"Chris Hogg" wrote in message
...
On Sat, 8 Jan 2005 22:18:03 +0000 (UTC), "Franz Heymann"
wrote:


If you were to crack a book on hydrodynamics you will find that
there
are essentially four different types of wave in an ideal
non-viscous
liquid

Capillary waves
Surface waves
Deep water waves
Solitons

(actually, there are also linear combinations of the first two)

Capillary waves are what you get if you induce a wave by vibrating
a
tuning fork witih one tine just touching the liquid
Surface waves are what you normally see on the surface of the ocean
before they break near the shore. That includes the so-called
"Giant
waves". These are simply distinguished by having a very large
amplitude of oscillation. In both those types of wave any "piece"
of
water moves vertically up and down.
In the case of a deep water wave, any *piece" of water executes a
circular motion, with the diameter of the circle equal to the depth
of
the water.
Both tidal waves and tsunamis are synonyms for deep water waves.
In the open ocean with a constant depth, a deep water wave is
essentially harmless and may pass practically unnoticed.

In the links I posted earlier (see below) and which seemed
authoritative, tsunamis were described as shallow water waves,
despite
propagating in deep water. I assumed this meant their
characteristics
place them in the 'shallow water wave' category, and didn't mean
they
only appear in shallow water. I am now confused.

I am confused as to why the first link calls them "shallow waves"
The waves which I called "deep water waves" are those with a speed of
sqrt(gh), and a rolling particle motion, where g is the graviational
acceleration and h is the depth of the ocean floor. Why he calls the
same waves shallow water waves and then produces a a calculation for
the speed in a 4 km deep ocean is beyond me. Shallow water waves
have, so to speak, no knowledge of the depth of the ocean. They are
the ones I called surface waves.
The same link also gives an incorrect impression of what is known as a
tidal wave. My view of what constitutes a tidal wave is not just the
normal "steady" progression of the tide, but occurs in those (few)
local geographical circumstances where the tidal rise enters a
constricted region in which a deep water wave can be excited.

http://www.fluidmech.net/tutorials/ocean/tsunami.htm
http://www.es.flinders.edu.au/%7Emat...lecture10.html

It only
becomes dangerous when it meets a sloping ocean floor, so that the
nornal circular motion can no longer be accomodated. It is at that
stage that it begins to "suck" in water ahead of it and begins to
pile
up in height.

That would certainly fit the descriptions coming from survivors, who
talked about the tide going right out very rapidly, before the crest
of the wave arrived. But I also heard that it doesn't always happen
this way, depending on whether a crest or a trough hits the shore
first. And wouldn't this also occur with any type of wave? They all
presumably have peaks and troughs.

It happens to surface waves as well, as witness the usual breakers on
a beach just before they break, but not to capillary waves. The
latter are purely surface tension effects and are independent of water
depth.

Incidentally, what I call surface waves are also known as gravity
waves.

Solitons are peculiar beasts in which certain specifically-shaped
transient surface disturbances can move unchanged in shape.

Franz