Local and Global Friction Factor in a Channel with V-Shaped Bottom


Civil Engineering, Urmia University


This paper presents an experimental research on the distribution of local friction factor, fb, and global friction factor, f, over the cross-section of a channel with V-shaped bottom, which typically occurs in sewers and culverts. Several series of experiments were conducted for measuring velocity and boundary shear stress. It is found that, Darcy-Weisbach, f, is more sensitive than other resistance coefficients such as Manning, n. Local boundary shear stress around wet perimeter, τb, and depth-averaged velocity, Ud, data are used to evaluate fb. The results illustrate that, the friction factor is shown to be a function of flow depth as well as being dependent upon the Froude number, Fr. Therefore, the local friction factor does not remain constant across the channel. Meanwhile, the common technique of assuming a constant friction factor over the entire section of the channel has been found to have theoretical justification. The results indicate that the global friction factor decreases as flow discharge increases and channel bed getting steeper in the experiments including subcritical and supercritical flow conditions. However, it can be seen that the effects of walls are very high at low Froude numbers. In general, the results indicate that the perturbations in the distribution of local friction factor are quite considerable in steeper channels as Froude number increases.