Investigation of mechanisms governing turbulence-induced initiation of motion in non-cohesive sediment layers

In this research, experiments were conducted to quantify how turbulence affects the initiation of three groups of quartz sands with consecutively increasing particle diameter. In the experimental setup, controlled acrylic paddles, a camera equipped with a macro lens and an acoustic Doppler velocimeter (ADV) were applied to generate turbulence, to record the precise moments of sediment initiation and to measure flow velocity and turbulence properties, respectively. Two categories of sediment initiation were characterized: sliding and suspension, with the onset of suspension occurring at higher turbulence intensities when compared to sliding. In addition, turbulent kinetic energy and Reynolds shear stress were identified as the most critical parameters for sediment initiation. The results suggested that the first and third groups required higher turbulent levels compared with the second group, due to variations in particle size and weight. Quadrant analysis showed that sliding events predominantly occurred during ejection and up-acceleration phases, while suspension events were mostly associated with ejections, which confirmed that initiation tends to occur in specific turbulent structures. Last, a single dimensionless parameter (<i>J</i>) that expresses the probability of sediment initiation under turbulent flow conditions, based on the ratio of turbulent kinetic energy to critical incipient shear stress, was proposed.