Abstract
This project aims to re-assess our understanding of the shape of fluvial bedload gravels by drawing together existing information on fluvial gravel shape. At its crux, however, is the interpretation of a large, high-quality set of measurements made on bedload gravels from the Sabeto River of western Viti Levu, Fiji. This work reveals that the apparent simplicity displayed by most studies of downstream rounding disguises a complex pattern of stepwise reversals to an angular state, the result of the splitting of cobble- and boulder-sized particles. Particle sphericity changes rapidly during the initial stages of transport. Along the Sabeto, this seems to be the result of attrition, with breakage generating the low and continuing presence of low sphericity particles in the system. Elsewhere, however, sphericity is a consequence of shape sorting and we speculate that rivers globally exist along a sorting–attrition continuum. The form of fluvial gravels is not what would be expected were sorting the dominant control on gravel form. Instead measurements of form display a complex relationship with roundness (and thus with breakage and abrasion). Fluvial gravels appear to evolve to a distinctive shape that may offer a means of distinguishing the products of riverine deposition.
Highlights
Shape, together with size and composition, is one of the fundamental properties of geological particles
The catchment lies in the south Pacific tropical cyclone belt and the river experiences frequent floods associated with cyclonic rainfall
This exerts a primary control on river geomorphology, it affects particle mobility, it influences the size distributions of fluvial sediments and it contributes to our understanding of the sedimentary processes that operate in rivers
Summary
The shape and origin of fluvial gravels have been studied for over a century, fundamental questions remain about how shapes evolve in the fluvial environment, what shapes tell us about sedimentary processes and whether particle shapes are environmentally diagnostic. At its crux is the provision and interpretation of a large, high-quality set of measurements made on bedload. This work has aimed to tackle these problems by drawing together and assessing existing information on fluvial gravel shape. At its crux is the provision and interpretation of a large, high-quality set of measurements made on bedload gravels from the Sabeto River of western Viti Levu, Fiji. These have added appreciably to the existing corpus of information on fluvial gravel shape, whilst providing the opportunity for a much more sophisticated assessment of gravel shape and its determining factors than has previously been possible
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