[1] Steep, rough channels occupy a large fraction of the total channel length in mountainous regions. Most sediment mobilized on hillslopes must pass through these streams before reaching lower-gradient channels. Steep channels have wide grain size distributions that are composed of finer, more mobile sediment and large, rarely mobile grains. The large grains can bear a significant portion of the total shear stress and thereby reduce the stress available to move the finer sediment. Conventional bed load transport equations often overpredict the sediment flux in steep channels by several orders of magnitude. We hypothesize that sediment transport equations overpredict the sediment flux because they do not (1) account for the stress borne by rarely mobile grains, (2) differentiate between highly and rarely mobile sediment, and (3) account for the limited availability of mobile sediment. Here we modify a conventional bed load transport equation to include these three effects. We use measurements of the flow, bed properties, and sediment flux in a small, steep flume to test this equation. We supply gravel at a constant rate through fields of regularly spaced immobile spheres and measure the bed coverage by gravel and sphere protrusion (the percent of the sphere that protrudes above the gravel deposit). For a given sphere spacing, the proportion of the bed covered by gravel increases and the sphere protrusion decreases with greater sediment supply. Thus bed coverage and immobile grain protrusion may serve as proxies for sediment availability in steep, rough streams. Unlike most transport equations that we tested, our modified bed load equation predicts sediment fluxes to within an order of magnitude of the measured values. Our results demonstrate that accurately predicting bed load transport in steep, rough streams may require accounting for the effects of local sediment availability (coverage by mobile sediment) and drag due to rarely mobile particles.
Sediment transport in the Erlenbach, a small stream with step-pool morphology in the canton of Schwyz, Switzerland, has been monitored for more than 20 years. During this time three exceptional events (events with high sediment yield and long return times that have a large effect on channel morphology) have impacted the stream and partly or completely rearranged the existing step-pool morphology. In the aftermath of the events, sediment transport rates at a given discharge and total sediment yield remained elevated for about a year or longer. For the last event, dated on the 20 June 2007, observations of boulder mobility and step destruction were used to interpret channel stability. Boulders with median diameters of up to 135 cm and estimated weights of more than 2·5 tons have moved during the 2007 event. Using hydraulic observations and shear stress calculations boulders up to 65 cm in diameter were predicted to have been fully mobile in peak conditions, even if form resistance and increased critical stresses needed for the initiation of motion in steep streams were taken into account. For two of the events, estimated peak shear stresses at the bed exceeded 1000 Pa, calculated both from observations of the flow hydraulics and from boulder mobility. This suggests that highly energetic flows occur relatively frequently in small, steep streams and that large boulders can be transported by fluvial processes in such streams. The observations have potential significance for hazard risk mitigation, stream engineering and restoration.
[1] Vegetation is ubiquitous in river channels and floodplains and alters mean flow conditions and turbulence. However, the effects of vegetation patches on near-bed turbulence, bed load transport rates, and sedimentation are not well understood. To elucidate the influence of emergent vegetation on local and patch-averaged bed load transport, we conducted a set of experiments in which we varied the mean flow velocity (U), total boundary shear stress (τ), or vegetation density between runs. We measured 2D velocity fields using Particle Imaging Velocimetry and bed load fluxes using high-speed video. Simulated rigid vegetation caused bed load fluxes to vary spatially by an order of magnitude, causing distinct scour zones adjacent to, and depositional bed forms between stems. These local patterns of sedimentation could impact recruitment and survival of other plants. Large bed load fluxes were collocated with high near-bed turbulence intensities that were three to four times larger than spatially averaged values. Higher vegetation densities increased the importance of inward and outward interactions, particularly downstream of vegetation. At the patch scale, greater stem densities caused either an increase or decrease in run-averaged bed load fluxes, depending on whether U or τ was held constant between runs. This implies that sedimentation in vegetation patches is not only a function of bed grain size, sediment supply, and vegetation density and species, but whether vegetation significantly impacts mean and local flow properties, which could depend on vegetation location. Commonly used bed load transport equations did not accurately predict average sediment fluxes in our experiments unless they accounted for the spatial variability in the near-bed Reynolds stress.
[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.
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