An investigation of three-dimensional turbulence structures at open-channel confluence with zero or ten degrees was conducted experimentally by using a flow visualization technique. Measurements were performed with the aid of a high-accurate image analysis method of particle image velocimetry by illuminating vertical and horizontal sections with a laser light sheet. Evolution of secondary flows for zero and ten-degree confluence showed a significant difference even though the difference of the angle is rather small. For example, increase of tributary channel discharge for a ten-degree confluence generates a large scale secondary flow that extends to the whole channel width, which was not predicted before the experiment. In addition, the extent of confluence effect in the downstream direction was also examined.
