Abstract: The confluence of water flows occurs in a great number of hydraulic systems, and its flow structure is often complicated but crucial to understanding the systems. To examine the flow behaviors of the confluence of an open channel and a tributary, this study uses the PIV technology to measure pollutant concentration distribution so as to overcome the limit of single point measurement, focusing on pollutant diffusion and concentration field in an asymmetrical intersection with different confluence angles. Results show that near the tributary outlet, surface pollutants are more uniform than those in deeper layers, and along with the further development of mixing between the two streams, pollutants in the deep layers become more uniform than those in the surface layer. At small confluence angles, a long and narrow contaminated zone is formed at the tributary side of the main channel, while large angles will shift this zone laterally further toward the other side. Higher pollutant concentration leads to a longer contaminated zone, and a change in pollutant concentration will greatly impact the location and width of the mixing layer. This study demonstrates that the PIV technology is satisfactory in measuring pollutant diffusion in channel intersection areas and thus is applicable to detailed studies of the pollutant concentration field.

Key words: PIV, intersection, pollutants, concentration field, open channel, transport and diffusion