Water in MotionWaves and water movementWaves are undulations in the surface of a water body. Most waves are created when kinetic energy is transferred to
water by the frictional stress of wind blowing over it. The resulting transfer causes a rise
in water level producing a wave crest, followed by the sinking
of the surface creating a wave trough. The
wave
length is the distance between successive crests. The time required
for successive crests to pass a point is the wave period.
The wave height is the distance
between the crest of the wave and the still water level. Wave height is
determined by (1) wind velocity, (2) duration of the wind, and (3) the
fetch. The fetch is the distance of
uninterrupted flow over an open water surface. An increase in any these
factors will increase wave height and length. [See these effects using the
"Savage Seas" Wave Machine.
The rise and fall of oscillatory waves
in an open water reflects the circular
motion of water particles. There is relatively little forward motion by a
water particle as a wave passes. It is simply the wave form and its energy that is transmitted across the
ocean surface. Water particles move in circular orbits that diminish with
depth. The radius of the circular path is greatest at the surface
and decreases with toward the bottom of the wave. Larger waves
exhibit larger orbital radii and extend to a greater depth than
smaller waves. At some point in deep water, the wave has no effect
on the motion of the water. Thus a zone of no wave motion
exists from the base of the wave to the ocean floor.
Swells are smooth, rounded waves that travel outward from a storm center or continue as broad undulations of the ocean surface after the wind dies down. The wave slope is expressed as the ratio of the wave height to wave length, ranging from 1:25 to 1:50. A wave will become unstable at slopes greater than 1:7 and will fall over itself, or break.
Figure 21.6 Plunging breakers. (Courtesy Naval Meteorology Program and Oceanography Command "Restless Sea") As a wave approaches the coast, a depth is reached offshore where the wave touches the ocean floor. The tug of the ocean floor changes the circular wave motion into an elliptical one; the water moves back and forth over the bottom as each wave passes. The friction imparted from the floor slows the wave base. At a depth of 1.3 times the wave length, the drag causes the top of the wave to rush forward, become unstable and break. Water in the breaking wave is transported toward shore as a wave of translation.
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Figure 21.5 Wave Characteristics (Courtesy
