Incipient Motion of Bed Material in a Channel with Varying Width and Vegetated Channel Walls

Abstract

This experimental study aims to investigate the characteristics of turbulent flow in channels with vegetated banks and varying channel width under the condition of the incipient motion of bed material. The natural reeds were used as emergent vegetation on the sidewalls of a laboratory flume. In total, nine experimental runs have been conducted with different experimental setups by using three different particle sizes of bed material and three different channel bed slopes. An Acoustic Doppler velocimetry (ADV) was used to acquire velocity components in three directions. The results of this study indicate that the streamwise velocities have the maximum and minimum values at the cross sections with the narrowest and widest width, respectively. When the aspect ratio is less than 5, the maximum velocity occurs below the water surface, due to presence of the secondary currents. It is found that, at all measurement points, the distribution of the Reynolds shear stress has a Z-shaped profile owing to presence of vegetation on the channel sidewalls. By extrapolating the profiles for flow velocity and Reynolds shear stress towards the surface of the channel bed, the near-bed incipient velocities and the corresponding shear stresses for the incipient motion have been determined. By increasing the channel bed slope, the estimated near-bed parameters for all particle sizes decreased, indicating the dominance of the gravity effect over the pressure gradient effect. It was also observed that the Shields method was invalid for assessing the incipient motion of bed material in the presence of vegetation on the sidewalls of a channel that has a varying width.