Tsunami

Tsunamis are long water waves generated by earthquakes, sliding masses, volcanic eruptions, asteroid impacts as well as nuclear explosions. The word tsunami (津波、つなみ) consists of tsu (津) character that means bay or harbor, and nami (波) character that means waves. So literary tsunami means big waves that occur at a bay or an harbor. The Japanese word tsunami has been adopted to replace the expression tidal wave to avoid confusion with the astronomical tides (because they are not in any way related to the tides of the Earth). Tsunamis are sometimes also called as seismic sea waves, although they can be generated by mechanisms other than earthquakes. Because tsunami occur suddenly and often without any warning, tsunamis are extremely dangerous to coastal communities.

Tsunamis are characterized as shallow water waves. These are different from the waves that usually happen on a beach, which are caused by the wind blowing across the ocean’s surface. Wind generated waves usually have period (time between two successive waves) of five to twenty seconds and a wave length of 100 to 200 meters. A tsunami can have a period in the range of ten minutes to two hours and wave lengths greater than 500 km.  A wave is characterized as a shallow water wave when the ratio of the water depth and wave length is very small (d/L ≤ 0.05). The acceleration of a shallow water wave is also equal to the square root of the product of the acceleration of gravity, g, (9.81 m/sec2) and the depth of the water, d.

The rate at which a wave loses its energy is inversely related to its wave length. Since a tsunami has a very large wave length, it will lose little energy as it propagates. Thus, in very deep water, a tsunami will travel at high speeds with little loss of energy. For example, when the ocean is 6100 m deep, a tsunami will travel about 890 km/hr, and thus can travel across the Pacific Ocean in less than one day.

As a tsunami leaves the deep water of the open sea and arrives at the shallow waters near the coast, it undergoes a transformation. Since the velocity of the tsunami is also related to the water depth, as the depth of the water decreases, the velocity of the tsunami decreases. The change of total energy of the tsunami, however, remains constant.

Furthermore, the period of the wave remains the same, and thus more water is forced between the wave crests causing the height of the wave to increase. Because of this “shoaling” effect, a tsunami that was imperceptible in deep water may grow to have wave heights of several meters or more.

If the trough of the tsunami wave reaches the coast first, this causes a phenomenon called drawdown, where it appears that sea level has dropped considerably.  Drawdown is followed immediately by the crest of the wave which can catch people observing the drawdown off guard. When the crest of the wave hits, sea level rises (called run up).  Run up is usually expressed in meters above normal high tide.  Run ups from the same tsunami can be variable because of the influence of the shapes of coastlines.  One coastal area may see no damaging wave activity while in another area destructive waves can be large and violent. The flooding of an area can extend inland by 300 m or more, covering large areas of land with water and debris. Flooding tsunami waves tend to carry loose objects and people out to sea when they retreat. Tsunami may reach a maximum vertical height onshore above sea level, called a run up height, of 30 meters. A notable exception is the landslide generated tsunami in Lituya Bay, Alaska in 1958 which produced a 60 meter high wave.

Because the wave lengths and velocities of tsunami are so large, the period of such waves is also large, and larger than normal ocean waves.  Thus it may take several hours for successive crests to reach the shore.  (For a tsunami with a wavelength of 200 km traveling at 750 km/hr, the wave period is about 16 minutes).  Thus people are not safe after the passage of the first large wave, but must wait several hours for all waves to pass. The first wave may not be the largest in the series of waves. For example, in several different recent tsunamis the first, third, and fifth waves were the largest.

 

source: The Influence of Building’s Height and Lay-out to Tsunami’s Run Up

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