Development of probabilistic prediction methods for graded sediment from discrete particle simulations
The aim of this work was to examine the behaviour of sediment, from the perspective of the grain scale mechanics, through computer simulation of grain interactions. A discrete particle model was developed, capable of manipulating the simultaneous motion of large numbers of grains through numerical integration of the equation of motion on an individual basis. Simulations of the critical entrainment shear stress of grains in the surface of single-sized sediment beds demonstrated a distributed nature of threshold values which was dependent upon the arrangement of the randomly deposited bed surface. Geometrical arguments were developed that indicated the existence of general critical entrainment shear stress distributions of both single and mixed-sized beds of a restricted grain size range. Effects of flow sheltering were examined, finding them to hold a significant influence over the effective critical entrainment shear stress distributions. Simulations of saltation trajectories revealed a transition in saltation behaviour associated with a densimetric Froude number, Fr 1.2, above which saltation was maintained indefinitely. Trajectory lengths were also investigated over a range of bed grain to saltating grain size ratios and were found to vary linearly with particle Froude number, to a first approximation, for d/D =1. The critical entrainment shear stress and trajectory length results were then incorporated into a probabilistic model which was used to predict fractional bed-load composition, providing valuable insight into the significance of grain scale effects upon large scale phenomena. The results reproduced some of the aspects of the partial transport regime identified by Wilcock (1997), showing evidence of a strong dependency on saltation trajectory length.