Author; John Sidles sidles@u.washington.edu
Time for Sidles to write! People first began to discover
why waves appear in sets when they built a wave tank which
launched an absolutely regular train of waves out of a
little wave-making machine at one end of the tank.
Trouble was, when this train of waves reached the other
end, it was no longer regular. It had organized itself
into tiny litle "sets". Hey! said the engineers. Our
wave machine must not be working smoothly. So they put a lot
of effort into making the tank and the machine absolutely
precise and regular. But no go .. the wave trains *still*
organized themselves into sets, no matter how carefully and
uniformly they were launched.
So the engineers sat down and studied the equations that
govern wave motion. They found that certain (very tiny and
usually neglected) nonlinear terms in the equations of
motion of deepwater waves act to transfer energy (very
slowly) from the leading and trailing edge of a set of
waves, toward the center waves. Thus the central waves get
bigger at the expense of leading and trailing waves.
As a result, even if the wind is blowing absolutely
uniformly, waves will still organize their energy into
sets. This organization happens slowly, which is why local
windswells are less-well-organized than swells that have
propagated thousands of miles. The mathematical name of the nonlinear
interaction that creates sets in water waves is the
"Benjamin-Feir Instability".
In any case, it is pretty cool that this obscure energy
transfer mechanism, acting over thousands of miles, creates
sets for us surfers!
The best place to learn more about waves is the very cool
and accessible paperback book "Waves and Beaches" by
Willard R. Bascombe. Take a copy on your next surfari!
The photos of Bascombe surfing a 20-ton WWII amphibious landing
vehicle, one mile offshore on a maxed-out 20' PNW beach break
swell, are more than worth the price!
Cawabunga ... JAS
The Math
Author; Timothy B. Maddux tbmaddux@gear.ucsb.edu
A linear gravity wave with a phase speed of 40 mph
(C = gT/2pi, so T = 2piC/g) has a period of 11 seconds.
A linear gravity wave with a phase speed of 75 mph has a
period of 21.5 seconds.
Feel free to check my math. We don't get many waves
over 20-second periods from relatively closer storms,
but we also don't get hurricane-force winds over the
long fetches and durations required to produce fully
developed seas.
Fetch/duration and wind speed, and that is all. The first
two are grouped together as they're really expressions of
the same thing: how long the wind acts over a wave as it
travels. Waves care nothing for distinctions between
reaching the end of a fetch or the wind dying off with time.
