Breakwaters have been built since long ago and the design criteria have arleady been almost established through successful and unsuccessful experiences all over the world. There still remain, however, a lot of detailed problems in strcutural design of breakwaters.
　This report deals with various problems related to the stability of breakwaters under wave action, such as the stability of vertical breakwater against sliding or overturn, stability of rubble mound as a foundation for vartical wall, stability of natural or artificial blocks in mound breakwater, dynamic behaviour of wave resistant structures and so on.
　One of the most important items to be studied is the relation between the practical design criteria and the irregularity of natural sea waves. Most experimental and theoretical investigations have so far been conducted with regular waves of constant amplitude and peirod. At present, however, more acurate examination is required to apply these fundamental studies to actual design of harbour and coastal structures.
　In this report are presented some experimental results using irregular waves in a wave basin. The authors devised an irregular wave generator, which consists of three generating plates drived with slightly different periods and supplies a certain
irregularity by superposing these elementary waves. The irregularity of wave height is approximated by a probability function similar to Rayleigh's distribution.
　Practical design formulas for wave force always contain the term of wave height. As breakwaters have to resist the biggest wave force caused by a wave train dur
ing a storm, the maximum wave height should be used in principle in wave force formulas. In some cases, however, the maximum wave height is probably replaced by the significant wave height or any other suitable value as an index representing the biggest wave force. Conventional design criteria are not exact in this respect. The most reasonable wave height to be used as an index has to be determined taking into consideration the irregularity of waves, pattern of structures and the accuracy of each formula. The formulas for wave pressure against a vertical wall and for required weight of blocks are examined in this report through model experiments under irregular wave action.
　In order to get a definite idea on the stability of breakwaters, it is indispensable to examine the behaviour of structures under wave action beyond the limit of stable state. The vertical breakwater, for example, will begin to slide when the wave force exceeds the critical resistance. The distance of sliding is a function of the magnitude and duration of wave force and the resisting capacity of the wall. Moreover, the total sliding distance during a storm depends on the occurrence of wave force in an irregular wave train. The authors have proposed a new concept of probable sliding distance and presented a formula for it basing on the experimental results in the irregular wave basin. The stability of vertical breakwater is quantatively evaluated by this method.
　The sliding or overturn of vertical breakwater and the degradation of armour block in mound breakwater are examples of dynamic behaviour as a rigid body in the unstable region. Another aspect of dynamic behaviour is found in elastic or elasto-plastic vibration of structural members. The main problem is the response of structures to periodic wave force and to impulsive breaking wave pressure. The authors discussed some problems in this field, such as the response of vertical wall
supported by rubble mound, behaviour of breakwater under hydro-dynamic pressuse due to earthquake, response of plates on elastic foundation, similarity of model experiments for impulsive wave force, etc.
　This report also includes some miscellaneous problems and is expected to serve as an informative guide to the structural design of breakwaters.