Recent Changes Of Erosion Datum In Poyang Lake And Their Effects On Hydrological Regime And Lake Bed Evolution

Poyang Lake is the largest freshwater lake in China,with rich fresh water and biological resources.In addition,it is one of the first wetlands selected for the list of Ramsar wetlands of international importance in China and becomes home to the Yangtze finless porpoise.Its ecological environment,hydrology and geomorphological changes have attracted the attention of scholars.It is Hukou station that is the only outlet of Poyang Lake connecting to the Yangtze.Its riverbed elevation determines whether the sendimentation and erosion of the riverbed could keep balanced,as well as the degree of erosion.So,it is the erosion base of Poyang Lake.Therefore,it is very important to study the change of erosion base level in Poyang Lake.The operation of Three Gorges Dam has been greatly decreasing the sediment discharge in middle and lower Yangtze areas and causing significant channel erosion since 2003.Particularly Zhangjiazhou South Waterway,at the confluence of Yangtze and Poyang Lake,the depth of eroded river bed reached 8-10 m,and the depth of eroded river bed reached 1.2 m at Hukou.Meanwhile,a rise in dry periods and a continous drop in the minimal water discharge have a great impact on household and agricultural water consumption.Therefore,it is urgent to explore the impact of erosion base level change on the water regime change of Poyang Lake.Combining with the hydrological data of Wuhe,Xingzi hydrological stations in Poyang Lake Basin and Hankou,Datong,Jiujiang hydrological stations in the Yangtze Basin,and the river bed elevation data at Hukou in multiple years,the disertation used the measured multi-beam 3D underwater terrain,3D laser-scanned shore slope terrain of aquatic-terrestrial transition zone,the particle size of sediments on the surface of the beach and of the bed,suspended sendiment concentration,sub-bottom sedimentary characteristics to analyze the characteristics of the erosion basis of Poyang Lake,and their effects on hydrological regime and the bed revolution.The main results and conclusions of this paper are as follows.(1)Characteristics of the erosion base changes of Poyang Lake.Between 1953and 2008,the elevation of the deep flood point of the river trough at Hukou hydrological station decreases continuously.In 2008,the altitude of the lowest point of the river bed was-0.78 m deep.In 2016,the elevation of it decreased to-4.28 m.According to Datong Station,compared with the annual flow from 1951 to 2002,the water level at the same discharge of Hukou-Datong whose average water discharge were 20000 m³s^-1,25000 m³ s^-1,30000 m³ s^-1,respectively decreased by 0.56 m,0.51 m and 0.43 m,respectively.This approximately quantified the erosion base changes.In addition,the shape of Hukou section changed,and the thalweg gradually shifted to the right bank.(2)The water level at the same discharge of Hukou-Datong is significantly and positively correlated with the sediment discharge of Ganjiang River and Fuhe River,of Hankou Station and Datong Station,and the water level at Jiujiang Station.This paper selects Ganjiang River as the effect of sediment changes from five rivers,Hankou Station as the effect of sediment discharge changes in the main stream of Yangtze River,and Jiujiang Station as the effect of the water level changes of the main stream.Then the multiple linear regression model,indicates that it is significant at the 5%significance level,and the regression results are good.The relative importance analysis shows,firstly,it is the channel erosion caused by the decrease of sediment discharge of the main stream that is the main reason for the declining erosion base of Poyang Lake,with the proportion of 66.3%;the second reason is the influence of the decreasing sediment discharge in Ganjiang River on the erosion base of Poyang Lake,with the proportion of 25.9%;the last reason is the weakening influence of sediment carrying and backflowing caused by the lower water level at Jiujiang Station on the erosion base of Poyang Lake,with the proportion of 7.8%.(3)The influence of the declining erosion base on the hydrological regime evolution of Poyang Lake is mainly manifested as follows.The water level at the same discharge of Hukou-Datong has a significantly positive correlation with the water level of Xingzi Station in October,and a negative correlation with the days of dry period.The declining erosion base causes the rapidly falling water level in the dry season,the advanced dry period,and the prolonged dry period.Compared with the period 1956-2002,the annual average water level of Poyang Lake in October from 2003 to 2018decreased by 1.95 m,and the annual average ten-day water level in late October decreased by 2.82 m.This directly shows that the water level of Poyang Lake decreases rapidly in a short period.The paper chooses the water level at the same discharge of Hukou-Datong Station when the average flow of Datong Station is 20000m³s^-1 as the independent variable and the water level of Xingzi Station in October of the dry season as the dependent variable to explore the influence of the changes of erosion base on the water level of Xingzi Station of Poyang Lake.The linear regression equation,indicates that it is significant at the 5%significance level,and the regression results are good.(4)The influence of the declining erosion base on the bed evolution at Poyang Lake is mainly manifested as follows.(1)The decrease of erosion base level increases the sediment carrying capacity and changes the sedimentary environment of Poyang Lake.From 1956 to 1999,the total net sediment deposition was about 259.27×10⁶ t,with an annual average deposition of 5.89×10⁶ t.From 2000 to 2019,the overall net erosion was 97.93×10⁶ t,with an annual average erosion of 4.9×10⁶ t.(2)The declining erosion base accelerates the sedimentation evolution of the beach in the aquatic-terrestrial transition zone by influencing the water level in dry season.There is a apparent difference between coarse and fine-grained.As for the beach in the aquatic-terrestrial transition zone in the aquatic-terrestrial transition zone,the reciprocal step change in 2019 was more significant than 2020,and the horizontal distance of sediment stratification in 2019 was shorter than 2020.(3)The declining erosion base enhances the sediment activity on the lake bed,and the head-ward erosion in the river entry channel.In short,in recent years,the erosion base level of Poyang Lake shows a downward trend,which is mainly affected by the sediment coming from Ganjiang River and the operation of the Three Gorges dam.The main reason is the decrease of sediment discharge in the main stream of the Yangtze River caused by the operation of the Three Gorges dam.The decrease of erosion base level has an impact on the water level of Poyang Lake in dry season,especially in October.In addition,the decline of erosion base level changed the beach sedimentary environment and the characteristics of lake bed retrogressive erosion in the water land transition zone,and then affected the evolution of lake bed in Poyang Lake.