Experimental Study On Flow Characteristics Of River Channel Around Different Flow Structures

The study of flow characteristics of river under spur dike started earlier.The study of flow around spur dike has always been a classical engineering research problem.There are many achievements in time-averaged flow field and turbulence characteristics,and its theoretical results have reached a mature stage.In the alpine Canyon area,due to the geological hazards such as collapse,landslide and debris flow,it is easy to produce loose bodies into the river to form deposits which are difficult to transport under general flow conditions.These deposits will increase the source of sediment,squeeze the flow path,and change the flow characteristics and sediment transport characteristics of the river.There are similarities between the effects of deposit and spur dike on river flow,such as the phenomenon of bypass and local scouring in the river,so the study of river flow characteristics under different structures can draw lessons from the research results of spur dike.In this paper,the flume model experiment and theoretical analysis are combined to analyze the different effects of different shape and scale structures on the natural flow field of rivers by flume experiments under different flow conditions with various generalized models of spur dikes and deposit,so as to explore the similarities and differences of the effects of spur dike and deposits on the flow field and to evolve the riverbed with different flow structures.The research lays the foundation.The water depth was measured by automatic water level meter and the velocity was measured by Particle Imaging Velocimeter(PIV).The effects of different flow structures on the river surface line,time-averaged flow field and flow deflection were studied.The main research results are as follows:(1)The variation of water surface line in different flow structures is analyzed.Generally speaking,whether on the left bank or the right bank of the river,the variation law of water level along the river is the same: although backwater occurs in the upper reaches of the river,the flow is relatively smooth,and the water level gradient along the river is almost unchanged,but trapezoidal spur dike have the greatest backwater effect on the water level,square spur dike take the second place,and the deposit is the smallest.When the water flow gradually diffuses around the flow structure,the phenomenon of hydraulic jump occurs,and the square spur dike hydraulic jump is the most violent,occurs at the earliest position,and dissipates energy by hydraulic jump is the largest.However,the hydraulic jump efficiency of each flow structure is small,which shows that the total energy of the cross section before jump is mainly dissipated by hydraulic jump,but mainly by the horizontal back-and-forth fluctuation of the water level of the cross section after jump.Yes.The location of hydraulic jump is not only related to the length of the structure along the river,but also to the blocking area: the longer the length of the structure,the larger the blocking area,and the lower the position of hydraulic jump.In the lower reaches of the river,the flow tends to be steady gradually.The trapezoidal spur dike has the greatest influence on the lateral gradient of water level,the square spur dike takes the second place,and the deposit is the smallest in the section where the flow around the structure has the greatest contraction(x*=0).In the downstream section close to the flow structure(x*=0.5),the trapezoidal spur dike has the first backflow region,which makes the horizontal gradient change greatly,the water level is the lowest on the left bank and the largest on the right bank.Near the reflux zone(x*=1.1),trapezoidal spur dike still have the greatest lateral impact on water level.From the left bank to the right bank,the water level first decreases to the minimum level,then increases to the level equivalent to the left bank,and slightly higher than square spur dike and deposit.(2)The time-averaged flow field distribution of different flow structures is analyzed.In backwater area and mainstream area,the flow field distributes evenly and moves forward along the direction of flow.In stagnation zone,the stagnation effect of square spur dike and trapezoidal spur dike on backwater flow is greater than that of deposit.In the backflow region,the backflow length of deposit is longer than that of square spur dike and trapezoidal spur dike,and lasts to the hydraulic jump area.The longitudinal velocity and transverse velocity distribution along the course and along the transverse direction of different flow structures are similar.The transverse velocity is less than its longitudinal velocity,and the order of the maximum value is: square spur dike is larger than trapezoidal spur dike,and is larger than deposit.It shows that near the river bed,square spur dike scours the bottom of the right bank riverbed most,trapezoidal spur dike is next,and deposit is the smallest.It is not difficult to find that the longitudinal velocities of different flow structures in their respective reflux zones are relatively small,and reverse velocities are generated,which is prone to the phenomenon of left silting and right scouring.The maximum flow velocity of spur dike in the main stream area of each flow layer around the structure is obviously larger than that of the deposit,and the maximum flow velocity of trapezoidal spur dike is slightly larger than that of square spur dike.Therefore,the influencing factors of the maximum flow velocity in the main stream area of each flow layer downstream of the structure are only related to the maximum blocking area.(3)The effects of different flow structures on the deflection of river flow are analyzed.For different structures,there are the same rules.From the direction of the flow,the flow in the upstream backwater area of the structure deflects to the right bank,and decreases with the development of the contraction section.After the contraction section,the flow in the downstream mainstream area deflects to the left bank,and the amplitude is not relatively stable.From the transverse analysis: the deflection of the same section is basically reflected in the gradual increase from the left bank to the middle of the flume,and gradually decrease from the middle of the flume to the right bank,and the deflection of the two ends of the left and right bank near the wall of the flume is very small,which is due to the fact that the structure far from the flow around is less affected by it.Vertical analysis: In the upstream main stream area of the structure,the deflection of the line flow in the middle of the flume to the right bank gradually decreases along the water depth from bottom to top.At the same time,for different flow structures,the maximum deflection angles of square spur dike,trapezoidal spur dike and deposit to the right bank are 71.7 degrees,69.3 degrees and 26.6 degrees,respectively.Therefore,the order of influence of different flow structures on river flow deflection is: square spur dike,trapezoidal spur dike and deposit.(4)This paper intends to conduct flume experiment under the same flow conditions with various generalized models of spur dikes and deposit,to analyze the different effects of structures with different shapes and scales on the natural flow field of rivers,and to discuss the necessity of studying deposits on the basis of spur dike.At the same time,the study of threedimensional flow characteristics under different structures can provide validation data for flood discharge,sediment erosion and siltation,river regulation and embankment protection in the upstream of bypass structures.It can provide theoretical support for the analysis and prediction of sediment incipience,transport and development process,as well as the evolution and change of riverbed.The research results can also provide technical support for the optimization of river regulation schemes affected by deposits in high mountain and gorge areas.