Most of the rivers in mountainous regions have cobble bed. The dynamic nature of rivers in coarse beds including changing in slop, cross section, and depth of rivers in different points make the study of them very complicated. The Turbulence structure and presence of non-uniform and unsteady flows make this object more complicated. In channel with coarse bed, sediment occupies noticeable proportion of flow depth that leads to a large resistance against the flow and affects the vertical velocity profiles. In this study, Turbulent flow characteristics in presence cobbles (isolated and cluster in path of flow) and its interaction with gravel bed including velocity profiles and location of maximum velocity, inner and outer boundary layer region, Reynolds stress distribution, turbulence intensity, and turbulent kinetic energy were studied. This experiment was conducted at Laboratory of Agriculture Faculty of Isfahan University of Technology in a 16 m long, 0.9 m wide, and 60 cm height flume with maximum discharge of 40 l/s and also in Plasjan river. Laboratory experiment was carried out in 2 series: - presentence of cobble on gravel bed with slop of -1/5% in different distances from wall and entrance flume - Cobble cluster among of gravel bed with -1.5% slop. the median sizes of grains in the flume in two series and river were 88 mm, 31 mm, and 25 mm respectively. Velocity profiles were taken in the reach that flow was fully developed. The measurement in flume and river were carried out by Adv. Results show that velocity profiles in cobble-bed in contrast to river condition have not been affected by aspect ratdo such as Dip. Velocity profiles in laboratory condition were divided into two sub-zones, that depend on distances from wall, position to cobbles, slop of near bed zone, and depths is located at the boundary between two area have different trends. Stream-wise turbulence central axis on cluster and leading edge have concave form, but before and after from cluster have convex form. While, vertical turbulence central axis on cluster, before and after from cluster have convex form. In addition, cobble in bed noticeably affects turbulence structure by increasing vorticity and turbulence near cobbles and makes changing in procedure and value turbulent flow characteristic. Vorticity, turbulence intensity component, kinetic energy values increased. The results of quadrant analysis show that ejection, the dominant event, was influenced by cobble height in cluster and isolated cobbles and it is reduced near the bed. After removing the effect of these height dominance again finds and participation shares for quadrant 1 and 3 are smaller than contribution sweep and ejection. Keywords: Gravel bed, cobble, turbulence flow, velocity distribution, quadrant analysis
