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The effects of bed-load movement on mean flow characteristics were experimentally investigated using particle image velocimetry (PIV). The gravel beds associated with open-channel flow are hydraulically rough, and two types of grain with different median diameters were used. The flow field and bed load motion images were simultaneously obtained via the particular experimental setup. The mass of sediment particles instantaneously moving on the bed, i.e., the sand-wave, was observed in the experiment. It was found that the sand-waves have an average friction angle of 11.2 • and heights ranging from 2.2 d 50 to 3.8 d 50 . The mean velocity profiles on the mobile bed can also be represented with logarithmic distribution with deviations occurred in the near wall region. Based on the Schultz-Grunow formula, a modified equation is developed in this study to better express the flow resistance factor in terms of the Reynolds number and the ratio of flow depth to roughness height.

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