Bedload abrasion and the in situ fragmentation of bivalve shells

Newell, Andrew J.; Gower, David J.; Benton, Michael J.; Tverdokhlebovs, Valentin P.. 2007 Bedload abrasion and the in situ fragmentation of bivalve shells. Sedimentology, 54 (4). 835-845.

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The aim of this study was to determine how Unio bivalve shells fragment within the channel of the Sakmara River (southern Urals, Russia). The Sakmara River has an abundant bivalve population and a highly variable flow regime which, at low flow, allowed much of the channel bed to be examined. A large data set of 1013 shells (Unio sp.) was examined and these were shown to have consistent patterns of orientation, aspect, shell abrasion, perforation and fracture. The close spatial relationship between areas of shell abrasion, shell perforation and shell fracture showed that they form part of a continuum whereby areas of abrasion evolve into perforations and perforations coalesce and enlarge into fractures. The mechanism of shell damage proposed is one of abrasion in place, whereby the shell remains stationary on the surface of the point bar and is impacted by bedload. Underpinning this process are the hydrodynamic properties of the bivalve shell, with consistency in the orientation and aspect of the valve in a flowing current producing consistency in the distribution of damage on the shell surface. Valves preferentially lie in a convex-up position and orientate in the flow such that the umbo faces upstream. The elevated, upstream-facing umbo region is exposed to particle impact and is the first to be abraded and perforated. The vulnerability of the umbo to perforation is greatly increased by the thinness of the shell at the umbo cavity. The in situ abrasion process is enhanced by the development of an armoured gravel bed which restricts valve mobility and maintains shells within the abrasion zone at the sediment-water interface. The in situ abrasion process shows that broken shells are not a reliable indicator of long distance transport. The study also raises the issue that tumbling barrel experiments, which are generally used to simulate shell abrasion, will not replicate the type of directionally focused sand-blasting which appears to be the principal cause of shell fragmentation in the Sakmara River.

Item Type: Publication - Article
Digital Object Identifier (DOI):
Programmes: BGS Programmes > Groundwater Management
ISSN: 0037-0746
Additional Keywords: Bedload, Abrasion, Bivalvia, Russia
NORA Subject Terms: Earth Sciences
Related URLs:
Date made live: 03 Aug 2009 11:07 +0 (UTC)

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