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Keywords:

  • bedload transport;
  • initiation of motion;
  • gravel-bed streams

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Evidence
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[1] Transport processes that lead to the initiation of bedload motion in gravel-bed rivers have not yet been clarified. We report patch- and grain-scale processes involved in the initiation of bedload motion in a natural gravel-bed stream as observed through a series of video experiments. With increasing flow strength, the phases of initiation of motion that have been identified are (1) within-patch grain instability (grain vibration, pivoting, and grain-scale rolling), (2) within-patch gyratory step-and-rest motion, and (3) general sediment motion involving downstream transport from an individual patch and the throughput of grains inherited from upstream.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Evidence
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[2] Laboratory and field studies have shown that several properties of gravel beds affect the entrainment of sediment. These include: grain protrusion and exposure [Fenton and Abbott, 1977; Raudkivi and Ettema, 1982]; friction angle [Buffington et al., 1992]; bed packing [Church, 1978]; bed relief [Laronne and Carson, 1976]; grain imbrication [Johansson, 1963]; grain clusters [Brayshaw et al., 1983]; and sand content [Wilcock, 1998]. Variation in channel topography and boundary shear stress lead to sorting of bed material and to the common formation of surficial patches of sand, granules and small pebbles that locally cover part of the armour layer [Lisle and Madej, 1992; Paola and Seal, 1995]. Patches are accumulations of fine sediment that are indicative of sediment supply and have considerable control over the nature of river-bed habitats [Townsend, 1989; Kondolf and Wolman, 1993], hydraulic roughness [Dietrich et al., 1989], and, importantly, the initiation and the texture of bedload [Garcia et al., 1999].

[3] The Shields entrainment function [Shields, 1936; Miller et al., 1977], defined for near-uniform grains, is the parameter most commonly used to determine the conditions at which particle motion is initiated under hydraulically rough flow typical of gravel-bed rivers. There is an inherent difficulty in defining initial motion for non-uniform bed materials, despite the application of a number of other methods such as (1) extrapolation of a transport relation to zero or a specified low reference value [Parker and Klingeman, 1982], (2) visual observation in a flume [Gilbert, 1914], (3) competence functions based on the largest mobile grain [Andrews, 1983], and (4) theoretical force balance equations [White, 1940]. Each of these methods has been deployed with variable and limited success in particular situations [Carson and Griffiths, 1985]. For example, the reference value and competence methods are thought to be more suitable for predicting reach-average incipient motion and the visual observation method is best applied to mobility studies of discrete sedimentary patches [Buffington and Montgomery, 1997]. For two decades, research on gravel-bed rivers has focused on the important role that an armour layer plays in initiation of motion and how its disruption at times of flood releases considerable amounts of fine bed material from the subsurface [Parker and Klingeman, 1982]. However, fine bed material is mobilized from patches prior to that being released from beneath an armour layer; it is therefore the initial source of bedload under conditions of initial motion [Laronne et al., 2001]. Indeed, because of a growing awareness of the areal segregation of many gravel-bed channels into patches of differing grain-size, patch dynamics are now receiving greater attention. It is increasingly apparent that they are important for the general assessment of bedload flux [Paola and Seal, 1995] as well as for hydro-ecology (fish habitat, flushing flows, and minimum flow requirements), especially in upland rivers [Gibbins et al., 2007].

2. Field Evidence

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Evidence
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[4] The processes that lead to motion of grains from bed patches have been observed through a series of video experiments conducted on the Tordera River, a perennial gravel-bed channel located in the Catalan Coastal Ranges, NE Spain. The experiments were carried out in the reach immediately upstream from a permanent sediment transport monitoring station [Garcia et al., 2000]. Images of the river bed were recorded by deploying a video camera vertically from a position about 1 m above the water surface. To avoid distortion of the images by refraction, a Perspex™ viewing box 2 m long by 1 m wide was placed on the water surface. Flow velocity was increased artificially in order to induce sediment motion either by concentrating the flow of water in the study area using sand bags or by gradually constricting water depth below the viewing box by pushing it towards the river bed. The viewing box was depressed at a rate of 4 mm s−1 through a distance of 100 mm, while maintaining near-parallelism between the plane of the Perspex™ window and that of the original local water-surface slope. The 3-dimensional rugosity of the channel bed, established by the cobble-boulder framework clasts that enclose each fine-grained patch, induces a highly complex flow structure characterized by significant upstream flow vectors near the bed, amongst other features. In this context, the dimensions of the viewing box were scaled to those of the bed patches under investigation rather than attempting to condition the flow over longer channel lengths. The accelerated flow under the box induced bedload motion and eventually led to extra-patch bedload transport. Within a field of view of 0.35 m by 0.45 m, flow velocity ranged from 0.3 to 0.4 m s−1 during the experiment. The video record of each patch that was subjected to treatment allowed subsequent analysis of the sequence of grain- and patch-scale processes (Movie S1 in the auxiliary material).

[5] The patch we show in the video-clip had dimensions of 0.13 by 0.06 m, a mean grain size of 2 mm and a sediment thickness between 0.05 and 0.08 m. The original flow depth was 0.35 m over the patch and 0.25 m over the upstream cobble. Initially, there was no movement; a patch is usually located in the lee of a protruding clast or a rib-like structure formed by several such clasts [Laronne and Carson, 1976] and flow separation protects the finer patch sediment from turbulence. The patch sediment had an open structure [Laronne and Carson, 1976], implying low interparticle friction, and little micro-relief (Figures 1a and 2a). As flow velocity was increased, instabilities occurred, including grain vibration [Schumm and Stevens, 1973], grain pivoting without significant displacement [Allan and Frostick, 1999] and irregular and occasional hopping of grains. Eighteen particles moved a mean distance of 0.05 m from the upstream side of the patch to its center (Figures 1b and 2b). During the next phase, we observed a giratory motion of material, the diameter of each gyre being about 0.025 m. During the first few seconds (11:59:49 to 11:59:51), three particles per second moved in saltation; from 11:59:52 onwards, the number of particles in saltation increased to eight per second while the indicative flow velocity was 0.30 m s−1 at 0.05 m above the surface of the upstream cobble that offers protection to the patch. By 11.59.55, velocity had increased to 0.32 m s−1 and the number of particles in saltation had increased to 32 per second, while a mean saltation jump of 50 mm was recorded (Figures 1c, 1d, and 2c). The gyratory motion was strong enough to have remobilized fish reds [Wilcock et al., 1996] and was complemented by increasingly frequent downstream ejections of single grains [Garcia et al., 1996] as turbulence swept the patch floor in response to the changing location of flow reattachment at the downstream end of the separated flow. At 11:59:56, flow velocity reached 0.35 m s−1 and the effects of the first major sweep of the patch were recorded, with more than 40 particles moved (Figure 1e). Such particle disturbance is intermittent and associated with turbulent burst-sweep events during which low-momentum fluid is ejected toward the outer regions of the flow to be replaced by high-momentum fluid that sweeps the bed [Grass, 1983]. The next and final phase was characterised by extra-patch movement: during this phase, general motion occurred within the patch and particles were exported downstream, with bedload movement occurring over an immobile armour. A few particles slid, many rolled, but saltation transported most grains downstream (Figures 1f and 2d) [cf. Drake et al., 1988]. The location of the transport pathway was fairly consistent and depended on the geometry of the coarse clasts that formed the downstream boundary of the patch, the exit path being identified as a saddle between two adjacent grains.

image

Figure 1. (a–f) Six images extracted from a video clip showing the processes of initiation of sediment motion in a river bed patch that eventually lead to bedload motion as flow strength increases. Flow is from right to left. Figure 1a (time - 11:59:39) shows a stable patch of fine material in the lee of a protruding cobble. Figure 1b (11:59:44) displays within-patch instability involving the vibration and small dispacements of individual grains (areas 1 and 2). Figure 1c (11:59:49) displays within-patch gyratory sediment motion in response to increased flow strength (area 1) and an increase in the number of rolling particles (area 2). Figure 1d (11:59:56) shows saltation of a few particles out of the patch (areas 1 and 2); pivoting of comparatively large grains in the downstream portion of the patch (area 3). In Figure 1e (11:59:56) bursts of sediment movement reflect increasingly active areas within the patch, including the portion previously protected in the lee of the upstream cobble (area 1). Sediment moves downstream within the patch (areas 2 and 3). Figure 1f (11:59:57) displays general sediment motion from and through the patch. This is the first net downstream flux signifying conventionally defined bedload under conditions traditionally considered as just above the threshold of motion.

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image

Figure 2. (a–d) Cartoon showing the typical movement of sand or granule particles in a sediment patch at sequential phases of increasing flow strength. Flow is from right to left. Figure 2a (time - 11:59:39), static; Figure 2b (11:59:44), in situ vibration and spasmodic single hops; Figure 2c (11:59:49), gyratory step-and-rest movement within the patch; Figure 2d (11:59:57), extra-patch displacement i.e., conventionally defined entrainment and bedload flux.

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3. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Evidence
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[6] Observations such as these provide, for the first time, an indication that bed material disturbance is considerable during rising flows and well ahead of any conventionally defined initiation of bedload [Wilcock and McArdell, 1997], following which there is an expectation of net downstream flux.

[7] The three phases of fine-grained bedload transport (Figures 1 and 2) were video-documented in the Tordera in a number of neighbouring patches having similar characteristics. (In a 50 m reach, as many as 124 patches were surveyed, an average occurrence of 0.44 m−2). That these phases condition the amount of downstream sediment transport in channels where patch bed character is similar to that of the Tordera is attested by a number of observations made elsewhere. First, a controlled laboratory flume experiment, in which fine-grained (sand, granule) patch sediments were colour-coded according to size-class (in a manner reminiscent of that deployed by Wilcock and McArdell [1993]), revealed not only the three phases of grain displacement with increasing flow, but also selective grain entrainment that produced the within-patch downstream coarsening observed in the field by Laronne et al. [2001] (I. Reid, personal communication, 2007). Second, an extended field study in Wood Brook, England, a comparatively steep step-pool channel (average slope = 0.033) with channel-bed character that has affinity with that of the Tordera, has revealed through the tracing of magnetically-tagged patch (i.e., pool) grains that 97 percent of fine-grained material of granule-small-pebble calibre is displaced within-pools by floods that exert values of peak specific stream power up to 55 W m−2, beyond which there is significant ‘export’ of grains downstream from pool to pool (and beyond). Indeed, during the largest event recorded, in which specific stream power reached a peak value of 165 W m−2, 64 percent of the tagged grains were ‘exported’ downstream from the pools in which they had been seeded [Dudley, 2007].

[8] There are a number of implications of this pre-flux movement. The patch material is hydraulically winnowed, smaller grains being observed to move to the upstream portion of a patch and into a position protected by the large armour layer clasts that define the upstream patch boundary, while coarser grains remain in the downstream portion where flow reattachment and impinging eddies are most likely to cause downstream ejection. Hydraulic rearrangement of the patch sediment also allows it to increase its packing density, so maximizing the angle of internal friction. These local movements of bed material – within-patch bedload – are likely to be a factor contributing to the considerable unpredictability of bedload in gravel bed rivers [Garcia et al., 1999; Laronne et al., 2001]. This unpredictability makes it difficult to estimate the consequences of sediment transport, such as the flushing efficiency of flow releases designed to maintain a healthy river ecology [Wiens, 2002; Crowder and Diplas, 2002] and the sedimentary processes that may threaten benthic life as flood flows wax and wane [Lancaster et al., 2006]. The pre-flux phases ultimately leading to conventionally-defined bedload transport are particularly relevant to models of initiation of motion, to evaluations of in-channel sediment supply of bedload and, hence, to the prediction of bedload motion during conditions prior to armour disruption.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Evidence
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

[9] Funds for the Tordera monitoring station were granted by the Spanish Ministry of Science and Education, grant AMB93-0418. Joan Estrany prepared the two figures. We thank Jordi Pérez, Sònia Papell, Mariona Mesull, and Cecilia Corrado for fieldwork assistance. Comments by two anonymous reviewers helped us to improve the clarity and completeness of the paper.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Evidence
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Evidence
  5. 3. Discussion
  6. Acknowledgments
  7. References
  8. Supporting Information

Auxiliary material for this article contains a series of video experiments conducted on the Tordera River.

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