A numerical study of single N-type tsunami drawdown processes at a geophysical scale

Inundations by tsunamis have caused tremendous disasters on coasts all over the world. Consequently, the characteristics of tsunami runup have attracted public and scientific curiosity, which have led to a better understanding of the characteristics of tsunami evolution and runup. On the other hand, the drawdown of tsunamis has less attracted public and scientific interest. Thus, in this study, we investigate the drawdown processes of tsunamis at a geophysical scale. We simulate tsunami propagation and drawdown using a numerical model for fully nonlinear, weakly dispersive, rotational, and turbulent flow. Considering typical geophysical scales, we examine the effects of the amplitude, period, and shape of the incident wave, and bathymetric slope. The simulations show that the maximum drawdown of single leading-elevation N-wave (LEN) occurs prior to runup phase, whereas the maximum drawdown of single leading-depression N-wave (LDN) occurs after the runup phase. In addition, it is observed that the physical processes to induce drawdown of LDN- and LEN-type tsunamis are different from each other. Counterintuitively, the maximum drawdown depth of LEN can be greater than LDN tsunamis. Although the physical procedures are different, the maximum drawdown depths of both N-type tsunamis follow power functional relationships with the surf-similarity parameter ξ (Battjes, 1974). A noteworthy point is their power-law exponent o f: ξ: The numerical simulations under the geophysical scales result in that the ratio of maximum drawdown depth to incident tsunami wave amplitude is proportional to −ξ^0.7 and −ξ^−0.4 for which [Fromula Presented] and [Fromula Presented], respectively, regardless of LDN or LEN. Based on this consistency, we propose empirical formulae to predict the maximum drawdown depth of single N-type tsunamis.