Direct Measurement of in-channel abrasion processes

P.A. Brewer¹, G.J.L. Leeks² and J. Lewin¹

¹ Institute of Earth Studies, University of Wales, Aberystwyth, Dyfed, SY23 3DB, UK.
² Institute of Hydrology. Staylittle, Llanbrynmair, Powys, SY19 7DB, UK.

Downstream trends in the bed sediment character of alluvial channels are attributed to hydraulic sorting and mechanical abrasion processes acting within the channel. Historically, abrasion processes have been isolated in the laboratory by use of abrasion tanks and tumbling barrels. However, laboratory abrasion rates are commonly lower than those observed in the field, and Schumm and Stevens (1973) proposed that "abrasion in place" processes may also operate within the channel. Thirty nine mobile and semi-mobile tracers were seeded in two selected reaches of the Plynlimon Experimental Catchments, to directly measure the abrasion of individual test clasts in the natural channel environment. Significant losses (~ 0.5g) sustained by tracers above the bed load trap, tracers fixed directly to bedrock, and tracers tethered confirmed the potential importance of over-passing bed load as an abrasion process. Weight losses sustained by tracers below the bed load trap (~ 0.05g) also indicated the potential of sandblasting as an additional "abrasion in place" process.

In Bogen, J. Walling, D. and Day, T.J. (Eds.), Erosion and sediment transport monitoring programmes in river basins. IAHS Publication 210, p21-30.