Downstream changes in bed meterial size and shape characteristics in a small upland stream: Cwm Treweryn, in South Wales

Bu çalışmanın amacı, Galler'in Brecon Beacons yöresinde küçük bir akarsu olan Cwm Treweryn'in yatağı boyunca yatak yükü (çakıl boyutundaki iri unsurlar (>32 mm)) malzemesinin şekil ve boyut özelliklerinin incelenmesidir, Bu amaçla, tali kollar dahil, yatak boyunca toplam 21 lokasyonda yatak yükünü oluşturan malzeme incelenmiştir. Her lokasyonda çapı (b-ekseni) 32 mm'den büyük olan toplam 100 çakıl örneği alınmıştır. Daha önce yapılan bir çok araştırma, aşınma ve seçici taşınmadan dolayı, akarsu yatağı boyunca (kaynaktan ağıza doğru) yatak yükünü oluşturan unsurların boyutlarının genellikle küçülme eğiliminde olduğu ve malzemenin yuvarlaklığının arttığını göstermiştir. Bununla birlikte, Cwm Treweryn nehrinde yapılan ölçümler, anılan modeldeki basitliğin aksine, yatak yükünü oluşturan unsurlar şekil ve boyut özellikleri bakımından daha karmaşık ve düzensiz bir değişim örneği göstermiştir. Yatak malzemesinin boyut özellikleri 21 lokasyonda oldukça geniş bir dağılım ve dolayısıyla zayıf bir seçici taşınma örneği göstermektedir. Bütün yatak boyunca, disk şekilli çakılların birinci ve bıçak sırtı şeklindeki çakılların da ikinci büyük oranda olması ise, havzada hakim litoloji olan Old Red kumtaşının yapısal ve litolojik özelliklerine bağlanmıştır. Sonuç olarak, önceki çalışmalarla karşılaştırıldığında, Cwm Traweryn nehrinde yatak yükünü oluşturan unsurların boyut ve şekilsel özelliklerinin (yuvarlaklık ve küresellik) akarsu yatağı boyunca küçük bir değişim gösterdiği saptanmıştır. Bu durum değişik faktörlerin biraradaki etkisine bağlanabilir. Bunlardan bazıları; yatak yükünü oluşturan malzemenin litolojik özellikleri (Old Red kumtaşı), akarsu boyunun kısalığı ve akarsuya yatak ve teraslardan eklenen yeni malzemedir.

Dağlık bir bölgede, kısa bir akarsu mecrası boyunca yatak malzemesi yükünün şekil ve boyut özelliklerinin değişimi: Cwm Treweryn Nehri, Güney Galler

The aim of this study is to investigate the downstream changes in bedload clast size and shape along the Cwm Treweryn stream, Brecon Beacons, South Wales. A total of 21 sites (including six tributary entry points) were used to evaluate Downstream changes in bedload size and shape (form, roundness and sphericity) characteristics. At each sites, 100 surface clasts with b axes (intermediate) greater than 32 mm were sampled. Many of the earlier studies found that, due to abrasion and sorting processes, particle size decreases and roundness increases in a downstreahl direction. However, in contrast to these studies, the Cwm Treweryn stream does not conform this simple dowhstream pattern, instead shows irregular and complex changes in both size and shape along the chan-nel. In terrflè of size characteristics, bed material at all 21 sites represented a wide range of size fractions and hence poor sorting. The dominance of disc-shaped clasts with blades the second most frequent shape along the entire channel length is attributed to the structural and lithological characteristics of the predominance of Old Red Sandstone. As a result, comparatively small downstream changes were determined in bedload clast size and shape (roundriëss, sphericity) and these changes are attributed to the combined effect of different factors, including the lithology of the bedload (Old Red Sandstone), the short river course, and occasional inputs of angular fresh material frorn the channel banks and bed.

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