Temporal And Spatial Variation Of Tidal Range In The Changjiang Estuary And Its Environmental Significance

The interaction between land and sea in the estuary area is strong,and various environmental factors influence each other.The changes of these factors are closely related to the changes of hydrodynamic conditions.Tidal waves are an important part of the hydrodynamic conditions of the estuary,and tidal range is an important indicator of tides.The tidal range in the estuary area is affected by factors such as offshore tidal,coastal topography and river runoff.The change mechanism is complex and has strong spatial and temporal distribution characteristics.Tidal flat is one of the main types of landforms in the estuary coastal area.Its erosion and sedimentation evolution and vegetation distribution are closely related to tidal and its changes.Studying the temporal and spatial changes of the estuary estuary can provide an important dynamic mechanism for the interpretation of estuarine saltwater intrusion,sediment movement,channel evolution,and bank scouring and silting.In this paper,we use the forecast tidal range of 66336 data from 1972-2018 in the Yangtze River estuary tidal station,and the short-term measured tidal data of some stations,combined with statistical analysis,wavelet analysis,MK test,harmonic analysis and other methods to analyze the sea level at sea level.The temporal variation of the tidal range in the Yangtze River estuary under the influence of factors such as rising.Using the sea chart data,measured tidal flat topography,tidal flat measured hydrological data,remote sensing image and other data,combined with MIKE21 hydrodynamic numerical model,Arc GIS software and other technical methods,the Yangtze River tidal dynamic model and Chongming Dongtan digital elevation model were constructed.Based on the above model,the spatial distribution characteristics of the tidal range in the Yangtze River estuary and its response to sea level and runoff changes are analyzed,as well as the influence of tidal range changes on the tidal flooding area.The main findings are as follows:(1)In addition to the common spring-neap tidal cycle,the tidal range of the Lushan station has a seasonal variation of 0.5 a and an interdecadal scale change with a period of 18.5 a.The 0.5 a cycle tidal range is about 19 cm,which is equivalent to 7.5% of the average annual tidal range.The annual maximum value of the monthly average tidal range occurs in March and September,and the minimum value appears in June and December;18.5 The a-cycle tidal range is about 16 cm,which is equivalent to 6.3% of the average annual tidal range.The maximum annual tidal range between 1972 and 2018 occurred in 1977,1996 and 2015,and the minimum appeared in 1986 and 2005..There is a significant corresponding relationship between the tidal range change in the middle and long period and the significant changes in environmental factors such as salt water intrusion in the Yangtze River estuary,changes in suspended sediment concentration,and evolution of underwater delta erosion and silencing,which may be related to the ecological environment of the Yangtze River estuary.Changes in factors have important potential impacts.According to the change of tidal range of 18.5 a,the tidal range of the Yangtze River estuary will show a significant decrease during 2015~2024,which may lead to a corresponding decrease in saltwater intrusion and seabed erosion activities.The concentration of suspended sediment in the Yangtze River estuary will also Correspondingly reduced.(2)In terms of space,the annual average tidal range of the Yangtze River estuary ranges from 1.9 m in the upper reaches of the South Branch system in the mouth to 3.9 m in the mouth gate.In the offshore waters,the tidal range near Nanhuizui is larger than that in Qidongzuo.However,due to the narrowing of the river channel and shallow water depth,the tidal range of some north branches is generally larger than that of the south branch.The results of harmonic analysis show that the half-day tide M2 is the dominant divergence in the Yangtze River estuary,and the variation of M2 tidal current is basically consistent with the tidal range change.(3)The change of runoff in the Yangtze River has a certain influence on the tidal range of the Yangtze River estuary.The tidal value model is used to simulate the tidal range changes of runoff 10000 m~3/s,30,000 m~3/s,50000 m~3/s and 70,000 m~3/s.When the runoff increases,the monthly mean tidal range of the area from the mouth to the whole decreases downward.Compared with the runoff of 10000 m~3/s,the average monthly tidal range decreases by about 2.3 cm and 5.3 cm,respectively.7.8 cm.The decline in tidal range gradually decreases from the upstream to the mouth,and there is no significant change from the outside.The increase of runoff caused the tidal wave deformation to increase,and the tidal asymmetry intensity of most areas in the mouth increased significantly with the increase of runoff.(4)Affected by factors such as global warming,the sea level in the East China Sea has shown a significant upward trend in recent years.The problem of land subsidence in the Yangtze River estuary has been very significant.Combined with sea level rise,it has caused a change in the relative water depth of the Yangtze River estuary.This will undoubtedly have a certain impact on tidal wave propagation and tidal range.According to the simulation results,after the average sea level rises by 1 m,the tidal range of the Yangtze River estuary from the mouth to the inward area increases,and the increase gradually increases from the mouth to the upstream,with an average increase of about 0.25 m.As the average sea level rises,the tidal effect increases,and the asymmetry intensity of the tidal pressure in the mouth decreases.(5)Taking Chongming Dongtan as an example,according to remote sensing image and numerical model analysis,the tidal flat vegetation coverage increased by about 27.5% in 2015 compared with 2005.In the case of sea level rise,beach surface erosion and reclamation,in the tidal range 18.5 a cycle change,the annual average tidal range maximum year 2015 intertidal zone inundation area is smaller than the average annual tidal range The year increased by 9.2% in 2005.In summary,the spatial and temporal changes in the tidal range of the Yangtze River estuary have reached a level that cannot be ignored,and its impact on the environment deserves attention.