Bedforms and vegetation covers influence the flow structure in open channels and rivers. However, a few studies have been reported on the interaction of vegetation over bedforms. The objectives of this study are to investigate the effect of vegetation cover over bedform and length of separation zone, determining turbulence, velocity and Reynolds stress components, Kolmograph energy spectrum, bursting phenomenon and Kelvin-Helmholtz instability in an open channel with dunes and vegetation cover over them. This experiment was conducted in a flume of 7 m long, 0.32 m wide and 0.36 m deep and flow discharge of 18 L/s in Hydraulic Laboratory of Isfahan University of Technology. Gravel with D 50 =10mm and grass as vegetation cover with height of 0.03 m were used in this study. The measurments carried out by ADV in 4 runs in laboratory: gravel bedform, vegetated bedform, gravel flat bed, vegetated flat bed and one run was conducted in Kaj river to compare lab and field results. The results showed that length of separation zone in vegetated bedform was more than gravel one and flow separation was obviously obsreved on vegetated bedforms in laboratory and field studies. Inflection points in velocity profiles located on top of the vegetation crest. Flow separation caused a shear layer with K-H instability where turbulence intensity increased along shear layer. Both gravel and vegetated bedforms increased friction factor. Maximum RMS velocity was changing along bedforms and maximum RMS(u’), RMS(v’), RMS(w’) occurred at crest or trough zones, where high turbulence intensities were prevalent. Value of the maximum Reynols stress occured at stoss zone in vegetated bedforms, while it occurred at trough for gravel bedforms. Negative Reynolds stress values were observed on vegetated bedform while no negative stress was observed over gravel bedform. The frequent phenomenon inside of vegetation cover was ejection while outside of vegetation the effect of sweep was dominant. Keywords: Bedform, vegetation cover, flow structure, Kelvin-Helmholtz instability, flow separation, quadrant analysis.
