Tsunami Loads on Infrastructure
Tsunami. The Japanese word for “harbor wave.”
Coastal bathymetry. The study and mapping of the submarine ocean floor in near-shore areas.
Inundation. The overflowing of water onto normally dry land.
Loading combinations. The summation of individual force components occurring simultaneously.
Tsunami forces on infrastructure
Existing design guidelines
While several design codes explicitly provide guidelines for flood-induced loads (UBC, 1997; ASCE, 2006; IBC, 2006), a survey of current design codes, design standards, and design guidelines indicates that limited attention has been given to tsunami-induced forces. Four pioneering design documents specifically account for tsunami-induced forces, namely: the Federal Emergency Management Agency Coastal Construction Manual, FEMA 55 (FEMA, 2003), which provides recommendations for tsunami-induced flood and wind wave loads; the City and County of Honolulu Building Code (CCH, 2000), which contains regulations that apply to districts located in flood and tsunami-risk areas; the Structural Design Method of Buildings for Tsunami Resistance (SMBTR) proposed by the Building Center of Japan (Okada et al., 2005), outlining structural design for tsunami refuge buildings; and Guidelines for Structures that Serve as Tsunami Vertical Evacuation Sites, prepared by Yeh et al. (2005) for the Washington State Department of Natural Resources to estimate tsunami-induced forces on structures. Recently, the Federal Emergency Management Agency published Guidelines for Design of Structures for Vertical Evacuation from Tsunamis, FEMA P646, (FEMA, 2008). This document focuses on high-risk tsunami-prone areas, and provides design guidance for vertical evacuation structures. Conservative assumptions have been incorporated in FEMA P646 to ensure safety and security for the public requiring shelter from tsunami flood waters.
Tsunami-induced force components
A tsunami wave imposes significant loading on structures. The parameters defining the magnitude and application of these forces include inundation depth, flow velocity, and flow direction. These parameters mainly depend on tsunami wave height and wave period, near-shore bathymetry, coastal topography, and roughness of the coastal inland. The inundation depth at a specific location can be estimated using various tsunami scenarios (magnitude and direction) and by numerically modeling coastal inundation. The estimation of flow velocity and direction, however, is much more difficult to quantify. Flow velocities can vary in magnitude, whereas flow directions can vary due to onshore local topographic features, as well as soil cover and obstacles. The force components associated with tsunami-induced flows consist of: (1) hydrostatic force, (2) hydrodynamic force, (3) buoyant and uplift forces, (4) impulsive force, (5) debris impact and damming forces, and (6) gravity forces. The reader is referred to Nistor et al. (2009) and FEMA P646 for a comprehensive review of the individual force components.
Hydrodynamic (drag) force
Buoyant and uplift forces
Debris impact and damming forces
Debris impacting a structure can cause accumulation of debris, as depicted in Figure 7, leading to a damming effect. The forces generated due to damming can be estimated from the hydrodynamic force (Equation 2) by replacing \( b \) with the width of the debris dam.
Drawdown of the tsunami-induced flooding can result in retention of water on structural flooring systems. This phenomenon imposes additional gravity loading on the structure, which must be considered in design.
Classic wave-breaking formulas are applicable for the case of wave breaking directly onto coastal structures, such as breakwaters, piers, and docks. Tsunami waves, however, depending on the near-shore bathymetry, tend to break offshore and approach the shoreline in the form of a rapidly moving hydraulic bore. Furthermore, inland infrastructure is generally not affected by the action of wave breaking occurring at the shoreline.
Tsunami flow velocity
The hydrodynamic force is proportional to the square of the flow velocity. Thus, uncertainties in estimating velocities result in large differences in the magnitude of the resulting hydrodynamic force. Tsunami inundation velocity magnitude and direction can vary significantly during a major tsunami. Current estimates of the velocity are crude; a conservatively high flow velocity impacting the structure at a normal angle is usually assumed. Also, the effects of run-up, backwash, and direction of velocity are not addressed in current design documents. A number of guidelines and researchers have proposed estimates of velocity for given tsunami inundation levels, such as Murty (1977), Camfield (1980), FEMA 55 (Dames and Moore, 1980), Kirkoz (1983), CCH (2000), Iizuka and Matsutomi (2000), Bryant (2001), and FEMA P646 (2008).
Tsunami-induced loading combinations
Appropriate construction and layout design of a structure located in a tsunami-prone area can reduce the risk of damage during a tsunami event. Tsunami forces increase proportionally with exposed area and nonstructural elements that remain intact during the impact of the tsunami-induced flooding. Therefore, it is prudent to orient buildings with the shorter side parallel to the shoreline. Further, structural walls should also be oriented, if possible, to minimize the exposed area. Exterior nonstructural elements located at lower levels should be designed with a controlled failure mechanism that is triggered by the initial impact of the tsunami. This concept, known as breakaway walls, reduces the amount of lateral load that is transferred to the lateral force resisting system of the structure. Conversely, however, breakaway walls may result in an increase in debris loading. The use of rigid nonstructural exterior components, while providing protection to buildings from flooding, increases the lateral loading.
Recent catastrophic tsunamis (2004 Indian Ocean Tsunami, 2007 and 2010 Solomon Islands Tsunamis, 2010 Chile Tsunami, 2011 Tohoku, Japan Tsunami) have emphasized the destructive power of tsunami-induced flooding as it propagates overland and impacts near-shoreline infrastructure. As a result, research has evolved to improve our understanding of the forces associated with tsunamis and the interaction between tsunami-induced flow and infrastructure. Currently, force components and loading combinations have been proposed to assess and design structures against tsunami forces. The force components include hydrostatic, hydrodynamic, buoyant and uplift, impulsive, debris impact and damming, and gravity. There is, however, uncertainty in both the estimation of the component forces, as well as the total tsunami load that should be considered. Future efforts, including experimental and analytical studies, are being directed toward a better understanding of the forces that should be considered in design of infrastructure located in tsunami-prone areas.
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