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A Closer Look

Rogue Waves
rogue wave is an unusually large wave wind direction, for example. Large rogue coming together to form a mountain-
A that appears with smaller waves. The waves have been reported from 21 to 35 m ous wave that lasts several minutes before
rogue wave has also been called a "freak" (69 to 114 ft) tall and have been observed to subsiding. Other processes, such as wave
wave. Whatever the name, it is generally almost capsize large ships. focusing by the shape of the coast or move-
one or a group of two or three waves that It is believed that a rogue wave is an ment by currents, may play a part in form-
are more than twice the size of the normal extreme storm wave. It probably forms dur- ing rogue waves. The source of rogue waves
surrounding waves. The rogue wave is one ing a storm from constructive interference continues to be a mystery and an active
or several very large "walls of water" and has (see p. 126) between smaller waves when topic of research. This much is known—
unpredictable behavior, not following the the crests and troughs happen to match, rogue waves do exist.

EXAMPLE 24.3
The position offshore where waves with the circular motion of water
particles will encounter the bottom can be determined from the slope
of the bottom and the wavelength (λ). If a beach gently slopes at
2.1 centimeters per meter and the wavelength is 12 meters, how far
offshore will wave base encounter the bottom?
The depth of wave base is one-half the wavelength.
1 _
λ = 12 m = λ
wave base
wave base = ? 2
1 _
= (12 m)

2
= 6 m
Slope is the ratio between the change in depth of the bottom and the
distance offshore; hence, the distance offshore where wave base will
encounter the bottom can be determined from the slope. FIGURE 24.19 The white foam is in the surf zone, which is
where the waves grow taller and taller, then break forward into a froth
_ _ _
ΔY
ΔY
cm

slope = 2.1 slope = ∴ ΔX = of turbulence. Do you see any evidence of rip currents in this picture?

m ΔX slope
ΔY = 6 m _ m _
cm

Convert m to :
m
ΔX = ? that are carried by local currents back to the ocean. The rest
cm _
_ 1m

2.1 m( 2 ) of the energy goes into the kinetic energy of water molecules,

1 × 10 cm
which appears as a temperature increase.
−2
2.1 × 1 0 Swell does not transport water with the waves over a distance,
_ but small volumes of water are moved as a growing wave is pushed
6 m

ΔX =

−2
2.1 × 10 to greater heights by the wind over the open ocean. A strong wind
= 2.9 × 10 m can topple such a wave on the open ocean, producing a foam-
2
topped wave known as a whitecap. In general, whitecaps form
when the wind is blowing at 30 km/h (about 20 mi/h) or more.
EXAMPLE 24.4 Waves do transport water where breakers occur in the surf
If the surf zone at the beach in example 24.3 is 165 m offshore, what is zone. When a wave breaks, it tosses water toward the shore,
the wave height of the breakers? (Answer: 2.6 m.) where the water begins to accumulate. This buildup of water
tends to move away in currents, or streams, as the water returns
to a lower level. Some of the water might return directly to the
Waves break in the foamy surf, sometimes forming smaller sea by moving beneath the breakers. This direct return of water
waves that then proceed to break in progressively shallower forms a weak current known as undertow. Other parts of the
water. The surf may have several sets of breakers before the water accumulated water might be pushed along by the waves, produc-
is finally thrown on the shore as a surging sheet of seawater. The ing a longshore current that moves parallel to the shore in the
turbulence of the breakers in the surf zone and the final surge surf zone. This current moves parallel to the shore until it finds
expend all the energy that the waves may have brought from a lower place or a channel that is deeper than the adjacent bot-
thousands of kilometers away. Some of the energy does work in tom. Where the current finds such a channel, it produces a rip
eroding the shoreline, breaking up rock masses into the sands current, a strong stream of water that bursts out against the waves
24-17 CHAPTER 24 Earth’s Waters 613