We focused on the effects of tides on storm surge deviations. According to previous studies, storm surge deviations depend on when the typhoon strikes in the tidal phase because the water depth of a bay varies with tides. The previous studies concluded that the dependence can be explained by the conventional formula. However, the results of our numerical simulations could not be explained quantitatively by the conventional formula and require a development mechanism for storm surges that are not expressed in the formula, to explain the dependency. Therefore, we focused on the water depth gradient, and confirmed from the numerical simulation results that the depth gradient greatly influenced the development characteristics of storm surges. Moreover, a new formula considering the depth gradient was derived, and the tidal dependence of storm surge deviations was investigated. Based on the new formula, two indexes that indicate the effect of the water depth gradient and the effect of tides on storm surges were derived. We examined the indexes to clearly understand the development characteristics of storm surges. Finally, the validity of the derived indexes was investigated by comparing the computed surge heights based on the indexes with the results of the numerical simulations.
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