Turbulent Velocity Profiles for Smooth and Rough Open Channel Flow

Velocity measurements, using a laser‐doppler anemometer, are carried out in a fully developed, rectangular, subcritical open channel flow on smooth and rough beds. The average roughness heights of the “rough” surfaces, used in the experiments, are 1 mm, 4 mm, 8 mm, and 12 mm. A model is presented for determining the reference level of the velocity distribution on “rough” surfaces. The shear velocities are determined from velocity profiles measured close to the bed. The calculated shear velocities show an increasing tendency as the wall roughness increases. In the fully turbulent inner region on a “smooth” wall, the coefficients in the logarithmic law‐of‐the‐wall distribution take values of 2.44 and 5.5; the former corresponds to 0.41 for the von Kármán constant. The velocity‐defect expression of Coles has a profile parameter of 0.1, rather than 0.55. For “rough” channels, the representation of the overall data in terms of law‐of‐the‐wall distribution seems reasonable; however, the velocity‐defect distribution is not satisfactory.