Morphological data obtained for 16 years from 1989 to 2004 around a submerged breakwater on the Niigata coast, which is 40 m in width and about 1500 m in alongshore length, were analyzed. The submerged breakwater and groins stopped beach erosion that had taken place on the seaward side of the existing detached breakwaters. They also stabilized the sandy beach behind the submerged breakwater.
　A scour of 1.5 m was generated shoreward of the submerged breakwater just after the start of the construction work of the submerged breakwater. The scour expanded with the breakwater extension. When the ends of the submerged breakwater reached near the tips of the first and second groins constructed before the extension of the breakwater, the scour deepened to about 3 m. However, in spite of the generation and development of the scour, the influence of the scour was limited. The beach shoreward of the scour was stable in the region protected by the submerged breakwater and groins.
　Although existing detached breakwaters induced beach erosion on the seaward side of the structures, accretion was observed seaward of the submerged breakwater.