Survival of the strong, slow, and dense: Field evidence for rapid, transport-dependent bed material abrasion of heterogeneous source lithology

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 a simple exponential function of transport distance and a rock-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. Field studies of abrasion commonly characterize downstream trends in bed material lithology, shape, and size, though trends often are complicated by tributary inputs. Furthermore, to model downstream abrasion, studies have generally relied on time-intensive laboratory tumbling experiments of bulk sediment mixtures taken in the field. Such labor-intensive methods limit the feasibility of characterizing variability in sediment abrasion potential. To overcome this limitation, we developed a new method for rapidly quantifying baseline bed material abrasion rate in the field via Schmidt Hammer Rock Strength (SHRS). We use this new method, along with measurements of gravel bar lithology, to quantify abrasion the Suiattle River, a basin in the North Cascades of Washington State dominated by a single coarse sediment source: 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 fails to reproduce observed downstream patterns in lithology and clast strength in the Suiattle, even when accounting for the heterogeneity of source material strength and systematic underestimate of abrasion rates by tumbler experiments. Incorporating transport-dependent abrasion into our model largely resolves this failure. While a simplified approach to characterizing abrasion is tempting, our findings suggest that sediment heterogeneity and transport-dependent abrasion are important controls on the downstream fate of coarse sediment in fluvial systems.