Engineering and Construction

Tsunami is a Japanese word that when translated into English means harbor wave and it is defined as an ocean wave that is created by a disturbance that vertically displaces a column of seawater.Tsunami can be generated by an oceanic meteorite impact, submarine landslide, or earthquake if the sea floor abruptly deforms and vertically displaces the overlying water. Earthquakes generated at sea-floor subduction zones are particularly effective in generating tsunamis. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium.

Tsunami is different from a normal ocean wave in that it has a long period and wavelength. For example, tsunami can have a wavelength in excess of 60 mi (100 km) and a period on the order of 1 h.

In the Pacific Ocean, where the typical water depth is about 13,000 ft (4000 m), tsunami travels at about 650 ft/sec (200 m/sec). Because the rate at which a wave loses its energy is inversely related to its wavelength, tsunami not only propagates at high speeds, it also can travel long transoceanic distances with limited energy losses.

The tsunami is transformed as it leaves the deep water of the ocean and travels into the shallower water near the coast. The tsunami’s speed diminishes as it travels into the shallower coastal water and its height grows. While the tsunami may be imperceptible at sea, the shoaling effect near the coast causes the tsunami to grow to be several meters or more in height. When it finally reaches the coast, the tsunami may develop into a rapidly rising or falling tide, a series of breaking waves, or a tidal bore.

Seiche is identical to tsunami, except that it occurs in an inland body of water, such as a lake. It can be caused by lake-bottom earthquake movements or by volcanic eruptions and landslides with- in the lake. Seiche has been described as being similar to the sloshing of water in a bathtub.

Because of the tremendous destructive forces, options to mitigate structural damage caused by tsunami or seiche are often limited. Some possibilities include the construction of walls to deflect the surging water or the use of buildings having weak lower-floor partitions that will allow the water to flow through the building, rather than knocking it down.