Artificial reefs have been constructed for protecting coast form erosion and for replacing detached breakwaters. The constrictions cause topographical change around them. Main sediment transport around an opening is bed load, but suspended sediment cannot be ignored. So the experiment using 3-ADV (Acoustic Doppler Velocity Meter) was carried out to measure the vertical mean velocity distribution at a center of opening.The result of experiment shows that the return flows direction was upward at onshore, and downward at an opening. The return flows turn alternately up and down. The amplitude of mean horizontal velocity was the maximum at the bottom. Closing to water surface, the amplitudes of velocity were getting to small.

Artificial reefs have been used to protect coastal erosion, and are now replacing detached breakwaters. When we establish a submerged breakwater or an artificial reef, it is known that a topography change

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