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

  • DaÛlÝk bir blgede, kÝsa bir akarsu mecrasÝ, +6 authors Tuncer DEMÜR
  • Published 2003

Abstract

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 downstream direction. However, in contrast to these studies, the Cwm Treweryn stream does not conform this simple downstream pattern, instead shows irregular and complex changes in both size and shape along the channel. In terms 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 (roundness, 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 from the channel banks and bed. Key words: Bedload, gravel-bed streams, particle shape, particle size, roundness, sphericity, Wales. …Z Bu alÝßmanÝn amacÝ, GallerÕin Brecon Beacons yšresinde kŸŸk bir akarsu olan Cwm TrewerynÕin yataÛÝ boyunca yatak yŸkŸ (akÝl boyutundaki iri unsurlar (>32 mm)) malzemesinin ßekil ve boyut šzelliklerinin incelenmesidir. Bu amala, tali kollar dahil, yatak boyunca toplam 21 lokasyonda yatak yŸkŸnŸ olußturan malzeme incelenmißtir. Her lokasyonda apÝ (b-ekseni) 32 mmÕden bŸyŸk olan toplam 100 akÝl šrneÛi alÝnmÝßtÝr. Daha šnce yapÝlan birok araßtÝrma, aßÝnma ve seici taßÝnmadan dolayÝ, akarsu yataÛÝ boyunca (kaynaktan aÛÝza doÛru) yatak yŸkŸnŸ olußturan unsurlarÝn boyutlarÝnÝn genellikle kŸŸlme eÛiliminde olduÛu ve malzemenin yuvarlaklÝÛÝnÝn arttÝÛÝnÝ gšstermißtir. Bununla birlikte, Cwm Treweryn nehrinde yapÝlan šlŸmler, anÝlan modeldeki basitliÛin aksine, yatak yŸkŸnŸ olußturan unsurlar ßekil ve boyut šzellikleri bakÝmÝndan daha karmaßÝk ve dŸzensiz bir deÛißim šrneÛi gšstermißtir. Yatak malzemesinin boyut šzellikleri 21 lokasyonda oldukca geniß bir daÛÝlÝm ve dolayÝsÝyla zayÝf bir seici taßÝnma šrneÛi gšstermektedir. BŸtŸn yatak boyunca, disk ßekilli akÝllarÝn birinci ve bݍak sÝrtÝ ßeklindeki akÝllarÝn da ikinci bŸyŸk oranda olmasÝ ise, havzada hakim litoloji olan Old Red kumtaßÝnÝn yapÝsal ve litolojik šzelliklerine baÛlanmÝßtÝr. Sonu olarak, šnceki alÝßmalarla karßÝlaßtÝrÝldÝÛÝnda, Cwm Traweryn nehrinde yatak yŸkŸnŸ olußturan unsurlarÝn boyut ve ßekilsel šzelliklerinin (yuvarlaklÝk ve kŸresellik) akarsu yataÛÝ boyunca kŸŸk bir deÛißim gšsterdiÛi saptanmÝßtÝr. Bu durum deÛißik faktšrlerin biraradaki etkisine baÛlanabilir. Bunlardan bazÝlarÝ; yatak yŸkŸnŸ olußturan malzemenin litolojik šzellikleri (Old Red kumtaßÝ), akarsu boyunun kÝsalÝÛÝ ve akarsuya yatak ve teraslardan eklenen yeni malzemedir. Anahtar kelimeler: Yatak yŸkŸ, akÝl aÛÝrlÝklÝ akarsu, akÝl ßekli, akÝl boyutu, kŸresellik, yuvarlaklÝk, Galler. Yerbilimleri, 28 (2003), 33-47 Hacettepe †niversitesi Yerbilimleri Uygulama ve AraßtÝrma Merkezi BŸlteni Bulletin of Earth Sciences Application and Research Centre of Hacettepe University T. Demir E-mail: tdemir@harran.edu.tr

Cite this paper

@inproceedings{blgede2003DownstreamCI, title={Downstream changes in bed material size and shape characteristics in a small upland stream: Cwm Treweryn, in South Wales}, author={Da{\^u}l{\'Y}k bir blgede and k{\'Y}sa bir akarsu mecras{\'Y} and boyunca yatak and malzemesi yknn and boyut zelliklerinin and de{\^u}i\ssimi Cwm and Treweryn Nehri and G. M. Galler and Tuncer DEM{\"{U}R}, year={2003} }