Birleflik Krall›k’›n Güney Galler bölgesi, Brecon Beacons yöresinde, Usk Nehri’nin kolu olan Cwm Treweryn akarsuyunda 1995 y›l›n›n ya¤›fll› geçen ilk döneminde, sepet-biçimli yatak yükü tuzaklar› kullan›larak yatak malzemesi yükü tafl›nma flekilleri incelenmifltir. Çal›flmada özellikle belirli oranlardaki çak›l boyut gruplar› ve çak›l flekillerinin tafl›nma üzerine olan etkileri (seçici tafl›nma) incelenmifltir. 2.5 ile 19.5 debiler aras›nda de¤iflen güçlü ak›nt›lar kaydedilmifltir. Oldukça iri boyuttaki yatak yükü hariç tutuldu¤unda, yakalanan çak›llar›n boyut özelliklerinin yüksek debi oranlar›yla çok az bir de¤iflme gösterdi¤i tespit edilmifltir. Bu durum yatak yükü tafl›nmas›n›n, yatakta hakim olan küçük boyuttaki çak›llar›n hareketleri fliddetli olarak belirlemektedir. Hareket eden yatak yükünün flekilsel özellikleri hemen civardaki yatak material yüküyle karfl›laflt›r›lm›fl olup, bulgular, iri çak›llar (32–64 mm) içerisinde disk flekilli materyalin oran› yatak yükünü teflkil eden malzemeye oranla iki kat daha fazla, küremsi olanlar›n oran› %5 daha fazla ve oval›ms› flekildeki çak›llar›n oran›n›n ise daha az oldu¤unu göstermifltir. Küçük bloklar (64–128 mm) içerisinde ise disk fleklindeki hareket eden materyalin oran› yatak yükünü teflkil eden malzemeye oranla %10 daha fazla, iri bloklar (> 128 mm) içerisinde ise egemendir. Ayn› flekilde, bu grup içerisinde küremsilerin oran› en fazla buna karfl›l›k yass› flekilli hiçbir iri blok hareket etmemifltir. Hareket eden materyal içerisinde, iri çak›llar grubunda küremsilerin ve iri bloklar içerisinde ise küremsi ve oval fleklindeki materyalin yüzde oran› artan debi ile birlikte artma göstermifltir. Boyanm›fl tafl deneyleri ile ilgili olarak, yass› ve disk fleklindeki tafllar eflit a¤›rl›klara sahip olmalar›na ra¤men küremsi ve ovalimsi tafllara oranla hareket mesafeleri daha k›sa olmufltur. Bulgular genel olarak gösteriyor ki, çak›l boyutunun üzerinde hareket etti¤i pürüzlü yatak yüzeyi elemanlar›na oranla daha büyük oldu¤u durumlarda, çak›l fleklinin seçici tafl›nma üzerindeki etkisi daha belirgin olmaktadir. Buna karfl›l›k, oldukça pürüzlü yatak yüzeyleri üzerinde, herhangi bir flekle sahip çak›l›n hareketi yatak yüzeyini oluflturan unsurlar›n hareketi ile s›n›rl›d›r.

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 m3 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.