الفهرس 
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Abstract
Sediment transport in open channels and rivers is considered one of the most important and complicated processes in hydraulic engineering. The flow characteristics in compound open channel are greatly affected by the geometry of both main channel and floodplain.
In this study, we demonstrated the effect of sediment transport on the flow characteristics in a non-prismatic compound channel with gradually enlargement floodplain. This study was conducted using the Discrete Phase Model (DPM), which is available in the Computational Fluid Dynamics (CFD) software ANSYS-Fluent. Moreover, the joint effect of the floodplain divergence and convergence angle and sediment particle size on the depth-averaged velocity (DAV) within non-prismatic compound channels was also investigated. Four sediment particle sizes of 1, 5, 50, and 500 µm and three sediment mass discharges of 5.19, 10.38, and 15.56 kg/s were considered during simulations. In addition, three floodplain divergence and convergence angles of 4.0, 6.3, and 〖11.3〗^° were taken into account. Two relative depths (dr) of 0.20 and 0.40 were also considered.
Results revealed that, the stream-wise flow velocity along a non-prismatic compound channel with gradually enlargement floodplain deceased along the main channel and floodplain subsections with decreasing the sediment particle size. Also, as the sediment mass discharge increased within the flow, the stream-wise flow velocity decreased, especially, near the channel outlet. Results also showed that, the lateral DAV gradient between the main channel and floodplain at the middle and the end sections of the divergence and convergence reach is much larger in case of larger divergence and convergence angle.
For simulation scenarios with diverging floodplain, the sediment particle size had a considerable impact on the lateral DAV, especially, in case of lower relative depth. As the sediment particle size increases, the distortion in the lateral DAV distribution curves increases.
However, the stream-wise DAV in the main channel and floodplain decreases, as the sediment particle size decreases, especially, in case of higher relative depth.
For simulation scenarios with converging floodplain, the effect of sediment particle of small sizes on distorting the lateral DAV was not substantial. While, as the sediment particle size increased and the floodplain convergence angle decreased, the stream-wise DAV within the compound channel decreased.
Generally, and based on the results, it can be concluded that, the effect of the secondary sediment phase on the primary water phase in a non-prismatic compound channel with gradually enlargement floodplain was more pronounced in case of small sizes and high mass discharge of sediments. Corresponding to the floodplain divergence effect, the variation in the lateral DAV gradient was more substantial in case of higher divergence angle and coarser sediment particles. Furthermore, the stream-wise DAV dip along the main channel and floodplain was more explicit in case of higher relative depth and finer sediment particles.
For the floodplain convergence effect, the effect of coarse size of sediment particles on decreasing the stream-wise DAV within the main channel was more pronounced in case of lower convergence angle and higher relative depth. However, the effect of sediment particles with small size on decreasing the main channel stream-wise DAV was more apparent in case of higher convergence angle and higher relative depth, especially, near the channel outlet.