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

Abstract

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 two-dimensional 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 of WGBs was oriented streamwise owing to the action of water work, but the gravels of SGBs were randomly poised. The distribution of bed roughness fluctuations was negatively skewed in the WGB and 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.

Water-Worked Gravel Bed: State-of-the-Art Review

“…In their study, owing to the water action, the WGB roughness structure was observed to be better organized than the SGB roughness structure, where gravels were randomly sorted. Akin to other rough-flow, in the study by Padhi et al [32], owing to the presence of gravels, the values of ū in both the WGB and SGB are small in the near-bed flow zone. However, they gradually increase with an increase in vertical distance, reaching a maximum at the free surface.…”

“…However, they gradually increase with an increase in vertical distance, reaching a maximum at the free surface. Moreover, Padhi et al [32] found that, close to the bed, the ū in the WGB is higher than that in the SGB, although the flow conditions in both the beds were alike. They argued that the well-organized roughness structure in a WGB makes the near-bed flow more streamlined than that in an SGB, inducing the ū to attain a higher magnitude in the former than in the latter.…”

“…In addition, the comparison of Π values in the WGB and SGB revealed that the values of Π remain the same in the velocity profiles of both the beds, suggesting that the water-work has an insignificant impact on the Π, which mainly governs the velocity profile in the outer layer. To ascertain the impact of the water-work on streamwise velocity more precisely, using a unimodal sediment mixture, Padhi et al [32] measured streamwise velocity in a WGB using a Particle Image Velocimetry (PIV) system and compared it with that in an SGB (Figure 8). In their study, owing to the water action, the WGB roughness structure was observed to be better organized than the SGB roughness structure, where gravels were randomly sorted.…”

“…To ascertain the impact of the water-work on streamwise velocity more precisely, using a unimodal sediment mixture, Padhi et al [32] measured streamwise velocity in a WGB using a Particle Image Velocimetry (PIV) system and compared it with that in an SGB (Figure 8). In their study, owing to the water action, the WGB roughness structure was observed to be better organized than the SGB roughness structure, where gravels were randomly sorted.…”

Water-Worked Bedload: Hydrodynamics and Mass Transport

“…The bed surface topography and the flow characteristics in water-worked and screeded gravel beds were explored by several researchers [1,[5][6][7][8][9][10][11][12]. However, a series of recent studies by Padhi et al [13][14][15] indicated a clear distinction in turbulence characteristics in water-worked and screeded gravel bed flows. Therefore, the research on water-worked beds, in addition to the related hydrodynamics and transport processes, being the topic of this Special Issue, demands further attention.…”