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.
Highlights
It has been proposed that the transport of coarse-grained material in mountainous streams occurs when flow strength – or bed shear stress – exceeds a grain-size-specific critical shear stress (Miller et al, 1977; Tucker and Slingerland, 1997; Church, 2006)
In the Swiss Alps, the critical shear stress values τc for entraining the D84 grain size range from ca. 20 Pa (Emme river) to ca. 90 Pa (Rhein and Simme rivers)
The shear stress values related to the mean annual water discharge Qmean range from ca. 15 to 100 Pa in the Alps and from 20 to >400 Pa in the Andes
Summary
It has been proposed that the transport of coarse-grained material in mountainous streams occurs when flow strength – or bed shear stress – exceeds a grain-size-specific critical shear stress (Miller et al, 1977; Tucker and Slingerland, 1997; Church, 2006) This has been documented based on flume experiments (e.g., Meyer-Peter and Müller, 1948; Dietrich et al, 1989; Carling et al, 1992; Ferguson, 2012; Powell et al, 2016) and field observations (e.g., Paola and Mohring, 1996; Lenzi et al, 2006; Mueller et al, 2005; Lamb et al, 2008), and related concepts have been employed in theoretical models (Paola et al, 1992; Tucker and Slingerland, 1997). We considered the mobilization of the D84 grain size as a priori condition – and
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