In surf zone, wave height and velocity of incoming waves change dramatically. It has been said that the sand transportin the near shore region is mostly occurred in the surf zone. Sawaragi and Iwata(1973) discovered a coherent structure of turbulent flow under plunging breaker, and they named it as ‘Horizontal roller'. They pointed out that a lot of sediment on the sea bottom is curled up towards water surface by the horizontal roller. Clarification of the turbulent structure in breaking waves make advances in developing the physical models of wave breaking and sand transport. But until now, many attempts to measure the velocity field in surf zone, with the LDV or EMC, have been prevented due to a large amount of bubble entrained by plunging of water jet. Nakaza et.al. (1995) succeeded in measuring of the velocity field contained a large amount of bubble under plunging breaker. In this study, by Nakaza's method utilizing a super-miniature proper current meter, the coherent structures in surf zone, particular the horizontal roller, are clarified experimentally. The generation and dissipation of large eddies in the water under plunging breaker are studied, applying Dean's stream function theory to discrete thepotential and velocity components of flow velocity from measured data.
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