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

2005; Volume: 14; Issue: 1 Linguagem: Inglês

1303-619X

Tuncer Demir Rory Peter Dominic Walsh,

Flood Risk Assessment and Management

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. Ozet: Birleflik Krall›k'›n Guney Galler bolgesi, Brecon Beacons yoresinde, Usk Nehri'nin kolu olan Cwm Treweryn akarsuyunda 1995 y›l›n›n ya¤›fll› gecen ilk doneminde, sepet-bicimli yatak yuku tuzaklar› kullan›larak yatak malzemesi yuku tafl›nma flekilleri incelenmifltir. Cal›flmada ozellikle belirli oranlardaki cak›l boyut gruplar› ve cak›l flekillerinin tafl›nma uzerine olan etkileri (secici tafl›nma) incelenmifltir. 2.5 ile 19.5 debiler aras›nda deiflen guclu ak›nt›lar kaydedilmifltir. Oldukca iri boyuttaki yatak yuku haric tutulduunda, yakalanan cak›llar›n boyut ozelliklerinin yuksek debi oranlar›yla cok az bir deiflme gosterdii tespit edilmifltir. Bu durum yatak yuku tafl›nmas›n›n, yatakta hakim olan kucuk boyuttaki cak›llar›n hareketleri fliddetli olarak belirlemektedir. Hareket eden yatak yukunun flekilsel ozellikleri hemen civardaki yatak material yukuyle karfl›laflt›r›lm›fl olup, bulgular, iri cak›llar (32—64 mm) icerisinde disk flekilli materyalin oran› yatak yukunu teflkil eden malzemeye oranla iki kat daha fazla, kuremsi olanlar›n oran› %5 daha fazla ve oval›ms› flekildeki cak›llar›n oran›n›n ise daha az olduunu gostermifltir. Kucuk bloklar (64—128 mm) icerisinde ise disk fleklindeki hareket eden materyalin oran› yatak yukunu teflkil eden malzemeye oranla %10 daha fazla, iri bloklar (> 128 mm) icerisinde ise egemendir. Ayn› flekilde, bu grup icerisinde kuremsilerin oran› en fazla buna karfl›l›k yass› flekilli hicbir iri blok hareket etmemifltir. Hareket eden materyal icerisinde, iri cak›llar grubunda kuremsilerin ve iri bloklar icerisinde ise kuremsi ve oval fleklindeki materyalin yuzde oran› artan debi ile birlikte artma gostermifltir. Boyanm›fl tafl deneyleri ile ilgili olarak, yass› ve disk fleklindeki tafllar eflit a¤›rl›klara sahip olmalar›na ramen kuremsi ve ovalimsi tafllara oranla hareket