Visualisation of the solitary grain sediment dynamics of paerticles of varying shape: Implications for sediment transport over coarse-gravel bed rivers

Shape, size, and density are fundamental properties controlling the hydrodynamic behaviour of sediment particles. Particle shape can play a significant role in bedload transport processes by controlling the nature of particle settling and near-bed motion. The aim of the experiments reported here is to investigate the influence of shape on the settling initial motion and transport of gravel-size particles.

Experiments, using strobe-light photography, were carried out with natural and artificial gravel-size particles of differing shape (sphere, rod, disc and blade), size and weight. Two types of experiment were undertaken in a 10 litre, water-filled rectangular tank. Firstly, particles were dropped, through water, onto a 30 inclined, smooth glass plate. A camera mounted outside the tanlq normal to the sloping glass, recorded the fall and movement of each particle. Particle velocities and trajectory paths were measured from the photographs by plotting successive centres of mass of the particle. A second set of experiments, using the same set-up as the first, but this time investigating the initial motion of particles of varying shape and size was also tested on two beds of differing roughness (7 and 14 mm). The bed was tilted until the test particle moved from its pocket of origin and strobe-light photographs were taken at the initiation of motion.

Results indicate that shape is an important particle characteristic that has a significant effect on settling rates and also the mode of transport. These effects increase with larger particle sizes. Departure from a spherical form leads to a decrease in its settling velocity. Experiments show, across the range of sizes tested that, when compared to a sphere of approximate equivalent weight and density, sphere and rod shaped particles tend to settle the fastest and move by rolling. Discs and blades showed slower settling rates and, in most instances, moved by sliding. Experiments carried out with irregularly shaped, natural particles show greater variability in settling behaviour and irregular patterns of motion. For every size group, sphere and rod shaped particles have lower critical angles of initial motion than blade and discshapes. Regardless of shape, greater bed roughness, or decreasing particle size results in an increase in the critical angle for motion. The implications of these results for bedload transport in river channels is briefly discussed.