Re: [Paddlewise] question for the hydrodynamics experts...

Gratytshrk_at_aol.com wrote:
> 
> Hi yall,
> I have a question about a bizarre (and somewhat terrifying experience) i had
> today. [snip] after one spectacular cast i happened to look up and see a
> huge freighter coming down the [narrow, shallow] channel. 
> [snip] While the ship was about 1/8 of a mile away all the water from
> near the sides began rushing out toward the ship.  I was in about 4 ft of
> water which drained to the bottom in about 20 seconds.  The force of the
> outrushing flow was greater than any undertow i have ever encountered.  
 [snip] Soon a wave of returning water and
> ship wake (which was at least 2-3 feet above my head from where i was sitting
> in the trough, I would estimate it at 6-8 feet) was rushing back toward the
> bank, as i was rushed toward the wave.  The outrushing water and inrushing
> wave collided to form a huge breaker which crashed right on my face.
[snip]
> After the ship passed i decided to call it a day and happened to notice
> another equally large tanker coming through the canal.  I decided to hide in
> a side canal and paddled back about 1/2 mile into a canal and thought i could
> hide around a couple of bends.  I was amazed to see the same effect occuring
> as the ship approached, [snip]

What you experienced was the "surge" portion of the ship's wake.  In brief: 
that surge produces a minimal vertical excursion of the water's surface *when
the water is very deep* (at least as deep as the "length" of the wave the
surge forms in deep water).  That's why you would not notice the surge if the
ship passed by in deep water.  OTOH, when the water is very shallow, the
leading part of the wave (the trough) arrives first, and because its "ideal"
form is impossible to maintain in shallow water, it "sucks" water back toward
the ship, to help form *the crest* of the wave (which arrives just behind the
trough).  Because the water is too shallow to sustain the crest,  a breaking
wave is formed, similar to surf, and the wave becomes akin to what happens in
a tidal bore.  What happened to you is sort of a small scale version of a
tsunami hitting a shallow, sloping beach.

For more about waves (and beaches), chase down Willard Bascom's classic work,
written for the lay person:  Waves and Beaches, Anchor Books, 1980 (revised
version), ISBN 0-385-14844-5.  Out of print, I am sure, but in any good
library's earth science section.  A treasure trove of stuff useful for ocean
paddlers, covering both waves on the water, and how coastal landforms are
affected by waves.  You'll be a better person, and have an improved sex life
if you read this!  <g>

-- 
Dave Kruger
Astoria, OR


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