Laboratory Observation of Turbulence and Wave Shear Stresses Under Large Scale Breaking Waves Over a Mild Slope

Laboratory measurements of turbulent flow and shear stresses in a surf zone under monochromatic breaking waves in a large-scale wave flume are presented. Waves with various surf similarity parameters were generated to break at nearly identical water depths over a 1/100 mild slope. Free surface elevations were measured along the plane beach. Velocities in a water column located at the breaking region were measured by acoustic Doppler velocimeters. Results from wave shoaling coefficient, breaking index, and images are inconsistent with the breaker type categorizations from the surf similarity parameter. This suggests that a modification of the surf similarity parameter for very mild slopes may be necessary. For the plunging breakers, the ensemble-averaged results indicate that intense turbulence kinetic energy was initially generated by large eddies at the surface then transported to the bottom frequently. The instantaneous results demonstrate that the intermittency of turbulence kinetic energy near the bed was caused by large eddies generated by wave overturning and relatively smaller obliquely descending eddies. Moreover, the magnitude of the wave shear stress (WSS) is one order of magnitude larger than that of the turbulent shear stress, indicating that the effects of the bottom slope, wave breaking, and bed friction are significant in the surf zone. The sign of the time-averaged WSS switched between positive and negative, depending on the occurrences of phase lag and phase lead of the vertical coherent velocity. It suggests that a change may be required in Zou et al.'s (2006, https://doi.org/10.1029/2005JC003300) theory on the formulation of wave-breaking induced WSS.