Velocity distribution characteristics in meandering compound channels with one-sided vegetated floodplains

Abstract

To discuss the effects of asymmetrically vegetated floodplains on the velocity distribution characteristics and discharge capacity of a meandering main channel, in this paper, continuous river bend models with smooth floodplains and one-sided vegetated floodplains were designed in a generalized way. Three-dimensional instantaneous velocity data were obtained with an Acoustic Doppler Velocimeter (ADV), and the velocity distribution characteristics and discharge capacities of the generalized models were compared. Compared with the smooth floodplain conditions, the one-sided vegetated floodplain conditions are associated with lower cross-sectional average velocities, lower ratios of main channel discharge to total discharge near the bending apexes with minimal extension of vegetated floodplain, but higher ratios near the bending apex with maximal extension of vegetated floodplain. The deflection degrees of depth-averaged velocities in the meandering main channel are related to the cross-sectional position, relative depth and floodplain vegetation. The depth-averaged velocities near the bending apexes are inclined towards the main channel concave sides, those in the upstream crossover area are generally inclined towards the vegetated side, and those in the downstream crossover area are obviously inclined towards the smooth side. For a given relative depth, the deflection degrees near the bending apexes are smaller than those in the crossover areas. The deflection degrees near the bending apexes with minimal extension of vegetated floodplain are smaller than those under smooth floodplain conditions, but the deflection degrees near the bending apex with maximal extension of vegetated floodplain are obviously larger than the corresponding values under smooth floodplain conditions. Near the bending apexes, the longitudinal depth-averaged velocities are higher close to the main channel convex side and lower adjacent to the concave side. When the relative depth is small, one-sided vegetated floodplain conditions feature larger transverse gradients of the cross-sectional longitudinal depth-averaged velocities near the bending apexes with minimal extension of vegetated floodplain. When the relative depth is large, the transverse gradients near the bending apexes with minimal extension of vegetated floodplain are all much smaller than the corresponding values of the smooth floodplain conditions. However, near the bending apex with maximal extension of vegetated floodplain, there are no significant differences concerning the transverse gradients of the cross-sectional longitudinal depth-averaged velocities under the both floodplain conditions. One-sided vegetated floodplain conditions significantly influence the longitudinal velocity distributions in the meandering main channel. For a given relative depth, the longitudinal velocities at the cross-sectional positions under one-sided vegetated floodplain conditions are lower than those under smooth floodplain conditions to varying extents.