Hydrodynamics of water-worked and screeded gravel beds: A comparative study

The turbulence characteristics within flows over water-worked gravel beds (WGBs) and screeded gravel beds (SGBs) were examined by measuring the instantaneous flow velocity field using a twodimensional particle image velocimetry system. To compare the responses of a WGB and an SGB to velocity and various turbulence characteristics, the flow Froude number was kept identical for both the beds that remained immobile. The roughness structures of both the beds were measured using a laser scanner. The results showed that the bed surface roughness was higher in the WGB than in the SGB. However, the longest axis of the gravels ofWGBswas oriented streamwise owing to the action ofwater work, but the gravels of SGBs were randomly poised. The distribution of bed roughness fluctuations was negatively skewed in theWGBand positively skewed in the SGB. Double averaging methodology was applied to analyze the flow parameters. In this paper, the vertical profiles of the double-averaged streamwise velocity and the turbulence parameters, specifically the spatially averaged (SA) Reynolds shear and normal stresses, form-induced shear and normal stresses, turbulent kinetic energy (TKE) and form-induced TKE fluxes, quadrant analysis of SA Reynolds shear stress, etc., are presented and analyzed critically by focusing on comparisons between a WGB and an SGB. A comparative study reveals that in the near-bed flow zone, the SGB underestimates the turbulence parameters compared to the WGB. Therefore, in order to represent the prototypical flow in laboratory, the experiments should be performed in a WGB.