Abstract
The effects of roughness on the flow structure in a gravel-bed channel were investigated by using particle image velocimetry. The bed was composed of gravel particles, of which the median diameter, d50, (i.e., the diameter at which 50% are smaller) ranged from 2.5 to 35.0 mm. The flow covered a wide range of Reynolds numbers (R=3,048–69,982). The experimental results showed that the logarithmic-law region of the velocity profile changed with the size of the roughness elements. Under the large-scale roughness condition, that is, h/d50=1.78 (the ratio of flow depth to median diameter), the mean velocity distribution deviated from the logarithmic law in the near-wall region. In addition, a new formula was established to calculate the Darcy-Weisbach friction factor for the gravel bed, expressed for the Reynolds number and the surface geometric parameters. On the basis of the boundary layer theory, the displacement thickness was introduced to estimate the limit of the roughness effect, and it was found to be g...
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