Experimental study of the effect of channel side slope trapezoidal channel over the erosion phenomenon in open-channel confluence

Channel-river confluences are an important issue in open channel studies. Variation in quantity and direction of velocity and discharge value causes the erosion of the bed and banks near the confluence. Adequate knowledge about the process of forming a scour hole can be helpful to determine the proper distance of protective structure from channel confluence and river bank. The dynamic of river confluence is as a function of the momentum ratio of reaching flows and the geometry of the channels confluence location. By considering this fact that these conditions do not exist in nature and usually the banks of rivers are included. The slope of the banks causes the changes in three dimensional flow patterns in the place of confluence in relation to septum with 90 degrees that can be the reason of changing the pattern of scouring.
The main objective of this study is to investigate the effect of side slope of the channel on the maximum depth of the erosion at the confluence with a main channel. First by using the dimensional analysis, dimensionless parameters are derived. Then, by building a physical model, the effect of various parameters, including side slope of the channel (Z), discharge ratio (Qr), downstream densimetric Froude number, and downstream Froude number, on the maximum depth of bed erosion (ds) in a perpendicular confluence is investigated. The experimental tests were conducted in a main flume (6 m length, 80 cm width and 50 cm depth), a tributary flume (5 m length, 24 cm width, and 50 cm depth). The bed slope of two channels is considered as horizontal. Changes are made to form the false floor and movable bed, and also the width of the main channel is decreased from 80 to 50 centimeters to enhance the ability of turning the main channel wall to a sloping shape. Main and tributary channels with wooden platform with 14 centimeter height and the width that equal to width of main and tributary channels are raised. The space between these three false floors is connected as experimental region and then is filled with sediment. The side slope of the main channel is considered as 45, 60 75 and 90 degree. For all experiments, the input discharge of the flume was regulated as 15 lit/s. By considering four wall side slopes, four of discharge ratios and four different flow depths, four Froude numbers and four densimetric Froude numbers are applied. To regulate the water surface at the end of main channel, we used stop logs with one centimeter height and by omitting each stop log, one Froude number is produced. The experiment was ended when the equilibrium condition for erosion and scouring hole is created. By considering this fact that the most equilibrium time will take in the slope of 45 degrees in comparison with the other slopes. The experiments with the slope of 45 degrees were done at the most and least of ratio discharge and finally, after six hours, 90 percent of scouring was produced. Therefore, for all the experiments, the experiment time is considered equal to six hours.
Immediately after the experiments are started, because of contacting the tributary and main channel flow, two rounding vortices are made. These vortices moved in contrary to each other. It is the main factor to start the scouring of bed materials in the place of confluence. Rounding vortices that are nearer to the place of confluence, act stronger because of the contraction of flow. The bed material in downstream confluence is shouted to the surface of water and moved to the downstream of the channel. When the discharge ratio is going to be increased, because of the increasing of momentum of tributary channel, the tributary flow enter the main channel with an increasing intensity and this make the vortices and down-flow stronger and causes bed scouring especially in the first stages of the experiment. Bed materials that are shooting to the surface of the flow, after becoming nearer to the water surface, they lose their energy and when they are falling, the secondary flow transported them to the downstream confluence and by settling these particles, we could see little by little sediment bar.
The results indicate that the depth of scour at confluence increases with increasing the side slope of the channel, discharge ratio, densimetric Froude number, and Froude number, whereas it decreases as flow depth increase. The best performance in decreasing scour (equal to 46% in comparison with side slope of 90 degrees) belongs to side slope of 45 degrees and the discharge ratio of 0.2. The derived relationship for predicting the maximum depth of scour at the confluence shows that scour depth is more related to Froude number and downstream densimetric Froude number than the other parameters (i.e. discharge ratio, side slope of the channel and relative depth). The best performance in decreasing the scour was belong to side slope of 45 degrees and the discharge ratio of 0.2 which was equal to 46 percent. Since the side slope of 75 degrees is close to slope with 90 degrees, it doesnt show the appropriate operation, especially in high discharges ratio and it has 9 percent decreasing of scouring. In the present study a dimensionless equation was presented which can be applied for prediction of scour depth at river confluences with the variable side slope.