Laboratory Measurements of Turbulence Under Shoaling Waves

Understanding the sediment transport in surf and swash zone is essential for numerous coastal projects and applications. Many time-dependent models parameterize the sediment transport. While some can be quite successful in predicting the evolution of the bed shape, our understanding of the sediment suspension mechanisms is still rather poor. When the waves approach the shore and break, the energy of the waves is lost to the generation of turbulence and currents, the entrainment of bubbles, and a fraction of the energy from breaking waves is transferred to suspend the sediment particles. A further investigation of the turbulence caused by breaking waves, resulting in the suspension of the sediment particles from the beach bed, is therefore necessary to improve our understanding of the suspension processes. We have used a Particle Image Velocimetry (PIV) technique to study the temporal and spatial structure of the turbulence in the bottom boundary layer. The experiments were performed in the large linear wave flume at the Air-Sea Interaction Laboratory at the University of Delaware. Continuous linear monochromatic deepwater waves were mechanically generated and sent to break naturally on an adjustable planner Plexiglas beach. The beach angle was adjusted to 1:2.6, 1:3.2, or 1:4.6 and a number of wave amplitudes were utilized for each of the beach angle. The PIV field of view was also arranged parallel to the beach bottom and in the vertical plane. The measurements were taken after reaching a quasi-steady state. Ensemble phase averages are used to obtain mean and turbulent fields. Time evolution is studied to investigate the formation and evolution of the turbulent structures and hairpin vortices within the bottom boundary layer. The results are discussed in the context of sediment transport.