W A V E S
The Steepness of a Wave
Jerry Dennis & Glenn Wolff
The steepness of a wave never exceeds a 1:7 ratio of height to length. Thus a wave seven feet from crest to crest cannot be more than
one foot high. Whitecaps are formed when wind stronger than about
20 feet per second (13.6 miles per hour) pushes small steep waves to
that 1:7 limit. When the limit is reached the waves become unstable
and break into a froth of turbulence.
In a whitecap or breaking wave, particles of water fall down the
crest and make actual progress. But in ordinary waves, water particles
move only in well-defined circular orbits, essentially remaining in place
while the wave passes through them. This motion is one of the oddest
and least understood features of a wave. Waves are energy passing
through water, visible only on the surface, but they have roots
extending downward a distance equal to one-half the wave's length. As
the energy passes it sets particles of water spinning in circular orbits,
like rollers on a conveyor that spin when a crate passes over them but
remain seated in place. The orbit of the water particles is large at the
surface, where the diameter is equal to the height of the wave, but
diminishes quickly with depth. Beneath the final, smallest orbit at the
bottom of the wave there is no motion at all.
This difficult concept can be tested by filling a plastic bottle with
water so that it barely floats. Place the bottle in waves next to a
stationary object and note how it moves slightly forward with every
crest and slightly backward with every trough. The orbiting water
particles within each wave follow the same swaying motion.
Excerpts I, II, III & IV:
When waves enter shallow water they undergo dramatic changes.
COPYRIGHT 1996 GLENN WOLFF/JERRY DENNIS/HARPER COLLINS -- ALL RIGHTS RESERVED