Simulation of the Effect of Bed Discordance on Flow Pattern at the River Confluence by Flow-3D Model

The dynamics of flow in river confluences are complex; in particular, immediately at the downstream of the junction, the flow creates a separation zone with secondary re-circulation patterns. This study aims at examining the influence of the relative bed discordance (z/y3) and discharge ratio (Qr=Q2/Q3) on flow dynamics in river confluence by Flow-3D model. To achieve this goal four discharge ratios (Qr= 0.2, 0.33, 0.5, 0.67) and four bed discordance ratios (z/y3=0, 0.1, 0.2, 0.3) were considered in a 90 degree junction. In this research, the parameters such as separation zone dimension, flow pattern, mixing layer, water level and velocity pattern were studied. The results showed that changing bed level affects the flow pattern in the junction; so that the dimensions of the created separation zone was different for concordance and discordance bed level. The separation zone near the bed observed only for same bed level junction and it was not occurring at the unequal bed level. By increasing the bed discordance, ration the width and length of separation zone increases and decreases, respectively. Also, the water level in the main channel increased (at the upstream of the junction) and in the tributary flume decreased with increasing the bed level. In addition, the energy turbulence at the confluence increases by increasing the bed level that its amount in unequal bed level junction (z/y3=0.3) is 1.34 times of equal bed level.