Shape and size characteristics of bedload transported during winter storm events in the Cwm Treweryn stream,Brecon Beacons,South Wales
Birleşik Krallık'ın Güney Galler bölgesi, Brecon Beacons yöresinde, Usk Nehri'nin kolu olan Cwm Treweryn akarsuyunda 1995 yılının yağışlı geçen ilk döneminde, sepet-biçimli yatak yükü tuzakları kullanılarak yatak malzemesi yükü taşınma şekilleri incelenmiştir. Çalışmada özellikle belirli oranlardaki çakıl boyut grupları ve çakıl şekillerinin taşınma üzerine olan etkileri (seçici taşınma) incelenmiştir. 2.5 ile 19.5 debiler arasında değişen güçlü akıntılar kaydedilmiştir. Oldukça iri boyuttaki yatak yükü hariç tutulduğunda, yakalanan çakılların boyut özelliklerinin yüksek debi oranlarıyla çok az bir değişme gösterdiği tespit edilmiştir. Bu durum yatak yükü taşınmasının, yatakta hakim olan küçük boyuttaki çakılların hareketleri şiddetli olarak belirlemektedir. Hareket eden yatak yükünün şekilsel özellikleri hemen civardaki yatak material yüküyle karşılaştırılmış olup, bulgular, iri çakıllar (32-64 mm) içerisinde disk şekilli materyalin oranı yatak yükünü teşkil eden malzemeye oranla iki kat daha fazla, küremsi olanların oranı %5 daha fazla ve ovalimsi şekildeki çakılların oranının ise daha az olduğunu göstermiştir. Küçük bloklar (64-128 mm) içerisinde ise disk şeklindeki hareket eden materyalin oranı yatak yükünü teşkil eden malzemeye oranla %10 daha fazla, iri bloklar (> 128 mm) içerisinde ise egemendir. Aynı şekilde, bu grup içerisinde küremsilerin oranı en fazla buna karşılık yassı şekilli hiçbir iri blok hareket etmemiştir. Hareket eden materyal içerisinde, iri çakıllar grubunda küremsilerin ve iri bloklar içerisinde ise küremsi ve oval şeklindeki materyalin yüzde oranı artan debi ile birlikte artma göstermiştir. Boyanmış taş deneyleri ile ilgili olarak, yassı ve disk şeklindeki taşlar eşit ağırlıklara sahip olmalarına rağmen küremsi ve ovalimsi taşlara oranla hareket mesafeleri daha kısa olmuştur. Bulgular genel olarak gösteriyor ki, çakıl boyutunun üzerinde hareket ettiği pürüzlü yatak yüzeyi elemanlarına oranla daha büyük olduğu durumlarda, çakıl şeklinin seçici taşınma üzerindeki etkisi daha belirgin olmaktadir. Buna karşılık, oldukça pürüzlü yatak yüzeyleri üzerinde, herhangi bir şekle sahip çakılın hareketi yatak yüzeyini oluşturan unsurların hareketi ile sınırlıdır.
Güney Galler'in Brecon Beacons yöresinde bulunan Cwm Treweryn akarsuyu'nda taşınan yatak yükü malzemesinin şekil ve boyut özellikleri
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.
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