Mechanism of sediment particle incipient motion governed by turbulent structures in open channel flows

The critical condition for sediment incipient motion is a foundational issue in the field of fluvial hydraulics. Recent studies on turbulence have uncovered the existence of turbulent structures across various scales in open-channel flows, including very-large-scale motions (VLSMs), large-scale motions, and small-scale motions. Nevertheless, research investigating sediment incipient motion through the perspective of turbulent structures is still in its early stages, and substantial controversy remains regarding the dominant turbulence scales and their underlying mechanisms. In the present work, 100 sets of sediment incipient motion experiments are conducted using a particle image velocimetry (PIV) system. A continuous wavelet transform is employed to examine the transient characteristics of turbulent structures across various scales during the sediment incipient motion. The findings indicate that, under the present experimental conditions, VLSMs dominate up to 76.0% of sediment incipient motion events. Additionally, theoretical analysis suggests that the fluctuating drag force is primarily governed by the streamwise fluctuating velocity and the increase in turbulent kinetic energy (TKE). As sediment particles approach the critical condition for incipient motion, the intensity of VLSMs substantially escalates. This accounts for 74.0% of the streamwise fluctuating velocity and 59.3% of the increase in streamwise TKE, establishing VLSMs as the predominant turbulent structure responsible for sediment incipient motion.