Diffusive Properties of Bed Load Moving Sediments at Short Time Scales

In this paper bed load sediment motion within the intermediate range of scales, corresponding to particle motion between two rest periods, is investigated. Moreover, sediment diffusion at the earliest stages of motion of entrained particles is analyzed. On this aim some experimental trials were carried out to observe motion of sliding, rolling and jumping sediments over a mobile bed for weak bed load transport conditions. A particle tracking velocimetry technique was used to track sediments; nearly 700 intermediate trajectories were collected for the analyzed experimental condition and each one was validated superimposing the extracted trajectories on played images of the experiments. A detailed description of particle velocity as a function of time is presented. It clearly shows that entrained particles experience an acceleration phase which duration is relevant with respect to the mean duration of intermediate trajectories. The sediment diffusive process is strongly affected by this unsteady phase of motion resulting in a diffusive process faster than ballistic along the streamwise direction.