This study investigated the influence of a giant tsunami by the earthquake along the Nankai Trough on the benthic environment in terms of heavy-metal pollution in Osaka Bay as a pilot ocean. The high-resolution tsunami-tide and particle tracking simulations demonstrated that the marked resuspension area of zinc "hot spots" was locally formed in the nearshore ocean around landfills. After the tsunami, the zinc was transported by the tidal current and widely redeposited in the bay. The benthic environment and ecosystem can be improved only in areas around the "hot spots" where the zinc concentration decreases.
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 in Japan, 2013). Giant tsunamis can disturb marine sediment and form muddy seawater in nearshore areas. Marine sediment can be resuspended and transported by not only the tsunami current but also by the tidal, wind-forced, and density currents in the coastal ocean. Because the marine sediment in the coastal oceans around megacities contains heavy metals and cysts of harmful algae (e.g., Alexandrium spp. ), the resuspension of marine sediments can induce multiple forms of marine pollution, such as harmful red tides and heavy metal contamination in extensive areas (Hayashi et al., 2015). Marine pollution can threaten the marine ecosystem in coastal areas and lead to fishery losses, for example, by shellfish poisoning (Kaga et al., 2012). However, no study has evaluated the exposure or risk to such marine hazards caused by tsunamis (hereafter, tsunami marine hazard).
The importance of coastal habitats (coral reefs, mangroves, tidal wetlands, sea grass beds, etc.) for reducing disaster risk has received global attention in recent years. For example, mangroves can act as natural barriers to tsunamis (Barbier, 2006). The Hyogo Framework for Action (HFA) in 2005 was the first global agreement to recognize the importance of sustainable ecosystems and environmental management for disaster risk reduction (Sudmeier-Rieux et al., 2013). The concept of an ‘Ecosystem Approach to Disaster Risk Reduction (Eco-DRR)’ has been proposed on the basis of the idea that regulating ecosystem services is crucial to protecting coastal areas from tsunamis and storm surges, as well as to enhancing the resilience of human society (Gupta and Nair, 2012). These studies have focused on the potential protecting role of coastal ecosystems, but they have hardly considered or evaluated their damage.
