| The tidal reach of the Yangtze River is located at the intersection of the four major spheres of the earth.It is the most active area where the geosphere,oceans,and atmosphere interact with each other.It is also the most active area and environment fragile zone for human life and various economic activities.The geomorphology of the tidal-fluvial reaches of the Yangtze River and the characteristics of erosion and silting along the Yangtze River from the top to the bottom must be discussed in depth.Moreover,studying about the interrelationship among the characteristics of the dynamic changes of runoff and sediment,changes in land and water,and geomorphological changes is valued.It will not only provide insights into change regularities of large river estuary in the background of sand reduction,but also play an active role on shipping,flood control,floodplain exploitation and wetland conservation and so on.In addition,these studies can provide a scientific basis for the economic and social layout and planning in the estuary area.Based on the above considerations,we studied the geomorphology changes of river courses in the tidal reach of the lower reaches of the Yangtze River,changes of hydrology and sediment load in drainage basin,and changes in tides.Our aims as follow,and the first one is to study the features of river bed erosion and silting for long-distance and long-period in the tidal reach of the Yangtze River.And,the second is to analyze the characteristics of hydrology,sediment load and tidal changes in the tidal reach of the Yangtze River.Another is to discuss the response of the river bed evolution to the runoff and sediment load variation and tidal range and interdecadal variation in the Yangtze River tidal reach.In order to achieve these goals,we collected long-term data of hydrology,sediment load,tides,and channel terrain data of 4 different years in the tidal reach of the Yangtze River.In the past 50 years,the sediment load in the Yangtze River basin has decreased continuously the rate of siltation in the tidal reach,and it shows the interdecadal variation characteristics of strong siltation,weak siltation,erosion and siltation.The river bed erosion and silting rate and the amount of erosion and silting respectively decreased from 8.78 cm/yr and 1 billion 568 million m~3 in period of 1963-1972 to 1.08cm/yr and 298 million m~3 in period of 1972-2002,and then to-5.03 cm/yr and-6.02billion m~3 in period of 2002-2013.At the same time,the sediment reduction in river basins have significant difference effects on the erosion and silting in different river types.In particular,the erosion of the tidal reach is mainly concentrated in bifurcated channel(the rate of erosion and silting is-6.46 cm/yr)and straight channel(the rate of erosion and silting is-3.08 cm/yr),and the silting mainly in the micro-bend branching channels(the rate of erosion and silting is 0.59 cm/yr)after the Three Gorges Dam was built.The reduction of sediment discharge is the main reason for the adjustment of erosion and silting in the Yangtze River.There is a highly positive correlation between erosion and silting rate and sediment load in most sections of the Yangtze River as well as the whole the tidal reach of the Yangtze River,the rate of erosion and silting decreases with the decrease of the amount of sediment load.According to the fitting correlation curve,this paper calculated that when the average annual sediment load of Datong station is about 300 million,the whole tidal river section is generally in the state of erosion and siltation.The correlation between the average sediment load of Datong station and the erosion and silting rate of three different types of river channels in different periods is indicated that the response of erosion and silting of different type of channel to sediment load reduction in watershed shows different characteristics.Thereinto,sediment load reduction in the basin has the greatest influence on the bend branching channel,followed by the straight branching channel,and has the smallest influence on the micro bend branching river.After the construction of the Three Gorges Dam,the tidal dynamic variation in the upper and lower reaches of the tidal current limit in dry season(Zhenjiang)is significant.The tidal range and duration of high tide in the lower section of the tidal reach of the Yangtze River increased significantly compared with the upper reach,resulting in more erosion in the lower section of the channel along the river channel than in the upper section.The Three Gorges Dam impounds water in flood season and draws off water in dry season.This running mode of the dam led to decrease the flood and dry season dominant discharge and increase by 13.36%and 11.62%respectively in the frequency of the flood and dry season dominant discharge.It induced more water diversion into branch.Furthermore,in the background of continuous reduction of sediment load in the river basin,the main and branch channel were scoured synchronously in the braided reach of central bar for tidal reach after the dam construction,and the erosion rate of branch is greater than the main.The average erosion rate of branching reaches were 3times that of the main branch.The average erosion and silting rate of main and branch branches has changed from 14.49 cm/yr and 24.14 cm/yr during the 1963-1972 year period to-1.60 cm/yr and-4.83 cm/yr during the 2002-2013 year period. |
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