Laboratory Measurements of Velocity Field Structure in the Surf Zone by LDV

A series of measurements have been made to clarify the structure of internal velocity field within the surf zone by using a laser-Doppler velocimeter. The measurements establish that the velocity field within the surf zone is clearly divided into two regions of the transition zone and the bore establishment zone. Further, it is found that the velocity field within the bore establishment zone consists of the upper layer and the bottom layer. The former is characterized by the existence of large scale eddies containing most of entrained air bubbles and small scale turbulence associated with them, whereas the latter is characterized by the coexistence of the small scale turbulence originated from the upper layer and the bottom originated turbulence. The mechanism of onshore-offshore movements of the 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 offshoreward and the latter onshoreward. Relative magnitude of these two factors depends upon the offshore wave steepness.