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 (Formula presented.) 28,502), and subcritical flows (0.14 (Formula presented.) 0.52) over large roughness elements, where (Formula presented.) and (Formula presented.) 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 (Formula presented.) 50 mm. These obstacles yield a relative submergence 1.45 (Formula presented.) 4.41 and a constant spacing ratio (Formula presented.) across all experimental runs, where (Formula presented.) and e denote the normal flow depth and the center-to-center spacing between cylinders, respectively. The relative amplitude of the surface profiles, ((Formula presented.)), was analyzed and found to correlate strongly with (Formula presented.), (Formula presented.) and (Formula presented.). The results reveal very high values of the Darcy friction factor, f, which follows scaling laws of the form (Formula presented.), with (Formula presented.), independent of a, and (Formula presented.), where (Formula presented.) is closely linked to a. Scaling relationships for the Manning roughness coefficient, (n), were also investigated and are reported herein. © 2025 by the authors.