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The behaviour of velocity profiles and shear velocity for non-uniform flow in gravel-bed rivers is studied, with the objectives: (a) to test a new method of shear velocity estimation in gravel-bed rivers that is based on boundary layer parameters, and to compare it with the log law and parabolic law; (b) to consider the influence of flow non-uniformity on the outer layer region of velocity profiles; and (c) to investigate the effect of aspect ratio on velocity profiles. For the primary study river, mid-channel velocity profiles were analysed with relative submergence ranging from 9.7 to 33.3 in channel sections with aspect ratios ranging between 16.2 and 50. Velocity profiles deviated from the log law in the outer region due to flow nonuniformity or pressure gradient effects, and the vertical extent of the inner region was variable. Estimates of shear velocity using the boundary layer parameters (δ * and θ) compared well with estimates from the log law. In a second study river, boundary-layer parameter estimates of shear velocity compared well to shear velocity estimates from linear extrapolation of Reynolds stress profiles.

The behaviour of the shear velocity along a gravel-bed channel is investigated experimentally in the presence of a negative pressure gradient (accelerating flow). Different methods of estimation of the shear velocity, derived from vertical profiles of the mean longitudinal point velocity, are examined and a new method is proposed. Results show that the proposed method of estimation is comparable to the St Venant and Clauser's methods. At a specific cross section, for constant bottom slope and relative roughness, shear velocity increases with discharge. Augmentation de la vitesse de cisaillement dans un canal à lit de gravier Résumé Le comportement de la vitesse de cisaillement a été étudié expérimentalement en présence d'un gradient de pression négatif (écoulement accéléré) le long d'un canal à lit de gravier. Différentes méthodes d'estimation de la vitesse de cisaillement, reposant sur différents profils de vitesse longitudinale, ont été étudiées et une nouvelle méthode est proposée. Les résultats indiquent que la méthode proposée mène à des résultats similaires à ceux déduits des méthodes de Clauser et de St Venant. Pour une section transversale donnée, pour une pente du lit constante et une rugosité relative constante, la vitesse de cisaillement augmente en fonction du débit.

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