Re: [Paddlewise] Wave Speed - (totally trivial question!)

Peter Osman wrote:

> Can anyone tell me if all (sea) waves travel at roughly the same hoizontal
> speed (relative to the current of course). I've been trying to observe if
> this is the case for some time and am coming to the conclusion that they do
> but am not sure if they speed up when approaching shore?

[The following is based on Bascom's "Waves and Beaches," 1980 edition, pages
32-34 and 69-73.]

The answer to your question depends on whether you are dealing with "deep water
waves," or "shallow water waves."

For waves in "deep water" -- water so deep the waves do not "feel" the bottom
(depth is greater than half the wavelength), the translational velocity is
proportional to the square root of the length, and directly proportional to the
period.

This means that swell in deep water running with a 12-second gap between crests
will travel twice as fast as swell with a 6-second period, for "ideal" waves. 
Real waves will be close to this.

When these same waves approach shore, however (the depth is less than half the
wavelength), where we are more likely to be paddling, they become "shallow
water waves," whose translational velocity is essentially **independent** of
period (and hence wavelength).  In "shallow water," their speed is proportional
to the square root of the depth of the water, and does not depend (much) on
their period or wavelength.

This means that the 14-second swell and the 7-second swell will approach the
surf zone with essentially the same speed.  NOTE that these period values will
change some as the waves steepen up as they reach shallow water and begin to
"peak up" preparatory to breaking in the surf zone. 

Some wave velocities, as a function of period:  6 seconds (21 miles/hr); 12
seconds (42 miles/hr); and [the longest period swell ever recorded] 22.5
seconds (78 miles/hr!!).  All these are for "deep water."  But, even the
6-second swell has to be in at least 92 feet of water to be a "deep water
wave."  The 12-second swell needs 369 feet of depth!

My apologies if this is more than you wanted to know.  <g>

-- 
Dave Kruger
Astoria, OR
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