Port-resolving, tsunami, and tidal simulations to locate "tsunami vortexes" for safe vessel evacuation planning

Vortexes induced by tsunamis, "tsunami vortexes", pose a potential danger for ship evacuation, and their prediction is important to determine their role in navigation and safe vessel evacuation. This study simulated tsunami-induced vortexes in Osaka Bay and investigated their spatial and temporal scales using a port-resolved, high-resolution tsunami model based on a scenario for the largest tsunamigenic earthquake expected to occur along the Nankai Trough in the near future. Vortexes induced by tidal currents were also evaluated using a realistic tidal simulation for comparison with the magnitudes of the tsunami vortexes. The tsunami simulation identified the dominant areas of the tsunami vortexes characterized by the magnitude of the vorticity. Unified maps of tsunami speeds and vortexes showed the difficult areas for ship evacuation and indicated that these areas can persist after the approach of the leading wave around the entrances to port areas. The areas of tsunami vortex formation were largely deduced by analyzing the vorticity fields in the tidal simulations, because the magnitude of tsunami-induced vorticities was closely correlated with that of the tide-induced vorticities. The combination of tide and tsunami simulations can identify areas of tsunami vortex formation and provide valuable information to guide measures for safer vessel evacuation.