The Impact Of Human Activities On Stream Flow And Sediment In The Yangtze River Estuarine Area

River systems play a significant role in the transport of water and sediment from land to the ocean,which must be seen as a key pathway for material transfer on the Earth.In the world,the influence of the human activities on the flow and sediment fluxes into the sea has always been attached great foeus in the intemation research,which directly and indirectly influenced the water resources and environment,geological,and waterway.The research,of which the study area is the Yangtze River Estuarine area,combines with geography,hydrology,sediment dynamics,etc.,uses the statistical analysis,such as trend,jumping and wavelet cycle analysis,and mathematical model to detect the trends of runoff and sediment in the mainstream and transporting into the estruary of the Yangtze River,and quantitatively analyze the contribution rate of climate change and human activities on the sediment discharge at the Datong hydrological station.And then,the hydrological and sediment model of the Yangtze River estuary was established to discuss the influence of water diversion project on the hydrodynamic conditions in the dry season,and predict the future of human activities impact on the water and sediment transport of the study area.On the basis of about sixty years annual runoff and sediment load at three hydrological stations in the mainstream,the study indentified the characteristics of runoff and sediment load.From 1950-2013,the main cycle of the annual runoff and seiment load at the three stations had good consistency,but the wet season period had not good correlation with the sandy period.Compared with the pre-TGD period,the dry season runoff increased and the monthly sediment load decreased at the mainstream of the Yangtze river in the pos-TGD period(2003-2013).The climate changes was the main reason for the interannual variations of the runoff,and the trend changes of the runoff and sediment load were attributed to human activities,especially the Three Gorge Dam(TGD),which was the main reason for the increasing of dry season runoff,but the effect weakened with the distance along the downstream direction from TGD.The runoff and sediment load at Datong station,from which the stream flow and sediment flux into Yangtze River Estuary,had obvious interannual variation.From 1950-2013,the maximum values of the monthly runoff and sediment load were 4.5 times and 31 times of the minmum values,respectively.The sediment at Datong station was mainly composed by suspend fine-sediment.The median endothelial cushion grain-size(Dso)became gradually smaller.In the period analysis,after 2003,the annual and flood season runoff at Datong station were in dry period,and in 20 years time scale,the annual and flood season sediment load was in less sand period.From 1950-2013,the dry season runoff was in significantly increasing(P<0.05),of which the abrupt change point was 2003,and annual and flood season sediment load was in significantly decreasing trend(P<0.05),with the abrupt change points of the annual sediment load were 1996.The change trends of water and sediment flux into the estuary were attributed to the human activities,and the study quantitively analyzed the contribution rate of human activities on change trend of sediment flux into the estuary.From 1954 to 2013,the contribution rates of precipition,water yield capacity and sediment concentration were 3.95%,-0.11%and 96.16%respectively,and indifferent periods,the sediment concentration was the main factor resulting in the decrearing of the sediment load at Datong station.The sediment concentration was influenced by human activities in Yangtze River Basin,and the contration rate of the reservoir construction resulting in the decreasing of sediment load at Datong station was about 85%.The human activities influced the sediment concentration in the Yangtze River resulting into the decreasing of the sediment load,and reservoir construction was the main reason.And then,based on MIKE 21 hydrodynamic and mud model,we constructed the 2-dimensional(2-D)hydrodynamic and mud model of the study area.On the basis of the investigation of the water diversion projects in the study area,we set different scenarios with combination between the discharge of Datong with the water diversion in the dry season.In the dry season,under the premise of constant tidal condition from the sea,as the discharge of Datong station decreased,the tidal level dropped and tidal range increased.In the same flow condition at Datong station,the instantaneous tidal current limit could be traced as far as the upper reach of the estuary during neaping tide.On the basis of the future climate change and human activities influences on stream flow and sediment flux into the estuary,we researched the impact of future human activities on flow and sediment transport in the study area.In 2050,the influence of human activities on flow and sediment transport in wet season(Octorber)was more than that in the dry season(February).In the dry season,compared with scenoria l,the human activities in scenoria 2 had no consistent influence on the tidal level and tidal range in different area along the estuary,resulting in the uncertainty change of the tidal current limit,the flux of stream flow and suspended sediment into sea would decrease,but the ratio of the flow of South and North Branch would be balanced and the ratio of the sediment of South would have an advantage,especially in the spring tide.In the wet season,in scenoria 2,the tidal level dropped and tidal range increased,resulting in the tidal current limit moving to the upstream,the flux of stream flow and suspended sediment into sea would decrease,and the diversion ratio of the flow discharge and sediment transport of the south branch would have an advantage.