Incipient motion of sediment in riverbed plays an important role in many river engineering projects such as design of stable channels and determination of riverbed resistance. The interaction between these fine sediment covers and vegetation patches under turbulent flow is very complex process. In this experimental study, the effect of submerged vegetation patch with individual plants on incipient motion of sediment has been examined. Experiments have been carried out in a rectangular flume that is 15 m long, and 0.9 m wide. Criterion for incipient motion of sediment in presence of vegetation patch was determined as continues scouring process around plant sheath, leading to considerable traort of sediment from the end of vegetation patch. Results showed that vegetation patch in 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 patch completely change vertical distribution of velocity and Reynolds stress pattern and leads to negative Reynolds stress values in zones with negative velocity gradient. Through the vegetation patch, streamwise velocity profiles are S-shape, showing two turning points at 0.15 and 0.5-0.6 water depths. In presence of vegetation, threshold cross-averaged streamwise velocity is 20-29 percent smaller than that for without vegetation, but because of occurrence of preferential path around the sheath section, the near the bed streamwise velocity is increased. Also, vegetation increase turbulence intensity, encouraging sediment motion. Key Word Incipient motion, Reynolds stress, Shields parameter, turbulence, vegetation patch.
