Controls on the rates and products of particle attrition by bed-load collisions

Kimberly Litwin Miller,Douglas Jerolmack

doi:10.5194/esurf-9-755-2021

Abstract

Abstract. River rocks round through the process of impact attrition, whereby energetic collisions during bed-load transport induce chipping of the grain surface. This process is also important for bedrock erosion. Although previous work has shown that impact energy, lithology, and shape are controlling factors for attrition rates, the functional dependence among these quantities is not settled. Here we examine these factors using a double-pendulum apparatus that generates controlled collisions between two grains under conditions relevant for bed-load transport. We also determine the grain size distributions (GSDs) of the attrition products. Two experimental results appear to support previous treatments of impact erosion as brittle fracture: (i) mass loss is proportional to kinetic energy, and this proportionality is a function of previously identified material properties; and (ii) attrition-product GSDs are well described by a Weibull distribution. Chipping results from the development of shallow and surface-parallel cracks, a process that is distinct from bulk fragmentation that occurs at higher energies. We suggest that Hertzian fracture is the dominant mechanism of impact attrition for bed-load transport. We also identify an initial phase of rapid mass loss in which attrition is independent of energy and material properties; this is a shape effect associated with removal of very sharp corners. The apparent universality of both mass loss curves and attrition-product GSDs requires further investigation. Nonetheless, these findings are useful for interpreting the contribution of in-stream attrition to downstream fining and the production of sand resulting from bed-load transport of river pebbles.

Highlights

Traveling downstream in a typical river, one observes river sediments becoming rounder in shape (Sneed and Folk, 1958; Adams, 1978) and smaller in size (Sternberg, 1875; Ferguson et al, 1996)
While there is a debate over whether mechanical breakdown by impact attrition or hydraulic sorting caused by relative transport rates is responsible for fining patterns (Kodama, 1991; Ferguson et al, 1996; Gasparini et al, 1999; Lewin and Brewer, 2002), it is generally agreed that impact attrition is the chief mechanism producing the rounding of sediments (Kuenen, 1956; Sneed and Folk, 1958; Schumm and Stevens, 1973; Litwin Miller et al, 2014; Szabó et al, 2015; Novak-Szabo et al, 2018)
With the schist on three occasions, the entire block more or less split in two, fracturing at weathering planes. In both cases, fracturing occurred at a pre-existing weak region of the rock that appeared to be associated with chemically weathered surfaces

Summary

Introduction

Traveling downstream in a typical river, one observes river sediments becoming rounder in shape (Sneed and Folk, 1958; Adams, 1978) and smaller in size (Sternberg, 1875; Ferguson et al, 1996). Attrition is often called “abrasion” in the geological literature, but that term is avoided here since abrasion has a mechanically distinct meaning in the comminution literature (Novak-Szabo et al, 2018). It is the process whereby river sediments are worn away due to energetic collisions with other grains and the channel bed during transport (Kuenen, 1956; Kodama, 1994b). Impact attrition by saltating bed-load particles is a significant, and in many cases dominant, contributor to the erosion of bedrock river channels (Sklar and Dietrich, 1998, 2004)

Methods

Results

Discussion

Conclusion