Response of open-channel flow to a sudden change from smooth to rough bed

The turbulence characteristics in open-channel flow owing to a sudden change from smooth (upstream) to rough (downstream) bed are investigated experimentally using a Particle Image Velocimetry system. The upstream flow had a shear Reynolds number of 4.86, characterizing the hydraulically smooth flow, while the downstream flow had a shear Reynolds number greater than 70, characterizing the hydraulically rough flow. The Reynolds stress results reveal that for a given vertical distance, both the Reynolds shear and normal stresses in the downstream bed increase with the streamwise distance as compared to their upstream values. Their peaks appear at a distance of one-fifth of the flow depth from the bed. In addition, the bed shear stress enhances with the streamwise distance. The stress contours corroborate that the formation of a roughness-induced layer over the downstream bed thickens with the streamwise distance. The third-order correlations reveal that an arrival of slowly moving fluid streaks associated with an outward Reynolds stress diffusion prevails in the flow on the upstream bed, while an inrush of rapidly moving fluid streaks associated with an inward Reynolds stress diffusion governs the near-bed flow zone in the downstream bed. These results are in conformity with those obtained from the turbulent kinetic energy (TKE) fluxes and the bursting events. With regard to the TKE budget, the peaks of TKE production and dissipation rates appear near the downstream bed and are greater than those in the upstream bed. However, in the downstream bed, an enhanced negative TKE diffusion prevails near the bed.