Spatially averaged dissipation rate in flows over water-worked and screeded gravel beds

The characteristics of the spatially averaged (SA) turbulent kinetic energy (TKE) dissipation rate are studied experimentally in flows over water-worked gravel beds (WGBs) and a screeded gravel bed (SGB) measuring the instantaneous velocity field by the Particle Image Velocimetry system. To study the response of water work roughness to the TKE related parameters, the flow Froude numbers in both the WGB and SGB were maintained identical in the experiments. Owing to the action of water work, the surface gravels in the WGB are more spatially organized than those of the SGB, where they are randomly oriented, resulting in a higher roughness in the WGB than in the SGB. Examination of the second- and the third-order velocity structure functions reveals the evidence of an inertial subrange in both the WGB and SGB. Kolmogorov’s two-thirds and four-fifths laws are preserved within the inertial subrange, providing an accurate estimation of the TKE dissipation rate in both the beds. However, owing to the higher roughness in the WGB than in the SGB, the TKE and the scaling law parameters are greater in the former than in the latter case. Furthermore, to examine the effects of the shear Reynolds number on the TKE dissipation rate and the scaling law parameters, two additional experiments with different shear Reynolds numbers were conducted with the WGBs. The comparative results suggest that the SA TKE dissipation rates estimated from Kolmogorov’s two-thirds and fourfifths laws increase with an increase in the shear Reynolds number in the WGBs.