Study On Evolution Mechanisms And Trends Of Bars-channels Of The Taipingkou Waterway After The Impoundment Of The Three Gorges Reservoir

The Taipingkou waterway is the main obstacle sub-reach in the middle reaches of the Yangtze River.The Jingzhou Yangtze River Highway Bridge is above it.There are two navigation bridge holes in the North branch and the original South branch.However,the original South branch was seriously deposited around year 2000 and the Yanglinji point bar moved down and squeezed to the North branch after year 2012.The conflict between this natural evolution process and the location of the navigation bridge holes is very obvious.With the construction of anti-collision facilities for non-designed navigation bridge holes,the conflict has eased but unsolved.The main work of this paper:firstly,based on the finite volume method with the HLLC approximate Riemann solver and the Hybrid LTS/GMaTS scheme,this paper developed an efficient hydro-sediment-morphodynamic model for the Taipingkou waterway.And this model has been well verified by ideal case,benchmark experimental dam-break datas and the measured datas of the Taipingkou waterway.Secondly,based on the long-term datas of water and sediment flux in Shashi station,this paper analyzed the characteristics of new water and sediment conditions in the Taipingkou waterway after the impoundment of the Three Gorges reservoir.Thirdly,based on the model of this paper and the measured datas for more than ten years after the impoundment of the Three Gorges reservoir,this paper completely analyzed and successfully reproduced the key evolution processes of bars-channels and the conversion processes of the navigation condition in the Taipingkou waterway after the impoundment of the Three Gorges reservoir.Fourthly,based on the latest measured topography of the Taipingkou waterway,six possible future conbinations of income flow and sediment conitions were considered and this paper studied the possible future evolution trend.Furthermore,this paper proposed realignment suggestions.Fifthly,this paper studied the stability of the bradied channel and simulated the formation and development process of the ditch.The concrete conclusions are as follows.1)The Sanders LTS scheme has a potential instability.This paper introduced modification to avoid spatial variations in time step at wet/drt front or at dynamic/static front and then stable results were obtained under different LTS level.Combined with GMaTS to solve equations governing bed materials(i.e.,the morphodynamic part),the Hybrid LTS/GMaTS scheme proposed in this paper greatly improved the computational efficiency of hydro-sediment-morphodynamic model(for the application of the Taipingkou waterway:the reduction in computation time was 82%-91%s and a speed-up of 5.6-10.6 was achieved).2)The analysis of long-term datas of water and sediment flux in Shashi station shows that after the impoundment of the Three Gorges reservoir,the upstream sediment discharge has been greatly reduced,the duration of median discharge increased while the durations of flood discharge and dry discharge decreased,and the riverbed of the Taipingkou waterway has been obviously cut down and coarsened.The analysis of river evolution and study of numerical simulations show that:the new water and sediment condition and the enhancement of the water flow in the south channel are the causes.The central section of the Lalinzhou point bar is in the dominant position and the Sanba channel bar is a responser which is easily affected by any change in the Taipingkou waterway.The Taipingkou waterway carried out a global evolution processes following the key line "the central section of the Lalinzhou point bar is continuously washed down and therefore the lower section of the Lalinzhou point bar is growing bigger and higher,and the planar shape of the edge of the Lalinzhou point bar is gradually smooth year by year".The two shifts of the dominating branch between the North and South branches both surrounded this key line and the trend of the river evolution was good for the South branch.However,the North branch became the transitional main branch because the the big flood process of year 2005 and the abnormal storm flood of year 2010 delayed the development of the South branch.The series of channel improvement projects implemented in the Taipingkou waterway played its due role as a guardian,but it is difficult to reverse the evolution trend of the shift of the dominating branch from the North branch to the South branch.According to the analysis,the Taipingkou waterway may have initially completed the adjustment of the bars and channels in the past ten years after the impoundment of the Three Gorges.And it is trying to establish a new balance.3)The possible future evolution trend indicates that the Yanglinji point bar will continue to move down and squeeze the 2#branch and gradually shrink(for a long time,it may exist in the form of underwater bar in the entrance of the North branch).The 1#branch has temporary navigation potential.If the Yanglinji point bar is removed,the inflow of the North branch will be improved.The research suggests that it can rely on dredging to maintain the navigation of the north branch and the piers of the bridge should be reinforced timely in case of emergency.Renovating or rebuilding the bridge is a relatively long-term remediation plan.Excavation of the Yanglinji point bar may be beneficial to the north branch,but it is necessary to be verificated by the physical model.4)The study of the stability of the braided channel shows that the formation and development and the formation position of the ditch are closely related to the characteristic flow rate.And the magnitude of the characteristic flow rates of the formation and development of the ditch will depend,to some extent,on the planar shape of the channel bar.