## 4 Discussion
### Wave and Tide Surge Interaction  
To determine the relative effects of the current and water level, three sets of numerical experiments were conducted to assess the model performance and to analyze the mechanism of wave water level interaction: (a) Full Coupled run (Run-Full): SWAN was driven by atmospheric forcing, current and water level to simulate wave height (H\({}_{\text{Full}}\)). The results showed that both the current and water level affect waves during a typhoon process. (b) Current coupled only run (Run-CO): SWAN was driven by atmospheric forcing and current to simulate wave height (H\({}_{\text{CO}}\)). (c) Water level coupled only run (Run-WO): SWAN was driven by atmospheric forcing and water level to simulate wave height (H\({}_{\text{WO}}\)). (d) No Coupled run (Run-NC): neither current nor water level is provided, SWAN is driven by only atmospheric forcing to simulate wave height (H\({}_{\text{NC}}\)). The wave height variations were determined by subtracting the wave heights of the No Coupled run from the other cases. In nearshore areas, tides can have significant effect on short waves. First, wave heights are modulated along a tidal cycle because tide-induced water level variations shift the cross-shore position of the surf zone [30]. Second, in coastal zones subjected to strong tidal currents (e.g., tidal inlets), tidal currents can substantially affect the wave field [31]. A tidal wave is a progressive wave. During the flood phase, the direction of wave propagation is consistent with the current direction, which is not conducive to wave growth; thus, the tidal current flattens the wave amplitude and reduces the wave height. However, during the ebb phase, as and when the current increases, wave steepness increases with an increase in wave height together with a decrease in wavelength. In the Taiwan Strait area, interaction between tides and waves was evident. According to the simulations (a-d of Figures 10-13), wave height oscillated within the tidal period and this oscillation was most obvious in the nearshore area. Waves break with shoaling and wave height was affected by both the current and the water depth. Wave height was affected markedly by the storm surge current, which was opposite to the direction of wave propagation at the Gulei and Douweigang buoys during the passage of Typhoon Dujuan. The wave height variations were larger in the nearshore areas where the wave height was low. The maximum wave height variation was 0.98 m at the Huangqi buoy during the passage of Typhoon Dujuan; the oscillation amplitude was up to 0.40 m, which is comparable with the mean value. In the northwest of the Taiwan Strait (i.e., at the Huangqi and Beishuang buoys), where the tidal range is comparable with water depth, the wave height was affected mostly by water level. The high (low) tide was accompanied by higher (lower) wave height. Based on the DFT method (right-hand column of Figures 9-12), the numerical simulation results indicated that waves were affected mostly by the current in the middle of the Taiwan Strait, where water depth is \(>\)20 m. The contribution of current to wave-surge interaction is approximately 0.04 m, whereas the contribution of water level is \(<\)0.01 m. Conversely, it was found that the water level was more important than the current in coastal areas when interaction occurred. The contribution of current to wave-surge interaction is up to 0.18 m at Beishuang station, whereas the contribution of water level is 0.08 m at the Gulei and Beishuang stations.