Macroturbulent structure of open-channel flow over gravel beds

The turbulent structure of open-channel flow over a mobile gravel bed was i nvestigated in an 8 m long, 0.3 m wide, and 0.3 m deep tilting flume. A flo w visualization technique was used and complemented by measurements of flow velocity fluctuations near the bed. The experiments reveal that turbulent flow consists of a sequence of large-scale eddies with a vertical size clos e to the flow depth, an average length equal to four to five depths, and a width of about two depths. The downstream motion of these eddies causes qua siperiodic fluctuations of the local flow velocity components. The developm ent of longitudinal troughs and ridges on the mobile bed and preferential t ransport of bed particles along troughs are related to the effect of the ed dies. The experimental results indicate that the depth-scale eddies are an important turbulence mechanism contributing to sediment transport.