geosc040, apr 3; lecture 21 waves, tsunami come on baby take the wave (love) train the o'jays marine...

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

Apr 3; Lecture 21 Waves, Tsunami

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

Wind Blowing over the Ocean Generates Waves 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.

Wave Generation by Wind 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 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.

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)

Waves and Water Depth Deep-water waves, Shallow water waves

Deep water waves: 1) S = L / T, 2) S = 1.56 T, 3) L =1.56 T 2 (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 The Equations we know and love

Deep water waves: 1) S = L / T, 2) S = 1.56 T, 3) L =1.56 T 2 (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 The Equations we know and love

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

Most ocean waves have a period between 1 and 10 seconds

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

Wave Interference 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 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

Waves bend! Diffraction

Wave Diffraction

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 H22 m! Wavelength L Wave Height H

Wave Generation by Wind How Big is Big? Wavelength L Wave Height H 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 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

Wind Waves --the highest, confirmed wind-wave height observed is about 34 meters!! Ugh! 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!!

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

Come on Baby Take 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). 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)

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

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

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

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 piru.alexandria.ucsb.edu/~tierney Wavelength (L) and speed (S) decrease while period T remains constant S = L/T

a) Speed is: 0.64 m/s b) Speed is: 156.0 m/s c) Speed is: 0.156 m/s d) Speed is: half the wavelength e) Speed is: 31.2 m/s Deep water waves: 1) S = L / T, 2) S = 1.56 T, 3) L =1.56 T 2 (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?

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

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

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

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

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 piru.alexandria.ucsb.edu/~tierney Wavelength (L) and speed (S) decrease while period T remains constant S = L/T

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)

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

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

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)

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

Tsunami are Shallow Water waves!

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

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.

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.

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

Andaman-Nicobar Earthquake Magnitude 9.0 Dec. 26 2004

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

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

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 T 2 (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

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

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

A.Speed is: 0.64 m/s and depth is 1.42 m B.Speed is: 156 m/s and depth is 7800 m C.Speed is: 0.156 m/s and depth is 0.156 m D.Speed is: 156 m and depth is 156 m/s E.Speed is: 15.6 m/s and depth is 156 m Deep water waves: 1) S = L / T, 2) S = 1.56 T, 3) L =1.56 T 2 (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

A.Waves with period of 10 seconds B.Wave with wavelength of 156 m C.Wave with speed of 1.56 m/s D.Wave with period of 20 seconds E.None; they will all arrive at the same time. Deep water waves: 1) S = L / T, 2) S = 1.56 T, 3) L =1.56 T 2 (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?

geosc040, apr 3; lecture 21 waves, tsunami come on baby take the wave (love) train the o'jays marine...

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