In the aftermath of the 2011 Tohoku Earthquake and Tsunami Japanese tsunami protection philosophy now dictates that coastal defences should prevent the land that they protect from being flooded under a Level 1 event (with a return period in the order of about 100 years). To ascertain the overtopping mechanism and leeward inundation heights of tsunamis as they hit coastal structures, the authors conducted physical experiments using a dam-break mechanism, which could generate bores that overtopped different types of structures. Three different types of structures were considered, namely a wall of “infinite” height, a coastal dyke, and a vertical tsunami wall. The results show that the velocity of the tsunami bore is crucial in determining whether the structure will be overtopped or not, and thus it is imperative to move away from only considering the tsunami inundation height at the beach.

Following the 2011 Tohoku Earthquake and Tsunami, Japanese tsunami protection guidelines stipulate that coastal defences should ensure that settlements are shielded from the coastal inundation that would result from Level 1 tsunami events (with return periods in the order of about 100 years). However, the overtopping mechanism and leeward inundation heights of tsunami bores as they hit coastal structures has received little attention in the past. To ascertain this phenomenon, the authors conducted physical experiments using a dam-break mechanism, which could generate bores that overtopped different types of structures. The results indicate that it is necessary to move away from only considering the tsunami inundation height at the beach, and also consider the bore velocity as it approaches the onshore area. The authors also prepared a simple, conservative method of estimating the inundation height after a structure of a given height, provided that the incident bore velocity and height are known.