Propagation And Barrier Mechanism Of The River Regime Adjustment In The Middle And Lower Yangtze River

The Yangtze River is the largest river of China,and is also the only waterway transport corridor linking eastern,central,and western China.It is the urgent problem to be solved in the middle and lower Yangtze River that how to carry out comprehensive management to effectively control the favorable river regime and to improve the adverse river regime.So the Yangtze River whose waterway,harbor,and aquatic environment are excellent can facilitate the discharge transportation and the stability of river regime and bank line.Since the establishment of the nation,the bank collapse which endanger the safeties of embankments and the river regimes changing fiercely had all been systemically regulated.At the same time,the in-depth analysis about riverbed evolution and had been carried out too,which enriched the understanding about the regularities of the channel adjustment.Among them,many scholars have recognized that the upstream river regime adjustment may be the most important reason for the downstream riverbed evolution,whose influence and propagating range may include relatively long reaches.But some scholars believe that the upstream river regime adjustment may not continue to propagate downstream,there are some river reaches can block this propagation effect,so the adjustment of the upstream river regime can be limited to a certain range.Given in the Yangtze River is characterized by staggered distribution of bifurcated river and single-thread river,whether the adjustment of the upstream bifurcated river can pass through the single-thread river and propagate to the lower reaches will affect the stability of the overall river regime.The existence of the barrier river reaches can divide the Yangtze River into several scopes,wthin where river regime adjustment will not affect its downstream reach,making the regulation target and the regulation strategy are relatively simple.Therefore,it is of great theoretical and practical guiding significance to analyze the propagation and barrier factor of river regime adjustment and to define the definition,characteristics,causes and mechanism of the barrier river,on the basis of systematically combing the long-flow-path and long-time measured evolution data.The paper mainly discusses the following questions:(1)The defination of barrier river reaches.Through the systematic summary of propagation and barrier phenomenon in the middle and lower Yangtze River,classifying and analyzing the propagation and barrier elements,it is found that the occurrence time,mode and cycle of the river regime adjustment between the upstream and downstream is not in a one-to-one correspondence.This non-correspondence is not due to different river types or different water and sediment conditions,but instead,is driven by the in-between river reach itself,whose channel morphology and composition characteristics can maintain its own equilibrium of sediment transport no matter the changes in the water and sediment conditions,and can stabilise the position of the main stream line and provide a steady inflow condition for the entrance of the downstream reaches after the upstream river regime adjusts,so as to prevent the upstream river regime adjustments from affecting the downstream channel evolution.This kind of reach can be defined as barrier river reach.(2)The characteristics of barrier river reaches.Through comparing and analyzing the similarities and differences in aspects of plane,transverse and longitudinal morphology,as well as the stability of riverbank and the erosion resistance of riverbed between the barrier and non-barrier river reaches,the characteristics of the barrier river reaches can be summarized.Based on the theory of dissociative vortex and its induced velocity at the head of the spur dike,the change of the velocity distribution before and after the node deflecting flow was analyzed,and the mechanism of the node deflecting flow was clarified.Through analyzing the change regularity of standard deviations of average hydrogeometric coefficient along the flow-path inside river reach,it was found that there must be a cross section with a standard deviation greater than 15 in non-barrier reach,but they were all less than 15 in barrier reach,where the continuous length of cross sections whose hydrogeome-tric coefficient less than 4 was needed at least 3200 m.Based on the Fukuoka method,the stability of the river bank with binary structure was evaluated.From a view of both river banks,the critical hanging length of upper cohesive layer should be greater than the actual retreat distance of lower sandy layer in the barrier reach,making no serious bank collapse to widen the channel significantly.(3)The cause of barrier river reach.Basis on the influencing factors of main stream line migration,the empirical formula of plane position of main stream line was established.Though analyzing the differences of the migration modes between the barrier and non-barrier reaches,the characteristic flow interval was divided.The timing characteristics of duration days of the flow interval were analyzed by the wavelet transform method.The barrier reaches can always maintain the second-line models because they have advantages in material composition of river perimeter and river morphology.It was found that the duration days series of non-barrier reaches had shorter main cycles and a number of sub-cycles with strong signals,whose adjustment frequency of river regime were higher;but the barrier reaches series had longer main cycles and their signal vibrating strengths of sub-cycles were not very strong,therefore the adjustment frequency is lower.(4)The mechanism of barrier river reach.Through deducing the calculation formula of the bending radius of flow dynamic axis,it was found that under different flow conditions,the constraining force of the channel boundary was always greater than the migration force of the flow dynamic axis.The dynamic response model of riverbed in the propagation process of the main stream pendulum wave showed that,after the upstream river regime adjusted,the accelerated speed of the main stream pendulum wave in the non-barrier reach oscillated obviously,and the oscillation amplitude was increasing from upstream to downstream.The oscillation amplitude of accelerated speed of the main stream pendulum wave gradually attenuated in the barrier reach,and the fluctuation of pendulum wave may stagnate somewhere to block the propagation process from upstream to downstream.(5)Classification and application of barrier river reach.The influencing factors of the thalweg migration were extracted as:ratio of flow or duration above and below floodplain,relative displacement of thalweg at the entrance,intensity of node deflecting flow,ratio of river width to water depth,relative width of floodplain,and Shields number.Through establishing empirical relationships between the observed thalweg migratory distance and the limit indicator of thalweg migration,the barrier degree can be divided as:4 barrier river reaches;5 transitional river reaches transforming from barrier to non-barrier;10 transitional river reaches transforming from non-barrier to barries;and 8 non-barrier river reaches.For the first kind of reaches,the man-made engineering should be prevented from damaging the natural barrier properties.For the second kind of reaches,the loss of barrier properties due to adverse changes should be prevented.For the third kind of reaches,appropriate measures should be taken to eliminate potential causes of damage.For the forth kind of reaches,river regulation and planning should be systematically carried out from upstream to downstream.