Kevin's Journal: What is a tsunami?

If you're at the beach, and you happen to see that the sea water has suddenly receded from the coastline so much and so quickly that even fishes and other sea creatures were left stranded without water on the seabed, RUN!!!

With the death toll as of now at 66,000 and rising, it's really a very depressing and unforgettable way to mark the end of the year 2004.

What makes a tsunami so deadly?

In deep water, the energy of a tsunami is constant, a function of its height and speed. Thus, as the wave approaches land, its height increases while its speed decreases. A tsunami has a very long wave length (in the order of 100 km), which makes it act as a shallow-water wave. Since the speed of a shallow-water wave is ${\sqrt{g d}}$ , where $g$ is the gravitational acceleration and $d$ is the water depth, a tsunami in the open ocean can obtain a speed of about 700 km/h. While in deep water a person at the surface of the water would probably not even notice, but the wave can increase to a height of 30 m and more as it approaches the coastline and compresses.

More about it's definition here.

I could be wrong, but, from the above description, it seems that by the time anyone's able to guess that a tsunami is about to hit them, it may already be too late to really run very far.

Given that a tsunami travels at about 200 m/s in the open ocean (yes, that's 200 metres per second, although it starts losing speed but gaining height as it hits the shore), and a bullet fired from a weapon, depending on the bullet's shape, calibre, and various other factors, flies at about 180 - 1500 m/s... well you can imagine the rest...when a wall of water hits you at speed, not much difference from flying into a concrete wall...

Just when you think things cannot get any scarier, there is the megatsunami...