Advance, Retreat, and Halt of Abrupt Gravel‐Sand Transitions in Alluvial Rivers

Blom, Astrid; Chavarrías, Víctor; Ferguson, R.; Viparelli, E.

doi:10.1002/2017gl074231

Cited by 58 publications

(55 citation statements)

References 37 publications

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“…Yet if the controls change so slowly that the stream can keep pace with them, the stream continuously finds itself in a state of quasi‐equilibrium, in which the proposed formulations can be applied. This is illustrated by Blom et al [] for the case of an abrupt gravel‐sand transition.…”

Section: Discussionmentioning

confidence: 91%

The equilibrium alluvial river under variable flow and its channel‐forming discharge

et al. 2017

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When the water discharge, sediment supply, and base level vary around stable values, an alluvial river evolves toward a mean equilibrium or graded state with small fluctuations around this mean state (i.e., a dynamic or statistical equilibrium state). Here we present analytical relations describing the mean equilibrium geometry of an alluvial river under variable flow by linking channel slope, width, and bed surface texture. The solution holds in river normal flow zones (or outside both the hydrograph boundary layer and the backwater zone) and accounts for grain size selective transport and particle abrasion. We consider the variable flow rate as a series of continuously changing yet steady water discharges (here termed an alternating steady discharge). The analysis also provides a solution to the channel‐forming water discharge, which is here defined as the steady water discharge that, given the mean sediment supply, provides the same equilibrium channel slope as the natural long‐term hydrograph. The channel‐forming water discharge for the gravel load is larger than the one associated with the sand load. The analysis illustrates how the load is distributed over the range of water discharge in the river normal flow zone, which we term the “normal flow load distribution”. The fact that the distribution of the (imposed) sediment supply spatially adapts to this normal flow load distribution is the origin of the hydrograph boundary layer. The results quantify the findings by Wolman and Miller (1960) regarding the relevance of both magnitude and frequency of the flow rate with respect to channel geometry.

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Section: Discussionmentioning

confidence: 91%

The equilibrium alluvial river under variable flow and its channel‐forming discharge

et al. 2017

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“…The channel response timescale or system relaxation time reflects how fast a reach responds (with respect to channel slope, channel width, and the bed surface texture) to changes in the controls (Blom et al, ; De Vries, ; Howard, ). As the channel response timescale is generally long, there exists a subset of relatively short‐term control changes that do not appreciably affect the equilibrium state.…”

Section: Introductionmentioning

confidence: 99%

“…This implies that short‐term periodicity in controls produces negligible to limited associated channel response (e.g., Chatanantavet & Lamb, ; Howard, ; Viparelli et al, ). On the contrary, if the channel response timescale is much shorter than the timescale of changes of the controls (e.g., for slow control changes or in a small, highly mobile reach), channel geometry keeps pace with the changing controls (i.e., a quasi‐equilibrium state; Figure c; Blom, Chavarrías, et al, ; Howard, ).…”

Section: Introductionmentioning

confidence: 99%

“…These models provide tools for rapid assessment of the quasi‐equilibrium river geometry and the long‐term river response to natural changes in controls and river training works (e.g., De Vriend, ). A limitation of these models is that they typically only apply to quasi‐normal flow segments (e.g., Blom et al, ; Blom, Arkesteijn, et al, ; Blom, Chavarrías, et al, ; De Vries, ) or conditions with a spatially variable channel width and a steady water discharge (Bolla Pittaluga et al, ; Li et al, ). As such, these models cannot be applied to backwater reaches, for example, upstream of a river mouth.…”

Section: Introductionmentioning

confidence: 99%

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The Quasi‐Equilibrium Longitudinal Profile in Backwater Reaches of the Engineered Alluvial River: A Space‐Marching Method

et al. 2019

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An engineered alluvial river (i.e., a fixed‐width channel) has constrained planform but is free to adjust channel slope and bed surface texture. These features are subject to controls: the hydrograph, sediment flux, and downstream base level. If the controls are sustained (or change slowly relative to the timescale of channel response), the channel ultimately achieves an equilibrium (or quasi‐equilibrium) state. For brevity, we use the term “quasi‐equilibrium” as a shorthand for both states. This quasi‐equilibrium state is characterized by quasi‐static and dynamic components, which define the characteristic timescale at which the dynamics of bed level average out. Although analytical models of quasi‐equilibrium channel geometry in quasi‐normal flow segments exist, rapid methods for determining the quasi‐equilibrium geometry in backwater‐dominated segments are still lacking. We show that, irrespective of its dynamics, the bed slope of a backwater or quasi‐normal flow segment can be approximated as quasi‐static (i.e., the static slope approximation). This approximation enables us to derive a rapid numerical space‐marching solution of the quasi‐static component for quasi‐equilibrium channel geometry in both backwater and quasi‐normal flow segments. A space‐marching method means that the solution is found by stepping through space without the necessity of computing the transient phase. An additional numerical time stepping model describes the dynamic component of the quasi‐equilibrium channel geometry. Tests of the two models against a backwater‐Exner model confirm their validity. Our analysis validates previous studies in showing that the flow duration curve determines the quasi‐static equilibrium profile, whereas the flow rate sequence governs the dynamic fluctuations.

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“…Examination of gravel-sand transitions along downstream river profiles indicates that the mode of bed-material transport may switch abruptly from near threshold (gravel bedded) to suspension (sand bedded) (Miller et al, 2014;Venditti et al, 2015Venditti et al, , 2010Singer, 2010Singer, , 2008Blom et al, 2017), and hydraulic considerations have suggested that susceptibility to suspension increases rapidly as grain size decreases across the gravel to sand range (Lamb and Venditti, 2016). On the other hand, recent compilations of global datasets have been used to suggest that rivers exhibit a continuum of transport states -from near threshold through to full suspension -and that bank-full Shields stress varies smoothly with grain size, slope, and particle Reynolds number (Parker et al, 2007;Wilkerson and Parker, 2010;Li et al, 2015;Trampush et al, 2014)).…”

Section: Introductionmentioning

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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Advance, Retreat, and Halt of Abrupt Gravel‐Sand Transitions in Alluvial Rivers

Cited by 58 publications

References 37 publications

The equilibrium alluvial river under variable flow and its channel‐forming discharge

The equilibrium alluvial river under variable flow and its channel‐forming discharge

The Quasi‐Equilibrium Longitudinal Profile in Backwater Reaches of the Engineered Alluvial River: A Space‐Marching Method

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

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