ON THE EFFECT OF BREAKWATERS AGAINST TSUNAMI

Abstract

As well known the coast of Japan is often attacked by tsunami. As a kind of disaster preventive facilities, breakwaters have been built or are now under construction in several ports in order to protect the harbour area by diminishing the energy of rushing water and by reducing the water level elevation inside the basin. The biggest work of this sort is found at the port of Ofunato, Iwate Prefecture. The effect of breakwaters against such long-period wave as tsunami was investigated by the authors, using electronic computers. The principle is to solve the equations of motion and of continuity. Not only for the stationary state but also for the transient state. One of the most important items to be studied is the response of basin water to the incoming tsunami, of which the period is variable in a wide range from several minutes to more than one hour. The result of calculation has clarified the behaviour of tsunami affected by breakwaters located in a bay open to the outer sea. The computing method, together with an approximate method divised from fundamental investigations made in advance, has also been applied to several other ports, supplying a plenty of data for harbour planning and design.