MAR 110: Lecture 14 Outline – Ocean Waves
MAR 110 LECTURE #14
Ocean Waves
Figure 19.1 Do Ocean Surface Waves Present a Hazard?
“ ....a picture is worth a thousand words”
Tsunamis - giant shallow water waves – this one
devastating the city of Lisbon Portugal in 1755, which was
already suffering from fast spreading fires caused by the
earthquake that triggered the wave. Most of the city and
much of its population had been completely destroyed.
(NG)
Figure 19.3 Ocean Swells
Ocean swells from distant ocean storms grace
a California beach – surfer’s paradise!. (ItO)
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MAR 110: Lecture 14 Outline – Ocean Waves
Figure 19.4 The Destructive Power of Coastal
Ocean Waves (Santa Catalina Island California)
(???)
Figure 19.5 Open Ocean Chaotic Waves
In the open ocean waves are coming from all directions
with varying intensities, creating a chaotic sea surface
(left). Waves with different wavelengths (rights) when
combined form an irregular wave profile. (ItO, LEiO)
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MAR 110: Lecture 14 Outline – Ocean Waves
Figure 19.6 A Simple Surface Wave
A wave is a periodic (or repeated) sea surface
distance that is defined by its wave length L and its
wave height H. (LEiO)
Figure 19.7 Ocean Waves – a Constant Competition
Gravity restores a sea surface disturbance – most often
caused by wind – towards flat-ocean equilibrium. Overshoots
keep the cycle going in which the potential energy of
elevation is converted to kinetic energy of motion and back
again. (??)
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MAR 110: Lecture 14 Outline – Ocean Waves
Figure 19.8 The Wave Form Propagates
Waves move horizontally –they “propagate” away from their
generation sites. (LEiO)
Figure 19.9 Wave Mechanics
Surface waves In the open ocean propagate horizontally
(carrying their energy), HOWEVER, the water parcels
associated with the passing wave DO NOT propagate! Rather
they move in relatively small circles “in place”. (LEiO)
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MAR 110: Lecture 14 Outline – Ocean Waves
Figure 19.10 Wave Propagation Speed
The time it takes for a full wave with a wave length L
to pass a fixed spot is called the wave period T. The
wave speed Cp is the ratio of the wave length to the
period as shown above. (LEiO)
Figure 19.11 Wave Spectrum
Waves are classified or sorted in a spectrum
according to their wave periods , from the
very smallest period “capillary” waves ( withT
less than 0.1 sec) to “ocean swell” (T about
15 sec) to tsunamis (T about 20 min). The
amount of energy in the particular wave
classes is determined by the typical wave
heights H in the class. The forces that cause
the particular waves are indicated above.
(ItO)
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MAR 110: Lecture 14 Outline – Ocean Waves
Figure 19.13 Deep Water Waves
A wave is classified as a Deep Water Wave, if the water
depth D is greater than half of its wave length L. Below the
depth = wave length, the water motion associated with this
wave virtually disappears. Note the decreasing size of the
wave parcel orbits. (LEiO)
Figure 19.14 Deep Water Wave Speed
Deep water wave speed depends on wavelength. (LEiO)
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MAR 110: Lecture 14 Outline – Ocean Waves
Figure 19.15 Shoaling Waves
As deep water waves propagate into water shallower (depths
D less than their wave length L), the ocean bottom
interacts with the wave and the waves are transformed, as
they shoal. The shoaling waves slow down and wave heights
increase. (NH)
Figure 19.16 Shallow Water Waves
Shallow water waves do not have the sinusoidal
shape of deep water wave because the particle
orbits are forced into narrowing ovals (bottom) as
they move towards shore instead of the normal
circular orbits. As the water gets shallower the
wave height increases until gravity forces it to
break (top) (LEiO)
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MAR 110: Lecture 14 Outline – Ocean Waves
Figure 19.17 Shallow Water Wave Speed
Shallow water wave speed depends on depth. (LEiO)
Figure 20.2 Wave Energy Transmission & Wave Groups
Deep ocean waves with slightly different wavelengths
superimpose to form wave groups with speeds (Cg) that are
half that of individual wave speeds (Cp). Because the
individual waves move faster than the wave group and
energy (Cp = 2 Cg), individual waves are seen to (a) appear
at the back of the wave group (b) propagate through the
slower-moving wave group; and finally (c) disappear at the
head of the wave group.
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MAR 110: Lecture 14 Outline – Ocean Waves
Figure 20.3 Deep Water Wave Energy Transmission
Deep ocean waves with slightly different wavelengths form
wave groups – which are distinguished by their wave height
envelope; which contains the wave energy. The wave group
speed for deep water waves (Cg) is the wave speed Cp.
The form of the deep water wave group speed - given by the
relation above - indicates that the longer wave length, deep
water waves travel faster than shorter wave length waves.
Figure 20.4 Shallow Water Wave Energy Transmission
The shallow water wave group speed Cg is equal to the wave speed Cp and
thus depends on only the local water depth D, according to the above.
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MAR 110: Lecture 14 Outline – Ocean Waves
Figure 20.6 Open Ocean Wind Wave Generation and Dispersion
(Upper) In the wind-generation region, the ocean is rough and chaotic
because waves with all different wavelengths superimpose and interfere with
each other (lower left). As the waves propagate toward the right and away
from the generation zone, the waves sort according to wave group speed.
The longer wavelength/longer period waves known as “swell” (lower right)
“race” out ahead of the shorter wavelength/shorter period waves.. (ItO)
Figure 20.7 The strong winter storms in the Southern Ocean
generate waves that have a long time (& distance) to sort
themselves out according to wave length before they strike the
southwest facing shores of California.
(NH, ItO)
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