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This is significant in coastal regions. This phenomena occurs because of the force from the seabed as it becomes shallower. This slows down the wave: the shallower the water, the slower the wave. Even when tsunamis have only a small amplitude less than a meter they can shoal up to many meters high as they hit shallow water. When a tsunami hits shallower coastal waters, the trough or base of the wave contacts the beach floor. As a consequence, the leading edge of the tsunami slows dramatically due to the shallower water, but the trailing part of the wave is still moving rapidly in the deeper water.
The wave is compressed and its velocity slows below 80 kilometers per hour. Its wavelength diminishes to less than 20 kilometers and its amplitude is magnified many times.
By looking at the fundamental wave characteristics of tsunamis in deep and shallow water, we can understand why shoaling happens. Shoaling is an increase in wave amplitude that happens when water waves not just tsunamis go from deep to shallow water — particularly at the coast.
Tsunamis have a small amplitude in deep water often much less than a metre , but they can shoal up to many metres high in shallow waters. In the Boxing Day tsunami, there were waves 15—30m high.
Surf waves also undergo shoaling — they have a greater amplitude than the ocean swells they form from. They shoal much closer to the coastline than tsunamis, and the proportional increase in height of their wave crest height is much less. Shoaling happens because waves experience force from the seabed as the water gets shallower. This slows down the wave — the shallower the water, the slower the wave. As waves slow down, they start to bunch together, so they have a shorter wavelength than before.
Having a shorter wavelength means that the waves get higher.
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