Waves
Neptune's Horses by Walter Crane of 1892
One of the major deities in Greek mythology, Poseidon was the supreme ruler of the seas. The Romans called him Neptune. An
awesome, unruly, and powerful god, Poseidon was associated with storms, earthquakes, and some other violent
forces of nature. When angry, he could stir the sea to a fury. But he could also calm the raging waters with just a
glance. One of his titles, Enosichthon (Earth-shaker), reflected his ability to cause earthquakes by striking the
earth and mountains with his trident. Another name for Poseidon was Hippios (lord of horses), and the god
presented horses as gifts to various individuals.
Poseidon rode the waves in a swift chariot drawn by golden sea horses. He used his mighty trident not only to
provoke earthquakes and stir ocean waves but also to raise new land from beneath the sea or cause existing land
to sink below the waters. Although often helpful to humans—protecting sailors at sea, guiding ships to safety, and
filling nets with fish—Neptune could be a terrifying figure as well. Quick to anger, he directed his fury at anyone
who acted against him or failed to show proper respect.
Read more: http://www.mythencyclopedia.com/Pa-Pr/Poseidon.html#ixzz1dKdjjyn0
Waves
• Transport energy over a body of water
• Can cause coastal erosion
Waves
Origin and Type
• Wind waves – “wind-generated”
• Seiche – “pressure-generated”
• Tsunami – “seismic-generated”
• Tide – “gravity-generated”
Wave Terminology
Still water Height
line
•
•
•
•
•
Still water line – level of ocean if it were flat w/o waves
Crest – highest part of wave
Trough – lowest part of wave
Wave height (H) – vertical distance between crest and trough
Amplitude – distance between crest and still water line
– ½ the wave height
• Wavelength (L) – horizontal distance from each crest or each trough
– Or any point with the same successive point
• Steepness = Height (H)/length (L)
Wave Parameters
• Period (T) – the time it takes for two successive waves to
pass a particular point
• Frequency (f) – the # of waves that pass a
particular point in any given time period
Deep Water Wave
Motion
• Waves transmit energy, not water mass
• Water particles move in orbits
• Diameter of orbits decrease with depth
• Particle motion ceases at ½ wavelength
Waves
Types
• Deep Water
– H2O depth is >1/2
wavelength
• Shallow Water
– Depth is <1/20
wavelength
Orbital Motion
• When under a wave crest
– Water moves up and
forward
• When under a trough
– Water moves down and
back
• Thus, water particles do not
move forward
– They move in circles
Waves
Velocity
• Velocity (V) = wavelength (L)/Period (T)
• For example: – If T= 10 sec; L = 100 m
– Then 100 m/10 sec. = 10 m/sec
• The longer the wavelength, the faster the wave travels
Waves
Generation
• Generated by Force
– wind or air
• Generated at storm centers
• Separation of waves by
differing rates of travel -
dispersion
– Sea Wrinkles  ripples  wind waves  swells
Waves
Size
• Factors determining size:
º Fetch – Distance wind blows over open ocean
º
Wind velocity – Speed of wind
º
Duration – Length of time wind blows
Waves
Dispersion
• Wave train – a set of waves
with the same wavelength
• Individual waves move faster
than wave train
– wave dies out at front of train
while new waves form at rear
• Wave train travels half the
speed of individual wave
Wave Interference
• Combination of waves
– Build from one another = Constructive
– Cancel each other = Destructive
Shallow Water Waves
1. Swell feels bottom at depth < ½ wavelength
2. Wave crest peaks and wave slows
3. Orbits progressively flatten at depth
4. Wave height (H) increases and wavelength (L) decreases
5. Wave breaks when H/L ratio > 1/7
6. Just above seafloor particles move in back-and-forth motion
Breaking Waves
Breaking waves
Types
• Depends on steepness
• Type of Breakers:
(a) Spilling
– Gentle slopes
– Surf gently rolls over
the front
(b) Plunging
– Moderately steep slope
– Wave curls forming a
tunnel
(c) Surging
– Steep slope
– Rolls onto beach
Waves
Characteristics
• Refraction
– Bending of waves
– Waves approach shore at angle and drags
– Feels bottom first and slows
• Diffraction
– As waves pass through small opening, waves spread
Seiches
• Rise and fall of water level
• Occurs in enclosed basins and/or lakes
• Stationary point (node)
• Standing wave oscillates
Tsunamis
•Giant sea waves generated by earthquakes
Tsunami
Causes
• Earthquake
• Volcanoes
• Landslides
• Meteor impact
Tsunami
Characteristics
• Energy passes through entire
water column
• Long periods (T)
– T = 10-20 min.
• Small Height (H)
– H = 1-2 m
• long wavelengths (L)
– L = 100-200 km
• Shallow water wave
• Deep wave base
• Travel at great speeds
– c = 200 m/s
Earthquake-Generated Tsunami
• Usually associated with subduction zones
– Water is displaced suddenly
22
Tsunami
Crest and Trough
Historical Tsunamis
1883 Krakatau
Today
• Explosive volcano
• Large landmasses fell into the ocean
• 36,000 people killed by tsunami
Earthquake-Generated Tsunami
• Chile, South America (1960)
–
–
–
–
9.5M, largest EQ ever recorded
3rd wave 11 m (30’) high; 1 hour period
909 died; 834 missing
Tsunami Warning System in place
25
Lessons from Chile
• Hawaii
– 61 died
26
Chile, 1960
• Japan
– 181 deaths
27
Landslide-Generated Tsunami
• As large mass of land falls into ocean, a huge volume of
water is displaced
28
Landslide-Generated Tsubnami
• Lituya Bay, Alaska (1958)
– 150-m high wave
– Stripped vegetation
– Ancient tree trim shows
previous occurrence
29
Historical Tsunamis
1946 Pacific Tsunami
• Aleutian Island Earthquake
• Killed 165 people in Hawaii
• Resulted in the creation of
the 1st “tsunami warning
system”
Historical Tsunamis
2004 Indonesian Tsunami
• 9.0 M earthquake
• Deadliest tsunami ever recorded
• >260,000 people killed
Japan Tsunami
• Tsunami – The Great Wave
• Aerial View of Japan Tsunami
Tsunami Dangers
Drowning
Severe abrasion by dragging
Thrown against solid objects
Carried out to sea in outgoing wave
Hit by debris
House, cars, trees, rocks
33
Tsunami Hazard Mitigation
• Land Use Zoning
– Build to elevations above flood potential
– Structures engineered to resist erosion and scour
– Streets and buildings built perpendicular to shore
– vegetation
34
The Pacific Tsunami Warning System
Two Steps
1. Tsunami Watch
– Issued when an earthquake > 7.0M is detected in the Pacific Ocean
2. Tsunami Warning
– Significant Tsunami is identified
35
Tsunami Prediction
• Pressure sensor on
ocean floor detects
changes in wave height
• Transmit signal via
satellite
36
Surviving a Tsunami
1.
2.
3.
4.
If you feel an EQ when near the coast, get to high ground
Do not return to shore after initial wave
Never go to the shore to watch a tsunami
An unexpected rise or fall of sea level may indicate an
impending tsunami
37