Prediction of sediment transport in step‐pool channels

Abstract

[1] In mountainous drainage networks, sediment mobilized on hillslopes must first pass through steep streams before reaching lower-gradient channels. The bed of steep channels is typically composed of large, relatively immobile boulders and finer, more mobile gravel. Most sediment transport equations overpredict sediment flux in steep streams by several orders of magnitude because they do not account for the stress borne by immobile grains and the limited availability of the more mobile sediment. We previously developed and tested (in flume experiments) a sediment transport equation that accounts for these two effects. Here we modify the Parker (1990) bed load equation to include the resistance borne by steps and selective transport of the relatively mobile sediment using a range of hiding functions. We test a number of resistance equations and hiding functions, combined with our modified and the original Parker equations, against measured flow and sediment transport in three steep channels. Our modified sediment transport equation generally predicts the transported sediment volumes to within an order of magnitude of the measured values, whereas the unmodified equations do not. The most accurate sediment flux predictions were obtained from using our modified equation, combined with a hiding function that calculates highly selective transport of the relatively mobile sediment. Furthermore, this hiding function has a critical Shields stress that is similar to those reported for lower gradient channels. The effects of the immobile steps on flow and sediment transport are not adequately captured by simply increasing the critical Shields stress to values reported in steep streams.

Period and amplitude of bedload pulses in a macro-rough channel

“…The form drag caused by boulders increases with the number of boulders, resulting in lower shear stresses available at the bed for sediment entrainment (Bathurst, 1978;Lenzi et al, 2006;Yager et al, 2007Yager et al, , 2012. Hence, the presence of boulders decreases the sediment transport capacity (Yager et al, 2007(Yager et al, , 2012; Ghilardi and Schleiss, 2012). The effect of boulders on flow conditions (i.e., bed shear stress and bed resistance equations) and thus on sediment transport capacity can be accounted for via several morphological parameters, such as their protrusion, their cross section, the bed surface area occupied by them, the distance between boulders and a drag coefficient (Bathurst, 1978;Canovaro et al, 2007;Yager et al, 2007;Pagliara et al, 2008;Yager et al, 2012).…”

“…Boulders that act as macro-roughness elements endure a significant part of the total stress and disrupt the flow by altering the channel roughness (Wohl, 2000;Yager et al, 2007;David et al, 2011). The form drag caused by boulders increases with the number of boulders, resulting in lower shear stresses available at the bed for sediment entrainment (Bathurst, 1978;Lenzi et al, 2006;Yager et al, 2007Yager et al, , 2012. Hence, the presence of boulders decreases the sediment transport capacity (Yager et al, 2007(Yager et al, , 2012; Ghilardi and Schleiss, 2012).…”

“…Hence, the presence of boulders decreases the sediment transport capacity (Yager et al, 2007(Yager et al, , 2012; Ghilardi and Schleiss, 2012). The effect of boulders on flow conditions (i.e., bed shear stress and bed resistance equations) and thus on sediment transport capacity can be accounted for via several morphological parameters, such as their protrusion, their cross section, the bed surface area occupied by them, the distance between boulders and a drag coefficient (Bathurst, 1978;Canovaro et al, 2007;Yager et al, 2007;Pagliara et al, 2008;Yager et al, 2012). The presence of hydraulic jumps downstream of steps or boulders is another parameter clearly linked to the channel-bed morphology, as described by several authors Wilcox and Wohl, 2007;Endreny et al, 2011;Nitsche et al, 2011).…”

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

“…Although not a state variable, form drag is physically justifiable because larger clasts that protrude higher into the flow (e.g., stable boulders) tend to account for a disproportionate amount of the total stress through drag, turbulence generation and pressure gradients. 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.…”

sedFlow – a tool for simulating fractional bedload transport and longitudinal profile evolution in mountain streams

“…bends and irregular channel width), may be summarised as macro-roughness, which reduces the energy available for the transport of sediment. If macro-roughness is not accounted for in steep channels, bedload transport capacity may be greatly overestimated (Rickenmann, 2001Yager et al, 2007;Badoux and Rickenmann, 2008;Chiari and Rickenmann, 2011;Nitsche et al, 2011Nitsche et al, , 2012Yager et al, 2012). To correct for macroroughness, Nitsche et al (2011) suggested the use of a reduced energy slope, which represents a fraction of the real gradient, and which is based on a flow-resistance partitioning approach of Rickenmann and Recking (2011) and Nitsche et al (2011).…”