Numerical Simulation of Flow and Sediment Structure in Confluence of Rivers

Confluence is the hydraulic singularity where two or more channels (or rivers) converge in a single channel downstream. In fluvial networks, stream channel confluences produce significant changes in flow, sediment transport and water quality. Although over the last 60 years, the synergy between laboratory tests and field measurements has provided valuable information about the complex hydro-morpho-sedimentary processes acting in river confluence zones, but less numerical study as a cost-effective tool has been conducted in this area. The objective of the present paper is to investigate in detail the changes in the hydro-morpho-sedimentary processes occurring in confluence zone using a numerical model. Therefore, a depth-averaged, two-dimensional, unsteady-flow model, which is named CCHE2D, has been applied for channel confluence and bifurcation applications. In the absence of field data, experimental data for channel confluence collected in the laboratory were used to verify the simulated results. Experiments have been performed in a confluence flume where the main channel is 8.5 m long and 0.50 m wide. A 4.9 m long and 0.15 m wide tributary channel has been connected at an angle of 90°. Three discharge scenarios were tested. Numerical model was performed as the same geometry, flow and sediment data which have been used in experimental model. The simulated water surface and bed profiles in the confluence zone compared well with the experimental data. The statistical analysis showed that in confidence level of 99%, numerical model with averaged coefficient of determination (R2) about 0.85 and Mean Percent Absolute Error (MPAE) about 2.675 can predict water surface profile, with R2 about 0.90 and MPAE about 15.2 can predict longitudinal profile of sediment deposit, with averaged R2 about 0.82 and MPAE about 18.2 can predict longitudinal profile of bed erosion, and with averaged R2 about 0.99 and MPAE about 12.4 can predict maximum depth of sediment deposit or maximum depth of bed erosion at confluence zone in comparison with experimental data.