The influence of particle shape on bedload transport in coarse-bed river channels

Abstract

This thesis investigates the influence of bed material shape on sediment transport in gravel-bed rivers. The approach involves a combined series of field and laboratory experiments. Magnetic tracing experiments were carried out at three experimental sites in two Pennine gravel-bed streams. The specific aim of these experiments was to quantify the selective transport of different shapes of coarse river gravel and determine their spatial sorting within a natural stream channel. A total of 900 tracers in three size groups (32- 64 mm, 64-128 mm and greater than 128 mm) and four shape classes (spheres, blades, rod and discs) were prepared for each of the three sites. In die laboratory, tilting table experiments were carried out to clarify the mechanistic behaviour of different particle shapes, sizes and orientations on a variety of artificial and naturally formed bed roughnesses. Using strobe-light photography visualization experiments were undertaken with natural and artificially-moulded gravel-size particles of differing shape, size and weight in order to investigate the influence of shape on settling, grain impact, initial motion and transport paths of gravel-size particles. Results of the magnetic tracing experiments showed that there was both size and shape selectivity in bedload transport. Preferential movement occurred in die small and medium particle size classes with tracers concentrated along the channel thalweg. Sphere-shaped particles were transported the greatest distance and in greatest numbers. Rods also moved preferentially, while discs showed a lesser degree of transport and blades hardly moved at all. Results from the tilting table experiments highlight the importance of roundness as well as particle form and particle orientation in continuing thresholds of entertainment. In terms of size, friction angle was found to depend on the ratio of the diameter of the test particle to be moved to that it rests upon (d/D). Shape and orientation were found to be important parameters influencing friction angles. On a given bed roughness and for a constant size non-spherical test particles showed greater friction angles than spherical ones. A very clear difference was found in friction angle distibutions between sphere, transverse rod and other flat-shaped particles, namely, blades with parallel and transverse orientations, disc, and rod with parallel orientations. Visualisation experiments indicated that shape is an important particle characteristic that has a significant effect on settling rates and also the mode of near bed transport. These effects increase with greater particle sizes. The departure of a particle from a sphere leads to a decrease in its settling velocity, Experiments, across a range of test sizes showed that when compared to a sphere of 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 flian blade and disc-shapes. Regardless of shape, greater bed roughness, or decreasing particle size results in an increase in the critical angle for motion.