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Article

Observations of Nearbed Turbulence over Mobile Bedforms in Combined, Collinear Wave-Current Flows

1
Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton Waterfront Campus, European Way, Southampton SO14 3ZH, UK
2
Channel Coastal Observatory, National Oceanography Centre, Southampton SO14 3ZH, UK
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HR Wallingford, Howbery Park, Wallingford, Oxfordshire OX10 8BA, UK
4
Van Oord UK Ltd., Bankside House, Henfield Road, Small Dole BN5 9XQ, UK
*
Author to whom correspondence should be addressed.
Water 2020, 12(12), 3515; https://doi.org/10.3390/w12123515
Received: 20 October 2020 / Revised: 30 November 2020 / Accepted: 10 December 2020 / Published: 14 December 2020
(This article belongs to the Special Issue Sediment Transport in Coastal Waters 2.0)
Collinear wave-current shear interactions are often assumed to be the same for currents following or opposing the direction of regular wave propagation; with momentum and mass exchanges restricted to the thin oscillating boundary layer (zero-flux condition) and enhanced but equal wave-averaged bed shear stresses. To examine these assumptions, a prototype-scale experiment investigated the nature of turbulent exchanges in flows with currents aligned to, and opposing, wave propagation over a mobile sandy bed. Estimated mean and maximum stresses from measurements above the bed exceeded predictions by models of bed shear stress subscribing to the assumptions above, suggesting the combined boundary layer is larger than predicted by theory. The core flow experiences upward turbulent fluxes in aligned flows, coupled with sediment entrainment by vortex shedding at flow reversal, whilst downward fluxes of eddies generated by the core flow, and strong adverse shear can enhance near-bed mass transport, in opposing currents. Current-aligned coherent structures contribute significantly to the stress and energy dissipation, and display characteristics of wall-attached eddies formed by the pairing of counter-rotating vortices. These preliminary findings suggest a notable difference in wave-following and wave-opposing wave-current interactions, and highlight the need to account for intermittent momentum-exchanges in predicting stress, boundary layer thickness and sediment transport. View Full-Text
Keywords: wave-current interactions; boundary layers; coherent turbulence structures; bedforms; sediment dynamics wave-current interactions; boundary layers; coherent turbulence structures; bedforms; sediment dynamics
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MDPI and ACS Style

Kassem, H.; Thompson, C.E.L.; Amos, C.L.; Townend, I.H.; Todd, D.; Whitehouse, R.J.S.; Chellew, E. Observations of Nearbed Turbulence over Mobile Bedforms in Combined, Collinear Wave-Current Flows. Water 2020, 12, 3515. https://doi.org/10.3390/w12123515

AMA Style

Kassem H, Thompson CEL, Amos CL, Townend IH, Todd D, Whitehouse RJS, Chellew E. Observations of Nearbed Turbulence over Mobile Bedforms in Combined, Collinear Wave-Current Flows. Water. 2020; 12(12):3515. https://doi.org/10.3390/w12123515

Chicago/Turabian Style

Kassem, Hachem, Charlotte E.L. Thompson, Carl L. Amos, Ian H. Townend, David Todd, Richard J.S. Whitehouse, and Elizabeth Chellew. 2020. "Observations of Nearbed Turbulence over Mobile Bedforms in Combined, Collinear Wave-Current Flows" Water 12, no. 12: 3515. https://doi.org/10.3390/w12123515

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