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Journal ArticleDOI

Flow fields in tightly curving meander bends of low width-depth ratio

13 Nov 2009-Earth Surface Processes and Landforms (John Wiley & Sons, Ltd.)-Vol. 35, Iss: 2, pp 119-135
TL;DR: In this article, two subsets of these bends were examined in relation to their patterns of cross-stream flow relative to the channel boundary, and three-dimensional near bank full flow fields were presented for one bend with a meander pool; inward shifting of the maximum velocity filaments and limited sediment supply are proposed as mechanisms for the development and maintenance of these features.
Abstract: Upland swamp channels with low width/depth ratios (w/d), armoured beds, minimal sediment loads, tightly curving bends and an absence of point bars provide a striking contrast to the flow characteristics of larger channels with higher w/d ratios. Two subsets of these bends were examined in relation to their patterns of cross-stream flow relative to the channel boundary. The first, with mean w/d = 2·0 and gentle barforms, exhibited even velocity distributions at bend entrances but developed vertically stacked pairs of maximum velocity filaments (MVFs). Cross-stream circulation increased with decreasing curvature before essentially ceasing in the tightest bend due to the conservation of angular momentum and reduced vertical velocity differentials; bed friction has more limited influence in narrow deep channels relative to bank friction. In the second subset of bends, with larger w/d (mean 4·8) and much steeper barforms, the MVFs were laterally paired and strongly helical flow was partly driven by the vertical confinement of flow due to large, stable barforms at the bend entrances. In one bend, the velocity profile became inverted immediately past the apex and caused helical flow to abruptly reverse. Point bars in relatively wide bedload channels appear to greatly distort secondary flow patterns. In narrow, deep, sediment-starved channels, separation zones against the convex and/or the concave bank deliver the flow confinement that would otherwise be provided by point bars or concave-bank benches. In these channels, separation zones are important for protecting both the channel bed and banks from scour. Three-dimensional near bankfull flow fields are presented for one bend with a meander pool; inward shifting of the MVF and limited sediment supply are proposed as mechanisms for the development and maintenance of these features. These flow data in narrow and deep peatland channels demonstrate very different flow patterns and morphological characteristics relative to the more commonly studied wide, shallow channels with more abundant sediment. Copyright © 2009 John Wiley & Sons, Ltd.
Citations
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Journal ArticleDOI
TL;DR: In this paper, the authors examined three hydrodynamic processes in sharp bends with fixed banks and discussed their morphological implications: secondary flow saturation, outer-banks cells, and inner-bank flow separation.
Abstract: The migration rate of sharp meander bends exhibits large variance and indicates that some sharply curved bends tend to stabilize. These observations remain unexplained. This paper examines three hydrodynamic processes in sharp bends with fixed banks and discusses their morphological implications: secondary flow saturation, outer-banks cells, and inner-bank flow separation. Predictions from a reduced-order hydrodynamic model show that nonlinear hydrodynamic interactions limit the growth of the secondary flow. This process is called the saturation of the secondary flow. For outer-bank cells and inner-bank flow separation, the analysis relies on experimental findings from flume studies in channels with fixed and mobile beds. The experiments reveal that outer-bank cells exist near steep as well as shelving banks and amplify with increasing steepness and roughness of the outer bank, and especially with increasing curvature. The effects of flow separation at the inner bank are found to be strongly conditioned by flow-sediment interactions, which lead to an increased scour depth near the outer bank and increased velocities near the toe of that bank. Overall the results suggest that secondary flow saturation and outer-bank cells tend to inhibit meander migration, whereas inner-bank separation may enhance migration. The relative importance of these three hydrodynamic processes depends on hydraulic, geometric, and sedimentologic conditions, which is consistent with the large variance in observed migration rates. The results suggest that large shallow rivers have the most dynamic meandering behavior, while the occurrence of stabilized meanders seems to be favored in narrow rivers.

140 citations

Journal ArticleDOI
TL;DR: In this article, the authors combined interpretation of 3D seismic imagery of submarine sinuous channel belts in offshore West Africa with observations from a range of outcrop analogues, and recognized five main architectural elements of slope channel belts: lateral-accretion packages (LAPs), channel-bend mounds, levees, non-turbiditic mass-transport deposits (MTDs), and last-stage channel-fills.

139 citations

Journal ArticleDOI
TL;DR: In this article, the authors synthesize these observations, in the context of their direct knowledge of submarine channels, to derive an overview of submarine channel flow dynamics, and process-oriented intra-channel architecture models for low and high latitude systems.

100 citations

Journal ArticleDOI
TL;DR: In this paper, the authors report on successful laboratory experiments that elucidate flow structure in one constant-width bend and a second bend with an outer-bank widening, with both a flat immobile gravel bed and mobile sand bed with dominant bedload sediment transport.
Abstract: There is a paucity of data and insight in the mechanisms of, and controls on flow separation and recirculation at natural sharply-curved river bends. Herein we report on successful laboratory experiments that elucidate flow structure in one constant-width bend and a second bend with an outer-bank widening. The experiments were performed with both a flat immobile gravel bed and mobile sand bed with dominant bedload sediment transport. In the constant-width bend with immobile bed, a zone of mainly horizontal flow separation (vertical rotational axis) formed at the inner bank that did not contain detectable flow recirculation, and an outer-bank cell of secondary flow with streamwise oriented rotational axis. Surprisingly, the bend with widening at the outer bank and immobile bed did not lead to a transverse expansion of the flow. Rather, flow in the outer-bank widening weakly recirculated around a vertical axis and hardly interacted with the inner part of the bend, which behaved as a constant-width bend. In the mobile bed experiment, downstream of the bend apex a pronounced depositional bar developed at the inside of the bend and pronounced scour occurred at the outside. Moreover the deformed bed promoted flow separation over the bar, including return currents. In the constant-width bend, the topographic steering impeded the generation of an outer-bank cell of secondary flow. In the bend with outer-bank widening, the topographic steering induced an outward expansion of the flow, whereby the major part of the discharge was conveyed in the central part of the widening section. Flow in the outer-bank widening was highly three dimensional and included return currents near the bottom. In conclusion, the experiments elucidated three distinct processes of flow separation common in sharp bends: flow separation at the inner bank, an outer-bank cell of secondary flow, and flow separation and recirculation in an outer-bank widening.

90 citations


Cites background from "Flow fields in tightly curving mean..."

  • ...turbulent correlations and the turbulent dissipation rate. Blanckaert (2011) Sharply-curved laboratory flume with flat...

    [...]

  • ...Inner bank flow separation and recirculation Frothingham and Rhoads (2003) Sharply-curved bend on the Embarras...

    [...]

References
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Journal ArticleDOI
TL;DR: In this paper, a system of grain-size nomenclature of terrigenous sediments and sedimentary rocks is introduced wherein fifteen major textural groups are defined on the ratios of gravel, sand, silt, and clay.
Abstract: A system of grain-size nomenclature of terrigenous sediments and sedimentary rocks is introduced wherein fifteen major textural groups are defined on the ratios of gravel, sand, silt, and clay. Further subdivision of each class is based on the median diameter of each size fraction present. Next, the mineral composition of terrigenous sedimentary rocks is considered. A triangular diagram is used to define eight rock types (orthoquartzite, arkose, graywacke, and five transitional types) based on the mineralogy of the silt-sand-gravel fraction and ignoring clay content. The writer contends that the current practice of calling all clayey sandstones "graywackes" is not valid, inasmuch as it represents a confusion of texture with composition. It is suggested that sedimentary rocks may be best defined by the use of a tripartite name, based on the following pattern-(grain size): (textural maturity) (mineral composition).

1,225 citations

Journal ArticleDOI
TL;DR: In this paper, a stability analysis of meandering and braiding perturbations in a model alluvial river is described, and a perturbation technique involving a small parameter representing the ratio of sediment transport to water transport is used to obtain the following results.
Abstract: A stability analysis of meandering and braiding perturbations in a model alluvial river is described. A perturbation technique, involving a small parameter representing the ratio of sediment transport to water transport, is used to obtain the following results.Under appropriate conditions, the existence of sediment transport and friction are necessary conditions for the occurrence of instability in the flow and on the bed; thus instability is not inherent in the flow alone. An Anderson-type scale relation for longitudinal instability is obtained for meandering. A relation estimating the number of braids and differentiating between meandering and braided regimes is derived. These relations are independent of sediment transport.

497 citations

Journal ArticleDOI
TL;DR: In this paper, the effect of topographically induced velocity changes on the cross-stream flow pattern was analyzed in a channel with a constant bottom topology, where the velocity component near the bed and the pattern of boundary shear stress can be estimated by assuming fluid acceleration to be small.
Abstract: In a channel with bed topography that does not vary in the downstream direction, a secondary circulation composed of outward flow at the surface and inward flow near the bottom extends across the entire width. If the curvature is constant, the cross-stream velocity component near the bed and the pattern of boundary shear stress can be estimated by assuming fluid accelerations to be small. Unfortunately, this procedure cannot be used in analyzing the flow through natural river meanders, or through channels with downstream constant bottom topography but with rapidly changing curvature. In these latter cases, effects arising from bed- and bank-induced momentum changes must be accounted for. Evidence for a substantial topographically induced alteration in the cross-stream flow pattern relative to that for the analogous constant bottom topography case is provided through new analyses of several sets of laboratory and field data. Shoaling over the point bar in the upstream part of the bend is shown to force the high-velocity core of the flow toward the pool. This is accomplished by a convective acceleration-caused decrease in the cross-stream water surface slope and a resulting dominance of the vertically averaged centrifugal force. The primary effect is a velocity component toward the outside or concave bank throughout the flow depth over the upstream, shallow part of the point bar and an outward component of boundary shear stress in this region. The channel curvature-induced inward component of boundary shear stress consequently is confined to 20 or 30% of the channel width at the pool. Outward transfer of momentum over the point bar, as manifested by a rapid crossing of the high-velocity core from the inside bank to the outside one, contributes to an enhanced decrease in boundary shear stress along the convex side of the stream as the top of the bar is approached. Forces arising from topographically induced spatial accelerations are of the same order of magnitude as the downstream boundary shear stress and water surface slope force components, so they must be modeled as zero-order, not first- or second-order, effects.

423 citations

Journal ArticleDOI
TL;DR: The occurrence of the outer-bank cell is shown to be not just due to flow instability, but also to kinetic energy input from turbulence, which shows that turbulence plays a minor role in the generation of the centre-region cell, which is mainly due to the centrifugal force.
Abstract: Secondary currents are a characteristic feature of flow in open-channel bends. Besides the classical helical motion (centre-region cell), a weaker and smaller counter-rotating circulation cell (outer-bank cell) is often observed near the outer bank, which is believed to play an important role in bank erosion processes. The mechanisms underlying the circulation cells, especially the outer-bank cell, are still poorly understood, and their numerical simulation still poses problems, not least due to lack of detailed experimental data. The research reported herein provides detailed experimental data on both circulation cells in an open-channel bend such as found in nature. Furthermore, the underlying dynamics are investigated by simultaneously analysing the vorticity equation and the kinetic energy transfer between the mean flow and the turbulence. This shows that turbulence plays a minor role in the generation of the centre-region cell, which is mainly due to the centrifugal force. By accounting for the feedback between the downstream velocity profile and the centre-region cell, a strongly simplified vorticity balance is shown to yield accurate predictions of the velocities in the centre region. For strong curvatures, however, a fully threedimensional flow description is required. Due to the non-monotonic velocity profiles, the centrifugal force favours the outer-bank cell. Moreover, terms related to the anisotropy of the cross-stream turbulence, induced by boundary proximity, are of the same order of magnitude and mainly enhance the outer-bank cell. Both mechanisms strengthen each other. The occurrence of the outer-bank cell is shown to be not just due to flow instability, like in the case of curved laminar flow, but also to kinetic energy input from turbulence.

281 citations

Journal ArticleDOI
TL;DR: In this paper, the authors studied the downstream bed load transport in Muddy Creek, a sand-bedded meandering river with equilibrium bottom topography, and found that a zone of maximum sediment flux shifted across the channel from near the inside bank in the upstream part of the bend toward the pool at the minimum radius of curvature.
Abstract: Bed load transport in Muddy Creek, Wyoming, a sand-bedded meandering river with equilibrium bottom topography, was found to consist of a zone of maximum sediment flux that shifted across the channel from near the inside bank in the upstream part of the bend toward the pool at the minimum radius of curvature. Significant net cross-stream transport continued even through the crossings between the bends. The downstream bed load transport field for the bend which was studied in greatest detail was the same as that computed from bed form migration measurements and can be predicted from appropriate boundary shear stress data and the Yalin bed load equation. The zone of maximum bed load transport followed an outward-shifting region of maximum boundary shear stress, although in the downstream end of the bend the sediment transport maximum tended to stay closer to the centerline than the boundary shear stress maximum due to particle size influences. Net cross-stream transport varied with particle size and was effected by three major processes: topographically-induced, near-bed, cross-stream flow; trough wise flow along obliquely oriented bed forms; and rolling or avalanching of particles on bed form lee faces plus rolling or mass sliding on a steep cross-stream point bar side slope. Coarse particles were carried outward over the top of the point bar by a near-bed cross-stream flow that was induced by downstream shoaling. These particles then rolled and slid on the point bar side slope and eventually were carried toward the outer bank by troughwise transport along oblique dunes. In the upstream part of the bend, fine particles were carried inward by the channel curvature-induced, near-bed flow and by troughwise transport along lee faces of oblique bed forms. Fine particles and coarse particles crossed paths on top of the point bar, where weak troughwise flow moved just the finer particles toward the convex bank, and on the point bar face, where coarse particles rolled against the secondary circu- lation that was carrying the finer particles. Net cross-stream bed load transport was toward the pool and was on average about 10% of the downstream bed load transport. The Engelund equation predicts reasonably well the general pattern of net cross-stream transport through the meander, but it does not account for the substantial troughwise transport caused by oblique bedforms at several locations in the bend. Our observations suggest that equilibrium bed topography occurs when there is net outward transport into the outward shifting zone of maximum boundary shear stress.

271 citations