2023 Volume 44 Issue 4 Pages 131-147
In Japan, the management of coastal land is under the jurisdiction of several agencies, and each agency is set up in accordance with provisions of relevant laws. The port area is managed on the basis of the Port Law, the farmland near the coastline including the coastal forest is managed by the Forest Law, and the other areas are controlled by the Seacoast Law. At the same time, the academic system is separated corresponding to these works. Accordingly, the local optimization method has been widely employed in solving the issues in each area and the comprehensive view regarding the management of the extensive area considering the overall sediment budget has been lacking, resulting in the disharmony in coastal zone. In this study, the conservation of the coastal zone in view of the comprehensive management was considered, taking a coast with approximately 40 km length between Oarai and Kashima Ports facing the Pacific Ocean. When an offshore breakwater is constructed on such a coast, beach erosion often occurs on nearby beaches of the breakwater by the formation of a wave-shelter zone associated with the extension of the breakwater because of the occurrence of longshore sand transport from outside to inside the wave-shelter zone. Both bathymetric survey data and aerial photographs in the study area collected since 1979 were analyzed. It was found that 7.9×106 m3 of sand was deposited between 1979 and 2011 inside Oarai Port, resulting in the mean shoreline recession of 23 m in the other areas. It was concluded that it is important to consider the conservation of the entire area instead of local optimization at a local coast in view of geomorphological viewpoint.
