Precise velocity profile measurements were carried out in a fully developed, subcritical and supercritical flow on a smooth and a rough bed in a flume with a rectangular cross section. In both cases different approaches were used to calculate the friction velocity, u*, and comparisons were made. The influence of the reference level (hypothetical bed level) has been discussed. Friction velocities calculated from the velocity profile are very sensitive to the hypothetical bed level. For a smooth bed the determination of the reference level does not present any difficulties. However, for a rough bed this level shows dependency on the channel slope.

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