The coastal areas in western part of Japan suffered from high wave action and storm surge generated by Typhoon 9918 on September 24, 1999. The Yatsusiro-kai and Suo-nada bay areas were heavily damaged mainly by overtopping waves. The storm surge level and offshore wave heights at the moment of typhoon attac]elng were numerically estimated for the devastated region. The wave overtopping rates at the sea walls were calculated applying the Boussinesq wave transformation and overflow model. The estimated wave overtopping rates behind sea walls agreed with those obtained from field observation. The numerical estimation and field survey demonstrated that the coastal houses were damaged when the overtopping rate became greater than 0.1m3/m/s.
Typhoon 9918 was generated in the eastern China Sea on September 23, 1999. Initially the typhoon proceeded slowly in north eastern direction. Figure 1 shows the route of Typhoon 9918 in the coastal area of Kyushu island, Japan. The speed became more rapid when the typhoon approached to the west part of Kyusyu island (one of the main islands of Japan). The typhoon forward speed became fastest at the mouth of Yatsusiro-kai bay indicated in Fig. 1. The maximum speed was about 60km/hr. The radius of circle in the figure represents the typhoon radius z0 derived from the variation of atmospheric pressure distribution. The larger circle indicates that the typhoon intensity becomes smaller. The atmospheric pressure at the center of the typhoon was 935hPa at 03:00hrs on September 24, 1999 when the wind maximum in the Yatsusiro-kai bay. Severe damages on sea walls and inundation were induced by the storm surge and high waves generated mainly by the strong winds of the typhoon in the Yashusiro-kai (Y) and Suoh-nada (S) bays. The field survey was carried out just after the typhoon damages on the areas.
