A storm surge model is coupled with a wave model and then the storm waves and surges in Suo-nada and Yatsushiro-kai Bays, Japan, caused by Typhoon 9918, are hindcasted with the coupling model. The effect of water surface elevations and currents on the wave height and period in these bays is examined. The influence of the wave age dependent drag coefficient on the magnitude of the storm surges is also investigated.
The atmospheric forcing in the simulations of waves and storm surges, generated by typhoons, is usually simplified as pressure fields are often assumed to have concentric circular distributions. Most wave models neglect the effect of water surface elevations and currents due to astronomical tides and storm surges. The astronomical tides and the variation of the surface drag coefficient with the wave state are also ignored in most storm surge models. Such simplifications, however, considerably underestimated both the wave height and the storm surge in the west region of Suo-nada Bay, Japan, caused by Typhoon 9918 (BART, 18th typhoon in 1999) (Hashimoto and Maki, 2000; Kawai et al., 2000; Mitsunaga et al., 2003). Fig. 1 shows that the typhoon track passed on the northwest side of Yatsushiro-kai Bay with a central pressure of 945hPa and progression speed of 40km/h, and then crossed Suo-nada Bay with 950hPa and 50km/h. Such a violent typhoon has affected these bays about once a decade. Typhoon 9918 brought severe waves and surges to the coasts of these bays near the time of a high tide to trigger the most tremendous coastal disaster in these four decades in Japan. The observed maximum significant wave height and storm surge were approximately 3.5m and 2.1m respectively at Kanda, locating at the west end of Suo-nada Bay.
