Turbulent Flow Characterization over the Gravel Bed

Abstract

The double-averaged momentum equations are used as a natural basis for the hydraulics of rough-bed open channel flows, especially with small relative submergence. To study the double-averaged turbulence characteristics in flows over a gravel-bed, a uniform flow over a gravel-bed was run in a laboratory flume and the velocities were measured at different verticals by a vectrino probe. The result reveals that the local time-averaged velocity profiles can be split into three typical classes, namely log, S-shaped and accelerated. In the log class velocity profile fits a logarithmic curve over almost the entire measured profile. S Shaped refers to profiles that can be encountered in wake of macro-roughness bed elements due to flow separation on the lee side. The accelerated profile located in the vicinity of top of the macro-roughness elements and the velocities are slightly higher than log classes but rapidly drop to zero below crest. It never extends to zero bed level above the crest they are not influenced by the bed heterogeneity. The logarithmic law of velocity profile is preserved above the roughness crest and velocity below the crest follows a polynomial. The field of gravel-bed appears to be near Gaussian matches with the work of previous researchers. The Reynolds shear stress is the main governing stress across the flow depth with a damping in the vicinity of gravel bed due to decrease in turbulence level. The form induced stress is prominent below the roughness crest.