Processes of initiation of motion leading to bedload transport in gravel‐bed rivers

García, Carmen; Cohen, Hai; Reid, Ian; Rovira, Albert; Úbeda, Xavier; Laronne, Jonathan B.

doi:10.1029/2006gl028865

Cited by 32 publications

(34 citation statements)

References 30 publications

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“…Protuberances on the bed have a significant influence on flow resistance through form drag (Brayshaw et al, 1983;Hassan and Reid, 1990;Robert, 1990Robert, , 1997Lawless and Robert, 2001) and also induce a complex and variable hydraulic environment, including local reversal of the boundary shear stress vector Garcia et al, 2007). A heterogeneous near-bed hydraulic environment has a significant impact on the presence and behaviour of benthic organisms that reside on the substrate (Carling, 1992;Lancaster et al, 2006;Rice et al, 2008) and on the entrainment of bed material (Schvidchenko and Pender, 2001).…”

Section: Potential Impact On Sediment Transport Of Topographic Changementioning

confidence: 99%

Topographic disturbance of subaqueous gravel substrates by signal crayfish (Pacifastacus leniusculus)

2010

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Section: Potential Impact On Sediment Transport Of Topographic Changementioning

confidence: 99%

Topographic disturbance of subaqueous gravel substrates by signal crayfish (Pacifastacus leniusculus)

2010

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“…As flow strength was increased, entrainment began with small, occasional movements of isolated grains. However, in all our experimental runs, a step-change was clearly observed as flow was increased, whereby this occasional single-grain movement was replaced by general movement of the majority of surface grains, mimicking direct observations made in the field, where the bed has been water-lain and naturally conditioned (Garcia et al, 2007). We used the start of this latter, widespread movement of grains as our definition of the entrainment threshold.…”

Section: Entrainment Measurements and Analysismentioning

confidence: 99%

Stabilization of fine gravels by net‐spinning caddisfly larvae

Johnson

Reid

Rice

et al. 2009

Earth Surf Processes Landf

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We examined the impact of Hydropsychidae caddisfly larvae on the incipient motion of two sizes of narrowly graded fine‐gravel (4–6 and 6–8 mm). This impact was assessed relative to the collective impact of other abiotic and biotic processes that are potentially important conditioning agents of fine‐gravels. Trays of gravel were placed in the River Soar, Leicestershire, UK, where they were colonized to natural densities by caddisfly larvae. Identical trays that were surrounded by a 250 µm mesh were also deployed, preventing colonization but allowing field conditioning of sediments, including minor reworking of grains and biofilm development. After 21 days in the river, trays were removed to a laboratory flume where grain entrainment stresses were established. In addition to the colonized and conditioned treatments, critical shear stresses were measured for identical sediments that were not placed in the river (laboratory gravels). Gravels that were colonized by Hydropsychidae required significantly greater shear stresses for entrainment than conditioned trays (p ≤ 0·002), however, there was no significant difference between conditioned and laboratory gravels. This implies that the presence of caddisfly can be a more important influence on fine‐gravel stability than some conditioning processes. Shields parameter was compared across treatments and across the two gravel size‐fractions using two‐way ANOVA. No significant differences or interactions were observed, indicating that 4–6 mm gravel was stabilized to a similar degree as 6–8 mm gravel by conditioning and colonization processes. Our results extend earlier studies in two important ways: (1) entrainment stresses were established for fine gravels that were colonized at natural densities, under natural stream conditions; and (2) the caddisfly effect was measured relative to both field‐conditioned and unconditioned laboratory controls. The temporal and spatial distribution of silk‐spinning caddisfly larvae suggests that they have the potential to influence fine‐sediment mobility in many rivers, worldwide. Copyright © 2008 John Wiley & Sons, Ltd.

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“…For instance, bed material entrainment in gravel bed rivers is often controlled by patches of fine sediments. Several phases have been described in the literature as occurring in this type of sedimentary microfeature [Garcia et al, 2007], and recent technical developments Vericat et al, 2007Vericat et al, , 2008 have aided in examining the interaction between hydraulics and bed load under such marginal transport conditions (i.e., very low bed load transport rates). These rates are also very sensitive to changing hydraulic conditions [Church and Hassan, 2005].…”

Section: Comparison With Nonuniform (Flume and Field) Sediment Datamentioning

confidence: 99%

Theoretical development on the effects of changing flow hydraulics on incipient bed load motion

Recking¹

2009

Water Resources Research

174

247

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[1] Several decades of flume and field measurements have indicated that in rough turbulent flows the critical Shields stress increases with increasing slope and associated decreasing relative depth. This result contradicts the usual consideration of a decreased critical Shields value on very steep slopes because of increased gravitational effects. However, recent studies have demonstrated that these experimental results could be reproduced with a force balance model if the classical logarithmic velocity profile was replaced with a velocity profile that was more compatible with available velocity measurements over gravel beds. These measurements indicate the existence of a roughness layer that is a zone of almost constant velocity close to the bed, whose properties (mean velocity and turbulence) depended on the flow's relative depth. Unfortunately, velocity profile measurements for low relative depth associated with steep slopes are scarce, and it is still difficult to include such flow properties in a force balance model. Flow resistance data (on the basis of depth average velocity measurements) are very common and cover a wide range of slopes and relative depths. In this paper these data are used to fit a velocity profile including a roughness layer. When used in a force balance model for incipient motion, it adequately reproduced a data set composed of 270 critical Shields values measured in a flume with near-uniform sediments. The relevance of this research to field problems is discussed using a data set composed of 92 critical Shields stresses obtained from field measurements. Finally, a model is proposed for field applications taking into account the slope effect.

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Processes of initiation of motion leading to bedload transport in gravel‐bed rivers

Cited by 32 publications

References 30 publications

Topographic disturbance of subaqueous gravel substrates by signal crayfish (Pacifastacus leniusculus)

Topographic disturbance of subaqueous gravel substrates by signal crayfish (Pacifastacus leniusculus)

Stabilization of fine gravels by net‐spinning caddisfly larvae

Theoretical development on the effects of changing flow hydraulics on incipient bed load motion

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