Shape and Size Characteristics of Bedload Transported During Winter Storm Events in the Cwm Treweryn Stream, Brecon Beacons, South Wales


Abstract: Bedload transport in the Cwm Treweryn stream, a tributary of the Usk in the Brecon Beacons, South Wales, was investigated during a wet winter period in early 1995 using basket-type bedload traps and the tracing of painted clasts. The Cwm Treweryn is a typical mountain stream of the region with its gravel-bedload, flashy regime, high gradient and pool-riffle sequences. In particular, the study explored - for a range of categories of clast size - the nature of any influence of clast shape on bedload movement. Competent flows ranging from 2.5 to 19.5 m^3 s^{-1} (the latter close to bankfull flow) were monitored. The size composition of trapped bedload clasts varied little with peak flow magnitude except for the very largest clast fraction, suggesting that bedload transport was strongly influenced by movement of the dominant small cobble fraction. Shape composition of trapped bedload was compared with sampled bed material from the reach immediately upstream. For the large gravel fraction (32-64 mm), discs were twice as frequent and spheres 5% more frequent than in the upstream reach material. Rods were greatly under-represented. Discs were 10% over-represented in the small cobble fraction (64-128 mm) and dominant (though not over-represented) in the large cobble fraction (> 128 mm), in which spheres were distinctly over-represented and blades absent. The transported material was rounder and somewhat more spherical than the reach material. The percentage of spherical large gravel and spherical and rod-shaped small cobble clasts increased with peak discharge, whereas discs of all sizes tended to decline with peak discharge. Blades are over-represented in medium events but not moved in smaller competent events, and are under-represented at very high flows. In painted-clast experiments, blades - and to a lesser extent - discs moved shorter distances than spheres and rods of similar weight. Overall, the results suggest that the influence of clast shape on selective transport becomes much clearer with increase of clast size relative to the bed roughness elements. On rougher beds, on the other hand, the apparent behaviour of any particular shape may be the result of the behaviour of other shapes.

Keywords: bedload, gravel-bed streams, particle shape, particle size, sphericity, Usk river, Cwm Treweryn stream, Wales

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