Exploring the size ratio impact on an intruder segregating in bedload transport

Overview & Synopsis

Vertical size segregation in bedload transport impacts the sediment rate and the river bed morphology. To better account for this process in sediment transport models, it is essential to understand the mechanisms acting at the grain scale. This paper focuses on the behavior of a single large particle called the intruder, segregating upwards in a granular bed made of smaller mono-dispersed beads, during bedload transport. Experiments were carried out in a narrow flume, where the distance between the sidewalls was of the order of the intruder diameter. The latter was initially buried a few diameters below the bed surface. While supplying the flume with constant rates of small beads and water, we used a high-speed camera and image analysis techniques to monitor both the trajectory of the intruder and the velocity of the granular bed. We performed approximately 300 experiments with six size ratios (large to small) ranging from 1.25 to 3.0 to get statistically representative intruder trajectories. This extensive and unusual data set allows for a more thorough validation of the observations made in the work of Rousseau et al. [Phys. Rev. Fluids 7, 064305 (2022)]. Specifically, our results highlight the transition in the intruder behavior occurring around a size ratio of 2. Below this value, the intruder spatial trajectory exhibits a greater variability and gets steeper as the size ratio increases. Above the value of 2 and whatever the size ratio, the spatial trajectory is almost linear with a constant slope, and the time taken by the intruder to reach the bed surface increases with the size ratio. In addition, our extensive data set reveals insights into the intruder kinematics. Notably, we observed that the intruder undergoes a size-ratio-dependent streamwise velocity lag with respect to the bulk flow. Based on these observations, we propose a simple kinematic model which predicts the segregation duration of an intruder in bedload transport, incorporating a parameter representing the linear trajectory and a drift coefficient characterizing the streamwise velocity lag. Both parameters introduce a size ratio dependency based on our statistical analysis.