INTRODUCTORY
OCEANOGRAPHYINTRODUCTORY
OCEANOGRAPHY
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In the final part of the lesson on ocean waves, we will explore the reasons and mechanisms by which ocean waves change direction. For shoaling waves, this may (and usually does) occur before waves break.
Often waves approaching shore will change their direction. There are three independent ways that this may happen.
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Changes in wave direction and the definition of wave rays |
Waves that encounter a solid vertical surface (such as a seawall) will abruptly change direction without much loss of energy. Though not as simple as a ball bouncing off a wall, the reflection of a wave obeys the same principles, where the angle of incidence "qi" is equal to the angle of reflection "qr". When the angle is zero (as measured from a line perpendicular to the reflecting surface), reflection may generate standing waves (which we will discuss in detail in the next chapter) which are the product of two waves with the same period (or length) traveling in opposite directions. Sometimes reflection can create very dangerous conditions when the reflected wave interferes constructively with an incoming wave, creating a wedge with nasty characteristics. Very large waves, such as Tsunamis, may reflect a good portion of their energy off the continental shelf and slope, even though the wave also will break as surf in shallow water.
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Some more thoughts about wave reflection |
Diffraction also involves bending of wave rays, but now that bending is around solid objects such as breakwaters or other harbor structures and is not the result of changes in wave celerity. When striking the edge of a solid surface, any point on a wave may be the source from which energy can propagate in all directions. As shown in Fig. 10.22, a wave train that approaches a harbor entrance in what appears to be a non-threatening angle may bend (diffract) around the edge of the protective barrier and a substantial amount of wave energy will be directed into the protected harbor.
What would happen in the harbor behind a breakwater between two adjacent inlets if diffraction occurred in both inlets? Look at Fig. 10.23.
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A spy story |
Waves
also may bend as they shoal (because c a
d) if they approach the beach at an angle. If you have
ever flown along a coastal region of the ocean in an airplane you may
have noticed a series of nearly parallel swell crestlines (a "wave
train") moving at an angle toward a straight coastline. At some
distance away from the coast the crestlines may approach at an angle
of more than 45 degrees, but as they get closer to shore they appear
to bend so that the crestlines are more parallel with the coastline
by the time they reach the surf zone (as shown on the right in Fig.
10.20a). They bend (refract) because the portion of a crestline that
reaches shallow water first is slowed (remember that c a
d), but the portion still in deeper water continues at its regular
celerity. Therefore, the crestlines appear
to "wheel around" until most of its length is approaching
the coastline nearly head-on (i.e., nearly parallel with the line of
the coast).
Wave trains also may approach an irregular coastline (with headlands and bays) head-on (as shown below). The portion of the wave train that first encounters the shallow water of the headland will slow and both the crestlines and the wave rays will turn toward the headland and quickly break in the headland surf zone (in fact, it is this kind of wave action that erodes the headland in the first place). That portion on either side of the headland will continue for some time at its deeper water celerity without refracting. Often there are bays between two headlands (as shown below).
If we advance the waves toward shore, the wave rays will bend toward shallow water (toward the headlands) as shown below:
Look also at Fig. 12.13 below (from the book) to see that these wave rays and crestlines in a little more colorful way (note that the orientation of the coastline is 180 different than my drawings above.
Note that during this refraction into the bay, the area between two wave rays and two adjacent wave crests increases. What this means is that the wave energy per unit area is greatly decreased and will be small when the train enters the surf zone of the bay.
Do you see, therefore, that if you were caught at sea in a small boat by a storm and could not make it to a safe harbor, it would be better to anchor in the bay than near the headland?
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Some more thoughts about wave refraction |
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