MAY 2011
Oceanography Ch # 8
Waves and Water Dynamics
Largest waves are located at Maverick, Half Moon Bay, result of:
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Shoreline abrupt rise
Energy concentrated (wave Refraction)
8 – 1. Causes of WAVE FORMATION.
A disturbing force which releases energy.
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Wind generated waves are created along any interface between different density fluids.
Examples: air- water interface, air – air, water – water.
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Mass movement (splash)
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Sea floor movement (seismic) – Tsunamis
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Gravity - tides
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Human activity - explosions
8 – 2. Wave movement.
Energy is transmitted by cyclic movement thru matter. Particles in the matter oscillate, transmitting
energy from one particle to another. Simple progressive movement may be longitudinal, transverse or
orbital (combination). Orbital waves are on the ocean surface.
8 – 3. Wave Characteristics.
Idealized, a wave is simple, uniform, transmits energy from a single source.
Wave Terminology: (see p. 234 for diagrams).
Crest; Highest point of wave
Trough; lowest part
Still water Level; midway between crest and trough.
Wave Height ; Vertical distance between crest and trough
Wave Length; crest to crest.
Wave Steepness (the ratio of wave height to wave length); if steepness exceeds 1/7, it breaks. Indicates
the max wave height. Maximum Wave height observed is 120 ft. in a hurricane.
Wave Period; time taken for I complete wave to go by an object
Wave Frequency; the # of wave crests passing an object in a unit of time.
Wave Speed (Measured by dividing Wave length in meters by Period in Seconds). The longer the wave
length, the faster it can travel.
Wave Base is generally below one half the wave length where the orbital wave motion dies out. That is
about ½ the wave length.
Deep water Waves – have no interference from the ocean floor.
Shallow water Waves (Tsunamis and Tides). Their speed is influenced by gravitational acceleration and
water depth.
In deeper waters, the wave travels faster. Wave speed = Wave length divided by wave period.
Otherwise called Celerity
Transitional Waves have characteristics of both deep and shallow waves. Wave length ranges from 2 to
20 X water depth.
8 – 4. Wind generated wave development.
Ripples (Capillary waves), Ripples catch more wind produce Gravity waves. Wave height
increases become pointed and troughs rounded –(Trochoidal Wave form)
“Sea” – the area where the wind is blowing. The amount of energy depends on wind speed,
duration and fetch. The factors which determine the amount of energy in a wave are: Wind speed,
duration and fetch.
Wind Scale used is the Beaufort Scale 0 to 12 (Calm to hurricane).
A Swell is a long crested wave, forms as wind speed diminishes’
Wave trains are groups of waves.
Rogue Waves . up to 20X normal height, have abnormal shapes. Explained by the Constructive
interference theory. Each year about 10 super ships disappear without a trace. Rogue waves?
Theory # 1. Created when low and hi frequency components of ordinary ocean waves interact and
channel their combined energies into a narrow frequency band.
Theory # 2. Waves caused by destructive wave interference (a storm driven wave move against a strong
ocean current) which cause the wave to steepen, shorten and grow. Interference patterns may be
constructive or destructive.
8 – 5. How do Waves change in the Surf Zone?
Physical changes: Surf is the zone of breaking waves. As waves come into shallow water, they feel the
bottom. Their speed and wavelength decrease, while their height and steepness increase, causing the
wave to break. In deep waters, waves may be unstable due to high energy. The surf will be rough,
choppy and irregular.
8 – 6. Breakers and Surfing
Spilling Breakers result from gently sloped sea floor. A turbulent mass of water and air runs down the
front slope of the water.
Plunging Breakers: The curling water crest moves over an air pocket on a moderately sloped beach.
Surging Breakers: When the sea floor slope changes abruptly, energy is compressed over a short
distance and the wave surges forward breaking at the shore line. A surfer balances between gravity and
buoyancy. Max speed is about 40 km/hr.
Wave Refraction –when waves approach the shoreline at a slight angle, the part of the wave which
touches the bottom first, causes the crest to be refracted.
Wave Diffraction: When energy is transferred around a barrier that impedes forward motion,
diffraction occurs.
Wave REFLECTION – a seawall can reflect waves back into the ocean with little loss in energy.
STANDING WAVES – are produced when waves are reflected back at right angles to a barrier. A wedge
forms due to reflection and constructive interference.
8 –76. How are Tsunamis are created? sudden changes in the sea floor topography. Their wave length
can exceed 200km and speed in excess of 700 km/hr.
Splash waves may be caused by volcanic eruptions, vertical faulting, avalanches or /and meteoric
impacts below water.
Historic Tsunamis – mostly in the Pacific. (1946 –Hawaii; Alaska -1964 )
Warning system installed in 1946.
8 – 8. Can Power be Harnessed from Waves (Energy Source)?
Concerns – Significant problems to be overcome
Destruction from wave force.
Production primarily during storm conditions
Presence of many structures would have a major environmental impact
Can alter sand deposition/ erosion along the coast
Geography- requires a head land. See map p. 273.
10 megawatts of power can be extracted along 1 km of coastline. (Australia, New Zealand, S. Africa, and
S.America)