Short communication: Field data reveal that the transport probability of clasts in Peruvian and Swiss streams mainly depends on the sorting of the grains

Abstract

Abstract. We present field observations from coarse-grained streams in the Swiss Alps and the Peruvian Andes to explore the controls on the probability of material entrainment. We calculate shear stress that is expected for a mean annual water discharge and compare these estimates with grain-specific critical shear stresses that we use as thresholds. We find that the probability of material transport largely depends on the sorting of the bed material, expressed by the D96∕D50 ratio, and the reach gradient but not on mean annual discharge. The results of regression analyses additionally suggest that among these variables, the sorting exerts the largest control on the transport probability of grains. Furthermore, because the sorting is significantly correlated neither to reach gradient nor to water discharge, we propose that the granulometric composition of the material represents an independent, yet important control on the motion of clasts in coarse-grained streams.

Quantification of sediment fluxes and intermittencies from Oligo–Miocene megafan deposits in the Swiss Molasse basin

“…Moreover, the similarity between the $$$ \xi $$$ values across the three fans suggest that these are decoupled from the source lithology (Appendix I), the climatic and tectonic boundary conditions (Sections 2.2 and 5.2) and probably also the size of the catchments (Schlunegger et al., 1998; Spiegel et al., 2001). Our results suggest that alluvial fan sediments of the SMB have self‐similar grain size distributions and sorting properties (Schlunegger et al., 2020). Such characteristics were also documented for the coarse‐grained deposits of the Pobla basin adjacent to the Spanish Pyrenees and the Bermejo basin of Argentina (Whittaker et al., 2011 and Harries et al., 2019 respectively), implying similar underlying transport processes on each fan system.…”

Comparison of three grain size measuring methods applied to coarse-grained gravel deposits

“…Grain size distributions, percentiles values thereof and grain shapes are essential to quantify the dynamics and processes of sediment transport in rivers (e.g., Dade and Friend, 1998;Church, 2006;Petit et al, 2015). Grain size distributions additionally allow a classification of the sorting of a grain assemblage (e.g., Inman, 1952;Rice and Church, 2010;Schlunegger et al, 2020) and help to characterize the morphologies and bedforms of coarse-grained fluvial deposits (e.g., Lane, 1955;Brayshaw, 1984;Leopold, 1992;MacKenzie et al, 2018). Research in these fields has mainly focussed on material >2 mm, which is commonly referred to as the 'coarse-grained fraction' of the clastic material, and called gravel or conglomerate for unconsolidated or lithified material, respectively (Wentworth, 1922).…”

Automated grain sizing from uncrewed aerial vehicles imagery of a gravel‐bed river: Benchmarking of three object‐based methods

“…Another example is provided by the methods for predicting the threshold of incipient motion and transport rates (e.g., Hodge, Richards, & Brasington, 2007;Parker, Klingeman, & McLean, 1982;Recking, 2013Recking, , 2016. When studying bedload transport in mountain rivers-whose sediment deposits are often prone to grain sorting processes, such as bed armouring and sizedependent selective transport-accurate quantification of local grain size distributions is particularly important for assessing bedload transport mechanisms (e.g., Schlunegger, Delunel, & Garefalakis, 2020).…”