Depth-averaged Velocity Distribution in Compound Channel with Vegetated Floodplains

Vegetated floodplains frequently occur in natural rivers. The presence of vegetation plays an essential role in bank stabilization and ecological restoration. The hydraulic resistance due to vegetation on the floodplain typically causes a further reduction of flow velocity and increases the velocity difference between the main channel and the floodplain. As a consequence, a strong lateral shear layer leads to the exchange of mass and momentum between the main channel and floodplain which in turn affects the overall channel conveyance and certain fluvial processes. The goal of this study was to simulate flow velocity in compound channels with vegetated floodplains. The present paper builds on previous work by Tang and Knight and applies it to simulate flow on a vegetated floodplain. This model includes the effects of bed friction, drag force, lateral turbulence and secondary flows, via four coefficients ƒ, C􀭈, λ and Γ, respectively. To validate the 2D model, flume experiments have been conducted in straight compound channel with vegetated floodplains. Six series of tests were undertaken at 2 different bed slopes (S􀬴) and three depth ratios (Dr). The results of a comparison between calculated and measured values of the velocities showed that Colebrook-White equation cannot correctly predict values of friction factor (ƒ) and thus use of Nudging’s equation would be recommended. Five eddy viscosity models were selected from the literature and the best model was introduced, through the best-fit to the observed data.