| three main anabranching channels in the Jingjiang reach is one of main channels connecting the middle Yangtze River(Jingjiang)and Dongting Lake.Since 1950s,the hydraulic projects such as artificial cutoffs in the lower Jingjiang River,Gezhouba Dam,and Three Gorges Reservoir have been implemented successively,the amount of water-sediment diversion in three main anabranching channels from Jingjiang River decreased annually.caused a serious negative impact on the recent adjustment of the Jingjiang-Dongting Lake relationship.The formation and evolution of three main anabranching channels in the Jingjiang reach,the change of water and sediment and temporal change of local erosion and deposition are one of the direct manifestations of the evolution of river-lake relationship.It is of great significance to study the changes of water-sediment exchange and connectivity in rivers and lakes.Hydrological data,channel topography,and remote sensing imagery were used to analyze temporal change of runoff and sediment load,channel morphology at the inlet and tail zone in 1951-2016.Both grid and cross-section topographical methods were applied to calculate temporal change of local erosion and deposition amount in 2003-2011 and 2006-2009,respectively.Considering the impact of the water level drop in the inlet zone,utilizing the daily average water level and discharge of five hydrological stations in 2003-2018,Establish a amount of water-sediment diversion in Ouchi,Hudu,Songzi River from Jingjiang River empirical formula to predict changes in the discharge of the three rivers in the future.The main results of this study are as follows:(1)During 1951-1980,the sediment load and runoff decreased by 63.4%and 64.8%,respectively.the artificial cutoffs hydraulic projects in the lower Jingjiang River accelerates the process of water and sediment diversion and the number of days of dewatering in the Ouchi River.During the same period,inter-annual changes in runoff and sediment load in the Songzi River were relatively stable.After 1980,the decreasing trend of runoff slowed down and entered a relatively stable period in the Ouchi River.However the amount of sediment load in Songzi River decreased rapidly,from 50.98×106 t to 36.11×106 t,about 29.1%from 1981-1990 to 1991-2000.After the impoundment of the Three Gorges Project,the balance of water and sediment in the three rivers was broken.On the one hand,the decline in sediment transport was aggravated,and on the other hand,the attenuation of runoff was delayed.(2)In 1984-2000,the inlet zone was gradually developed from several scattered bars to a single sandbar,and developed obliquely northward.After the north and south sub-branches of the tail channel section in the eastern branch were blocked,the tail channel section continued to deposit and extend outwardly.The growth rate in 1984-2015 was 8.1 m2/a,and the narrowed rate of river width of the Ouchi River inlet and tail channel section in the eastern branch was 3.7 m/a and 3.6 m/a,respectively.In 1987-2018,the area of Yanggezhou Island at the inlet zone of Songzi River decreased by 0.4 km2(a reduction of 4.2%),but the variation of the channel morphology of the inlet zone was small.In 1984-016,the sandbanks at the confluence of the Songzi River and the Hudu River were heavily silted,and three independent sandbanks will gradually accumulate as a whole,(3)In 2003-2011,the Ouchi River entrance area and the partial tributary diversion section were all washed away.The local section of the Central Branch of the Songzi River-West Branch,the East Branch and the West Branch is dominated by siltation.The Songzi-Huduhe,Zhongzhi-Xizhi confluence sections washed 1.65×106 m3 and 2.14×106 m3,respectively.In 2006-2009,Dongzhi was divided into 4 river sections with golden gates,Yinjiazhou and Nanxian as geographical nodes.The erosion and siltation alternated and the east branch flushing reached 15.8×104 m3.In 2006-2009,the Ouchi River washed a total of 40.2×104 m3.The Songzi River has a total siltation of 682.7×104 m3,and the sediment volume of the Xizhi River section is relatively large,reaching 89%of the total siltation.(4)discharge diversion of the Three Major Outlets is a function of the water level in the inlet area and the water-level ratio along the three outlet rivers.The simplified formulae of discharge diversion are obtained to further modify these formulae with considering the water level drop at the inlet area,and channel width.In 2017-2018,the measured data in Guanjiapu and Kangjiagang stations of Ouchi River were used to verify the predicted vales,8.9%and 14.8%of relative error,respectively,8.7%error in Mituoshi station of Hudu River,and 4.3%and 7.2%in Xinjiangkou and Shadaoguan stations of Songzi River,respectively.In summary,these empirical formulae are in good agreement with the measured data,which can be used to estimate the change of discharge diversion of the Three Major Outlets River affected by water level dropping of the Jingjiang River. |
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