IMPROVEMENT OF COMPOSITE BREAKWATER ON SOLID BOTTOM AGAINST SEVER TSUMAMIS

Recent diversification in the use of coastal areas has increased the importance of tsunami prevention measures in the areas. To prevent disasters, it is important to set up various facilities, including tsunami breakwaters and tide embankments, and to improve systems to ensure the safety of people escaping from tsunamis with larger waves than predicted when designing the facilities. Previous studies showed that coastal and port structures are effective to decrease damage caused by tsunamis. These structures are, however, easy to overturn or slide, since they are designed without considering tsunamis as external forces on them. The North Breakwater in Okushiri Port damaged by the tsunamis of the Hokkaido Nanseioki Earthquake in 1993 is an example of a damaged composite breakwater (Photo 1). Recently, a composite breakwater has become popular as a port structure to cope with enlargement of ports and weak ground of coastal areas. It has two tiers: a caisson functioning as an upright breakwater on top, and a mound (rubble-mound foundation) functioning as a sloping breakwater on the bottom. Though a composite breakwater is frequently used as a tsunami breakwater, damage to it by wave forces larger than the external forces predicted for its design has not been sufficiently taken into account. In most cases, an incident wave of a tsunami on the composite breakwater is broken up due to the influence of the water depth,to become a tsunami.Tsunamis have the largest destructive force on a breakwater.