Form drag in rivers due to small‐scale natural topographic features: 1. Regular sequences

Kean, Jason W.; Smith, J. Dungan

doi:10.1029/2006jf000467

Cited by 72 publications

(171 citation statements)

References 20 publications

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“…To test the threshold-limiting idea indirectly, we consider the relative mobility of bed and bank materials as a function of grain size. We do not consider vegetation explicitly; however, we note that the reported range for erosion thresholds in vegetated bank materials is comparable to that of mud-sand mixtures (Kean and Smith (2006). It is important to point out that Shields 30 stress is not the relevant parameter for cohesive materials, where particle weight does not adequately describe resistance to motion.…”

mentioning

confidence: 99%

“…We acknowledge, however, that other factors unaccounted for in our simple analysis must also play a role. For example, form drag due to roughness on multiple scales (grains, bed forms, bars, meanders ) 5 can drastically change the effective bed stress (Kean and Smith (2006)). We suspect that proper accounting of flow resistance would reveal a stronger signal of near-threshold organization.…”

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confidence: 99%

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Evidence of, and a Proposed Explanation for, Bimodal Transport States in Alluvial Rivers

Dunne¹,

Jerolmack²

2017

Preprint

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Abstract. Gravel-bedded rivers organize their bankfull channel geometry and grain size such that shear stress is close to the threshold of motion. Sand-bedded rivers on the other hand typically maintain bankfull fluid stresses far in excess of threshold, a condition for which there is no satisfactory understanding. A fundamental question arises: Are bed-load (gravel-bedded) and suspension (sand-bedded) rivers two distinct equilibrium states, or do alluvial rivers exhibit a continuum of transport regimes as some have recently suggested? We address this question in two ways: (1) re-analysis of global channel geometry datasets, 5 with consideration of the dependence of critical shear stress upon site-specific characteristics (e.g. slope and grain size); and (2) examination of a longitudinal river profile as it transits from gravel to sand-bedded. Data reveal that the transport state of alluvial river-bed sediments is bimodal, showing either near-threshold or suspension conditions, and that these regimes correspond to the respective bimodal peaks of gravel and sand that comprise natural river-bed sediments. Sand readily forms near-threshold channels in the laboratory and some field settings, however, indicating that another factor, such as bank cohesion, must be 10 responsible for maintaining suspension channels. We hypothesize that alluvial rivers adjust their geometry to the thresholdlimiting bed and bank material -which for gravel-bedded rivers is gravel, but for sand-bedded rivers is mud (if present) -and present tentative evidence for this idea.

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confidence: 99%

mentioning

confidence: 99%

Evidence of, and a Proposed Explanation for, Bimodal Transport States in Alluvial Rivers

Dunne¹,

Jerolmack²

2017

Preprint

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show abstract

“…We acknowledge, however, that other factors unaccounted for in our simple analysis must also play a role. For example, form drag due to roughness on multiple scales (grains, bed forms, bars, meanders) can drastically change the effective bed stress (Kean and Smith, 2006). We suspect that properly accounting for flow resistance would reveal a stronger signal of near-threshold organization.…”

Section: Discussionmentioning

confidence: 99%

Evidence Of, and a Proposed Explanation For, Bimodal Transport States in Alluvial Rivers

Dunne

Jerolmack

2017

Geological Society of America Abstracts With Programs

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Abstract. Gravel-bedded rivers organize their bank-full channel geometry and grain size such that shear stress is close to the threshold of motion. Sand-bedded rivers, on the other hand, typically maintain bank-full fluid stresses far in excess of threshold, a condition for which there is no satisfactory understanding. A fundamental question arises: are bed-load (gravel-bedded) and suspension (sand-bedded) rivers two distinct equilibrium states, or do alluvial rivers exhibit a continuum of transport regimes as some have recently suggested? We address this question in two ways: (1) reanalysis of global channel geometry datasets, with consideration of the dependence of critical shear stress upon site-specific characteristics (e.g., slope and grain size); and (2) examination of a longitudinal river profile as it transits from gravel to sand bedded. Data reveal that the transport state of alluvial riverbed sediments is bimodal, showing either near-threshold or suspension conditions, and that these regimes correspond to the respective bimodal peaks of gravel and sand that comprise natural riverbed sediments. Sand readily forms near-threshold channels in the laboratory and some field settings, however, indicating that another factor, such as bank cohesion, must be responsible for maintaining suspension channels. We hypothesize that alluvial rivers adjust their geometry to the threshold-limiting bed and bank material, which for gravel-bedded rivers is gravel but for sand-bedded rivers is mud (if present), and present tentative evidence for this idea.

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“…Other authors proposed empirical expressions of the skin shear stress τ s as partition of the total shear stress τ b [4,10,11,16,17]. Most of these expressions are formulated as a function of the form factor η/λ.…”

Section: Introductionmentioning

confidence: 98%

Extension of the skin shear stress Li’s relationship to the flat bed

Smaoui

Ouahsine

2011

Environ Fluid Mech

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A proper estimation of the skin shear stress τ s is necessary for a proper evaluation of sediment flux at the sediment-fluid interface. Several empirical formulas of the skin shear stress have been proposed in the literature for rippled bed as function of the factor form η/λ (η and λ are respectively the height and wavelength of the bedform). These formulas express that in the presence of bedform, τ s is a partition of the total shear stress τ b . In contrast, when the bottom is flat, τ s is exactly equal to τ b . Based on in situ measurements, Li (J Geophys Res 99:791-799, 1994) has proposed a new formula of τ s depending on u * /η (u * is the friction velocity, u * = √ τ b /ρ), but not as a function of η/λ. This formulation appears physically more realistic, but does not cover the flat bottom range. The purpose of this short note is therefore the extension of the Li's expression to this type of bottom.

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Form drag in rivers due to small‐scale natural topographic features: 1. Regular sequences

Cited by 72 publications

References 20 publications

Evidence of, and a Proposed Explanation for, Bimodal Transport States in Alluvial Rivers

Evidence of, and a Proposed Explanation for, Bimodal Transport States in Alluvial Rivers

Evidence Of, and a Proposed Explanation For, Bimodal Transport States in Alluvial Rivers

Extension of the skin shear stress Li’s relationship to the flat bed

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