Statistical Analysis of Detached Breakwaters in Japan

Construction of detached breakwaters is one of the main countermeasures against beach erosion in Japan. The number of breakwaters has been rapidly increasing, with about 2500 constructed as of the end of 1981. This study aims at the investigation of the effect and stability of detached breakwaters along Japan’s coasts on the basis of survey of 1552 breakwaters constructed by the Ministry of Construction. The existing conditions of the dimensions of the detached breakwaters are statistically analyzed; the optimal dimensions for sand deposition behind a breakwater are proposed, and the critical conditions for advance of the shoreline facing an opening of breakwaters are investigated. Furthermore, the relations between the scattering rate of concrete blocks and various conditions such as bottom slope, the depth at the breakwater, the offshore distance of the breakwater and the weight of the blocks are studied. It is concluded that, in order to prevent scattering, the weight of the blocks should be at least 1.5 times heavier than that calculated from the Hudson formula.