Flow over rough mobile beds: Friction factor and vertical distribution of the longitudinal mean velocity

Abstract

The main objective of the present study is to identify the impacts of bed mobility on the vertical profile of the mean longitudinal velocity and on resistance in flows over water-worked beds of poorly sorted mixtures of sand and gravel. Water-worked beds with sediment transport are explicitly distinguished from immobile beds with imposed sediment feed. Flows with different equilibrium sediment transport rates are generated in a laboratory flume. The initial bed mixtures featured combinations of sand and gravel modes. Data collection included instantaneous velocities measured with Laser Doppler Annemometry. Wall similarity, in the sense of Townsend (1976), is assumed. The parameters of the formulae are discussed within three scenarios comprising different definitions of u* and ks combined with different conceptions of the Von Krmn constant ( flow independent or flow dependent). It is shown that the parameters of the formulae that express the velocity profile vary with the Shields number and with the initial bed composition. The variation is independent of the adopted scenario, except in what concerns the formulation of hydraulic smoothening in the presence of sand sizes, which depends on the definition of ks. © 2012. American Geophysical Union.