Impacts of turbulent flow over a channel bed with a vegetation patch on the incipient motion of sediment

In this experimental study, the effect of a submerged vegetation patch with individual plants on the incipient motion of sediment has been examined. Results showed that a vegetation patch in the channel bed affected the incipient motion process of sediment and resulted in significant impacts on flow velocity, turbulence intensities, turbulent kinetic energy, and Reynolds stress distribution. The presence of vegetation patches completely change vertical distribution of velocity and Reynolds stress pattern and leads to negative Reynolds stress values in zones with negative velocity gradient. In the presence of a vegetation patch, threshold cross-averaged streamwise velocity is 20% smaller than that without vegetation, but because of the occurrence of the preferential path around the sheath section, the near-bed streamwise velocity is the same as without vegetation. Also, vegetation increases turbulence intensity, thus encouraging sediment motion. In the presence of a vegetation patch, the Shields parameter is, on average, one and half times that of without vegetation over the bed.