In this study the effect of flat crest dune and vegetation patch on the transverse mixing coefficient is presented and compared with bare-bed case. Also, profiles of velocity, turbulence intensities and Reynolds stress distributions in longitudinal, transversal and vertical direction along with transverse mixing coefficient and full mixing length are investigated for both cases. Experiments were conducted in a channel in Isfahan University of Technology with a length of 8 m, a width of 40 cm and a height of 60 cm where the maximum flow rate was 40 lit/s. The median particle diameter was11.2 mm and vegetation patch was simulated by an artificial grass with a height of about 4 cm, using different dimensions. ADV was used to collect velocity data and to measure EC meters was applied. Experiments were conducted in two cases: investigating the effect of vegetation and flat-crest dune on the transverse mixing coefficient and with no vegetation patch on the bed. The results showed that the presence of vegetation patch on the bed increases flow velocity compared to no vegetation case. The maximum velocity occurs near the water surface, however, the maximum turbulence intensity occurs at the top of the vegetation. The presence of vegetation patch on the bedform increase the force of secondary flow, leading the reduction of the transverse mixing coefficient. The coefficient of transverse mixing decreased in the presence of vegetation in three injections over the bedform, while the full mixing length and increased. Keywords : Vegetation patch, Transverse mixing length, flat-bed crest dune
