Study On The Evolution Of Straight Rivers Under Different Upstream Sediment Conditions

In recent years,strong human activities have had a significant impact on the channel evolution and the laws of water and sediment transport in natural rivers.Especially the construction of large and medium-sized hydraulic engineering has greatly changed the conditions of incoming water and sediment.Changes in the incoming flow process are mainly reflected in changes in the runoff process.The medium flow discharge has a longer duration,the low flow discharge increases,the relative flow amplitude of the runoff process and the flood peak variation coefficient decrease.On the other hand,the reservoir blocks a large amount of sediment,the amount of sediment coming from the upstream channel is reduced,the flow is in an unsaturated state,and the river bed will be scoured.This change in water and sediment conditions will break the equilibrium of the downstream river bed and cause river bed reconstruction process.The main focus of this paper is the change of sand conditions in the upstream.In this paper,a simple straight river channel is selected as the research object.Based on the previous data analysis and theoretical research,the generalized flume experiment and numerical simulation are used as research methods to control the straight channel under different sand conditions.The main research results of this paper are summarized as follows:1)When the sand conditions change in the upstream of the straight channel,an important phenomenon in the evolution of the channel is the downward movement of the alternate bars.As the amount of sand coming from the upstream decreases,the downward movement speed of the alternative bars is correspondingly slowed down.The study found that the length of siltation at the end of the bars is positively correlated with the upstream sediment.The deposition of the bar at the most upstream of the river will compress the mainstream of the nearby river channel,which in turn will intensify the downward movement of the head of the bar downstream and accelerate the movement of the bar.This phenomenon mainly occurs when the alterna bars meet end to end and the bars are close to each other,and have a certain relationship with the width of the bars.When the upstream sediment is reduced,the speed of the sedimentation of the bar at the most upstream of the river section decreases,which slows down the movement speed of head of the bars.2)The area of the bars in the straight river channel also shows different rules with the sediment coming from the upstream.When the amount of sediment coming from the upstream is large,the area inside the river moves down while the area increases,and the size of the bars increases.However,changes to the area of a single bar will not fully comply with this rule.For example,some of the bars may be scoured and the area is reduced rapidly,but the total area of the bars in the river is generally positively correlated with the size of the upstream sand.3)In the longitudinal form of the river,when the upstream sand is large,the upper reaches of the river are usually in a deposition state.With the decrease of sand from the upstream,the riverbed gradually changed from deposition to scouring,and the riverbed scouring under the clear water was the largest.Beside the riverbank boundary with strong anti-erosion properties,the mainstream of the river channel is constrained of the river bank and form a deep trough nearby.The riverbed elevation in the deep trough is positively correlated with the upstream sand: When the sand is large in the upstream,there will be some sedimentation in the deep trough,so that the deep trough is correspondingly shallow.With the decrease of the amount of sand coming from the upstream,the riverbed elevation in the deep trough gradually decreases,and the deep trough becomes deeper.4)It reveals another possible reason for the formation of the “inverted-set” phenomenon in the straight river channel.In the study of this paper,it is found that in the process of the moving down alternative bars of the straight river channel,if the deposition of the bars' tail is not close to the river bank,the original deep trough does not disappear,a "downset" will be formed between the bars and the river bank.