Suppose a deep water wave 6 inches high and 500 miles crest to crest.
Approaching shallow water it may reach 20 feet in height, in some cases
100. Such a wave would not appear to be threat in deep water, but it's
approach can not be ignored in shallow water, as it will become quite
steep, break and is likely to become a bore with a near vertical wall.

In general, in deep water, it is wave steepness, not height that is the
major threat. That a wave that approaches ultimate steepness is in fact
in danger of plunging, due to the mass of water at the top traveling
faster than the lower part of the wave. This implies that a really steep
wave is in the process of slowing. That if the slowing is consistent
from top to bottom, it will not break. That wind may make the top blow
off, not necessarily in the same direction as the wave travel, the
effect being similar to a breaker.

Taking just the factors above, one can construct a myriad of wave forms.
The wave base can be slowed down by several known and perhaps unknown
factors, nearness of the bottom, opposing current, internal waves,
thermoclines and salinity and other water differences near the surface,
interaction with other waves.

In general wind waves do not have long crests, a crest that stretches
from horizon to horizon. Such waves are inherently short crested and in
my opinion long crests are directly related to long period waves and
vice versa. Short crested waves can be zigzagged around, even in slow
boats. That waves that show signs of breaking and are of such a size as
to endanger the boat should be skirted around.

No 2 wave crests travel at the same speed. Slower waves are always being
overtaken by faster ones. If waves would just behave and stay in step
like marchers in a parade, a sailor's life would a lot simpler. The same
speed implies the same wave period.

There is very little known about the end of the crests and how they
behave (ignoring breaking crests). It would seem that they travel at the
same speed as the rest of the crest, but perhaps they don't and this
explains some of why crests tend to die out, although the majority
opinion is that crest disappearance is cause by interaction with other
waves.

In shallow water things get very complicated, chaotic appearing. Waves
of different height will interact with the bottom and slow at different
speeds, assuming they were of the same speed and in step before, which
is never the case anyway.

If all this seems a little more than a sailor can bear, then keep in
mind that most of this apparent chaos is not visible most of the time.
But, on the other hand, because of this one can be lulled into a false
sense of security.

I was examining the latest graphic charts of generally high waves and
the area in Southern Oregon and Northern California was one of the
places where high waves were shown. Hawaii is a place where things are
not as calm as you might hope.

This is a URL to some further reading.

http://www.cassiopedia.org/wiki/index.php?title=Freak_wave

More later.

Mike

Capt. Mike Maurice
Beaverton Oregon(Near Portland)

http://lettres-histoire.ac-rouen.fr/histgeo/vagues.htm

I missed this page from the previous posting. This has some graphical
presentations.

Mike

In shallow water things get very complicated, chaotic appearing. Waves
of different height will interact with the bottom and slow at different
speeds, assuming they were of the same speed and in step before, which
is never the case anyway.

This is a good description of what can happen in the Gulf. It's an easy body
of water to underestimate & every year several boaters do to their demise.
Instead of riding swells your getting hit by smaller irregular steep waves,
sometimes from different directions, that you often go through instead of
over.  On the other hand the really huge waves don't seem to develop.

Carl
Scout 30
Hudson Beach, FL