Laboratory measurements of bed shear stress under spilling and plunging regular waves

The bed shear stress induced by breaking-wave-generated vortices on a plane slope was investigated for spilling and plunging regular waves. Three-component, three-dimensional (3C3D) velocity measurements were conducted from the free surface to the bottom in the inner surf zone using a Volumetric Three-Component Velocimetry (V3V) system. The 3C3D measurements were supplemented by high resolution two-dimensional (2D) velocity measurements inside the bottom boundary layer obtained using a Particle Image Velocimetry (PIV) system. The bed shear stress was determined from the measured velocities in the viscous sublayer using the velocity gradient method. The measured data were used to study the evolution of the large eddies and the characteristics of their induced bed shear stresses. The results showed that, in both spilling and plunging waves, the basic three-dimensional (3D) structure of the large eddies was a coherent vortex loop consisting of two counter-rotating vortices with strong downward momentum. Breaking wave vortices impinged on the bottom under the wave crest in plunging waves and induced large bed shear stress fluctuations when the wave-induced (mean) flow was shoreward, while they reached the bottom later in spilling waves and mainly affected the negative bed shear stresses after flow reversal. When the boundary layer was not disturbed by breaking wave turbulence, the near-bed velocity profile was similar to that observed in non-breaking waves in the laminar/transitional turbulent flow regime. The viscous sublayer and most of the buffer layer were developed by the time of the wave crest and wave trough phases, but a logarithmic region did not always exist. It was found that the instantaneous velocities did not generally conform to a law-of-the-wall profile during vortex impingement and the Spalding wall function underpredicted the bed shear stress in some cases but overpredicted in others. It was also found that the fluid shear stress measured at only a small distance above the bed grossly overestimated the instantaneous bed shear stress, and that a good correlation between the two parameters was observed only when sweep type events occurred in the bed region.