Experimental observations for the vertical distributions of velocity profiles and corresponding Eulerian mass transport under spilling breakers in outer and inner surf-zone boundary layer are presented. A particular efficient and non-intrusive high-resolution particle image velocimetry (PIV) is employed to measure the flow on a slope of 1/20. Particle velocities within the viscous boundary layer with thickness of 1mm were measured simultaneously over ten points. The velocity overshooting and velocity gradient are clearly observed in the viscous bottom region. The variations of velocity profiles in the outer and inner surf zone were examined and discussed. It is found that the near-bed mass transport is still onshore in the outer surf zone but change to offshore in the inner surf zone. Combined with previous observations, it is suggested that the near-bed mass transport is onshore from the shoaling region to the outer surf zone, but changes to offshore from the outer to the inner surf zone, which implies the amassment of sediment in the borderline.
Understanding boundary layer flow in the surf zone is important to geophysical, oceanic, coastal, and environmental researchers because of its significance for energy dissipation and sediment transport. Surf-zone turbulence is generated primarily in the crest region through wave breaking and secondarily by shear stress in the wave bottom boundary layer (WBBL). The turbulence produced in the crest region stretches into the interior region below the wave trough during the breaking process (Lin and Hwung 1992; Ting and Kirby 1996; Christensen et al. 2002). Turbulence may further penetrate into the WBBL and change the internal characteristics of flow motion. However, few studies have addressed boundary-layer turbulence as well as the effect of the interaction between the turbulence generated by these two sources in the crest region and WBBL (Christensen et al. 2002).
