Water and sediment movement in the vicinity of linear sand banks : the Norfolk Banks, southern North Sea
Water and sediment movement in the vicinity of linear sand banks : the Norfolk Banks, southern North Sea
Self-recording current meter data have revealed patterns of water movement in the vicinity of the Broken Bank, southern North Sea. The M2 harmonic is dominant throughout the area; the M4 component is, however, more distinctive at locations adjacent to the banks within the present study area. This may be indicative of local (topographic) generation of M4 tidal currents.
Tidal currents are strongly rectilinear, creating narrow tidal ellipses. The axis of the Broken Bank is, on average, orientated at approximately 20° in an anticlockwise sense to the direction of the dominant flow.
Along the eastern flank of the Broken Bank residual flow is directed towards the southeast and is, therefore, largely parallel to the axis of the Broken Bank. Flow along the western flank is more variable, but directed generally towards the northwest. In the swale between the Broken and Well Banks, the observed near-bed residual current direction is in an offshore (northeasterly) direction.
Within the present study area, it has been found that linear wave theory does not reproduce accurately wave-induced orbital velocities observed near the sea bed.
Concurrently recorded hydrodynamic data reveal that sea bed profiles, interpreted from near-bed photographs, are not controlled explicitly by either waves or currents. Rather, that observed ripple dimensions exhibit some agreement with formulae derived for both wave- and current-dominated conditions. The bed roughness 'felt' by the flow is related here to the ripple height and the angle made between the ripple crests and the direction of tidal current flow. This relationship improves considerably on the performance of existing roughness formulae. Rates of ripple migration have been used to investigate the ability of existing predictive sediment transport formulae to reproduce observed transport rates. A predictive formula has been presented which improves significantly on the performance of existing formulae.
Regional sediment transport patterns have been determined using formulae developed as part of the present investigation. These patterns reveal that this area is highly active; critical erosion velocities may be exceeded by up to 77% of the time. Currents along each flank of the Broken Bank are equally capable of transporting sediment. Sediment movement is directed predominantly towards the southeast on the eastern flank and towards the northwest on the western flank.
University of Southampton
Powell, Huw Joseph
59a56c43-f5d9-4f3e-9abd-e1330c114572
1996
Powell, Huw Joseph
59a56c43-f5d9-4f3e-9abd-e1330c114572
Powell, Huw Joseph
(1996)
Water and sediment movement in the vicinity of linear sand banks : the Norfolk Banks, southern North Sea.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
Self-recording current meter data have revealed patterns of water movement in the vicinity of the Broken Bank, southern North Sea. The M2 harmonic is dominant throughout the area; the M4 component is, however, more distinctive at locations adjacent to the banks within the present study area. This may be indicative of local (topographic) generation of M4 tidal currents.
Tidal currents are strongly rectilinear, creating narrow tidal ellipses. The axis of the Broken Bank is, on average, orientated at approximately 20° in an anticlockwise sense to the direction of the dominant flow.
Along the eastern flank of the Broken Bank residual flow is directed towards the southeast and is, therefore, largely parallel to the axis of the Broken Bank. Flow along the western flank is more variable, but directed generally towards the northwest. In the swale between the Broken and Well Banks, the observed near-bed residual current direction is in an offshore (northeasterly) direction.
Within the present study area, it has been found that linear wave theory does not reproduce accurately wave-induced orbital velocities observed near the sea bed.
Concurrently recorded hydrodynamic data reveal that sea bed profiles, interpreted from near-bed photographs, are not controlled explicitly by either waves or currents. Rather, that observed ripple dimensions exhibit some agreement with formulae derived for both wave- and current-dominated conditions. The bed roughness 'felt' by the flow is related here to the ripple height and the angle made between the ripple crests and the direction of tidal current flow. This relationship improves considerably on the performance of existing roughness formulae. Rates of ripple migration have been used to investigate the ability of existing predictive sediment transport formulae to reproduce observed transport rates. A predictive formula has been presented which improves significantly on the performance of existing formulae.
Regional sediment transport patterns have been determined using formulae developed as part of the present investigation. These patterns reveal that this area is highly active; critical erosion velocities may be exceeded by up to 77% of the time. Currents along each flank of the Broken Bank are equally capable of transporting sediment. Sediment movement is directed predominantly towards the southeast on the eastern flank and towards the northwest on the western flank.
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Published date: 1996
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Local EPrints ID: 459374
URI: http://eprints.soton.ac.uk/id/eprint/459374
PURE UUID: 0a8d7150-cc1d-4a26-b05b-17e8033a8e32
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Date deposited: 04 Jul 2022 17:09
Last modified: 16 Mar 2024 18:30
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Author:
Huw Joseph Powell
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