Hazard assessment for future tsunami inundation partly sheltered by an installed finite hybrid defense system comprised of an embankment, moat, and coastal forest in Onbetsu, Japan

Considering the limitations of tsunami mitigation functions and disadvantages of a coastal forest, a hybrid defense system (HDS) combining a forest with an embankment and a moat that strengthens existing coastal forests was installed in the Onbetsu district of Hokkaido Prefecture in Japan as a mitigation measure for a future large tsunami. This study demonstrated the effectiveness of installation of an embankment (E), moat (M), and vegetation (V) from seaward in Onbetsu compared with the former mitigation structure of only vegetation (VVV). The EMV structure can provide a large sheltering zone just behind the structure, thus increasing the time for people to evacuate. Because the tsunami arrival time at the coast is around 30 min and the inundated area is around 3 km inland in a Level 2 tsunami, it will be slightly difficult for all the residents to evacuate horizontally to outside the inundation area. This study also proposes not using an east-side route for evacuation because repelling the current by EMV from a small creek can greatly decrease the time for a safe horizontal evacuation. Thus, the safety and availability of vertical evacuation buildings are evaluated considering the tsunami inundation characteristics. • Tsunami mitigating effect of a hybrid defense system in the Onbetsu district was evaluated. • A large sheltering zone can increase the time for people to evacuate. • East-side route is relatively dangerous because of the repelling current from the mitigation system. • Even a small creek can greatly decrease the time for a safe horizontal evacuation. • Safety and availability of vertical evacuation buildings are evaluated.