Flow turbulence over fixed and weakly mobile gravel beds

Characteristics of turbulence structure in quasi-2D flows with static and weakly mobile gravel beds are presented. Three sets of measurements with acoustic Doppler velocimeters in an irrigation canal were used: two with subcritical bed shear stress (static beds) and one with the bed shear stress tau(o) close to critical tau(oc) (weakly mobile bed). The analyses included vertical distributions of local mean velocities, turbulence intensities, turbulent shear stresses, velocity auto- and cross-spectra, the quadrant method, and high-order velocity moments. A number of properties of turbulence intensities, high-order moments, streamwise bursting parameters, and velocity spectra appeared to be similar fur all three flows, but some properties were different. The most important one was an observed reduction in the von Karman constant for the flow with weakly mobile bed. Comparison of these results with other studies and analogies with drag-reducing flows suggest that at tau(o)/tau(oc) approximate to 1 the drag on the bed for a given granular material should be minimized.