Empirical assessment of theory for bankfull characteristics of alluvial channels

Abstract

We compiled a data set of 541 bankfull measurements of alluvial rivers (see supporting information) and used Bayesian linear regression to examine empirical and theoretical support for the hypothesis that alluvial channels adjust to a predictable condition of basal shear stress as a function of sediment transport mode. An empirical closure based on channel slope, bankfull channel depth, and median grain size is proposed and results in the scaling of bankfull Shields stress with the inverse square root of particle Reynolds number. The empirical relationship is sufficient for purposes of quantifying paleohydraulic conditions in ancient alluvial channels. However, it is not currently appropriate for application to alluvial channels on extraterrestrial bodies because it depends on constant-valued, Earth-based coefficients.

Evidence of, and a Proposed Explanation for, Bimodal Transport States in Alluvial Rivers

“…−1 * , and a similar result was found by Trampush et al (2014). Li et al (2015) concluded that the notion of a constant formative Shields stress for either gravel-or sand-bedded channels was not supported by the data.…”

“…slope, or grain size (Lamb et al (2008); van Rijn (2016))) is explicitly accounted for. Moreover, while previous data compilations found that bankfull Shields stress increases systematically with decreasing grain size (Li et al (2015); Trampush et al (2014)), one may readily find data that contradict this trend. Channels formed by seepage erosion in sand (Devauchelle et al (2011)) are observed to transport sand as bedload and, like gravel bedload rivers, cluster approximately at the threshold of motion.…”

“…The large, global datasets utilized in this paper are identical those used by Trampush et al (2014) and Li et al (2015). They were subsequently combined with a longitudinal profile from the Sacramento River (Singer (2010)), river channel cross sections on the Kosi Megafan (Gaurav et al (2015)), and channels formed by seepage erosion in the Apalachicola ravines in Florida 20 (Devauchelle et al (2011)).…”

“…Examination of gravel-sand transitions along downstream river profiles indicates that the mode of bed-material transport may switch abruptly from near-threshold (gravel-bedded) to suspension (sand-bedded) (Miller et al (2014); Venditti et al (2015); Venditti et al (2010);Singer (2010);Singer (2008)), and hydraulic considerations 15 have suggested that susceptibility to suspension increases rapidly as grain size decreases across the gravel to sand range (Lamb and Venditti (2016)). On the other hand, recent compilations of global data sets have been used to suggest that rivers exhibit a continuum of transport states -from near threshold through to full suspension -and that bankfull Shields stress varies smoothly with grain size, slope and particle Reynolds number (Parker et al (2007); Wilkerson and Parker (2010); Li et al (2015); Trampush et al (2014)). Importantly, this new presentation of the data suggests that there is no range in phase space 20 where rivers cluster near the ground state of a constant threshold Shields stress (Fig.…”

“…We address these questions by re-analysis of existing data. We revisit the global data compilations of Li et al (2015) and Trampush et al (2014), and argue that natural rivers appear to exhibit bi-modal transport states corresponding…”