LECTURE NOTES:
OCEANOGRAPHY (MARSC 100),
SNYDER, L.
Chapter 10: Waves
What are waves? All waves are movement of energy through a medium (air, rock, water)
Series of vibrations or undulations in a medium
Wave types: ocean, sound, light, seismic
Water Waves: Movement of energy through water (kinetic)
Rising & falling of water (lake, ocean, coffee cup)
Occurs through entire medium, Non-breaking
Breaking waves (surf): Last gasp of waves that often travel 1000’s of miles to spend their energy
on beach
Wave Formation Produced by: Generating force (water disturbing) and Restoring (water calming)
force
Natural Disturbing forces: Winds, storms, earthquakes, gravity
Restoring forces: Surface tension/Cohesion (Sm. waves), gravity (Lg. waves), Coriolis effect
Wave type examples:
Wave type
Capillary (water ripples)
Wind waves (Sea swell)
Tsunami(Seismic sea)
Generating force
Wind
Wind
Vertical movement of seafloor
Restoring force
Cohesion of water molecules
Gravity
Gravity
Ocean Wave Motion:
Transfers energy from H2O particle to H2O particle with a slight forward displacement
Causes H2O particles to move in circles (orbits)
Transmits wave energy across ocean surface
Waveform moves, not each water particle (molecule) - Orbital Wave
Ocean Wave Characteristics
Wave crest: highest part (peak)
Wave trough: lowest part (valley)
Wave height (H): vertical distance between crest & trough
Wavelength (L): horizontal distance between 2 successive crests or troughs
Wave period (T): Time for one wave to move a distance of one wavelength (A to B)
Wave frequency: Number of waves passing fixed point per second
Wavelength Determines Size of Water Particle Orbits
Decrease in orbit diameter with depth
Size of orbit at surface = wave height
Wave motion (orbit) nearly absent at a depth of ~½ of wavelength = Wave Base
Deep-Water & Shallow-Water Waves
Deep Water Waves: Don’t interact with seafloor, Circular water molecule orbits
Traveling at depths that are > ½ the wave's wavelength
Examples: Wind & Capillary waves
Shallow Water Waves: Do interact with seafloor; Flattened, elliptical orbits
Traveling at depths that are Shallower than (<) 1/20 wavelength
Examples: Seiches, Tsunamis, & Tides (b/c wavelengths are so long, these waves are
never in water depths that are > 1/20 the wave’s wavelength)
As a wave approaches the shoreline: Circular orbits become elliptical, the Wavelength shortens
(bunches up), & the Wave slows down
The longer the wavelength, the faster the wave energy moves (speed)
•Wave Velocity Formulas:
1. Deep water: C = L/T
C = speed (meters/second)
L = wavelength
T = time or period (sec.)
2.
Shallow water: C = sq root 3.1d
C = speed (m/sec)
d = depth (meters)
Wave velocity example: C (Speed) = L/T (wavelength / period-time)
C = 233 meters/12 sec.
C = 19.4 m/sec.
Deep & Shallow water wave characteristics:
Deep Water Waves (wind or capillary waves): Wavelength to 600 m (2000 ft), Speed to
112 km (70 mph)
Shallow Water Waves (Tsunami, tides): Wavelength to 200 km (125 miles), Speed to 760
km (470 mph)
Wind waves: Gravity waves formed by transfer of wind energy into surface waters
Height range: 2 cm to 3 m
•Wavelength range: 60 to 150 m
•Capillary waves form 1st (< 2cm)
•Wind friction on ocean surface = H2O piles up & gravity moves it forward slightly & forms wave
crest
Surface tension restores smoothness
Capillary Waves Become Wind Waves When their wavelength > 1.73 cm (0.68”):
Gravity (atmos. pressure) > than surface tension
Gravity Becomes dominant restoring Force
Piled-up water flows “downhill” (under influence of gravity)
Wave will grow if: Wind continues to blow or if Wave remains in water deeper than ½ its
wavelength
Swell: When wind slows (away from storm), Wave crests become rounded & regular, then the
waves Separate (dispersion) into groups w/ similar speed & wavelength
Creates smooth undulation of sea surface (Swell)
Can travel 1000’s of km across ocean basins, Break as surf on a distant shore
Wave Trains: Progressive groups of swell of similar origin Traveling together across ocean
Leading wave’s energy: ½ transferred to undisturbed H2O ahead (forward motion) & ½
transferred to wave behind (maintains motion)
Factors Affecting Wind Wave Development
Wind duration:
•If too short waves will not develop
•Wind strength (intensity or wind speed)
•Size of fetch: Distance that wind blows over surface w/out interruption
•In same direction
Wind Waves Change as Approach Shore
“Feel” bottom when H2O depth = ½ wave’s wavelength
Circular H2O particle orbits change to flattened ellipses, Wavelength decreases, Period (time)
remains same
Crests become peaked, not rounded, Wave height increases, Wave slows down (due to friction)
Wave breaks when ratio of wave height to H2O depth is 3:4
Surf: Turbulent mass of water rushing toward shore after wave breaks
Surf Zone: Area between breaking waves & shore
Wave Refraction: Waves approaching shore at an angle will SLOW & bend
Wave lines bend toward nearest point of shore
Waves bend in shallow water
Wrap around cliffs/headlands
Energy dispersed in bays
Wave diffraction:
Wave changes shape & direction
Wave train is interrupted as it moves around an obstacle
Gap in breakwater or between islands
Generates smaller secondary waves
Wave Reflection:
Waves bounce off vertical barriers
Move from obstacle in direction it came, Interfere w/ incoming waves
Barrier EX: Bluffs, Seawalls, Jetties
Standing waves can result w/in enclosed waters (harbors & bays)
Newport (“The Wedge”): Reflected wave overlaps original wave, producing The Wedge
Tsunami (Japanese):
Seismic sea waves created by seafloor movement
Earthquakes, Landslides, Meteors, Volcanoes, Glaciers
Narture of Tsunami
Shallow water waves: b/c wavelength is very large compared to water depth (Waves in water
shallower than 1/20th the wavelength)
Very long wavelength (typical = 200 km or 125 mi )
Very small height as move out from source
Only 1-2 m high in open ocean
Vessels on open sea wouldn’t notice one
Very long period: 5-20 min
Travel very fast: Up to 500 mph (Can cross Pacific Ocean in 10 hrs.)
Earthquake Generated Tsunami
Plates move against each other
1 plate subducts below
Crust catches, forces build up, 1 plate can tear free
Lurches forward = Earthquake
Dec. 2004 Sumatra: ~700 miles of plate boundary ripped
Wave Height (Amplitude) Determined by Amount of Seafloor Displacement:
1.
Energy from earthquake causes vertical movement of crust
2.
Displaces water
3.
Wave moves out from earthquake epicentre (up to 500 mph)
Sumatra: Crust uplifted ~15m (50ft.) & Displaced hundreds of cubic km of water
Tsunami Travel Long Distances w/ Little Energy Loss
Change radically at shoreline: Slow down because of friction with seabed
•Wavelength shortens dramatically (H2O “piles up”): Huge increase in wave height
•1st wave on shore may be trough or crest
Trough: Looks like super-low low tide
Crest: Like rapidly rising tide or big wave
Low-lying Areas Along Coast: At serious risk when a rapid series of waves rush onshore
Energy transferred to smaller waves
In Dec. 2004, Many People Had TIME to Escape, But Did Not: Most were not warned (no
warning system)
Recent Pacific Tsunami: 1946, 1957 Aleutian (Hawaii); 1960 Chile (Chile & Hawaii); Prince
William Sound (Alaska); Japan has had numerous, significant tsunami throughout recorded
history
Indian Ocean (NW Sumatra 26 Dec. 2004): Max. wave ~15m (49ft), Fatalities > 250,000
In response to Pacific tsunami: detection & warning system established
U.S.A (NOAA), Chile, Japan, Australia, U.N.
Satellites, tide guages, alert locals
Warning: sirens, radio, t.v., education
No such program exists for Indian Ocean