Unravelling the role of pocket geometry in the entrainment of coarse sediment particles: Insights from IMU-based analysis

• A new analytical framework is proposed based on the rolling mode of particle entrainment. • The concept of impulse acceleration is introduced to characterize the onset process. • Hydrodynamic forces are extracted from IMU-derived net forces through force analysis. • Drag and lift coefficients are quantified and compared with existing experimental data. • Findings provide new insights into coarse-particle entrainment in low-slope gravel-bed rivers. The entrainment of coarse sediment particles in gravel-bed rivers governs bed stability, channel evolution, and sediment transport. Understanding how local bed geometry influences the onset of motion for such particles remains a persistent challenge, owing to the complexity of near-bed turbulence and three-dimensional particle dynamics. This study investigates the role of pocket geometry in particle entrainment using a Smart Sediment Particle (SSP) instrumented with an inertial measurement unit (IMU). Controlled flume experiments were conducted over a fixed spherical bed to capture triaxial accelerations and angular velocities during entrainment and motion. Results show that, under fully submerged conditions, variations in flow depth exert little influence on entrainment thresholds, whereas pocket geometry governs both the onset and subsequent rotational behaviour. The saddle configuration requires lower critical velocities but produces stronger rotational impulses, while the grain-top configuration demands higher velocities yet leads to more sustained tumbling. The IMU-based approach reveals the coupling between hydrodynamic forcing and rotational dynamics, providing new insights for physically based sediment-transport modelling in coarse-bed rivers.