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Measurements of the semi-diurnal drag coefficient over sand waves

Measurements of the semi-diurnal drag coefficient over sand waves
Measurements of the semi-diurnal drag coefficient over sand waves
Simultaneous measurements of tidal flow and pressure gradient over a 10 km distance have been used to estimate the frictional drag coefficient over sand waves in the southern North Sea. The measurements were made over a 27-day period in October–November 1988 at 52°10?N, 3°46?E, within a field of essentially two-dimensional sand waves approximately 3 m high and of wavelength 250 m.

The M2 drag coefficient for depth-averaged flow normal to sand wave crests is found to be 2.95 × 10?3, in good agreement with values used in numerical models of the region. The uncertainty in this value is estimated to be of the order of 10%, primarily due to possible errors in the phase of the flow relative to the pressure gradient.

The time series of daily-averaged semi-diurnal (Z2) drag coefficients over the 27-day period shows surprisingly little correlation with nearbed wave orbital velocities. The time of highest waves is associated with a decrease in drag coefficient rather than the increase predicted by most wave-current interaction theories and some previous observations. It is suggested that this behaviour is caused by stratification near the bed due to sediment resuspension under high waves. Predictions using theGlenn andGrant (1987, Journal of Geophysical Research,92, 8244–8264) theory provide qualitative support for this hypothesis, though the magnitude of the predicted effect is smaller than observed.

The relative insensitivity of the drag coefficient to wave conditions suggests that incorporating simple wave-current algorithms into numerical models may be misleading.
0278-4343
437-456
Huntley, D.A.
c0ae6ddd-cb02-432a-a3e8-f4e7dfe161e7
Nicholls, R.J.
4ce1e355-cc5d-4702-8124-820932c57076
Liu, C.
b571ad53-5bc1-44a4-9128-35f73e9d742d
Dyer, K.R.
2a824785-333e-4a16-90c0-e1a48a3a95dd
Huntley, D.A.
c0ae6ddd-cb02-432a-a3e8-f4e7dfe161e7
Nicholls, R.J.
4ce1e355-cc5d-4702-8124-820932c57076
Liu, C.
b571ad53-5bc1-44a4-9128-35f73e9d742d
Dyer, K.R.
2a824785-333e-4a16-90c0-e1a48a3a95dd

Huntley, D.A., Nicholls, R.J., Liu, C. and Dyer, K.R. (1994) Measurements of the semi-diurnal drag coefficient over sand waves. Continental Shelf Research, 14 (5), 437-456. (doi:10.1016/0278-4343(94)90097-3).

Record type: Article

Abstract

Simultaneous measurements of tidal flow and pressure gradient over a 10 km distance have been used to estimate the frictional drag coefficient over sand waves in the southern North Sea. The measurements were made over a 27-day period in October–November 1988 at 52°10?N, 3°46?E, within a field of essentially two-dimensional sand waves approximately 3 m high and of wavelength 250 m.

The M2 drag coefficient for depth-averaged flow normal to sand wave crests is found to be 2.95 × 10?3, in good agreement with values used in numerical models of the region. The uncertainty in this value is estimated to be of the order of 10%, primarily due to possible errors in the phase of the flow relative to the pressure gradient.

The time series of daily-averaged semi-diurnal (Z2) drag coefficients over the 27-day period shows surprisingly little correlation with nearbed wave orbital velocities. The time of highest waves is associated with a decrease in drag coefficient rather than the increase predicted by most wave-current interaction theories and some previous observations. It is suggested that this behaviour is caused by stratification near the bed due to sediment resuspension under high waves. Predictions using theGlenn andGrant (1987, Journal of Geophysical Research,92, 8244–8264) theory provide qualitative support for this hypothesis, though the magnitude of the predicted effect is smaller than observed.

The relative insensitivity of the drag coefficient to wave conditions suggests that incorporating simple wave-current algorithms into numerical models may be misleading.

Full text not available from this repository.

More information

Published date: April 1994
Organisations: Energy & Climate Change Group

Identifiers

Local EPrints ID: 353135
URI: https://eprints.soton.ac.uk/id/eprint/353135
ISSN: 0278-4343
PURE UUID: 94bb17a5-cc03-47d0-8516-9ac367b63a0d
ORCID for R.J. Nicholls: ORCID iD orcid.org/0000-0002-9715-1109

Catalogue record

Date deposited: 18 Jun 2013 12:37
Last modified: 06 Jun 2018 12:44

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