Re: [Paddlewise] Tsunamis

On Thu, Dec 30, 2004 at 08:45:34AM -0800, Steve Holtzman wrote:
> I was watching a special on the Weather Channel the other day that mentioned
> that the height of most Tsunami's in deep water on the open ocean is only
> about 10 - 20 cm. As a result on a boat, you wouldn't even know it. However,
> the wave is traveling at speeds of up to 400 mph and the wave length can be
> 200 miles. That is a lot of water! When it reaches the shallow water at
> shore, it will typically rise to a wave height of 10 - 30 meters.

BTW, and I should have mentioned this in my earlier followup: this snippet
gives one way to appreciate the incredible amount of energy being unleashed.

The kinetic energy of a mass M moving at velocity V is 1/2 MV^2  (one-half
times M times V-squared).  Consider a V on the order of 400 MPH as mentioned
above: V-squared will be huge.

Now consider that the wave decelerates as it comes onshore.  Sure, some of
the energy will be dissipated in losses (friction, turbulence, etc.) but a
large chunk of it won't be.  So...if V decreases by a lot, say by a factor
of 10, but the energy doesn't just disappear (and it doesn't), then that
means that M has to grow by enough to compensate...and that means a very
large M will be involved.  (e.g. if the velocity decreases by a factor of
10, then the mass increases by a factor of 100)

BTW: this is a _very_ crude way of looking at the problem, but I figured
that trying to get into hydrodynamics is probably not necessary: doubly so
when all these antiseptic calculations don't begin to hint at the human
tragedy which is still taking place as all of us sit wherever we are today.

--Rsk
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