Effect of sediment transport on flow characteristics in non-prismatic compound channels

Sediment transport in open channels and rivers is considered one of the most important and complex processes in hydraulic engineering. In this study, we investigated the effect of sediment transport in non-prismatic compound channels on flow characteristics using the Computational Fluid Dynamics (CFD) software ANSYS-Fluent. Contrary to many previous numerical studies, the momentum exchange between both primary (water) and secondary (sediment) phases was considered by activating the Discrete Phase Model (DPM). The aim was thus, to investigate the behavior of particles from a Lagrangian viewpoint and a discrete perspective. This enabled to monitor the momentum lost or gained by transported sediment particles that were injected in the continuous phase. Moreover, the influence of discrete phase trajectories on the continuum continuous phase was considered. Four sediment particle sizes 1, 5, 50, and 500 µm and three sediment mass discharges 5.19, 10.38, and 15.56 kg/s were considered during simulations of the non-prismatic compound channel. Results showed that the stream-wise velocity of flow deceased along the length of the main channel and floodplain for sediments with small diameter. No considerable change was noted for larger particles. Sediment mass discharge had a considerable effect on flow velocity. As sediment mass discharge increased, flow velocity decreased. The flow velocity at the channel outlet was 0.52 and 0.50 m/s corresponding to sediment mass discharge of 5.19 kg/s and 15.56 kg/s, respectively. Results revealed that flow turbulence was attenuated in the presence of sediments and the degree of attenuation increased with increasing sediment mass discharge in flow, especially near the free channel surface. Moreover, suspended sediment concentrations were higher in the presence of gradual enlarged floodplain as compared to the straight one. Maximum concentrations were 13 and 190 kg/m3 in the gradual enlargement reach for sediment diameter 1 and 5 µm, respectively, and 4.5 and 170 kg/m3 in the same reach for the prismatic compound channel, respectively. Generally, and based on the results, it can be concluded that the effect of the secondary sediment phase on the primary water phase was more pronounced in case of small diameters and high mass discharge of sediments. In addition, the joint effect of floodplain width enlargement and sediment particle size on suspended sediment concentration was more pronounced for fine sediments than that for coarser sediment along the compound channel.