The Contribution of Abrasion and Size-Selective Sorting to Downstream Fining in a Tropical Montane Stream

Abstract:

Quantifying the effect of abrasion vs. size-selective transport on downstream diminution of grain size and mass is a long-standing question in fluvial systems. While some authors have emphasized sorting by size-selective transport as the dominant fining mechanism in various rivers, others demonstrated the effectiveness of abrasion in certain fluvial systems. We present a synthetic grain-scale model in which we combine a recently developed geometric abrasion model (the so-called ‘box equations’ [1]) with a simplistic selective deposition rule. Box equations are capable to describe the evolution of both the shape and the size of the particles during abrasion, as opposed to previous models which only dealt with the size (or alternatively, the mass) diminution. We adapt our synthetic model to numerically simulate the downstream grain size and shape evolution in a short tropical river in Puerto Rico where we conducted a detailed field study. By switching off abrasion and selective deposition separately in the numerical model, the individual effects of these two processes can be examined. Based on our simplistic model we deduce that 1/3 of the mass of the grains may be lost only by abrasion in the examined river system.

[1] Domokos, G., and G. W. Gibbons (2012), The evolution of pebble size and shape in space and time, Proc. R. Soc. A, 468(2146), 3059–3079, doi:10.1098/rspa.2011.0562.