Effects of Island Topography on Storm Surge in Taiwan Strait during Typhoon Maria

In July 2018, Super Typhoon Maria moved in the northwestward direction, passed by the northern tip of Taiwan Island, and severely impacted the coasts along Fujian and Zhejiang Provinces, China. In this paper, the storm surge and wind waves induced by Typhoon Maria are numerically simulated using a weather research and forecasting (WRF) model and the wind-surge-wave modeling suite [semi-implicit cross-scale hydroscience integrated system model (SCHISM) - wind wave model III (WWMIII)]. Numerical results are compared against available field measurements, including winds, atmospheric pressures, storm tides, and wave parameters. Using the model results, the significance of waves on modulating storm surges during Typhoon Maria is examined. Wind waves contribute significantly to surge heights in the Taiwan Strait and in nearshore waters. The models are then employed to conduct numerical experiments by reducing the topographic heights of Taiwan to 25% of their original values so as to investigate their effects on wind fields, surges, and currents. From these results, we observe that reduced topography weakens the wind intensity on the eastern side of the island while intensifying the wind on the other side of the island by up to 10 m/s, which is due to the terrain-induced blocking and channeling effects. The scenario with reduced topography also shows elevated surge heights on the right-hand side of typhoon landfalling coasts but slightly attenuated surge in the Taiwan Strait. Storm surge tends to increase the southwestwardly flux via the Taiwan Strait with maximum current velocities increased by approximately 0.5 m/s, compared with the case induced by astronomical tides only. The reduced island topography slightly weakens the southwestwardly current, decreasing maximum flux by approximately 16%, relative to the hindcast simulation using the original island topography. The results in this study indicate that the presence of island influences the propagation of surge wave with elevated surge height and storm-induced flux in the Taiwan Strait, this blocking effect weakens with reduced island topographic heights; meanwhile, the terrain-induced channeling effect, which alters the typhoon circulation and further impacts the surge pattern, is prone to forming in the case of high island topography.