Survival of the Strong and Dense: Field Evidence for Rapid, Transport‐Dependent Bed Material Abrasion of Heterogeneous Source Lithology. Issue 6 (2nd June 2022)

• Record Type: Journal Article
• Authors: Pfeiffer, Allison M.; Morey, Susannah; Karlsson, Hannah M.; Fordham, Edward M.; Montgomery, David R.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 6(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-06-02
• Subjects: Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2021JF006455
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: Bed material abrasion is a major control on the partitioning of basin‐scale sediment fluxes between coarse and fine material. While abrasion is traditionally treated as an exponential function of transport distance and a lithology‐specific abrasion coefficient, experimental studies have demonstrated greater complexity in the abrasion process: the rate of abrasion varies with clast angularity, transport rate, and grain size. Yet, few studies have attempted to assess the importance of these complexities in a field setting. Here, we develop a new method for rapidly quantifying baseline abrasion rate in the field via Schmidt Hammer Rock Strength. We use this method, along with measurements of gravel bar lithology, to quantify abrasion in the Suiattle River, a basin in the North Cascades of Washington State in which sediment supply to the channel is dominated by recurrent debris flows from a tributary draining Glacier Peak stratovolcano. Rapid downstream strengthening of river bar sediment and a preferential loss of weak, low‐density vesicular volcanic clasts relative to non‐vesicular ones suggest that abrasion is extremely effective in this system. The standard exponential model for downstream abrasion, using single‐lithology abrasion rates fails to reproduce observed downstream patterns in lithology and clast strength. Incorporating heterogeneity in source material strength as well as transport rate‐dependent abrasion largely resolves this failure. Further work is needed to develop a comprehensive quantitative framework for the dependence of bed material abrasion on grain size and transport rate. Plain Language Summary: As rocks transport along a riverbed, they hit one another and small pieces break off. This process is referred to as bed material abrasion. The rate of abrasion helps to control the ratio of sediment transported as gravel along the river bottom, and sediment transported in suspension (making the water turbid). In this study, we introduce a new method for estimating abrasion rate using a simple field measurement of rock strength. We then apply this method to the Suiattle River, which drains a stratovolcano that produces rock of varying strength. We use our field measurements to test the standard, simple mathematical approach for modeling bed material abrasion, and find it does a poor job of matching our field observations of how the riverbed rock mixture changes along the length of the channel. Finally, we test a more sophisticated model for abrasion that accounts for how rapidly grains transport. These model results better match our field observations, though we suggest that further work remains to improve models for bed material abrasion. Key Points: We present a new method for rapidly quantifying baseline abrasion rate in the field via Schmidt Hammer Rock Strength Abrasion is extremely effective at this site due to vesicular volcanic rocks, yet easy to underestimate using simplistic sampling approaches We invoke transport‐dependent abrasion to help explain the rapid downstream loss of the weakest grains