Critical Shields values in coarse‐bedded steep streams

Abstract

Critical Shields values ( τc*) suitable for specific applications are back‐calculated from representative bed load samples in mountain streams and a flow competence/critical flow approach. The general increase of τc50* (for the bed D50 size) as well as τc16* and τc84* (for the bed D16 and D84 sizes) with stream gradient Sx and also the stratification of τc50* by relative flow depth and relative roughness are confirmed. Critical Shields values τc16* are shown to exceed τc50* by about threefold, while those for τc84* are nearly half of τc50*. However, it remains unclear to what extent physical processes or numerical artifacts contribute to determining critical Shields values. Critical bankfull Shields values ( τcbf*) back‐computed from the average largest particles mobile at bankfull flow DBmax,bf approach τc16* at steep gradients and τc84* at low gradients and therefore increase very steeply with Sx. The relation τcbf*=f(Sx) is stratified by bed stability (D50/DBmax,bf) and predictable if bed stability can be field categorized. Noncritical Shields values ( τbf50*) computed from bankfull flow depth and the D50 size differ from τc50* and τcbf*. Only in bankfull mobile streams where D50/DBmax = 1 can τ*cbf, τc50*, and τbf50* be used interchangeably. In highly mobile streams, substituting τcbf* by τbf50* overpredicts the DBmax,bf size by up to fivefold and underpredicts DBmax,bf by the same amount in highly stable streams. A value of 0.03 is appropriate for τcbf* only on low stability beds with Sx ≅ 0.01, but overpredicts DBmax,bf by 30‐fold on highly stable beds with Sx ≅ 0.1. Differences in field and computational methods also affect critical Shields values.

Gravel threshold of motion: a state function of sediment transport disequilibrium?

“…The τ * c(q s ) model is consistent with previous interpretations that high sediment supply corresponds to low thresholds of motion, and vice-versa (Recking, 2012;Bunte et al, 2013). In the τ * c(q s ) model (Eq.…”

“…Second, because the best estimate of a given variable is usually its average, there is a tendency to attribute variability to measurement noise and uncertainty, even when that variability may be real, understandable, and important to system dynamics (Jerolmack, 2011;Buffington and Montgomery, 1997;Chen and Stone, 2008). Third, a broadly applicable model for the temporal evolution of τ * c has arguably not been developed, although progress has been made (Recking, 2012;Bunte et al, 2013;Wilcock and Crowe, 2003). Next in this section, I summarize previous work on τ * c controls, suggest ways that evolving τ * c may influence gravel-bed river morphodynamics, and then propose specific hypotheses to be explored with a new model for τ * c evolution.…”

“…Recent work also suggests that sediment transport can affect τ * c , with higher rates of upstream supply corresponding to more mobile sediment and lower τ * c (Recking, 2012;Bunte et al, 2013). If transport rate influences τ * c then there must be feedback because, by definition, τ * c influences transport rate (Eq.…”

“…Mechanistically, mobile grains impacting stationary grains have been shown to dislodge and entrain grains into the flow (Ancey et al, 2008). Bunte et al (2013) interpreted that lower τ * c corresponded to looser beds caused by higher rates of sediment supply from upstream, and noted that the stability of bed particles can be qualitatively assessed in the field while doing pebble counts. Yager et al (2012b) demonstrated that in-channel sediment availability varied inversely with the degree of boulder protrusion, indicating preferential filling of topographic lows by mobile sediment.…”

“…Form drag corrections have been incorporated into many transport models to enable reasonable transport rates to be calculated using τ * c values typical of systems without form drag (e.g., Rickenmann and Recking, 2011;David et al, 2011;Yager et al, 2012a). Conversely, another common approach (and that taken here) is simply to use higher τ * c (e.g., Bunte et al, 2013;Lenzi et al, 2006), treating τ * c as a physically meaningful fitting parameter to predict transport. Using field data, Schneider et al (2015) recently compared gravel transport predictions based on (a) form drag corrections and (b) higher reference stresses.…”