AbstractAs coastal areas are very valuable for mountainious Japan, many countermeasures against beach erosion have been taken. The length of seawalls for beach erosion amounts to 5,600 kilometers, which is much longer than the length of seadikes for flood tide, which is 2,900 kilometers. In addition, 10,000 groins and 2,300 detached breakwaters have already been constructed for beach erosion. Especially, we used many detached breakwaters for the last ten years. Fig.1 showes the increase of total length of seawalls and seadikes for the last twenty years.
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