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Wave-induced coherent turbulence structures and sediment resuspension in the nearshore of a prototype-scale sandy barrier beach

Wave-induced coherent turbulence structures and sediment resuspension in the nearshore of a prototype-scale sandy barrier beach
Wave-induced coherent turbulence structures and sediment resuspension in the nearshore of a prototype-scale sandy barrier beach

The suspension of sediments by oscillatory flows is a complex case of fluid–particle interaction. The aim of this study is to provide insight into the spatial (time) and scale (frequency) relationships between wave-generated boundary layer turbulence and event-driven sediment transport beneath irregular shoaling and breaking waves in the nearshore of a prototype sandy barrier beach, using data collected through the Barrier Dynamics Experiment II (BARDEX II). Statistical, quadrant and spectral analyses reveal the anisotropic and intermittent nature of Reynolds’ stresses (momentum exchange) in the wave boundary layer, in all three orthogonal planes of motion. The fractional contribution of coherent turbulence structures appears to be dictated by the structural form of eddies beneath plunging and spilling breakers, which in turn define the net sediment mobilisation towards or away from the barrier, and hence ensuing erosion and accretion trends. A standing transverse wave is also observed in the flume, contributing to the substantial skewness of spanwise turbulence. Observed low frequency suspensions are closely linked to the mean flow (wave) properties. Wavelet analysis reveals that the entrainment and maintenance of sediment in suspension through a cluster of bursting sequence is associated with the passage of intermittent slowly–evolving large structures, which can modulate the frequency of smaller motions. Outside the boundary layer, small scale, higher frequency turbulence drives the suspension. The extent to which these spatially varied perturbation clusters persist is associated with suspension events in the high frequency scales, decaying as the turbulent motion ceases to supply momentum, with an observed hysteresis effect.
Coherent turbulence structures, Sediment resuspension, Wavelets
0278-4343
78-94
Kassem, Hachem
658efa7a-a02c-4b29-9d07-5d57e95a4b51
Thompson, C.E.L.
2a304aa6-761e-4d99-b227-cedb67129bfb
Amos, C.L.
d0a18a13-bccd-4fdc-8901-aea595d4ed5c
Townend, I.H.
6c669d5b-c9ff-45f4-93f9-f8a29329c20a
Kassem, Hachem
658efa7a-a02c-4b29-9d07-5d57e95a4b51
Thompson, C.E.L.
2a304aa6-761e-4d99-b227-cedb67129bfb
Amos, C.L.
d0a18a13-bccd-4fdc-8901-aea595d4ed5c
Townend, I.H.
6c669d5b-c9ff-45f4-93f9-f8a29329c20a

Kassem, Hachem, Thompson, C.E.L., Amos, C.L. and Townend, I.H. (2015) Wave-induced coherent turbulence structures and sediment resuspension in the nearshore of a prototype-scale sandy barrier beach. Continental Shelf Research, 109, 78-94. (doi:10.1016/j.csr.2015.09.007).

Record type: Article

Abstract


The suspension of sediments by oscillatory flows is a complex case of fluid–particle interaction. The aim of this study is to provide insight into the spatial (time) and scale (frequency) relationships between wave-generated boundary layer turbulence and event-driven sediment transport beneath irregular shoaling and breaking waves in the nearshore of a prototype sandy barrier beach, using data collected through the Barrier Dynamics Experiment II (BARDEX II). Statistical, quadrant and spectral analyses reveal the anisotropic and intermittent nature of Reynolds’ stresses (momentum exchange) in the wave boundary layer, in all three orthogonal planes of motion. The fractional contribution of coherent turbulence structures appears to be dictated by the structural form of eddies beneath plunging and spilling breakers, which in turn define the net sediment mobilisation towards or away from the barrier, and hence ensuing erosion and accretion trends. A standing transverse wave is also observed in the flume, contributing to the substantial skewness of spanwise turbulence. Observed low frequency suspensions are closely linked to the mean flow (wave) properties. Wavelet analysis reveals that the entrainment and maintenance of sediment in suspension through a cluster of bursting sequence is associated with the passage of intermittent slowly–evolving large structures, which can modulate the frequency of smaller motions. Outside the boundary layer, small scale, higher frequency turbulence drives the suspension. The extent to which these spatially varied perturbation clusters persist is associated with suspension events in the high frequency scales, decaying as the turbulent motion ceases to supply momentum, with an observed hysteresis effect.

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More information

Published date: 15 October 2015
Keywords: Coherent turbulence structures, Sediment resuspension, Wavelets
Organisations: Ocean and Earth Science, Coastal & Shelf Research, Energy & Climate Change Group, Faculty of Engineering and the Environment

Identifiers

Local EPrints ID: 381868
URI: http://eprints.soton.ac.uk/id/eprint/381868
ISSN: 0278-4343
PURE UUID: bc6f9916-bb1a-4978-9af3-95cb8a8a3f32
ORCID for Hachem Kassem: ORCID iD orcid.org/0000-0002-5936-6037

Catalogue record

Date deposited: 24 Sep 2015 08:24
Last modified: 18 Feb 2021 17:22

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