The coastal areas in the western part of Japan suffered from high wave action and storm surge generated by the typhoon No. 18 on Sep. 24，1999. The Yatsusiro-kai and Suo-nada bay areas were heavily damaged by flooded water and overtopped waves. The inundation caused by the storm surge gave the twelve deaths in the Matsuai located at the innermost part of the Yatsusiro-kai bay. Meanwhile significant wave overtoping induced to the composition of storm surge and high wind waves made heavy erosion on the landward pavement in reclaimed grounds and catastorophic damages to houses built on the landward. Such disasters caused by the composition of storm surge and high wave overtopping were observed at many seaside locations facing the both bays.
　A field observation of wave overtopping rates at a seawall with parapets was carried out in the Koyagawachi fishery port area located in the middle part of the Yatsusiro-kai bay. The observed restllts demonstrate that the first floors of houses standing behind the sea wall without detached breakwaters were inundated and destroyed with large hydraulic pressures of overtopped flows. The damages of a building located on the ground protected by the sea wal lwith a detached breakwater composed of wave energy dissipating blocks were slight. Numerical simulation of wave overttopping and hydraulic overflow pressure was carried out to investigate the wave dissipating effects by the detached breakwater and to propose the countemeasure to high wave overtopping. The estimated wave overtopping rates agree with the observed rates in the each damaged area with and without a detached breakwater in case that the abnomal tidal raise during autumn is assumed to be about 40cm. The hydraulic pressures derived from the overtopped waterlevel and flow speed may be reduced by the wave energy dissipating effect of detached breakwaters. The integrated shore protection method composed of sea wall covered with armor blocks and detached breakwater is recomended as a countermeasure to wave overtopping disaster in storm surge.