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[Charlemagne 8]

I don't mean to be insensitive as the disaster that has happened  in Southern Asia is horrible. Their problems are just beginning.
But I am stuck on a technicality: There are reports of the wave that hit Sri Lanka as travelling at 500 miles per hour. How is this possible? That is roughly the cruising speed of a 747 at 30,000+ feet. There would be nothing and no one left if a wall of water 40 feet tall hit at that speed.

CVIII

[IanG]

But I am stuck on a technicality: There are reports of the wave that hit Sri Lanka as travelling at 500 miles per hour. How is this possible? That is roughly the cruising speed of a 747 at 30,000+ feet. There would be nothing and no one left if a wall of water 40 feet tall hit at that speed.

The speed of a Tsunami is proportional to the depth of the water  - those (possibly exagerated) speeds are seen in mid-ocean, though the wave height is so low that shipping probably wouldn't even notice it.  As the wave moves into shallower water the height increases, but the speed decreases.  Fortunately, when the wave reaches land, some of its energy is reflected back out to sea, otherwise the disaster would be even worse.

Ian G.

[Charlemagne 8]

OK. Thanks.
That would be typical of our news services -- technically accurate but not in any meaningful way.

[GHammer]

But I am stuck on a technicality: There are reports of the wave that hit Sri Lanka as travelling at 500 miles per hour. How is this possible?

Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.

As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very small. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s/s) and the water depth - let's see what this implies: In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses.

[Charlemagne 8]

In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr.

But surely that doesn't mean that said wave hits the shore at that speed does it? That's Mach 0.67.  If that were the case, it would seem to me that you could expect devastation on the order of Hiroshima. You would think that there would be no bodies to bury. When a jet airliner hits the ocean at even a quarter of that speed, there are only tiny pieces left and those are metal and plastic.

Certainly the videos of that wave (those waves?) coming on shore don't indicate a speed of more than 50 mph, if that.

I'm unschooled in such things but it just doesn't seem logical.

CVIII

[GHammer]

Try this:

http://observe.arc.nasa.gov/nasa/exhibits/tsunami/tsun_physics.html

[gpvillamil]

It is the wave that moves - not the entire mass of water.

An analogy - when people do the "wave" at a stadium, the wave travels around - but the people stay in their seats.

[Charlemagne 8]

So the "wave" just causes the water level to rise? And then the "wave" travels along that risen water?
Man, this is just getting more confusing the more I find out.

BTW Gary, good link. WAY more information than I could assimillate.

CVIII

[GHammer]

So the "wave" just causes the water level to rise? And then the "wave" travels along that risen water?
Man, this is just getting more confusing the more I find out.

BTW Gary, good link. WAY more information than I could assimillate.

CVIII

NASA has a knack for that. One of the few places I go for science info.

[KingSparta]

there is some more htmls

http://observe.arc.nasa.gov/nasa/exhibits/tsunami/

also a quiz