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Tsunami Mechanics

my understanding of tsunamis is much like any large ocean wave breaking on the beach. In the open (read deep) ocean the wave energy extends down in the water but as the wave approaches shore the water depth becomes shallow and that energy has to go somewhere so it builds up high above the water and hence you have a breaking wave on the beach.
Here's what I learned years ago in Naval Science. The tsunami wave is spread out, not down. Because they originate with the displacement of seabed through earthquake, underwater land slide, or meteor strike, the volume of water involved in the displacement is enormous, but they typically start out only one to three feet tall with a wavelength of a hundred miles or more.

The speed of travel is affected by the depth of the water. In really deep water tsunamis travel very fast - on the order of 500 mph. One can pass by a ship and no one aboard would even notice. That stands to reason. It would take 10 minutes to go by, so call it a 3 foot rise over 5 minutes followed by a three foot fall over 5 minutes. When a tsunami travels over a depth gradient as it does when it approaches land, the front of the wave slows down more than the rear does since the rear is in deeper water than the front. The wave length shortens and the height increases as the water piles up on itself since momentum is conserved.

A tsunami hitting land does not look like the wall of water crashing down on the beach you see at the cinema. Rather, the period shortens to a mile or three (or 10) and it appears that the sea level simply rises astoundingly within a few minutes, and the flooding can proceed many miles inland as the moving water dissipates its momentum by friction and climbing the land.

So I've been told. In all my years at sea, I never saw one for myself.

DaCAP
 
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