Geology Reference
In-Depth Information
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The waves described above are the classic
plunging break-
ers
that crash onto shorelines with steep offshore slopes, such
as those on the north shore of Oahu in the Hawaiian Islands
(Figure 16.5b). In contrast, shorelines where the offshore
slope is more gentle usually have
spilling breakers,
where
the waves build up slowly and the wave's crest spills down
the wave front (Figure 16.5c). Whether the breakers are
plunging or spilling, the water rushes onto the shore and then
returns seaward to become part of another breaking wave.
What Would You Do
While swimming at your favorite beach, you suddenly notice
that you are far down the beach from the point where you
entered the water. Furthermore, the size of the waves you
were swimming in has diminished considerably. You decide to
swim back to shore and then walk back to your original star-
ing place, but no matter how hard you swim, you are carried
farther and farther away from the shore. Assuming that you
survive this incident, explain what happened and what you did
to remedy the situation.
The area extending seaward from the upper limit of the shore-
line to just beyond the area of breaking waves is conveniently
designated as the
nearshore zone.
Within the nearshore zone
are the breaker zone and a surf zone, where water from break-
ing waves rushes forward and then flows seaward as back-
wash. The nearshore zone's width varies depending on the
length of approaching waves because long waves break at a
greater depth, and thus farther offshore, than do short waves.
Incoming waves are responsible for two types of currents in
the nearshore zone:
longshore currents
and
rip currents.
Wave Refraction and Longshore Currents
Deepwater
waves have long, continuous crests, but rarely are their crests
parallel with the shoreline (
Figure 16.6). In other words,
they seldom approach a shoreline head-on, but rather at
some angle. Thus, one part of a wave enters shallow water
where it encounters wave base and begins breaking before
other parts of the same wave. As a wave begins to break, its
velocity diminishes, but the part of the wave still in deep
◗
◗
Figure 16.5
Wave Base and Breakers
Image not available due to copyright restrictions
Image not available due to copyright restrictions
A spilling breaker.
c
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