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Braid bar

About: Braid bar is a research topic. Over the lifetime, 44 publications have been published within this topic receiving 3964 citations.

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Book ChapterDOI
17 Mar 2009
TL;DR: In this paper, a detailed profile of mid-channel bar development in the Jamuna River, Bangladesh is presented, showing that the structure of flow is dominated by a simpler flow divergence over the bar head, flow convergence at the bar tail and flow usually parallel to the thalwegs in each distributary.
Abstract: Detailed vertical profiles of time-averaged flow velocities and sediment concentration were taken during threeperiods of mid-channel bar development in the Jamuna River, Bangladesh. Bar growth was initiated downstreamfrom a major flow convergence and generated a bar 4 km long and 1 km wide in a channel up to 15 m deep. Flowvelocities and the concentrations of sand-grade suspended sediment were quantified using an acoustic Dopplercurrent profiler (ADCP). Bed morphology was measured using echo-sounding and all positions were locatedusing a differential global positioning system (DGPS).These data reveal no evidence for channel-scale, coherent helical flow cells in either distributary around thebraid bar. Instead, the structure of flow is dominated by a simpler flow divergence over the bar head, flow convergenceat the bar tail and flow that is usually parallel to the thalwegs in each distributary. During the later stages ofbar growth, flow is directed over the bar top from one distributary towards the other as the bar begins to adopt amore asymmetrical morphology. In addition, large sand dunes migrate up the bar stoss side, producing an accretionarydune front at the bar head. These dunes are strongly linked to high suspended bed-sediment concentrationsas flow shallows on to the bar top. A shadow of low suspended bed-sediment concentration is located in the bar leeduring the early stages of bar growth, this also being a region of small sand dunes.The lack of coherent secondary flows, around large kilometre-scale bars, may be explained through the largewidth-to-depth ratio of these channels, the low curvature of the anabranches, the complexity of flow over the bartop as it interacts with flow in the anabranches and the significant influence of large-scale dune-bedform roughness.These factors suggest that current models for the processes of mid-channel bar creation, growth and preservation,derived from studies of smaller rivers, require substantial revision before application to kilometre-scale sand-braidbars.

41 citations

01 Jan 1984
TL;DR: In this paper, a comparison of modern gravelly fluvial deposits from rivers in the Canadian Rocky Mountains and Cambro-Ordovician deep-sea valley-fill conglomerates (Cap Enrage Formation, Quebec) reveal some similarities and differences.
Abstract: Detailed comparisons of modern gravelly fluvial deposits from rivers in the Canadian Rocky Mountains and Cambro-Ordovician deep-sea valley-fill conglomerates (Cap Enrage Formation, Quebec) reveal some similarities and differences. Braided channel deposits in both deep-sea and fluvial settings occur in laterally fining/thinning units within concave-up lenses bounded by major scour surfaces at the base. Vertical sequences are mainly fining-upward, less commonly coarsening, 5–10 m thick for deep-sea and 2–3 m thick for fluvial channel fills. Main channel deposits in both settings are dominantly structureless. Braid bar deposits in deep-sea and fluvial sediments occur in laterally coarsening/thickening units with flat bases and convex-up top surfaces. Vertical sequences vary depending upon locations within the bar complex and the type of braid bar. Generally, vertical sequences are fining-upward, 1–2 m thick, horizontally stratified or cross-bedded units. Planar tabular cross-stratification is more common in fluvial bars than in deep-sea bar deposits. Graded trough cross-stratification, graded horizontal stratification and irregular inclined cross-stratification are very common in deep-sea bar deposits. These features were not recognized in the fluvial conglomerates. Horizontal stratification consists of layers with different clast sizes or layers with alternating matrix-filled and open-work texture in fluvial conglomerates. Horizontal stratification consists of layers with different clast sizes or layers with alternating clast-supported and clast-dispersed texture in deep-sea conglomerates. Open-work texture was not observed in the deep-sea conglomerates. Grading types and gravel fabric patterns are perhaps the most useful criteria in the distinction of fluvial from deep-sea conglomerates. Fluvial conglomerates are mainly ungraded, with less common normal or inversely-graded beds. Deep-sea conglomerates are mainly normally graded, with less common ungraded conglomerates and rare inversely or complexly graded beds. Deep-sea channel conglomerates have a-axis flow-parallel, a-axis upstream imbricate fabrics. In fluvial channels the smaller clasts may also assume an a-axis flow-parallel, a-axis imbricate upstream pattern. However, the coarser clasts are generally aligned in fluvial channels with a-axis flow-transverse, b-axis imbricate upstream. Braid bar deposits are distinguished on the basis of imbrication: in fluvial deposits imbrications are b-axis upstream; in deep-sea deposits imbrications are either a-axis upstream or a-axis upstream and downstream (bimodal). A-axis orientations in bedding are quite variable in both fluvial and deep-sea bar deposits and are not very reliable.

41 citations

Journal ArticleDOI
TL;DR: Bar-top hollows in the upper part of the bar depositional sequence of the South Saskatchewan River have been recognized as a morphological element of sandy braided rivers as discussed by the authors.

41 citations

Journal ArticleDOI
TL;DR: In this paper, four braided stream facies are recognized on the basis of distinct lithologies and assemblages of sedimentary structures, including upward-fining conglomerate to sandstone and mudstone channel fill sequences.

37 citations

Book ChapterDOI
01 Jan 2007
TL;DR: In this paper, a 225 MHz GPR survey was conducted within a braided reach of the Wisconsin River near Spring Green, Wisconsin, USA, to characterize the subsurface architecture of a midchannel bar.
Abstract: The internal architecture of sandbars in modern braided streams has not been adequately documented, especially in medium-scale braided rivers. Identification of the architecture and development of an understanding of the formative processes for these macroforms is important for (1) understanding sedimentation in braided streams, (2) understanding reservoir and aquifer compartmentalization in ancient deposits, and (3) predicting the controls on deposition in similar settings. A 225 MHz GPR survey was conducted within a braided reach of the Wisconsin River near Spring Green, Wisconsin, USA, to characterize the subsurface architecture of a midchannel bar. A 20 20 m survey grid consisting of sixteen GPR transects oriented approximately in flow-parallel and flow-transverse directions was established on the bar. Three-dimensional analysis of the GPR profiles resulted in the interpretation of five major radar facies that represent depositional mechanisms that controlled bar growth and modification. Vertical accretion (aggradation) was the primary depositional mechanism for bar growth and was augmented by much smaller amounts of downstream accretion, lateral accretion, and upstream accretion. A channel fill pattern was also recognized and correlated between multiple profiles, and it provided evidence for two preexisting, independent macroforms that converged to form the studied bar. The work provides insight into bar morphology within sandy braided reaches that closely resembles that of similar GPR studies performed in both smaller and larger rivers and supports a scale-independent model for some aspects of bar growth and modification in sandy, braided rivers.

31 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
20212
20202
20192
20184
20171
20151