Turbulent and Subcritical Flows over Macroroughness Elements

The determination of 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 reports the surface deformation, Darcy and Manning’s friction coefficients obtained from measurements on steady, turbulent (6,058≤Re≤28,502) and subcritical flows (0.14≤Fr≤0.52) over large roughness elements, where Fr,Re is the flow’s Froude and Reynolds number. The experiments were carried out in a rectangular inclined flume with a train of half-cylinders at the bottom, whose radius fall into the range 20 mm≤a≤50 mm. These obstacles are such that 1.45≤hN/a≤4.41 and e/a=12.8 far all the experimental runs, with hN the normal flow depth and e the spacing between the cylinders. The relative amplitude (Δh/a) of the surface profiles was analyzed showing a close correlation with hN/a, Re and Fr. Our results lead to very high values of the Darcy friction factor f following scaling laws of the type f∝(hN/a)n^ with n^<0 independent on a, and f∝Reβ, where β<0 closely related to a. Scaling laws for the Manning roughness coefficient (n) were also studied and reported.