A flume experiment on the effect of constriction shape on the formation of forced pools

Thompson, Douglas; McCarrick, C. R.

doi:10.5194/hess-14-1321-2010

Cited by 42 publications

(14 citation statements)

References 33 publications

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“…By comparison, MacVicar and Roy () and Thompson and Fixler () show that log structures, which effectively narrow a channel by ≈0.40 and 0.50

\overset{w}{true}

, respectively, lead to pool development. Thompson and McCarrick () conducted flume experiments on pool development through obstruction‐driven processes like that proposed by Clifford (), and they set the obstruction length scale as 0.40

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of their experimental channel. Nelson et al () provide a systematic experimental evaluation of pool‐riffle development along a variable‐width setup, for which the narrowest channel location measured 0.60 w of the widest section.…”

Section: Introductionmentioning

confidence: 99%

Morphodynamics of a Width‐Variable Gravel Bed Stream: New Insights on Pool‐Riffle Formation From Physical Experiments

et al. 2018

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Field observations, experiments, and numerical simulations suggest that pool‐riffles along gravel bed mountain streams develop due to downstream variations of channel width. Where channels narrow, pools are observed, and at locations of widening, riffles occur. Based on previous work, we hypothesize that the bed profile is coupled to downstream width variations through momentum fluxes imparted to the channel surface, which scale with downstream changes of flow velocity. We address this hypothesis with flume experiments understood through scaling theory. Our experiments produce pool‐riffle like structures across average Shields stresses τ∗ that are a factor 1.5–2 above the threshold mobility condition of the experimental grain size distribution. Local topographic responses are coupled to channel width changes, which drive flows to accelerate or decelerate on average, for narrowing and widening, respectively. We develop theory which explains the topography‐width‐velocity coupling as a ratio of two reinforcing timescales. The first timescale captures the time necessary to do work to the channel bed. The second timescale characterizes the relative time magnitude of momentum transfer from the flowing fluid to the channel bed surface. Riffle‐like structures develop where the work and momentum timescales are relatively large, and pools form where the two timescales are relatively small. We show that this result helps to explain local channel bed slopes along pool‐riffles for five data sets representing experimental, numerical, and natural cases, which span 2 orders of magnitude of reach‐averaged slope. Additional model testing is warranted.

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“…By comparison, MacVicar and Roy () and Thompson and Fixler () show that log structures, which effectively narrow a channel by ≈0.40 and 0.50

\overset{w}{true}

\overset{w}{true}

Section: Introductionmentioning

confidence: 99%

Morphodynamics of a Width‐Variable Gravel Bed Stream: New Insights on Pool‐Riffle Formation From Physical Experiments

et al. 2018

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“…; MacWilliams et al. ; MacVicar and Roy, ; Thompson and McCarrick, ; MacVicar and Rennie, ; MacVicar and Best, ; Chartrand et al. ).…”

Section: Morphodynamic Evolution Metrics At the Scale Of A Channel Widthmentioning

confidence: 99%

“…The topographic filter ( Figure 1a) is modulated by feedbacks between the local bed topography and the flow, through volume conservation for the CV shown. Over surface areas of roughly w ′2 , spatial variations of bed topography and channel width regulate the average downstream flow velocity Ū x (Furbish et al 1998;Thompson et al 1999;MacWilliams et al 2006;MacVicar and Roy, 2007;Thompson and McCarrick, 2010;MacVicar and Rennie, 2012;MacVicar and Best, 2013;Chartrand et al 2018). Ū x , and in particular the downstream change in Ū x (i.e.…”

Section: T U Bmentioning

confidence: 99%

What controls the disequilibrium state of gravel‐bed rivers?

Chartrand

Jellinek

Hassan

et al. 2019

Earth Surf Processes Landf

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A river at equilibrium is described by a statistically‐stationary mean bed elevation profile that arises in response to steady supplies of relief, water and sediment. Outside of the profile shape, how is the equilibrium state of a river most reliably identified and rigorously defined? Motivated by a proposed link between equilibrium and physical processes, we use scaling theory to develop the dimensionless channel response number ξ=KUb/Up. ξ is a metric for the local disequilibrium state of gravel‐bed mountain streams, which reflects a balance between the rate of topographic adjustment Ub, and the rate of bed sediment texture adjustment Up. The coefficient K can take one of two forms depending on choice of length scale for topographic adjustment. We hypothesize that equilibrium occurs where and when ξ≈O(1), and consequently, disequilibrium is the more general state captured by conditions of ξ≉O(1). The rates Ub and Up are controlled by the mechanics of sediment deposition and entrainment at the local scale of the channel width. The extent to which either process regulates disequilibrium depends on the bed strength, which is set by the time‐varying grain size distribution and packing. We use flume experiments to understand ξ and find that in the limit ξ>>1, the time‐varying response of an experimental channel depends sensitively on the spatially‐averaged bed shear stress ratio τ/τref. When τ/τref≈1.5, Ub was the dominant control on disequilibrium. However, when τ/τref≈2.0, Up contributed more significantly to disequilibrium. These results suggest that after an upstream supply perturbation, the equilibrium timescale is governed by Up, which we show is consistent with expectations from linear damping theory. Our experimental test of ξ is promising, but inconclusive with respect to our hypothesis. This uncertainty can be readily addressed with numerical or additional physical experiments. © 2019 John Wiley & Sons, Ltd.

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“…An additional consideration for bedform spacing is exclusion length theory, which suggests that at a constriction or pool forming element there is a relatively constrained downstream distance where a non-pool bedform would likely not occur, generally on the order of 1-3 W b f based on statistical modeling and field observations [147][148][149][150][151]. Thompson (2001) suggests that using a fixed spacing can prohibit adjustment from environmental change [23].…”

Section: Pool-riffle Variationmentioning

confidence: 99%

Hydrogeomorphic Scaling and Ecohydraulics for Designing Rescaled Channel and Floodplain Geometry in Regulated Gravel–Cobble Bed Rivers for Pacific Salmon Habitat

Brown

2022

Water

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Societies are increasingly restoring and/or rehabilitating rivers below dams for keystone species such as salmon. A fundamental concept for rehabilitating river morphology below dams for salmon is that a rescaled version of the river corridor synchronized to the regulated flow regime can restore habitat quantity and quality. Downscaled and resized hydrographs have been shown to provide environmental benefits to fish communities including salmon as well as riparian vegetation communities. However, less research exists on how this can be achieved through the topographic rescaling of heavily modified and regulated river corridors. The goal of this paper is to review analytical methods to determine initial of size of rescaled channel and floodplain mesohabitat units in regulated gravel–cobble bed rivers for Pacific salmon (Oncorhynchus spp.) habitat using hydrogeomorphic scaling and ecohydraulics. Hydrogeomorphic flow scaling is the prediction of river morphology and geometry using empirical and analytical relationships. Ecohydraulic scaling refers to the use of ecohydrology, habitat suitability curves, and fish density relationships to determine the size of mesohabitat units for ecologically relevant flows. In practice, these are complimentary first order estimates of channel and floodplain configurations followed by iterative design in a hierarchical manner. This review advances the science of river design by synthesizing these complimentary ideologies for Pacific salmon habitat restoration in regulated rivers. Following the review, the layout of features is briefly discussed followed by a discussion of important considerations beyond the physical and topographic rescaling of river corridors for salmonid habitat restoration.

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A flume experiment on the effect of constriction shape on the formation of forced pools

Cited by 42 publications

References 33 publications

Morphodynamics of a Width‐Variable Gravel Bed Stream: New Insights on Pool‐Riffle Formation From Physical Experiments

Morphodynamics of a Width‐Variable Gravel Bed Stream: New Insights on Pool‐Riffle Formation From Physical Experiments

What controls the disequilibrium state of gravel‐bed rivers?

Hydrogeomorphic Scaling and Ecohydraulics for Designing Rescaled Channel and Floodplain Geometry in Regulated Gravel–Cobble Bed Rivers for Pacific Salmon Habitat

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