About the Architecture of Tsunami-Resistant Buildings

Architects and engineers can design buildings that will stand tall during even the most violent earthquakes. However, a tsunami (pronounced soo-NAH-mee), a series of undulations in a body of water that is often caused by an earthquake, has the power to wash away entire villages. While no building is tsunami-proof, some buildings can be designed to resist forceful waves. The challenge of the architect is to design for the event AND design for beauty — the same challenge faced in safe room design.

Understanding Tsunamis

Tsunamis are usually generated by powerful earthquakes underneath large bodies of water. The seismic event creates a subsurface wave that is more complex than when the wind simply blows the water's surface. The wave can travel hundreds of miles an hour until it reaches shallow water and a shoreline. The Japanese word for harbor is tsu and nami means wave. Because Japan is heavily populated, surrounded by water, and in an area of great seismic activity, tsunamis are often associated with this Asian country. They occur, however, all over the world. Historically tsunamis in the United States are most prevalent on the West coast, including California, Oregon, Washington, Alaska and, of course, Hawaii.

A tsunami wave will behave differently depending on the underwater terrain surrounding the shoreline (i.e., how deep or shallow the water is from the shoreline). Sometimes the wave will be like a "tidal bore" or surge, and some tsunamis don't crash onto the shoreline at all like a more familiar, wind-driven wave. Instead, the water level may rise very, very quickly in what is called a "wave runup," as if the tide has come in all at once—like a 100 foot high tide surge. Tsunami flooding may travel inland more than 1000 feet, and the "rundown" creates continued damage as the water quickly retreats back out to sea. 

What Causes the Damage?

Structures tend to be destroyed by tsunamis because of five general causes. First is the force of the water and high-velocity water flow. Stationary objects (like houses) in the path of the wave will resist the force, and, depending how the structure is constructed, the water will go through or around it.

Second, the tidal wave will be dirty, and the impact of debris carried by the forceful water may be what destroys a wall, roof, or piling. Third, this floating debris can be on fire, which is then spread among combustible materials.

Fourth, the tsunami rushing onto land and then retreating back to the sea creates unexpected erosion and scour of foundations. Whereas erosion is the general wearing away of the ground surface, scour is more localized—the type of wearing away you see around piers and piles as water flows around stationary objects. Both erosion and scour compromise a structure's foundation.

The fifth cause of damage is from the waves' wind forces.

Guidelines for Design

In general, flood loads can be calculated like for any other building, but the scale of a tsunami's intensity make building more complicated. Tsunami flood velocities are said to be "highly complex and site-specific." Because of the unique nature of building a tsunami-resistant structure, the U.S. Federal Emergency Management Agency (FEMA) has a special publication called Guidelines for Design of Structures for Vertical Evacuation from Tsunamis.

Early warning systems and horizontal evacuation have been the main strategy for many years. The current thinking, however, is to design buildings with vertical evacuation areas: instead of attempting to flee an area, the residents climb upwards to safe levels.

"...a building or earthen mound that has sufficient height to elevate evacuees above the level of tsunami inundation, and is designed and constructed with the strength and resiliency needed to resist the effects of tsunami waves...."

Individual homeowners as well as communities may take this approach. Vertical evacuation areas can be part of the design of a multi-story building, or it can be a more modest, stand-alone structure for a single purpose. Existing structures such as well-constructed parking garages could be designated vertical evacuation areas.

8 Strategies for Tsunami-Resistant Construction

Shrewd engineering combined with a swift, efficient warning system can save thousands of lives. Engineers and other experts suggest these strategies for tsunami-resistant construction:

1. Build structures with reinforced concrete instead of wood, even though wood construction is more resilient to earthquakes. Reinforced concrete or steel-frame structures are recommended for vertical evacuation structures.
2. Mitigate resistance. Design structures to let the water flow through. Build multi-story structures, with the first floor being open (or on stilts) or breakaway so the major force of water can move through. Rising water will do less damage if it can flow underneath the structure. Architect Daniel A. Nelson and Designs Northwest Architects often use this approach in the residences they build on the Washington Coast. Again, this design is contrary to seismic practices, which makes this recommendation complicated and site specific.
3. Construct deep foundations, braced at the footings. A tsunami's force can turn an otherwise solid, concrete building completely on its side, substantive deep foundations can overcome that.
4. Design with redundancy, so that the structure can experience partial failure (e.g., a destroyed post) without progressive collapse.
5. As much as possible, leave vegetation and reefs intact. They won't stop tsunami waves, but they can act as a natural buffer and slow them down.
6. Orient the building at an angle to the shoreline. Walls that directly face the ocean will suffer more damage.
7. Use continuous steel framing strong enough to resist hurricane-force winds.
8. Design structural connectors that can absorb stress.

What's the Cost?

FEMA estimates that "a tsunami-resistant structure, including seismic-resistant and progressive collapse-resistant design features, would experience about a 10 to 20% order-of-magnitude increase in total construction costs over that required for normal-use buildings."

This article briefly describes design tactics used for buildings in tsunami-prone coastlines. For details about these and other construction techniques, explore the primary sources.

Sources

• United States Tsunami Warning System, NOAA / National weather Service, http://www.tsunami.gov/
• Erosion, Scour, and Foundation Design, FEMA, January 2009, PDF at https://www.fema.gov/media-library-data/20130726-1644-20490-8177/757_apd_5_erosionscour.pdf
• Coastal Construction Manual, Volume II FEMA, 4th edition, August 2011, pp. 8-15, 8-47, PDF at https://www.fema.gov/media-library-data/20130726-1510-20490-1986/fema55_volii_combined_rev.pdf
• Guidelines for Design of Structures for Vertical Evacuation from Tsunami, 2nd edition, FEMA P646, April 1, 2012, pp. 1, 16, 35, 55, 111, PDF at https://www.fema.gov/media-library-data/1570817928423-55b4d3ff4789e707be5dadef163f6078/FEMAP646_ThirdEdition_508.pdf
• Tsunami-Proof Building by Danbee Kim, http://web.mit.edu/12.000/www/m2009/teams/2/danbee.htm, 2009 [accessed August 13, 2016]
• The Tech To Make Buildings Earthquake — and Tsunami — Resistant by Andrew Moseman, Popular Mechanics, March 11, 2011
• How to Make Buildings Safer in Tsunamis by Rollo Reid, Reid Steel