Fundamental issues in Japan’s management system of coast for preventing beach erosion

When an offshore or port breakwater is constructed on a coast, beach erosion often occurs on nearby beaches of the breakwater due to the wave diffraction effect of the breakwater associated with the formation of a wave-shelter zone, because longshore sand transport is triggered from outside to inside the wave-shelter zone. Similarly, when unidirectional longshore sand transport is blocked by a breakwater, beach erosion will occur downcoast. In these cases, longshore sand movement is the key factor. Another aspect is arisen from the management system of the land near a coast subject to such longshore sand movement. In Japan, the management of coastal land is under the jurisdiction of several agencies. When sand is transported alongshore across 2 management areas, the sand right belongs to the agency administrating the area to which the sand is deposited, and the agency administrating the area from which the sand originated has no right. Thus, this leads to uncoordinated solution to erosion problems, because longshore sand can freely move across the boundaries of coastal management areas. In this study, these issues were studied through real examples. Even though the accuracy of the predictive model of beach changes is increased, implementing fundamental changes of the coastal condition is difficult when this issue is unsolved as it is.