Large horizontal motion of moored large vessels and breaks of mooring ropes are mainly induced by long period waves traveling into harbor. Some countermeasures to prevent the cargo handling interrupt due to long period waves are considered in major ports facing Japanese oceans. One of the most popular countermeasures is
the construction of offshore breakwaters. Meanwhile it is said that the construction of long vertical-wall type breakwater may cause the reduction of seawater exchange between the outer and inner harbors. A permeable-type breakwater is often considered to keep the function of water exchange.
　The wave transmission coefficient of permeable-type breakwaters increases as the wave period increases. Therefore, the permeable-type breakwater has not been expected to be effective against long period waves. In new harbor planning, however, we have to satisfy the both targets of water exchange and prevention of long period waves.
　The paper demonstrates the harbor tranquility calculation for long period waves in a model harbor with a permeable breakwater. The calculation for various wave transmission coefficient Kt of long period waves shows that the allowable maximum level of the transmission coefficient exists for the reduction of the agitation due to long period waves. The target permeability for long period waves becomes important to employ a water-exchange breakwater as the countermeasure.
　Hydraulic experiments in a wave flume are carried out to investigate the performance of the permeable breakwaters for long swells. In the experiment, the rubble mound breakwater with the width at the sea surface level of 20m and 30m in the prototype and the caisson breakwater with water-exchange openings with 6.3% and 12.6% is studied. A numerical analysis employing the MARS (Multi-Advection Reconstruction Solver) method is also applied to simulate the performance of the breakwater in long swells and long period waves. The physical and numerical model tests demonstrate the transmission coefficient Kt varies for the type and width of breakwaters. The breakwater with the target transmission coefficient suitable to raise the harbor tranquility is proposed.