Numerical Modeling of Tsunami Amplification and Beachfront Overland Flow in the Ukai Coast of Japan

Tsunami amplification and overland flow characteristics have been investigated using numerical modeling in a case study of the Ukai coast during the 2024 tsunami event. The tsunami wave amplification from offshore Iida Bay to Ukai has been investigated by using a hydrodynamic model. The model has been successfully validated by comparing simulated tsunami inundation with observations in Ukai. Non-breaking tsunami amplification from model simulations shows a power law, with a correlation coefficient R2 of 0.97, leading to a 1.84-fold amplification at the breaking depth location. After wave breaking, tsunami amplification follows an exponential function of water depth, with a significantly slower increase rate compared to that before breaking. Tsunami travel time from the Iida Bay offshore boundary to Ukai is determined by comparing tsunami peaks at two different locations. A quick approximation of tsunami travel time using the averaged depth for shallow wave celerity results in an 8.5% error compared to hydrodynamic model simulations. Supercritical and subcritical flow characteristics in the beachfront area have been examined using a wave dynamic model. Based on the Froude number, beachfront overland flow on an asphalt ground surface with low friction results in fast supercritical flow and deeper inundation, which can have major impacts on coastal structures and sediment scour. Grass-covered ground lowers tsunami velocity to slower subcritical flow and lower the maximum inundation height which can reduce the tsunami damage. The findings will provide valuable support for coastal hazard mitigation and resilience studies.