Characteristics of sediment movement and river-bed morphology at mountainous stream confluence region

Confluences are commonly observed in the natural river network of China, in particular in the southwestern mountainous regions. It is important to study and understand the evolution of the stream morphology for better design and management of channel engineering and restoration in these regions. This study selected the confluence zone of Shenxigou stream and Baisha stream, which is located in Dujiangyan City, Sichuan, China, as the prototype to build up the down-scaling physical model in the laboratory. Experiments were carefully designed and tested to investigate the characteristics of sediment movement and channel bed morphology at this confluence zone. Meanwhile, the numerical simulations are conducted for the same confluence system and calibrated by the experimental tests, so as to investigate the details in the confluence region. The results show that the erosion pattern of the point bar at the downstream corner of conjunction is fairly dependent on the presence of tributary and the flow discharge ratio of the mainstream and tributary. Beside the well-known point bar, with combination of mainstream and tributary, an ecological-function pool-riffle-pool bed form is formed at the mainstream side of confluence. On the evolved bed, the surface sediment is well sorted with particle size parameters characterized by typical manners. With the assistance of simulated flow and bed shear stress, explanation can be well made on the sediment movement characteristics in the confluence.