Breaking from the average: Why large grains matter in gravel‐bed streams

MacKenzie, Lucy G.; Eaton, Brett C.; Church, Michael

doi:10.1002/esp.4465

Cited by 75 publications

(75 citation statements)

References 70 publications

(153 reference statements)

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“…The relationship between bedload transport rate and excess shear stress used herein can be described by a power law similar with similar exponent values to that used by previous authors (Parker, 1990;Wilcock and Crowe, 2003;Recking, 2010;Piedra and Haynes, 2011). Furthermore, given changes in bed stability in response to interflood flow duration are grade sensitive our results indicate the not only is predicting entrainment based on a single critical Shield's value inaccurate but also that the D 50 may not be the best grain fraction from which to estimate entrainment thresholds (MacKenzie et al, 2018). Furthermore, given changes in bed stability in response to interflood flow duration are grade sensitive our results indicate the not only is predicting entrainment based on a single critical Shield's value inaccurate but also that the D 50 may not be the best grain fraction from which to estimate entrainment thresholds (MacKenzie et al, 2018).…”

Section: Effect Of Inter-flood Duration On Bed Stabilitysupporting

confidence: 80%

The impact of inter‐flood duration on non‐cohesive sediment bed stability

Ockelford

Woodcock

Haynes

2019

Earth Surf Processes Landf

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Limited field and flume data suggests that both uniform and graded beds appear to progressively stabilize when subjected to inter‐flood flows as characterized by the absence of active bedload transport. Previous work has shown that the degree of bed stabilization scales with duration of inter‐flood flow, however, the sensitivity of this response to bed surface grain size distribution has not been explored. This article presents the first detailed comparison of the dependence of graded bed stability on inter‐flood flow duration. Sixty discrete experiments, including repetitions, were undertaken using three grain size distributions of identical D50 (4.8 mm); near‐uniform (σg = 1.13), unimodal (σg = 1.63) and bimodal (σg = 2.08). Each bed was conditioned for between 0 (benchmark) and 960 minutes by an antecedent shear stress below the entrainment threshold of the bed (τ*c50). The degree of bed stabilization was determined by measuring changes to critical entrainment thresholds and bedload flux characteristics. Results show that (i) increasing inter‐flood duration from 0 to 960 minutes increases the average threshold shear stress of the D50 by up to 18%; (ii) bedload transport rates were reduced by up to 90% as inter‐flood duration increased from 0 to 960 minutes; (iii) the rate of response to changes in inter‐flood duration in both critical shear stress and bedload transport rate is non‐linear and is inversely proportional to antecedent duration; (iv) there is a grade dependent response to changes in critical shear stress where the magnitude of response in uniform beds is up to twice that of the graded beds; and (v) there is a grade dependent response to changes in bedload transport rate where the bimodal bed is most responsive in terms of the magnitude of change. These advances underpin the development of more accurate predictions of both entrainment thresholds and bedload flux timing and magnitude, as well as having implications for the management of environmental flow design. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

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Section: Effect Of Inter-flood Duration On Bed Stabilitysupporting

confidence: 80%

The impact of inter‐flood duration on non‐cohesive sediment bed stability

Ockelford

Woodcock

Haynes

2019

Earth Surf Processes Landf

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“…We have offered an explanation for this remarkable finding based on hiding effects and sustained particle rebounds in heterogeneous sand beds (section 5.2). Interestingly, a predominant effect of relatively very coarse particles on the mobility of the bed was previously reported also for water-driven sediment transport (MacKenzie & Eaton, 2017), which led MacKenzie et al (2018) to challenge the long-standing assumption that d 50 is the best choice for the characteristic size of bed particles. Our study challenges this assumption also for wind-driven sediment transport.…”

Section: Discussionmentioning

confidence: 93%

Large Effects of Particle Size Heterogeneity on Dynamic Saltation Threshold

et al. 2019

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Reliably predicting the geomorphology and climate of planetary bodies requires knowledge of the dynamic threshold wind shear velocity below which saltation transport ceases. Here we measure this threshold in a wind tunnel for four well‐sorted and two poorly sorted sand beds by visual means and by a method that exploits a regime shift in the behavior of the surface roughness caused by momentum transfer from the wind to the saltating particles. For our poorly sorted sands, we find that these measurement methods yield different threshold values because, at the smaller visual threshold, relatively coarse particles do not participate in saltation. We further find that both methods yield threshold values that are much larger (60–250%) for our poorly sorted sands than for our well‐sorted sands with similar median particle diameter. In particular, even a rescaling of the dynamic saltation threshold based on the 90th percentile particle diameter rather than the median diameter cannot fully capture this difference, suggesting that relatively very coarse particles have a considerable control on the dynamic threshold. Similar findings were previously reported for water‐driven sediment transport. Our findings have important implications for quantitative predictions of saltation transport‐related geophysical processes, such as dust aerosol emission.

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“…In addition, since the critical Shields parameter is computed here for D 84 , Shields stresses and thus transport stage are lower than they might be if using D 50 , for instance. As a consequence, there is still sediment transport and possible morphological adjustments for

τ^{*} false/ τ_{cr}^{*}

<1, although of lower intensity (MacKenzie et al, ). It has now been established that bedload transport of poorly sorted sediment does not show a sharp transition between static and mobile conditions, but rather a pseudocyclic and progressive shift toward partial and finally full mobility (Bacchi et al, ; Wilcock & McArdell, ).…”

Section: Resultsmentioning

confidence: 99%

Steep Bedload‐Laden Flows: Near Critical?

Piton

Recking

2019

JGR Earth Surface

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Steep gravel‐bed rivers sometimes experience floods that dramatically rework river bed structure and topography. Hazard assessments and paleo‐event reconstructions require better knowledge of such phenomena. This paper explores morphodynamic evolution of steep channels carrying bedload‐laden flows, using a generic Froude‐scaled model. Bedload‐laden floods were introduced in a narrow flume and spread into a 5 times wider unconfined area with a 0.1 steep slope (m/m). Image analysis enabled measurements taken at an unprecedented level of accuracy on unconfined flows laden with bedload. A flow reconstruction procedure was used to compute depth, Froude (Fr) and Shields (τ*) numbers on millions of pixels based on a friction law and measurements of surface velocity, slope, and roughness. Despite the steep slope, Froude numbers proved to be mostly subcritical in all but the regions experiencing the most active sediment transport. Competent flows, identified by the transport stage higher than unity ( τ*false/τcr*>1), were near critical and seldom had Fr> 1.3–1.5. This demonstrates that, providing that bed width and structure can adjust, hydraulic features such as standing waves, hydraulic jumps, and lateral shock waves dissipate energy very efficiently in addition to adjusting channel features. These competent flows also tend to rework channels to approach the energy minimum of near‐critical flows. This hypothesis was postulated by Gordon Grant (1997, https://doi.org/10.1029/96WR03134, Wat. Resour. Res. 33(2):349‐358) but demonstrated here for the first time at this scale. Considering near‐critical flows during discharges high enough to be clearly competent in laterally unconfined reaches seems reasonable as a first approximation in steep channels.

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Breaking from the average: Why large grains matter in gravel‐bed streams

Cited by 75 publications

References 70 publications

The impact of inter‐flood duration on non‐cohesive sediment bed stability

The impact of inter‐flood duration on non‐cohesive sediment bed stability

Large Effects of Particle Size Heterogeneity on Dynamic Saltation Threshold

Steep Bedload‐Laden Flows: Near Critical?

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