Determination of the critical conditions of incipient notion of bed load in mountain rivers

Bartnik, W.

doi:10.1007/bfb0011184

Cited by 5 publications

(5 citation statements)

References 1 publication

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“…The critical Shields stress in the Erlenbach hiding function (

τ_{rsgo}^{*}

= 0.07) is within the upper range of values reported for lower‐gradient streams [e.g., Buffington and Montgomery , 1997].

τ_{rsgo}^{*}

generally increases with lower relative submergences and higher channel slopes [e.g., Ashida and Bayazit , 1973; Aguirre‐Pe , 1975; Graf and Suszka , 1987; Aguirre‐Pe and Fuentes , 1991; Bartnick , 1991; Shvidchenko and Pender , 2000; Shvidchenko et al , 2001; Mueller et al , 2005; Lenzi et al , 2006; Lamb et al , 2008; Mao et al , 2008]. The dimensionless critical stress is often higher in steep streams than in lower‐gradient channels because roughness elements bear a significant fraction of the total stress [e.g., Buffington and Montgomery , 1997] and/or low relative flow depths reduce the near‐bed turbulence intensities [e.g., Lamb et al , 2008].…”

Section: Discussionsupporting

confidence: 59%

Prediction of sediment transport in step‐pool channels

Yager

Dietrich

Kirchner

et al. 2012

Water Resources Research

123

158

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[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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“…The critical Shields stress in the Erlenbach hiding function (

τ_{rsgo}^{*}

= 0.07) is within the upper range of values reported for lower‐gradient streams [e.g., Buffington and Montgomery , 1997].

τ_{rsgo}^{*}

Section: Discussionsupporting

confidence: 59%

Prediction of sediment transport in step‐pool channels

Yager

Dietrich

Kirchner

et al. 2012

Water Resources Research

123

158

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“…Neill later retracted his results and stated that criticism from colleagues caused him to re-examine his data, which revealed measurement bias [Neill, 1968]. The original slope-dependent findings of Neill, however, have been reproduced subsequently for steep slopes in experimental [Ashida and Bayazit, 1973;Aguirre-Pe, 1975;Bathurst et al, 1984;Olivero, 1984;Graf and Suszka, 1987;Torri and Poesen, 1988;Aguirre-Pe and Fuentes, 1991;Picon, 1991] and field studies [Bartnick, 1991;Lenzi et al, 2006]. Detailed experiments by Shvidchenko and Pender [2000] and indicate that incipient motion is slope dependent even on low slopes (S < 0.01) and for small particles (Re p < 10 2 ), which suggests that a slope-dependent Shields stress is applicable for lowland rivers as well as steep mountain streams.…”

Section: Introductionmentioning

confidence: 98%

Is the critical Shields stress for incipient sediment motion dependent on channel‐bed slope?

Lamb¹,

Dietrich²,

Venditti³

2008

J. Geophys. Res.

442

728

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[1] Data from laboratory flumes and natural streams show that the critical Shields stress for initial sediment motion increases with channel slope, which indicates that particles of the same size are more stable on steeper slopes. This observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force. Processes that might explain this discrepancy are explored using a simple force-balance model, including increased drag from channel walls and bed morphology, variable friction angles, grain emergence, flow aeration, and changes to the local flow velocity and turbulent fluctuations. Surprisingly, increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical Shields stress are similar for flume experiments and natural streams, and significant variations in bed morphology in flumes is unlikely. Instead, grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed-roughness scale to flow depth (i.e., relative roughness). A model for the local velocity within the grain-roughness layer is proposed based on a 1-D eddy viscosity with wake mixing. In addition, the magnitude of near-bed turbulent fluctuations is shown to depend on the depth-averaged flow velocity and the relative roughness. Extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes.

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“…Researchers typically report sediment transport rates as a function of total bed shear stress ( τ b ) normalized by grain diameter ( D ), termed Shields stress:

τ^{*} = \frac{τ_{b}}{((), ρ_{s} - ρ) g D}

, where g is the acceleration due to gravity, ρ is the material density of water, and ρ s is the material density of sediment. Measurements at the onset of sediment motion reveal that grains require increased Shields stress to move in steeper channels in both the field [ Bartnick , ; Andrews , , ; Church and Hassan , ; Mueller et al ., ; Lenzi et al ., ; Whitaker and Potts , ; Mao et al ., ; Scheingross et al ., ] and laboratory flumes [ Ashida and Bayazit , ; Bathurst et al ., ; Olivero , ; Graf and Suszka , ; Torri and Poesen , ; Picon , ; Shvidchenko and Pender , ; Gregoretti , ; Prancevic et al ., ] (Figure ). In addition, the threshold Shields stress required to mobilize material in the field is higher relative to flume experiments for a given bed slope (Figure ).…”

Section: Introductionmentioning

confidence: 99%

“…These relationships were initially developed for application to lowland rivers [ Shields , ; Meyer‐Peter and Mueller , ; Miller and Byrne , ] and are inadequate in describing sediment transport in steep, coarse‐bedded environments, where bed load transport rates are typically an order of magnitude smaller than predicted [e.g., Rickenmann , ; Yager et al ., ; Mueller et al ., ]. Even the onset of sediment transport in steep channels requires significantly higher bed shear stresses than expected [ Bartnick , ; Mueller et al ., ; Lenzi et al ., ; Scheingross et al ., ]. There is debate as to whether the divergence between predictions and observations in steep channels is due to changes in granular stability, changes in flow hydraulics, or both [e.g., Church et al ., ; Yager et al ., ; Lamb et al ., ; Recking , ; Zimmermann et al ., ; Ferguson , ].…”

Section: Introductionmentioning

confidence: 99%

Particle friction angles in steep mountain channels

Prancevic

Lamb

2015

JGR Earth Surface

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Sediment transport rates in steep mountain channels are typically an order of magnitude lower than predicted by models developed for lowland rivers. One hypothesis for this observation is that particles are more stable in mountain channels due to particle-particle interlocking or bridging across the channel width. This hypothesis has yet to be tested, however, because we lack direct measurements of particle friction angles in steep mountain channels. Here we address this data gap by directly measuring the minimum force required to dislodge sediment (pebbles to boulders) and the sediment weight in mountain channels using a handheld force gauge. At eight sites in California, with reach-averaged bed angles ranging from 0.5°to 23°a nd channel widths ranging from 2 m to 16 m, we show that friction angles in natural streams average 68°a nd are 16°larger than those typically measured in laboratory experiments, which is likely due to particle interlocking and burial. Results also show that larger grains are disproportionately more stable than predicted by existing models and that grains organized into steps are twice as stable as grains outside of steps. However, the mean particle friction angle does not vary systematically with bed slope. These results do not support systematic increases in friction angle in steeper and narrower channels to explain the observed low sediment transport rates in mountain channels. Instead, the spatial pattern and grain-size dependence of particle friction angles may indirectly lower transport rates in steep, narrow channels by stabilizing large clasts and channel-spanning steps, which act as momentum sinks due to form drag.

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Determination of the critical conditions of incipient notion of bed load in mountain rivers

Cited by 5 publications

References 1 publication

Prediction of sediment transport in step‐pool channels

Prediction of sediment transport in step‐pool channels

Is the critical Shields stress for incipient sediment motion dependent on channel‐bed slope?

Particle friction angles in steep mountain channels

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