Recently, coastal disasters seem to increase due to storm waves exceeding the design wave for Japanese port’s and coastal facilities. However, it is still difficult to conclude the reason why such high waves occur with the long-term trend of climate change or the accidental occurrence in statistics. In this study, the recent wave overtopping disasters for long period swell are focused because almost all recent storm disasters are categorized into them, and their mechanism are investigated in order to suggest some countermeasures.
　The characteristics of wave transformation in shallow water for the long period swells, the occurrence probability of them should be distinguished from one of wind waves focusing their wave height, are discussed with the results of model experiments and numerical simulations and some countermeasures for wave overtopping of the long period swells are suggested. The Boussinesq model equipped the wave overtopping model are improved to estimate the distribution of wave overtopping volume in space and time along shoreline and expanded to evaluate the inundation with the drain model. It is confirmed that the model can present that the temporary mounds installed behind the revetments make wave overtopping volume pile up and the drainages equipped in the revetments discharge the stagnant water into the sea.