Laboratory measurements of large-scale near-bed turbulent flow structures under plunging regular waves

The turbulent velocity field associated with the breaking of plunging regular waves on a 3% plane slope was measured in a plane running parallel to the slope using a particle image velocimetry (PIV) system. The measurement plane was located within the wave bottom boundary layer. The horizontal distance from the point of incipient breaking to the center of the measurement area was approximately 12 times of the breaking depth. The same wave train was generated 36 times and in each trial three consecutive wave cycles were recorded at a sampling rate of 15 Hz. The measured velocity fields were separated into a mean flow and a turbulence component using the ensemble averaging technique. The impingement process of breaking-wave-generated vortices on the bottom was investigated. The results showed that the impact of a plunging wave vortex on the bottom was a highly transient and three-dimensional phenomenon. The vortex arrived at the bottom around the instant of maximum positive wave-induced velocity. The surge of turbulence continued for a time of about 2Hb/g, where Hb is breaker height and g is acceleration due to gravity. The impingement region was not stationary, but continued to travel onshore with an initial speed close to the wave celerity. The distributions of turbulent velocity fluctuations and related momentum fluxes depended on the types of vortices produced. Plunger vortices generated at incipient breaking in deeper water had the characteristics of a three-dimensional vortex loop with counter-rotating vorticity. Large apparent shear stresses were measured in the flow attachment and detachment zones in front and behind the vortex loop. Transverse vortices generated in the subsequent splash in shallower water produced an asymmetrical impingement pattern similar to that of an inclined jet; the downburst of turbulent fluid was deflected outward and shoreward resulting in large onshore fluxes of turbulence energy near the bottom. Large apparent shear stresses were measured in the impingement zone and wall jet region. The motions of glass spheres sliding along the bottom were investigated. It was found that the velocities of glass spheres impacted by downbursts could significantly exceed the wave-induced velocities. It was also found that glass spheres could be trapped by counter-rotating vortices and carried for considerable distances onshore. The measured data suggested that compared to spilling waves, downbursts in plunging waves would enhance onshore sediment transport.