This study investigated influences of tides on a giant tsunami generated by the greatest earthquake scenario along the Nankai Trough occurring in the near future by conducting a tsunami-tide simulation in Osaka Bay as a pilot shallow ocean. Our results demonstrated that the tsunami-tide interaction is significant in the strong flood and ebb tide due to tidal advection. The speed of the tsunami is enhanced (decreased) by advection of the flood (ebb) tide, leading to the early (late) approach of tsunami waves to coasts. More realistic tsunami-tide coupled simulation is essential to establish more effective hazard maps and evacuation scenarios.
The Japanese government has reported that a Nankai Trough earthquake will occur with approximately 70% probability within 30 years and cause a giant tsunami (Headquarters for Earthquake Research Promotion, Japan, 2015; see http://www.jishin.go.jp/main/chousa/15jan_kakuritsu/p01_hikaku.pdf). The speed of the tsunami plus the tidal currents may exceed 2 knots (approximately 1 m sec−1) at many ports on the Japanese coast facing the North Pacific. This speed could make it difficult to operate marine vehicles or ships and to escape from the port that the tsunami strikes according to the evacuation scenarios for ships and tsunami hazard maps (e.g., Hashimoto et al., 2010). As a result, drifting marine ships during the tsunami intrusion could devastate many planimetric features located around the ports (Kobayashi et al., 2005). However, current tsunami simulations have hardly considered tidal currents.
For the real-time prediction and reproducibility of a tsunami simulation, one essential issue is whether there is a need to incorporate the tidal forcing at the lateral boundary to improve simulation accuracy in the coastal oceans (Androsov et al., 2011). Prevention measures for tsunami hazards, such as port evacuation plans and hazard maps, should be based on accurate and realistic tsunami simulations, including tidal variations.