ROLE OF NEAR-BED TURBULENCE STRUCTURE IN BED-LOAD TRANSPORT AND BED FORM MECHANICS

The interactions between turbulence events and sediment motions during bed load transport were studied by means of laser-Doppler velocimetry and high-speed cinematography. Sweeps (u' > 0, w' < 0), which contrib ute positively to the mean bed shear stress, collectively move the maj ority of the sediment; primarily because they are extremely common. Ou tward interactions (u' > 0, w' > 0), which contribute negatively to th e bed shear stress and are relatively rare, individually move as much sediment as sweeps of comparable magnitude and duration, however, and much more than bursts (u' < 0, w' > 0) and inward interactions (u' < 0 , w' < 0). When the magnitude of the outward interactions increases re lative to the other events, therefore, the sediment flux increases eve n though the bed shear stress decreases. Thus, although bed shear stre ss can be used to estimate bed load transport by flows with well-devel oped boundary layers, in which the flow is steady and uniform and the turbulence statistics all scale with the shear velocity, it is not acc urate for flows with developing boundary layers, such as those over su fficiently nonuniform topography or roughness, in which significant sp atial variations in the magnitudes and durations of the sweeps, bursts , outward interactions, and inward interactions occur. These variation s produce significant peaks in bed load transport downstream of separa tion points, thus supporting the hypothesis that flow separation plays a significant role in the development of bed forms.