Effects of wave breaking are felt even at the sediment bed
When waves break, their organized motion is converted into random velocity fluctuations (turbulence). These turbulent fluctuations contribute to the transport of sediment particles, and consequently, lead to changes of the beach profile. The effects of these fluctuations should therefore be considered when trying to predict the long-term coastal evolution and to improve coastal safety. However, wave breaking effects on sand transport are often neglected in such predictions because the effects are complex and not well understood.
This motivated a new large-scale experiment in the CIEM wave flume in Barcelona (photo). The experiment addresses the effects of wave breaking on the velocity field and on sediment transport. A first study using these data focuses on the flow in the lower 10 cm of the water column: the wave bottom boundary layer. Specifically, the study examines whether the breaking wave can affect the flow close to the sediment bed.
The answer is yes: the turbulent fluctuations near the bed are much more energetic in the breaking region than under non-breaking waves. This means that wave breaking affects velocities over the complete water column, from water surface all the way down to the bed. The timing of these turbulence events arriving to the bed is related to the velocity field under the breaking wave. This timing is probably important for sediment transport.
Forthcoming work will relate the velocity and turbulence observations to the transport of sand particles and to the evolution of the beach profile. These results contribute to the understanding of coastal systems and to the design of coastal protection measures such as sand nourishments.
The full paper recently appeared in Journal of Geophysical Research: Oceans (link). This research is part of the UK/NL joint project ‘SINBAD’, funded by STW and EPSRC.
Published: 1st September 2016