Tsunami Forces and Effects on Structures
Harry Yeh and Nobuo Shuto
The 2004 Indian Ocean Tsunami claimed more than 220,000 lives. It was a low-probability high-consequence event. A similar disaster could strike elsewhere, particularly in the Pacific but also in Caribbean, Atlantic, and Mediterranean regions. Unlike in seismic ground shaking, there is usually a short lead-time precedes tsunami attack: from a few minutes for a local source to several hours for a distant source. Because mega-tsunamis are rare and because forewarning of these events is possible, the primary mitigation tactic to date has been evacuation. Hence, most efforts have focused on the development of effective warning systems, inundation maps, and tsunami awareness. This strategy makes sense from the standpoint of saving human lives. However, it does not address the devastating damage to buildings and critical coastal infrastructure, such as major coastal bridges, oil and LNG storage facilities, power plants, and ports and harbors. Failure in critical infrastructure creates enormous economic setbacks and collateral damage. The accelerating construction of critical infrastructure in the coastal zone demands a better understanding of design methodology in building tsunamiresistant structures. In some coastal areas such as low-elevation coastal spits or plains, evacuating people to higher ground may be impractical because they have no time to reach safety. In these situations, the only feasible way to minimize human casualties is to evacuate people to the upper floors of tsunami-resistant buildings. Such buildings must be designed and constructed to survive strong seismic ground shaking and subsequent tsunami impacts.
The primary causes of structural failure subject to tsunami attack can be categorized into three groups: 1) hydrodynamic force, 2) impact force by water-born objects, and 3) scour and foundation failure. Tsunami behaviors are quite distinct, however, from other coastal hazards such as storm waves; hence the effects cannot be inferred from common knowledge or intuition. Recent research has addressed tsunami forces acting on coastal structures to develop appropriate design guidelines, and mechanisms leading to tsunamigenerated scour and foundation failures.
This special issue is a compilation of 14 papers addressing tsunami effects on buildings and infrastructure. The four main groupings begin with two papers on tsunami force acting on vertical walls. Arikawa experimentally investigates the structural performance of wooden and concrete walls using a large-scale laboratory tank in Japan. Also using a similar large-scale tsunami flume but in the US, Oshnack et al. study force reduction by small onshore seawalls in front of a vertical wall. The second grouping focuses on tsunami force on 3-D structures. Arnason et al. present a basic laboratory study on the hydrodynamics of bore impingement on a vertical column. Fujima et al. examine the two types of formulae for tsunami force evaluation: the one calculated from flow depth alone and the other based on the Euler number. Lukkunaprasit et al. demonstrate the validity of force computation recommended in a recently published design guideline (FEMA P646) by the US Federal Emergency Management Agency. The other two papers look into the specific types of structures: one is for light-frame wood buildings by van de Lindt et al, and the other is for oil storage tanks by Sakakiyama et al.
The topic of debris impact force is the focus of the third grouping. Matsutomi summarizes his previous research on impact force by driftwoods, followed by the collision force of shipping containers by Yeom et al. Yim and Zhang numerically simulate tsunami impact on a vertical cylinder; this paper is included in this grouping because their numerical approach is similar to that of Yeom et al. As for the fourth grouping, Shuto presents field observations on foundation failures and scours, and Fujii et al. discuss the erosion processes of soil embankments. There are two more papers: those are the application of fragility analysis to tsunami damage assessment by Koshimura et al. and evaluation of an offshore cabled observatory by Matsumoto and Kaneda.
The topics presented here are undoubtedly in progress, and many revisions and improvements are still needed in order to achieve better predictability for tsunami effects on buildings and infrastructure. We hope you find the papers in this issue intriguing and the information useful, and become further interested in this important natural hazard. Lastly, we wish to express our appreciation to the authors for their timely contributions, and to the reviewers for their diligent and time-consuming efforts.