Experimental study on flow pattern and sediment transportation at a 90° open-channel confluence

Abstract

In this paper, the evolutions of flow pattern and sediment transportation at a 90° open-channel confluence with different discharge ratios (q*) of the tributary flow to the total flow were studied. The experimental result showed that the water surface profile at channel confluence was influenced by tributary mixing action. The vertical distribution of the time-averaged flow velocities correlated with the discharge ratio as well as with the flow regime. The increase of the discharge ratio q* induced a gradually decline of the time-averaged velocity upstream of the confluence; high and low velocity regions were generated as well and a back flow separation region was formed in the cross sections downstream. When the discharge ratio q* was relatively large, the development of the separation region was restricted. During scouring experiments with no supplementary sediment, the motion of the bed load was intermittent and fluctuated, while the bed load discharge initially increased and then decreased monotonically. When the water in the tributary was clear, the largest local scour appeared near the shearing surface downstream of the confluence. The bed load transport rate and the accumulative sediment transport amounted in time were recorded and analyzed for different discharge ratios q* in each cases. According to the results, the distinctive characteristics of a sharp-edged, channel confluence flow might be divided into six main zones. These experimental results are helpful for the analysis and regulation of shoals at a tributary mouth.