Three-dimensional model of compound channel with rigid and alluvial bed for determination of velocity distribution and discharge-stage relationship

There are numerous investigations, which carried out regarding solution of lateral distribution of flow velocity and flow discharge computation in compound channels. Most of these researches have been in the case of rigid beds. However, compound river channels have alluvial beds and hydraulically have many differences with the experimental channel with rigid beds. In the case of alluvial bed channels due to the interaction effect of flow and bed sediments in main channel, bed forms as dunes develop and change the riverbed roughness coefficient. Furthermore, the flow pattern in compound channels is essentially three-dimensional and hence it is better to use 3-D mathematical models for solution of their hydraulic problems. In this study using mathematical model of FLOW-3D, two and three-dimensional variations of flow velocity in straight compound channels with rigid and alluvial beds have been simulated and compared with the experimental data of Wallingford Hydraulic Research. The lateral distribution of velocities and stage-discharge curves from two large-scale trapezoidal chanals including FCF-A with rigid bed and FCF-C with alluvial bed were used in this study. The results showed that flow velocity contours and the lateral distribution of velocity obtained by FLOW-3D model have suitable agreement with the experimental data in both cases of main channel and floodplains. The results of these comparisons also showed that the flow rates of the FLOW-3D model are in great agreement with laboratory data, so that the maximum and average of the relative errors of these results for the channel with rigid bed were 5 and 2.7% and for channels with alluvial bed it was 6% and 3.8%, respectively. Finally, the results obtained from the FLOW-3D model with the results of a one-dimensional DCM, SCM, HEC-RAS,COH, WDCM, EDM and the results of the SSIIM model. This comparison highlighted the superior ability of the FLOW-3D model in flow hydraulic simulation of compound channels with rigid and alluvial beds.