Tsunami is the Japanese name given to large waves that sometimes devastated the shores and ports of Japan. A tsunami is a wave in the ocean but it is very different to normal waves.
Tsunamis have very long wavelengths. Crest to crest they measure between 10 and 500 km and they travel through the ocean at more than 700 km/h. Sometimes there appears to be just one wave but often there are multiple waves travelling a few minutes apart.
Wave height [amplitude] may not appear to be great in the open ocean but unlike normal waves the tsunami is moving the entire water column, all the way to the sea floor. The water depth therefore has a major influence on the behaviour and appearance of the wave. In addition because of the wavelength, the first sign of the arrival of a tsunami may actually be the sea level falling and bays appearing to empty.
In deep open water the wave is almost impossible to see although modern instruments can detect it. However, as the wave approaches shore and the water shallows it slows down. The wave rapidly bunches up as the faster rear sections catch up with the slower front sections resulting in the wave growing
in height the closer it gets to shore. This effect is enhanced if the near-shore sea bed provides a long gradual shallowing. Many tsunamis are barely distinguishable from normal sea waves but some turn into monsters rising 30 metres above the shore line. The damage along a shore line may vary because of the influence the local shape of the sea floor has on wave behaviour.
Bays and harbours that are funnel shaped also suffer more from a tsunami because they concentrate the effects. Damage in these areas is further increased by the sloshing backwards and forwards of the water, just like in a bathtub.