Open Channel Flow with Varying Bed Roughness

The distribution of velocity and boundary shear stress in a rectangular flume has been examined experimentally, and the influence of varying the bed roughness and aspect ratio assessed. The resistance of the channel bed was varied by means of artificial strip roughness elements, and measurements made of the wall and bed shear stresses. Dimensionless plots of both shear stress and shear force parameters are presented for different bed roughnesses and aspect ratios, and these serve to illustrate the complex way in which such parameters vary. The definition of a wide channel is also examined, and a graph giving the limiting aspect ratio for different roughness conditions is presented. The boundary shear stress distributions and isovel patterns are used to examine one of the standard side-wall correction procedures. One of the basic assumptions underlying the procedure is found to be untenable due to the cross channel transfer of linear momentum.