Quantifying contributions of very large-scale motions to the time variability of sediment entrainment

The influence of very-large-scale motions (VLSMs) on the entrainment of sediment particles on riverbeds remains to be better understood, particularly regarding their contribution to the overall entrainment variability. To further explore this problem, a synchronized flow-sediment measurement system was developed to simultaneously capture flow velocity fluctuations and the entrainment of sediment particles at the bed surface. Experiments were conducted under various flow conditions using three sets of uniform sands with distinctly different grain sizes. Frequency-domain coherence analysis and pre-multiplied spectral analysis were employed to investigate the interactions. The results reveal significant coherence between flow velocity fluctuations and the entrainment of sediment particles in the normalized frequency range of U/(fh)>10. Furthermore, spectral energy integration shows that VLSMs contribute approximately 33% of the total variability of the entrainment of sediment particles. These findings underscore the critical role of VLSMs in modulating sediment entrainment. This study offers quantitative insights into the VLSM contribution to the entrainment of sediment particles and provides experimental evidence supporting a deeper understanding of flow-sediment interactions driven by VLSMs.