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The sedimentology of river confluences

The sedimentology of river confluences
The sedimentology of river confluences
Channel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically-based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to the morphodynamic behaviour of bars, confluence scour downstream of braid bars, bend scour and major junction scour. Comparison with existing, largely qualitative, conceptual models reveals that none of these can be applied simply, although elements of each are evident in the deposits generated by the numerical simulation and observed in the geophysical data. The characteristics of the simulated scour deposits are found to vary according to the degree of reworking caused by channel migration, a factor not considered adequately in current conceptual models of confluence sedimentology. The alluvial architecture of major junction scours is thus characterised by the prevalence of erosion surfaces in conjunction with the thickest depositional sets. Confluence scour downstream of braid bar and bend scour sites may preserve some large individual sets, but these locations are typically characterised by lower average set thickness compared to major junction scour and by a lack of large-scale erosional surfaces. Areas of deposition not related to any of the specific scour types highlighted above record the thinnest depositional sets. This variety in the alluvial architecture of scours may go some way towards explaining the paradox of ancient junction scours, that while abundant large scours are likely in the rock record, they have been reported rarely. The present results outline the likely range of confluence sedimentology and will serve as a new tool for recognizing and interpreting these deposits in the ancient fluvial record.
Rivers, Confluence Scour, Bangladesh, delta, Numerical model
0037-0746
1-39
Sambrook Smith, Greg
96e64c6c-0e63-4f38-931c-22a8c53feea4
Nicholas, Andrew
667e4252-c02e-408e-a941-a0cbcb7486d6
Best, James
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Bull, Jonathan
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Dixon, Simon
4bcdae11-7f63-445b-978b-cfa4ae8ea43f
Goodbred, Steve
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Sarkar, Mamin
426f3df7-be1e-4897-b48e-e80701e5af68
Vardy, Mark
732de795-9a99-41c0-b5cc-19d5fa561171
Sambrook Smith, Greg
96e64c6c-0e63-4f38-931c-22a8c53feea4
Nicholas, Andrew
667e4252-c02e-408e-a941-a0cbcb7486d6
Best, James
14e6ea56-3d65-4d60-a474-d76189a085da
Bull, Jonathan
974037fd-544b-458f-98cc-ce8eca89e3c8
Dixon, Simon
4bcdae11-7f63-445b-978b-cfa4ae8ea43f
Goodbred, Steve
d7e605d2-f98b-4f39-86e9-15f5147199c0
Sarkar, Mamin
426f3df7-be1e-4897-b48e-e80701e5af68
Vardy, Mark
732de795-9a99-41c0-b5cc-19d5fa561171

Sambrook Smith, Greg, Nicholas, Andrew, Best, James, Bull, Jonathan, Dixon, Simon, Goodbred, Steve, Sarkar, Mamin and Vardy, Mark (2018) The sedimentology of river confluences. Sedimentology, 1-39. (doi:10.1111/sed.12504).

Record type: Article

Abstract

Channel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically-based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to the morphodynamic behaviour of bars, confluence scour downstream of braid bars, bend scour and major junction scour. Comparison with existing, largely qualitative, conceptual models reveals that none of these can be applied simply, although elements of each are evident in the deposits generated by the numerical simulation and observed in the geophysical data. The characteristics of the simulated scour deposits are found to vary according to the degree of reworking caused by channel migration, a factor not considered adequately in current conceptual models of confluence sedimentology. The alluvial architecture of major junction scours is thus characterised by the prevalence of erosion surfaces in conjunction with the thickest depositional sets. Confluence scour downstream of braid bar and bend scour sites may preserve some large individual sets, but these locations are typically characterised by lower average set thickness compared to major junction scour and by a lack of large-scale erosional surfaces. Areas of deposition not related to any of the specific scour types highlighted above record the thinnest depositional sets. This variety in the alluvial architecture of scours may go some way towards explaining the paradox of ancient junction scours, that while abundant large scours are likely in the rock record, they have been reported rarely. The present results outline the likely range of confluence sedimentology and will serve as a new tool for recognizing and interpreting these deposits in the ancient fluvial record.

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SambrookSmith_et_al_2018_accepted - Accepted Manuscript
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Accepted/In Press date: 30 May 2018
e-pub ahead of print date: 30 June 2018
Keywords: Rivers, Confluence Scour, Bangladesh, delta, Numerical model

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Local EPrints ID: 421540
URI: http://eprints.soton.ac.uk/id/eprint/421540
ISSN: 0037-0746
PURE UUID: c2d78f02-963f-45c7-97ef-a3a8481570cc

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Date deposited: 14 Jun 2018 16:30
Last modified: 07 Oct 2020 05:07

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Contributors

Author: Greg Sambrook Smith
Author: Andrew Nicholas
Author: James Best
Author: Jonathan Bull
Author: Simon Dixon
Author: Steve Goodbred
Author: Mamin Sarkar
Author: Mark Vardy

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