Foundation, Concrete and Earthquake Engineering

Tsunami, Tides and Wind-Generated Waves

The terms tsunami comes from the Japanese meaning harbor (“tsu”) and waves (“nami”). Approximately 190 event of tsunami have struck coast of Japan. According to ordinary English practice an s can be added to represent it in plural form.

The Greek historian Thucydides was the first to relate tsunami to submarine quakes, but understanding the nature of tsunami remained slim until the 20th century and sometimes it merged with tides and wind generated waves. Therefore it is very important to precisely distinguish them depending on their characteristics.

Wave Length

Everyday wind waves has a wave length (from crest to crest) of about 1000m (300ft). But, the tsunami have a wavelength of about 200km and sometimes 750km 750 km in the open ocean.


Wind generated waves have an amplitude of about 2m (7 ft) high. But, the tsunami have only about 1m(3ft).

Sea surface slope

The generated waves produce a steep slope of sea surface as it has relatively high amplitude and much small wave length. But, the tsunami waves generate a gentle slope due to its great wave length and small amplitude. This makes tsunamis difficult to detect over deep water.

Speed of approach

Tsunami travels at a speed of 700 kilometers per hour in deep ocean and easily pace with a Boeing 747 which can not be compared with other.

Depth of Disturbance

Breezes blowing across the ocean crinkle the surface into relatively short waves and create currents restricted to a shallow layer. Strong gales can whip up waves 30 meters or higher in the open ocean, but even these do not more deep water. Tides, which sweep around the globe twice a day, do produce current that reach the ocean bottom just as tsunami do.

Cause of Occurrence

Cyclones, hurricanes are particular form of meteorological storm which is generated by deep depression, result a storm surge which can be several meters above normal tide levels. The centre of depression has low atmospheric pressure. This surge when reaches ashore, it can inundate vast areas of land. Such a storm surge inundates Burma (Myanmar) in May 2008.

The tides are generated by the gravitational pull of the Moon or Sun.

The tsunamis are produced impulsively by an undersea earthquake or, much less frequently, by Volcanic eruptions, meteorite impact or underwater landslides.

Inland Movement of Water

Tsunamis, tides or strong storm all produce waves of water that move inland, but in case of tsunami the inland movement of water is much greater and lasts for long period, giving the impression of an incredibly high tide.

Special Feature of Tsunami

When tsunami becomes nearer to coast and travel over shallow depth, the wave is compressed due to wave shoaling and its forward travel slows below 80 Kmph (50 mph). it wave length diminishes to less than 20 Km (12miles) and its amplitude grows enormously, producing a distinctly visible wave. As the wave length still remains in the order of several km (a few miles), the tsunami may take minutes to reach its full height. Open Bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with steep break point.

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