Flow Velocity in Meandering Compound Channel under the Influence of Sinusoidal Change

Natural rivers are rarely in direct flow because of regulating the energy grade-line, and usually have a curved path to which it is referred to as "meandering channels". After the appearance of meandering rivers, with the passage of time and the lateral movement of the meanders, the external bending progression and the sinusoidal or curvature is increased. In the meandering channels, the curvature of the meandering sections with a dimensionless number can be defined as the sinusoidal which is the ratio of meander length of the main channel to the floodplain length. By increasing sinusoidal slope number, flow velocity and river discharge capacity decrease. As a result, the risk of flood has increased significantly and during floods the water level exceeds to the main river boundary and enters to the floodplains. In this case, due to the interaction between higher velocities in the main channel and the slower velocities in the floodplains and the exchange of the momentum between these two regions, the flow profile is constantly changing. In this research, the hydraulic characteristics of the flow including the depth-averaged velocity, the free surface water profile, longitudinal velocity distributions, ratios of transverse to longitudinal velocities in the central axis of the main channel and the mean velocity and flow rate of the main channel along the meandering compound channel have been investigated numerically, regarding the change in the sinusoidal ratio for Six types of channels with different sinusoidal ratios. In order to investigate the effect of the sinusoidal ratio in meandering compound channel on the hydraulic characteristics of the flow, the FLOW3D software is used, the software was also chosen so that the turbulence model with experimental data have better compliance. For this purpose, two RNG and k-ε turbulence models were used and the performance of these two models was investigated in simulating the important hydraulic characteristics of the flow, such as the flow velocity, and it was determined that the RNG turbulence model has a better accuracy than the k- ε turbulence model. In the following, this model was introduced as the final turbulence model for numerical simulation. Numerical simulation results show that by increasing the sinusoidal ratio of channel from 1 to 1.641, the mean velocity of the main channel section is decreased by 54% on average and the flow rate of the main channel decreases by the average of 38%. Also, by increasing the sinusoidal ratio, the maximum depth-averaged velocity decreases from 0.55 m/s to 0.38 m/s, and the maximum free surface height of the water rises from 0.305 m to 0.332 m in the outer bend of the CS1 cross section. Increasing the sinusoidal ratio causes the ratio of the transverse velocity to be increased longitudinally in the central axis of the main channel, so that its value in the most critical state reaches from zero to 0.4. As the sinusoidal ratio increases, the maximum length velocity moves towards the right side flood plain (internal bend) and decreases its value so that by increasing the sinusoidal ratio from 1 to 1.641, the maximum longitudinal velocity 0.55 m/s to 0.42 m/s and its position moves from the center of the main channel to the inner bend over the depth of the main channel overflow.