Impacts of Particle Density and Relative Submergence on the Sediment Entrainment Process: An Experimental Study Using a Smart Sediment Particle

Abstract

Quantifying the hydrodynamic forcing associated with sediment entrainment at the threshold of motion remains a fundamental challenge because bed–particle contact reactions are rarely measured concurrently. We conducted controlled flume experiments using a freely moving Smart Sediment Particle equipped with an inertial measurement unit (IMU) to record gravity‐compensated linear accelerations during the 0.15 s pocket‐exit onset window across six particle densities (1.19–2.095 g/cm ³ ) and fully submerged conditions ( 2–5). We then linked the onset‐window IMU response to effective (bounded) hydrodynamic drag and lift components by explicitly accounting for an admissible range of contact forces; the resulting forces and coefficients are interpreted as bounded quantities rather than uniquely determined hydrodynamic values. Results show that, once fully submerged, relative submergence has a negligible influence on the onset‐window effective linear accelerations and the inferred coefficients. In contrast, particle density exerts strong control: both streamwise and anti‐gravity effective accelerations increase with density. The midpoint (mean‐of‐bounds) effective coefficients cluster around 2.28 and 0.98 and vary only weakly with particle Reynolds number and Froude number within the tested high‐ range. Placing the measured thresholds in Shields space further indicates that highly exposed particles ( 0.91) can exhibit very low critical Shields numbers ( 0.007) at 10 ³ . Overall, the study provides onset‐resolved, physically defensible bounds on effective hydrodynamic forcing during pocket exit and highlights the value of paired IMU and independent contact‐force measurements in future work.