1. Geosc040, Apr 3; Lecture 21 Waves, Tsunami
Come on Baby Take the Wave (Love) Train The O'Jays
Waves, Mr. Probz
Thanks to Morgan G.
Marine Fishes: I Drink Alone - George Thorogood
M 8.2 Earthquake, Chili

2. Apr 3; Lecture 21 Waves, Tsunami

3. Quiz 2 April 14. Calculators will be provided –but I doubt you’ll need one
On-line Assignments 10 and 11 due next week.
Extra Credit Letter accepted until 11pm April 4

4. Wind Blowing over the Ocean Generates Waves
(b) Capillary waves become gravity waves as their wavelength exceeds 1.74 centimeters. These wind-induced gravity waves (wind waves) continue to grow as long as the wind above them exceeds their speed.
Capillary waves become gravity waves as their wavelength exceeds 1.74 centimeters. These wind-induced gravity waves (wind waves) continue to grow as long as the wind above them exceeds their speed.

5. Wave Generation by Wind
•These are, for example, the big, regular swells (T or L) that approach the west coast of the U.S. from far distant storms, providing outstanding surfing
•Most of the time storm winds generally blow across areas of relatively small fetch for relatively short periods; thus, fully developed seas rarely occur.
• Nonetheless, large storm systems are important wave generators.
Regular swell spacing (one dominant period) at long distances from storm or wind producing the waves. An example of long-period swells outrunning swells of shorter period

6. Three Main Factors in Wave Generation by Wind
1) Wind strength
(avg. or persistent wind velocity)
2) Wind duration
(time that wind blows in one general direction)
3) Fetch
(distance over which wind blows uninterrupted in one direction)

7. Waves and Water Depth
Deep-water waves, Shallow water waves…

9. The Equations we know and love…
Deep water waves: 1) S = L / T, 2) S = 1.56 T, 3) L =1.56 T2
(S is speed in m/s, T is period in sec, and L is wavelength in m.)
In deep water, what is the wavelength of a 10-second period wave?
a) 1.56 m
b) 15.6 m
c) 75 m
d) 156 m

10. The Equations we know and love…
Deep water waves: 1) S = L / T, 2) S = 1.56 T, 3) L =1.56 T2
(S is speed in m/s, T is period in sec, and L is wavelength in m.)
In deep water, how deep does a wave of 10-second period extend?
a) 10 m
b) 1.56 m
c) 5 m
d) About 75 m

11. Wave size and period. Are they all the same or is there a big one every so often --every 7th wave?

12. Most ocean waves have a period between 1 and 10 seconds

13. For deep water waves, that means a speed between
1.56 m/s and 15.6 m/s
Recall:
S = 1.56 T
Most waves have a period between 1 and 10 seconds

14. Wave Interference Waves can “interfere” with one another
Constructive interference means that waves interact to build crests or troughs higher or deeper than those of the original waves
Destructive interference means that waves cancel one another out
Such interactions produce characteristic patterns or “beats” --every fourth, fifth, sixth (etc.) wave is larger than the other waves in the group. Surfers learn how to capitalize on these patterns

15. Wave Interference Waves can “interfere” with one another
Constructive interference means that waves interact to build crests or troughs higher or deeper than those of the original waves
Destructive interference means that waves cancel one another out
Such interactions produce characteristic patterns or “beats” --every fourth, fifth, sixth (etc.) wave is larger than the other waves in the group. Surfers learn how to capitalize on these patterns

16. Wave Interference Waves can “interfere” with one another
Constructive interference means that waves interact to build crests or troughs higher or deeper than those of the original waves
Destructive interference means that waves cancel one another out
Such interactions produce characteristic patterns or “beats” --every fourth, fifth, sixth (etc.) wave is larger than the other waves in the group. Surfers learn how to capitalize on these patterns

17. Waves bend!
Diffraction

18. Wave Diffraction

19. Wave Generation by Wind How Big is Big?
Steepness of a wave flank does not exceed about 120 degrees
A “rule of thumb”: H/L is a ratio of 1/7 (remember this!)
A wave with L=156 m in our example above can have H≤22 m!
Wavelength L
Wave Height H

20. Wave Generation by Wind How Big is Big?
Steepness of a wave flank does not exceed ~ 120 degrees
A “rule of thumb”: H/L is a ratio of 1/7 (remember this!)
A wave with L=120 m (~ 10 s period) has max H of 20 m
Wavelength L
Wave Height H

21. Wind Waves
Waves “break” when oversteepened, thus the “whitecaps” observed in strongly wind-driven seas
Observations through time suggest maximum wind waves with L at approximately 800 meters; this is equivalent to T of nearly 23 sec and S about 36 m/sec (or 130 km/hr). Whew!!
--the highest, confirmed wind-wave height observed is about 34 meters!! Ugh!

22. Wave Train Big Waves Travel in Packs!
Swells: An example of long-period swells outrunning swells of shorter period

23. Come on Baby Take the Wave Train
An individual wave does not persist long distances across the ocean--wave trains form (waves travel as groups)
--the front waves continually die out because of the energy required to “spin up” water through which they pass
--orbital motion remains after wavetrain passage and waves are added to back of train to replace waves lost at front (individual waves last only as long as it takes to travel through the wave train)
--because of this, wave trains travel at a rate of 1/2 the velocity of individual waves (whoa! even though individual waves obey the equations we have studied).

24. The package of waves (train) moves slower than individual waves
Wave Train!
The package of waves (train)
moves slower than individual waves
wave trains travel at a rate of 1/2 the velocity of individual waves

25. Types of Waves Defined in terms of Water Depth
Only some of the molecules in the water column are involved in a deep-water wave…

26. Wave Sets in Shallow Water
Note: increase in wave height and more peak shaped waves as they shallow near beach

27. Shallow-Water Waves Speed is related to water depth d: S =(g d)1/2
Waves are slower in shallower water: d=10 m, s=10 m/s;
d=5 m, s=7 m/s; d=1 m, s=3.2 m/s
Wavelength (L) and speed (S) decrease while period T remains constant S = L/T
piru.alexandria.ucsb.edu/~tierney

28. What is the speed of a deep-water wave with 20-second period?
Deep water waves:
1) S = L / T, 2) S = 1.56 T, 3) L =1.56 T2
(S is speed in m/s, T is period in sec, and L is wavelength in m.)
What is the speed of a deep-water wave with 20-second period?
a) Speed is: 0.64 m/s
b) Speed is: m/s
c) Speed is: m/s
d) Speed is: half the wavelength
e) Speed is: 31.2 m/s

29. Deep water waves: 1) S = L / T, 2) S = 1.56 T, 3) L =1.56 T2
Wave height H is always ≤ L / 7
What is the wavelength of a deepwater wave with period of 10 seconds?
156 m.
22 m/s.
1.56 m.
7.8 m

30. Deep water waves:
1) S = L / T, 2) S = 1.56 T , 3) L =1.56 T2
(S is speed in m/s, T is period in sec, and L is wavelength in m.)
Waveheight H is generally: H ≤ L / 7
What is the maximum wave height for a deepwater wave with period of 10-second?
Roughly 22 m.
Roughly 22 m/s.
Roughly 1.4 m.
Roughly 7.8 m

31. Deep water waves: 1) S = L / T, 2) S = 1.56 T, 3) L =1.56 T2
Which of these will likely have the largest wave height?
Wave height H is always ≤ L / 7
Waves with period of 1 second
Waves produced in a storm with weak winds that lasted a short time
Waves produced in a storm with strong winds that lasted a long time
Waves produced by a school of parrot fish

32. Deep water waves:
1) S = L / T, 2) S = 1.56 T , 3) L =1.56 T2
(S is speed in m/s, T is period in sec, and L is wavelength in m.)
If the water depth is 500 meters, does a 10-second period wave qualify as a deepwater wave? (Why or why not?)
a) No, wavelength is 500 m and therefore water depth must be >> 500 m so it’s not a deep water wave
b) No, wavelength is 15.6 m and therefore water depth must be > 78 m, so it’s not a deep water wave
c) Yes, wavelength is 156 m and therefore water depth must be > 78 m, which it is so it’s a deep water wave
d) Yes, wavelength is 1.56 m and therefore water depth must be > 20 m, which it is so it’s a deep water wave

33. Shallow-Water Waves S = L/T
Speed is related to water depth d: S =(g d)1/2
Waves are slower in shallower water: d=10 m, s=10 m/s; d=5 m, s=7 m/s; d=1 m, s=3.2 m/s
Wavelength (L) and speed (S) decrease while period T remains constant
S = L/T
piru.alexandria.ucsb.edu/~tierney

34. Speed (or Celerity) of Shallow-Water Waves
Speed is related only to water depth d (not L or T as in deep-water waves)
S = (g d)1/2 thus, waves move slower in shallow water
At the shore the celerity of the base of a wave is slower than the wave crest (friction with the bottom slows the body
In words-- the crest moves somewhat faster than the body of a wave, and the wave “breaks” (when H=0.75d)
So a 3 meter wave breaks in 4 meters water depth
The slope of the bottom locally determines whether a “breaker” plunges (e.g., a “tube”) or spills (plungers characteristic of steep slopes, spillers of gradual slopes)

35. Shallow-Water Waves Speed is related to water depth d: S =(g d)1/2
Waves are slower in shallower water:

36. Wave Sets in Shallow Water
constant period & wavelength wave set approaching coast
Note: increase in wave height and more peak shaped waves as they shallow near beach

37. Speed (Celerity) of Shallow-Water Waves
Speed is related to water depth d
Waves are slower in shallower water
Wave “breaks” when H = 0.75 d
A 3 meter wave (H) breaks in 4 meters water depth (d)

38. When and Why Do They Occur and How Can We Protect Coastal Regions?
Tsunami
Reason for Concern
When and Why Do They Occur and How Can We Protect Coastal Regions?

39. Tsunami are Shallow Water waves!

40. What Causes Tsunami?
Any event that causes displacement of large amounts of water including:
Earthquakes and associated seafloor uplift or depression
Landslides or submarine slides/slumps
Volcanic eruptions
The resulting water motion creates a wave trough or crest that then moves in all directions away from the event--a tsunami

41. Tsunami Tsunami means “harbor wave.”
They are caused by reverse or normal faults that rupture in the shallow ocean crust.
Waves can travel at 800 km/hr in the open ocean. Once the waves reach shallow shorelines, they are slowed down and the water “piles up” forming large amplitude waves.
The shorelines around the Atlantic Ocean are at much smaller risk for tsunami damage than those of the Pacific Ocean because the major faults in the Atlantic Ocean are transform faults, which do not move vertically. (No vertical motion = no tsunami) An exception was the Lisbon earthquake of 1755, which caused tsunami damage on the east coast of the U.S., but, as we will see, volcano collapse & undersea landslides could be an issue.

42. How Fast Do Tsunami Travel?
Tsunami are “shallow-water” waves, even while traveling over the deep ocean! [Why is this?]
Shallow-water waves travel at velocities proportional to the square root of the gravitational constant times the water depth (yup, S=(gd)1/2)
So, if the average depth of the ocean is 3.8 km, a tsunami travels about 695 km/hr. For comparison, most normal wind-generated waves travel about 55 km/hr in deep water (a little faster than a car on Beaver Avenue).
In the deeper parts of ocean basins (the Pacific averages 4.8 km deep), tsunami could travel 800 km/hr.
Tsunami waves are “refracted” (change directions) as their velocities increase or decrease with changes in ocean depth.

43. Damage from Tsunamis
Hilo, Hawaii, 1960
Anchorage, Alaska, 1964

44. Andaman-Nicobar Earthquake
Magnitude 9.0
Dec

45. Andaman-Nicobar Earthquake
Magnitude 9.0
Largest since Alaskan earthquake of 1964 (9.2)
Large area of the seafloor in motion in subduction zone

46. The December 26th Tsunami
Model simulation of the tsunami. For eastern Indian Ocean only.
-red is the crest
-blue is a trough
Simulation from Delft Hydraulics

47. 1) Which of the following deep-water waves has the highest velocity?
Deep-water wave when d > L/2; Shallow-water wave” when d < L/20
Deep water waves:
1) S = L / T, 2) S = 1.56 T , 3) L =1.56 T2
(S is speed in m/s, T is period in sec, and L is wavelength in m.)
1) Which of the following deep-water waves has the highest velocity?
A. A wave with period of 10 seconds
B. A wave with a velocity of 15.6 m/s
C. A wave with a wavelength of 1.56 m
D. A wave with period of 15 seconds
E. A wave with a velocity of 16 m/s

48. Tsunami are “shallow-water” waves, even in the deep ocean. S=(gd)1/2

49. Which of the following is (are) important in wind-wave generation?
Fetch
Wind speed
Duration of wind persistence
All of the above
All but C

50. Deep water waves: 1) S = L / T, 2) S = 1.56 T, 3) L =1.56 T2
(S is speed in m/s, T is period in sec, and L is wavelength in m.)
Deep-water wave with 100-second period? What is the speed and depth of wave disturbance
Speed is: 0.64 m/s and depth is 1.42 m
Speed is: 156 m/s and depth is 7800 m
Speed is: m/s and depth is m
Speed is: 156 m and depth is 156 m/s
Speed is: 15.6 m/s and depth is 156 m

51. A storm at sea will generate swells of varying period and wavelength.
Deep water waves:
1) S = L / T, 2) S = 1.56 T, 3) L =1.56 T2
(S is speed in m/s, T is period in sec, and L is wavelength in m.)
Wave height H is limited to one seventh of the wavelength: H < L / 7
A storm at sea will generate swells of varying period and wavelength.
Which waves will reach shore first?
Waves with period of 10 seconds
Wave with wavelength of 156 m
Wave with speed of 1.56 m/s
Wave with period of 20 seconds
None; they will all arrive at the same time.