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Turbulence based measurements of wave friction factors under irregular waves on a gravel bed

Turbulence based measurements of wave friction factors under irregular waves on a gravel bed
Turbulence based measurements of wave friction factors under irregular waves on a gravel bed
Very few studies have quantified the wave friction factor, fw, for coarse sediments at field-scale. To address this shortcoming, high-frequency measurements of turbulence obtained within the boundary layer of irregular waves over gravel in the Delta Flume, have been used to calculate values of fw using different evaluation methods. In the field-scale laboratory experiments reported here, three velocimeters were deployed on the seaward side of a 4 m-high, 5 m-wide and 55 m-long gravel barrier subject to a JONSWAP spectrum of waves with significant wave heights ranging from 0.8 m to 1.3 m, and peak periods of 3.0 s to 10 s and offshore water depths ranging from 1.75 m to 3.75 m. The deployment area was essentially flat, with little or no predicted or observed sediment movement under the wave conditions investigated. The turbulent kinetic energy method was found to be the most suitable approach for calculating the bed shear stress, which can be related to fw. Wave friction factor values under the conditions tested here fell in the range 0.01 and 0.27. Although fw predicted by an existing equation agrees well with the mean measured fw value, the application of a new predictor for fw is recommended for improved parameterisation of skin friction over the range of relative roughness values encountered in this study. This approach combines the wave Reynolds number, wave steepness and relative depth to provide a simple expression to assist assessments of coarse sediment transport by waves for uses within a range of practical engineering applications.
BARDEX, Gravel, Bed roughness, Bed shear stress, Wave friction factor
0378-3839
39-47
Thompson, C.E.L.
2a304aa6-761e-4d99-b227-cedb67129bfb
Williams, J.J.
dc0386a7-654d-441c-bb19-f11c5fe49113
Metje, N.
87608f83-cb14-472a-a6e6-9bd78af44ee7
Coates, L.E.
72f9b4ca-215c-4517-8509-33816a3ddd2d
Pacheco, A.
7abfb34e-5ba1-4c35-b98d-36999f2df164
Thompson, C.E.L.
2a304aa6-761e-4d99-b227-cedb67129bfb
Williams, J.J.
dc0386a7-654d-441c-bb19-f11c5fe49113
Metje, N.
87608f83-cb14-472a-a6e6-9bd78af44ee7
Coates, L.E.
72f9b4ca-215c-4517-8509-33816a3ddd2d
Pacheco, A.
7abfb34e-5ba1-4c35-b98d-36999f2df164

Thompson, C.E.L., Williams, J.J., Metje, N., Coates, L.E. and Pacheco, A. (2012) Turbulence based measurements of wave friction factors under irregular waves on a gravel bed. Coastal Engineering, 63, 39-47. (doi:10.1016/j.coastaleng.2011.12.011).

Record type: Article

Abstract

Very few studies have quantified the wave friction factor, fw, for coarse sediments at field-scale. To address this shortcoming, high-frequency measurements of turbulence obtained within the boundary layer of irregular waves over gravel in the Delta Flume, have been used to calculate values of fw using different evaluation methods. In the field-scale laboratory experiments reported here, three velocimeters were deployed on the seaward side of a 4 m-high, 5 m-wide and 55 m-long gravel barrier subject to a JONSWAP spectrum of waves with significant wave heights ranging from 0.8 m to 1.3 m, and peak periods of 3.0 s to 10 s and offshore water depths ranging from 1.75 m to 3.75 m. The deployment area was essentially flat, with little or no predicted or observed sediment movement under the wave conditions investigated. The turbulent kinetic energy method was found to be the most suitable approach for calculating the bed shear stress, which can be related to fw. Wave friction factor values under the conditions tested here fell in the range 0.01 and 0.27. Although fw predicted by an existing equation agrees well with the mean measured fw value, the application of a new predictor for fw is recommended for improved parameterisation of skin friction over the range of relative roughness values encountered in this study. This approach combines the wave Reynolds number, wave steepness and relative depth to provide a simple expression to assist assessments of coarse sediment transport by waves for uses within a range of practical engineering applications.

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More information

Published date: May 2012
Keywords: BARDEX, Gravel, Bed roughness, Bed shear stress, Wave friction factor
Organisations: Coastal & Shelf Research

Identifiers

Local EPrints ID: 347124
URI: http://eprints.soton.ac.uk/id/eprint/347124
ISSN: 0378-3839
PURE UUID: 37537f16-4662-4400-98bd-3af8a0762fd3
ORCID for C.E.L. Thompson: ORCID iD orcid.org/0000-0003-1105-6838

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Date deposited: 16 Jan 2013 17:12
Last modified: 15 Mar 2024 03:05

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Contributors

Author: C.E.L. Thompson ORCID iD
Author: J.J. Williams
Author: N. Metje
Author: L.E. Coates
Author: A. Pacheco

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