Influence of cohesive clay on wave–current ripple dynamics captured in a 3D phase diagram
Influence of cohesive clay on wave–current ripple dynamics captured in a 3D phase diagram
Wave–current ripples that develop on seabeds of mixed non-cohesive sand and cohesive clay are commonplace in coastal and estuarine environments. While laboratory research on ripples forming in these types of mixed-bed environments is relatively limited, it has identified deep cleaning, the removal of clay below the ripple troughs, as an important factor controlling ripple development. New large-scale flume experiments seek to address this sparsity in data by considering two wave–current conditions with initial clay content, C0, ranging from 0 % to 18.3 %. The experiments record ripple development and pre- and post-experiment bed clay contents to quantify clay winnowing. The present experiments are combined with previous wave-only, wave–current, and current-only experiments to produce a consistent picture of larger and smaller flatter ripples over a range of wave–current conditions and C0. Specifically, the results reveal a sudden decrease in the ripple steepness for C0 > 10.6 %, likely associated with a decrease in hydraulic conductivity of 3 orders of magnitude. Accompanying the sudden change in steepness is a gradual linear decrease in wavelength with C0 for C0 > 7.4 %. Ultimately, for the highest values of C0, the bed remains flat, but clay winnowing still takes place, albeit at a rate 2 orders of magnitude lower than for rippled beds. For a given flow, the initiation time, when ripples first appear on a flat bed, increases with increasing C0. This, together with the fact that the bed remains flat for the highest values of C0, demonstrates that the threshold of motion increases with C0. The inferred threshold enhancement, and the occurrence of large and small ripples, is used to construct a new three-dimensional phase diagram of bed characteristics involving the wave and current Shields parameters and C0, which has important implications for morphodynamic modelling.
231-247
Wu, Xuxu
539e8018-671f-4bcd-a199-d600e0f7763e
Malarkey, Jonathan
47695191-3894-4aaa-ba15-e47cef46ac8c
Fernández, Roberto
1ab30fb4-2a06-4ee1-96bd-e4d1bb98bb61
Baas, Jaco H.
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Pollard, Ellen
d333135b-3d9d-4aa6-b425-75a9226c923e
Parsons, Daniel R.
b7b0ca12-8152-481c-9d26-642ad410a157
30 January 2024
Wu, Xuxu
539e8018-671f-4bcd-a199-d600e0f7763e
Malarkey, Jonathan
47695191-3894-4aaa-ba15-e47cef46ac8c
Fernández, Roberto
1ab30fb4-2a06-4ee1-96bd-e4d1bb98bb61
Baas, Jaco H.
b0769c25-0a5a-460d-9c7d-5b4d53e5d09a
Pollard, Ellen
d333135b-3d9d-4aa6-b425-75a9226c923e
Parsons, Daniel R.
b7b0ca12-8152-481c-9d26-642ad410a157
Wu, Xuxu, Malarkey, Jonathan, Fernández, Roberto, Baas, Jaco H., Pollard, Ellen and Parsons, Daniel R.
(2024)
Influence of cohesive clay on wave–current ripple dynamics captured in a 3D phase diagram.
Earth Surface Dynamics, 12 (1), .
(doi:10.5194/esurf-12-231-2024).
Abstract
Wave–current ripples that develop on seabeds of mixed non-cohesive sand and cohesive clay are commonplace in coastal and estuarine environments. While laboratory research on ripples forming in these types of mixed-bed environments is relatively limited, it has identified deep cleaning, the removal of clay below the ripple troughs, as an important factor controlling ripple development. New large-scale flume experiments seek to address this sparsity in data by considering two wave–current conditions with initial clay content, C0, ranging from 0 % to 18.3 %. The experiments record ripple development and pre- and post-experiment bed clay contents to quantify clay winnowing. The present experiments are combined with previous wave-only, wave–current, and current-only experiments to produce a consistent picture of larger and smaller flatter ripples over a range of wave–current conditions and C0. Specifically, the results reveal a sudden decrease in the ripple steepness for C0 > 10.6 %, likely associated with a decrease in hydraulic conductivity of 3 orders of magnitude. Accompanying the sudden change in steepness is a gradual linear decrease in wavelength with C0 for C0 > 7.4 %. Ultimately, for the highest values of C0, the bed remains flat, but clay winnowing still takes place, albeit at a rate 2 orders of magnitude lower than for rippled beds. For a given flow, the initiation time, when ripples first appear on a flat bed, increases with increasing C0. This, together with the fact that the bed remains flat for the highest values of C0, demonstrates that the threshold of motion increases with C0. The inferred threshold enhancement, and the occurrence of large and small ripples, is used to construct a new three-dimensional phase diagram of bed characteristics involving the wave and current Shields parameters and C0, which has important implications for morphodynamic modelling.
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esurf-12-231-2024
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Accepted/In Press date: 28 November 2023
Published date: 30 January 2024
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© 2024 Copernicus GmbH. All rights reserved.
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Local EPrints ID: 491768
URI: http://eprints.soton.ac.uk/id/eprint/491768
ISSN: 2196-6311
PURE UUID: 0f64588c-abc7-40f6-93a8-898efc0c6ab3
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Date deposited: 03 Jul 2024 17:22
Last modified: 12 Jul 2024 02:17
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Author:
Xuxu Wu
Author:
Jonathan Malarkey
Author:
Roberto Fernández
Author:
Jaco H. Baas
Author:
Ellen Pollard
Author:
Daniel R. Parsons
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