Small-scale changes in bed morphology and sediment transport under waves and currents
Small-scale changes in bed morphology and sediment transport under waves and currents
Understanding the behaviour of bedforms and the related sediment transport is a key issue in coastal science and engineering. The generation of bedforms and the transport of sediment are part of a highly complex interactive system, involving feedback between a turbulent flow and the bed morphology. The present study applies an integrated approach to the 'trinity' of: boundary layer structure; sediment transport; and bedform development (Best, 1993). In addition to the interactions between the bed and flow, the investigation contributes to the understanding of the combined-flow influence on sediment transport and bedform dimensions. The data considered in this study fall into two categories: strong currents, with no or low wave activity (INDIA, Day 51 and 58, respectively); and weak currents in the presence of large waves (Ralph). The INDIA data release to a tidal inlet channel (Ria Formosa, Portugal) and the Ralph data to an offshore coastal shelf site (Nova Scotia, Canada).
Sediment transport and bedform geometry respond to the maximum conditions over a wave orbit, with the influence of waves being detectable even when the mean enhancement of shear stress is low. The ratio of Hs/d influences the effectiveness of the wave-current ratio, in determining the dominant hydrodynamic regime on the bed. Shear stress is enhanced by both waves and bedforms, simultaneously, in a non-linear manner. The migration of bedforms also acts to increase shear stress, indicating that both shape and movement of bedforms affects boundary layer flow. Macro-turbulent structures were observed, with a strong local effect on the shear stress. These features relate to and migrate with the bedforms. This result has significant implications in terms of modelling flow over bedforms i.e. the use of a spatially-averaged shear stress (or roughness) may not be sufficient to account for this behaviour. On Day 51, the drag coefficient was of the order 0.0032. On Day 58, when larger bedforms are observed, the drag coefficients increased (0.0065). Observed roughness was 0.03m and 0.16m, on Days 51 and 58, respectively. Established models of bed roughness over-predict these values.
Small sub-aqueous dunes were observed at the INDIA field site. This result is significant, as established theory predicted large dunes at this location. However, predictions appropriate to bedform dimensions provide reasonable estimates of the observed heights and lengths, most notably the prediction of van Rijn (1993), for small sub-aqueous dunes. The Scotian Shelf site (Ralph) exhibits suborbital wave ripples, which demonstrate a greater dependency on wave orbital diameter than is accounted for within existing predictions of suborbital ripples.
University of Southampton
Hughes, Zoe Joanne
798bd979-2584-452f-a9c6-01ba906eff0c
2003
Hughes, Zoe Joanne
798bd979-2584-452f-a9c6-01ba906eff0c
Hughes, Zoe Joanne
(2003)
Small-scale changes in bed morphology and sediment transport under waves and currents.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
Understanding the behaviour of bedforms and the related sediment transport is a key issue in coastal science and engineering. The generation of bedforms and the transport of sediment are part of a highly complex interactive system, involving feedback between a turbulent flow and the bed morphology. The present study applies an integrated approach to the 'trinity' of: boundary layer structure; sediment transport; and bedform development (Best, 1993). In addition to the interactions between the bed and flow, the investigation contributes to the understanding of the combined-flow influence on sediment transport and bedform dimensions. The data considered in this study fall into two categories: strong currents, with no or low wave activity (INDIA, Day 51 and 58, respectively); and weak currents in the presence of large waves (Ralph). The INDIA data release to a tidal inlet channel (Ria Formosa, Portugal) and the Ralph data to an offshore coastal shelf site (Nova Scotia, Canada).
Sediment transport and bedform geometry respond to the maximum conditions over a wave orbit, with the influence of waves being detectable even when the mean enhancement of shear stress is low. The ratio of Hs/d influences the effectiveness of the wave-current ratio, in determining the dominant hydrodynamic regime on the bed. Shear stress is enhanced by both waves and bedforms, simultaneously, in a non-linear manner. The migration of bedforms also acts to increase shear stress, indicating that both shape and movement of bedforms affects boundary layer flow. Macro-turbulent structures were observed, with a strong local effect on the shear stress. These features relate to and migrate with the bedforms. This result has significant implications in terms of modelling flow over bedforms i.e. the use of a spatially-averaged shear stress (or roughness) may not be sufficient to account for this behaviour. On Day 51, the drag coefficient was of the order 0.0032. On Day 58, when larger bedforms are observed, the drag coefficients increased (0.0065). Observed roughness was 0.03m and 0.16m, on Days 51 and 58, respectively. Established models of bed roughness over-predict these values.
Small sub-aqueous dunes were observed at the INDIA field site. This result is significant, as established theory predicted large dunes at this location. However, predictions appropriate to bedform dimensions provide reasonable estimates of the observed heights and lengths, most notably the prediction of van Rijn (1993), for small sub-aqueous dunes. The Scotian Shelf site (Ralph) exhibits suborbital wave ripples, which demonstrate a greater dependency on wave orbital diameter than is accounted for within existing predictions of suborbital ripples.
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Published date: 2003
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Local EPrints ID: 464985
URI: http://eprints.soton.ac.uk/id/eprint/464985
PURE UUID: 306f123f-88ef-4cef-8f3b-ec4dbf48da79
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Date deposited: 05 Jul 2022 00:15
Last modified: 16 Mar 2024 19:52
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Author:
Zoe Joanne Hughes
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