The near-bed mass transport under surf-zone spilling breakers

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 lmm 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. These findings may further provide an explanation for the formation of a sand bar in the surf zone.