The flow structure around the bed, especially in gravel layer, is very important and needs to be clarified because the detailed topography of the bed exerts a significant influence on generation and dissipation of turbulent energy. A lot of channel hydraulics has considered smooth bed conditions but there are still many unsolved problems awaiting clarification for gravel bed rivers. This study investigates how the turbulence characteristics i.e. vertical distribution of longitudinal velocity,

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... ynolds stress distribution and effect of boundary layer development on velocity distribution can differ in a shallow gravel bed river with high effective roughness height when bed conditions change. The flume experiments using particle image velocimetry (PIV) were performed with varying surface roughness elements spacing and results were compared with the frictional resistance, mixing length and constant mixing length theories, which allow studying vertical components of velocity in the flow.