Modelling initial motion of non-spherical sediment particles on inclined and seeped beds

Abstract

Analytical and numerical solutions are developed to compute the threshold stress for the initiation of motion of sediment particles, from spherical to ellipsoidal, on inclined beds under boundary layer flows. The numerical model is based on the discrete element method and simulates the micro-mechanics of the sediment as an aggregate of rigid ellipsoids interacting by contact, while the analytical solution establishes the basic dynamics for initiation of motion. In both models, the coupling of grain motion with the flow dynamics is accomplished via drag, lift, buoyancy, and seepage forces under turbulent velocity fluctuations by a one-way technique, since incipient motion does not substantially modify the flow.

The comparison of both models and their extension to compute the threshold stress in terms of seepage, turbulent velocity fluctuations, bed inclination, and particle orientations provides a better understanding of the onset of sediment particles. The parametric analyses cover a wide range of natural and usual conditions for incipient motion, showing that most of the results fall within the variability range of the experiments. Additionally, these models are able to compute the initiation of motion for configurations not yet tested in the laboratory.