Abstract

t It is well known that pebble diameter systematically decreases downstream in rivers. The
contribution of abrasion is uncertain, in part because (1) diameter is insufficient to characterize
pebble mass loss due to abrasion and (2) abrasion rates measured in laboratory experiments cannot be
easily extrapolated to the field. A recent geometric theory describes abrasion as a curvature-dependent
process that produces a two-phase evolution: in Phase I, initially blocky pebbles round to smooth,
convex shapes with little reduction in axis dimensions; then, in Phase II, smooth, convex pebbles slowly
reduce their axis dimensions. Here we provide strong evidence that two-phase abrasion occurs in a
natural setting, by examining downstream evolution of shape and size of thousands of pebbles over
~10 km in a tropical montane stream. The geometric theory is verified in this river system using a variety
of manual and image-based shape parameters, providing a generalizable method for quantifying the
significance of abrasion. Phase I occurs over ~1 km, in upstream bedrock reaches where abrasion is
dominant and sediment storage is limited. In downstream alluvial reaches, where Phase II occurs,
we observe the expected exponential decline in pebble diameter. Using a discretized abrasion model
(the so-called “box equations”) with deposition, we deduce that abrasion removes more than one third of
the mass of a pebble but that size-selective sorting dominates downstream changes in pebble diameter.
Overall, abrasion is the dominant process in the downstream diminution of pebble mass (but not
diameter) in the studied river, with important implications for pebble mobility and the production of
fine sediments.

Citation

Miller, Szabo, Jerolmack and Domokos (2014): Quantifying the significance of abrasion and selective transport for downstream fluvial grain size evolution. Journal of Geophysical Research: Earth Surface. DOI: 10.1002/2014JF003156

This Paper/Book acknowledges NSF CZO grant support.