Abstract: Affected by geology and fluvial process, rivers in mountain area are featured with alternated wide and narrow channels. In this research, river model with wide and narrow alternated channels with varying width ratio was established through flume model test. The flow velocity in the upstream section was measured, and the rule of velocity variation was analyzed in comparison with uniform flow, hence obtaining the frictional velocity. Results reveal that velocity distribution in the upstream far from the wide and narrow alternated channel is in line with logarithmic distribution in general, but does not follow logarithmic distribution near the contraction segment; whereas in the wide and narrow alternated channel, the transverse flow velocity distribution conforms with exponential distribution. Moreover, the coefficient of transverse velocity distribution and the frictional velocity change regularly, and are related with the ratio of width and the distance from contraction segment.

Key words: wide and narrow alternated channels, ratio of width, flow characteristics, frictional velocity, logarithmic distribution, transverse velocity distribution