ABSTRACT
Shallow open-channel flows over very rough walls were investigated with Particle Image Velocimetry (PIV). The flow was fully resolved both within the roughness canopy and above so that the change of scale to 1D flow parameters could be evaluated. In particular, estimates of the double-averaged mean and turbulence profiles were obtained by minimizing and quantifying the errors introduced by usual limited spatial and temporal sampling. It is shown that poor spatial sampling can lead to erroneous vertical profiles in the roughness sublayer, critical in the determination of the friction velocity and the roughness layer height. To estimate the friction velocity, it is shown that the dispersive stresses are necessary. The roughness layer heights reveal values well below the usual more ad hoc estimates. The logarithmic law is shown to persist for submergence ratios at least as high as 0.3, while the roughness sublayer largely extends into it. A dependence of the roughness length on relative submergence is observed, but not for the displacement height.
