Analysis Of Abrupt Change On Lower Water Level Of The Transitional Reach From Gravel Bed To Sand Bed In The Middle Yangtze River And Its Mechanism

The transitional reach from gravel bed to sand bed is usually developed in the mountain-pass of large rivers,and there exist abrupt changes in the upper and lower riverbeds in it.The transitional reach in Middle Yangtze River is close to the lower reaches of the Three Gorges Reservoir,of which river bed composition changes significantly at Jiangkou.When a large-scale water conservancy project builds on the river,the downstream of the dam occurs varying degrees of adjustment due to the destruction of natural properties.Compared with other river reaches,the adjustment of the transitional reaches from gravel bed to sand bed is more special,the irregular pebble layer of the gravel river bed would make the distribution of the sediment quantity of scour-silt along the river,the river course and the coarsening process to constrain each other,resulting in the uneven decline of the lower water level.Consequently,the high gradient slope,large flow velocity and some other phenomenon may occur,leading to the deterioration of waterway conditions.Therefore,the changes of lower water level in the transitional reaches has always been given much attention.The recently measuring data illustrates that lower water level in Zhijiang-Jiangkou reach changed sharply around 2009.And the change of water level in the upper and lower reaches of river were different due to the particularity of riverbed composition:the lower water level of the gravel area in the upstream of Baofa Temple changed from uneven decline to uniform decline,while the water level of the downstream of Baofa Temple transit from uniform decline to uneven decline.This phenomenon has a significant impact to the shipping conditions,thus further investigation is necessary.As a basic factor affecting the change of lower water level,determining the erosion characteristics of riverbed is the key of investigating the abrupt change.After the impoundment of the Three Gorges Reservoir,the transitional reach in the downstream of the dam will inevitably go through the top-down erosion along the way.And it is worth noting that sand bed may exist backwards erosion because of the decreasing of water level and backwards transmission in Shashi.As backwards erosion occurred in the transitional reach before the impoundment due to the cut-off in the Jingjiang reach,it has the condition that the erosions along the river and backwards the river occur at the same time.At present,there are few studies on the existence of backwards erosion in the transitional section.The issues of the confluence of the alongshore erosions and backwards erosions are also rarely studied.These problems are the key for determining the causes of the abrupt change of the water level in the transitional reach,should be further analyzed.In this paper,the Zhijiang-Jiangkou reach is taken as the research object,and the existing data are used to analyze the phenomenon and mechanism of the water level mutation in the river section.And the results are listed below:(1)The decline of the lower water level in Zhijiang reach was less than that in Jiangkou reach,and the water level of the whole reach was significantly decreased from 2002 to 2014,this process is consistent with the water level descending in Shashi,with delaying for about 2 or 3 years,indicating that the decreasing of river water level was affected by the backwards erosion in Shashi,which spent approximately 2 years.Furthermore,it should be noted that the change of the water level in the river section occurred around 2009:the rate of the decline of the lower water level in Zhijiang reach became uniform,instead of increasing along the river as before,and the rate of decline of the lower water level in Jiangkou reach showed a uniform upwards trend before 2009,while the rate of decline increased dramatically in the reach between Baofa Temple and Xiacaojiahe after 2009.(2)After 2006,the Zhijiang-Jiangkou reach experienced alongshore erosion and backwards erosion at the same time,and around 2011,the alongshore erosion and backwards erosion crossed at the area near Xiacaojiahe,and the backwards erosion stagnated in the area between Baofa Temple and Xiacaojiahe.This mainly because the riverbed of Xiacaojiahe consists of coarse cobblestone,requiring a far greater flow rate than the flow rate causing the erosion.Based on the difference of the geological composition of the river section,the abrupt change of the lower water level in the Zhijiang reach was caused by the erosion of the bottle-neck part.The change in Jiangkou reach was caused by which the backwards erosion didn't reach to Baofa Temple,while the riverbed of Xiacaojiahe was undercut by the two kinds of erosion.It was due to the different composition of the riverbed between Baofa Temple and Xiacaojiahe.Considering the change of stream gradient,the Zhijiang reach still has a strong control effect on the lower water level,while the control effect to the downstream water level of Jiangkou reach is obviously increased.This demonstrates that in the process of erosion Jiangkou reach has become a new bottle-neck for the falling of lower water level.(3)The maximum erosion thickness can be predicted based on the equilibrium mechanism of the principle of river bed coarsening.The results indicate that the upstream area of Xiacaojiahe has basically formed an impact roughened layer,especially the bottle-neck like Lijiadu,Baofa Temple and Xiacaojiahe,while there is still a thick sandy layer near the branch of the beach and the branch of Xiaojia Embankment,The relationship between the gradient of Jiangkou reach and the size of incipient particle is obtained in the study of the mechanism of scouring mechanism.The result is consistent with the actual development process of backwards erosion,and the critical condition for the backwards erosion can be judged by this formula.According to the analysis of the process of erosion,it can be inferred that the decline of the water level in the river may be significantly decreased in the vicinity of Lijiadu,and the influence of its lower water level falling on the lower water level of Changmenxi should be concentrated.