Reliability design of composite breakwaters based on the deformation level will be one of the most important future design methodologies; however, it has not been applied to actual design since it is very difficult to take the various design conditions into account. In this study, we investigated the procedures of reliability design of composite breakwaters based on the sliding distance in order to introduce this design procedure to the practical design.
　The main conclusions of this paper are as follows:
1) Relations among the criteria for the design method based on sliding distance, such as the expected sliding distance, the average sliding distance for the design wave, and the exceedance probability for certain sliding distance, are greatly dependent on the distribution of extreme wave height and water depth.
2) The thresholds of exceedance probability for several sliding distances were proposed according to the importance of structures, as the rational performance design of composite breakwaters. This design method guarantees the structures for the same stability irrespective of design conditions, such as the distribution of extreme wave height and water depth.
3) A series of model experiments for investigating the characteristics of wave forces on horizontally composite breakwaters were conducted, and the new time series model of the wave force used for calculating the sliding distance were proposed.
4) Trial designs of actual breakwater in Kochi port were carried out to compare the cross section calculated from the current design and the performance design based on the sliding distance. The minimum caisson width by the performance design was 14% smaller than that by the current design.
5) Composite breakwaters designed by current design method in H.H.W.L. generally have higher stability, and the caisson width can be reduced by introducing the performance design based on the deformation level. Construction cost will be further reduced when the performance design based on the displacement due to mound failure is also established.