Impact of acceleration and deceleration rates on the entrainment and settling thresholds of a sandy bed

Understanding the impact of flow transients is critical for predicting sediment dynamics in environments characterized by high unsteady currents, such as macro-tidal systems and estuaries. In such contexts, the acceleration and deceleration rates investigated in this study (1.3 to 10.4 mm/s²) provide new insights into how flow unsteadiness affects the sediment transport cycle. The estimation of the threshold motion is very important for the evaluation of sediment transport. The image correlation method developed by Vah et al. (2022) for the detection of threshold motion in laboratory flume is considered. Two thresholds are detected with this method : the bedload (U_bl) and bedform (U_bf) thresholds. They were measured for the sand S₃₂₈ (D₅₀ = 328 µm) with different acceleration and deceleration ramps.

The stability observed in the threshold of motion for the studied sand can be attributed to an initial triggering of grain movement driven by the instantaneous local drag force. Furthermore, while the bedload threshold remains constant (0.31 m/s) for acceleration rates ranging from 1.3 to 10.4 mm/s², the settling threshold exhibits a strong dependence on flow deceleration. Specifically, at higher deceleration rates, grains maintain motion until reaching lower flow velocities (0.21 m/s), suggesting a significant hysteretic effect driven by particle inertia. Finally, the earlier onset of ripples under high acceleration—occurring at 0.32 m/s compared to 0.36 m/s at lower rates—suggests that fluid acceleration promotes morphological instability, thereby shortening the transition from a flat bed to bedform development.

Reference

Vah, M., Khoury, A., Jarno, A., & Marin, F. (2022). A visual method for threshold detection of sediment motion in a flume experiment without human interference. Earth Surface Processes and Landforms, 47(7), 1778-1789.