Shorelines and Shoreline Processes - The Changing Earth: Exploring Geology and Evolution - page 411

◗

Figure 16.4 Waves and Wave Terminology

Wavelength decreases

Off-
shore

Wave height increases

Nearshore

Wave height

Breaker

Wavelength

Crest

Trough

Surf zone

Wave base
= wavelength

Waves and the terminology applied to them.

a

b When swells in the deep water move toward the shore, the
orbital motion of water within them is disrupted as they enter
water shallower than wave base. Wavelength decreases while wave
height increases, causing the waves to oversteepen and plunge
forward as breakers.

c For scale, notice the surfers waiting for this wave that is
beginning to break.

ship Dawn was damaged by a 21-m-high rogue wave that
fl ooded more than 60 cabins and injured four passengers.

Broad, undulating deepwater waves are transformed
into sharp-crested waves as they enter shallow water. This
transformation begins at a water depth corresponding to
wave base—that is, one-half wavelength (

Shallow-Water Waves and Breakers Swells moving out
from an area of wave generation lose little energy as they
travel long distances across the ocean. In these deepwater
swells, the water surface oscillates and water moves in circu-
lar orbits, but little net displacement of water takes place in
the direction of wave travel (Figure 16.4a). Of course, wind
blows some water from wave crests, thus forming whitecaps
with foamy white crests, and surface currents transport water
great distances; but deepwater waves themselves accomplish
little actual water movement. When these waves enter pro-
gressively shallower water, however, the wave shape changes
and water is displaced in the direction of wave advance.

◗

Figure 16.5a).
At this point, the waves “feel” the seafl oor, and the orbital
motion of water within the waves is disrupted (Figure 16.4a).
As waves continue moving shoreward, the speed of wave
advance and wavelength decrease, but wave height in-
creases. Thus, as they enter shallow water, waves become
oversteepened as the wave crest advances faster than the
wave form, and eventually the crest plunges forward as
a breaker (Figure 16.4c). Breaking waves might be sev-
eral times higher than their deepwater counterparts, and
when they break, they expend their kinetic energy on the
shoreline.