THE INFLUENCE OF ROUGHNESS GEOMETRY AND SHIELDS PARAMETER ON FLOW RESISTANCE IN GRAVEL-BED CHANNELS

The spatial variability of bed particles of a gravel-bed channel is an alysed and treated experimentally in order to simulate the effects of the arrangement of coarse bed elements on the flow resistance law. For the studied bed patterns, characterized by the concentration Gamma of coarser elements arranged on the bed layer, a particle arrangement pa rameter alpha is proposed. The a parameter is useful for estimating th e intercept b(0) of the semi-logarithmic flow resistance law deduced b y flume measurements carried out for the hydraulic condition of large- scale and transition roughness. The differences between the experiment al friction factor parameter values and the ones calculated by the pro posed semi-logarithmic relationship are explained by the ratio between the Shields parameter and its critical value. The analysis shows that estimates of the friction factor parameter are not improved by introd ucing the Froude number into the flow resistance law. (C) 1997 by John Wiley & Sons, Ltd.