Friction velocity associated to a non-uniform flow and an intermediate scale roughness

This work consists of an experimental study of the resistance associated wi th non-uniform flows over cobble-bed channels, leading to an intermediate s cale roughness. Friction velocity is estimated for twelve cases of accelera ting and decelerating flows. Four estimation methods are compared: Reynolds stress, logarithmic law, parabolic law and a global approach. They are all in close agreement. The observed mean longitudinal velocity profiles are c onsistent with observations reported in the literature for non-uniform flow s over gravel-bed channels with small relative roughness. Accelerating flow s produce smaller friction velocities than decelerating flows, which is con trary to observations reported in the literature.