Abstract

Determining the friction coefficients for uniform flows over very rough bottoms is a long-standing problem in open-channel hydraulics and river engineering. This experimental study presents measurements of the surface deformation as well as Darcy-Weisbach and Manning friction coefficients for steady, turbulent (6058 <= R e <= 28,502), and subcritical flows (0.14 <= F r <= 0.52) over large roughness elements, where F r and R e denote the Froude and Reynolds numbers, respectively. The experiments were conducted in a rectangular, inclined flume with a train of half-cylinders mounted on the bed, with radii in the range 20 mm <= a <= 50 mm. These obstacles yield a relative submergence 1.45 <= h (N) / a <= 4.41 and a constant spacing ratio e / a = 12.8 across all experimental runs, where h N and e denote the normal flow depth and the center-to-center spacing between cylinders, respectively. The relative amplitude of the surface profiles, ( Delta h / a ), was analyzed and found to correlate strongly with h N / a , R e and F r . The results reveal very high values of the Darcy friction factor, f, which follows scaling laws of the form f proportional to ( h( N )/ a ) n <^> , with n <^> < 0 , independent of a, and f proportional to R e( beta) , where beta < 0 is closely linked to a. Scaling relationships for the Manning roughness coefficient, (n), were also investigated and are reported herein.