NHS evolution from 1958 to 1989.During 1958-1978,the Jiuduan Shoal migrates southward and the South Passage narrows down with the ebb flow intensity weakening,which leads to the significant progradation of the NHS together with the shape transformation of the cusp.During 1978-1989,the Jiangya Shoal grows larger and gets separated from the NHS eventually,which directly changes the geometry of the northern NHS and controls progradation-retreat and migration of the cusp by altering curvature of the upper South Passage.The evolution of NHS during 1989-2013 is controlled by human activities.(5)Similarities and couplings among the NHS's morphodynamics of different time scales Though the morphodynamics of the NHS of different time scales exhibit variant forms,they indicate coupled linear and non-linear processes constantly:The daily-scale morphodynamics of the NHS shows strong non-linear processes,which is dominated by tidal regime and wind weather alternatively.In the monthly scale,the intertidal flat of the NHS experiences evolution mode transition owing to the extinction of the tidal creek and significantly linear erosion and deposition.In the annual scale,the NHS experiences dramatically linear siltation as a result of estuarine engineering.The former processes depend on variation modes of the hydrology and sediment environment,which are governed by natural and artificial forcing.Besides,it is found that the non-linear processes are mostly dominated by natural factors,while the linear processes are mainly controlled by human activities or special events.Thereinto,the impacts of human activities have their particularities,which could not only induce abnormally linear processes but also mask the non-linear processes in NHS evolution.The long-term evolution of the NHS stems from superpositions of the morphodynamic processes in shorter time scales,among which the net changes during a cycle under normal conditions contribute the most.The intensity of the non-linear processes could be larger than the net linear changes in a given period,which deserve extreme attention in the scientific researches and resource exploitation relating to morphodynamics of the estuarine tidal flats.(6)Resource crisis and policy support of the NHS The decrease in actual quantity of tidal flat resource(decreased by 169 km2 between 1958 and 2013)directly reflect a degradation status of resource of the NHS.At the same time,deficiency in exploitation potentiality of the NHS indicated by the topographic feature desires concerns.The northern NHS experiences landward accretion and seaward erosion under the impacts of the siltation promotion project and the deep waterway project,and a 'steep slope9 structure,with slope approaching 10‰ at most,is formed between-2 and-3 m of elevation.The 'steep slope' significantly limits the resource exploitation in future,which allows just 80 km2 of tidal flat in the region landward to be reclaimed.Under the combined pressures of continuously decreasing riverine sediment supply from the Changjiang River,relatively intensive reclamation and regional sea level rise,the resource crisis of NHS would become more severe.Thus,it is urgent to conduct related measures,including 'comprehensive adjustment involving watershed and estuary','scientifically promoting siltation of tidal flats','keeping balance between reclamation and rebirth of tidal flats' and 'coping strategies to sea level rise',for sustainable utilization of the NHS.