Limited information on the velocity field within the surf zone is available at present, mainly because of the difficulty of velocity measurements. In this experimental study, a new velocimeter of laser-Doppler anemometry is used to make clear the structure of internal velocity field in the surf zone.
　As a result, spatial distributions of the asymmetries of orbital velocity profile, the turbulence intensity, the entrained air bubble density and Eulerian mean velocity are successfully revealed. Close examinations on the interrelationships between these factors show that velocity field in the surf zone is clearly divided into two regions, i.e., the transition zone and bore establishment zone. Further, it is found that velocity field within the bore establishment zone consists of the upper layer and bottom layer. The upper layer is characterized by the existence of large scale
eddies, associated with the small scale turbulence and entrained air bubbles, whereas the bottom layer is characterized by coexistence of the small sclae turbulence originated from the upper layer and the bottom originated turbulence. The mechanism of onshore-offshore movements of bottom sand can be better understood by decomposing the bottom velocity field into the Eulerian mean velocity and the asymmetrically fluctuating orbital velocity; the former tends to transport sand offshore-ward and the latter onshoreward.