waves, tides, currents john huth harvard university
TRANSCRIPT
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Waves, Tides, Currents
John HuthHarvard University
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Wave Parameters(Figure 7-1a)
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What Causes Waves?
• Wind
• Submarine disturbance
• Gravitational attraction of sun and moon (tides – very long wavelength waves)
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Motion of Water Particles Beneath Waves
(Figure 7-3b)
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Deep Water Waves(Figure 7-4a)
Waves do not interact with the seafloor
Orbits of the water molecules are circular.
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Shallow Water Waves(Figure 7-4b)
Waves interact with the seafloor are known as Orbits of the water molecules become elliptical.
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Characteristics of water waves
• Velocity depends on wavelength *or* water depth– Unlike sound or light – velocity is independent of
wavelength for these
• Waves become unstable when height is 1/7th of wavelength – whitecaps (120 degree interior angle)
• Longer wavelength waves hold more energy• Depth for “shallow” versus “deep” is about 2
times wavelength
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2
gLV
gdV
Deep
Shallow
g
d
L
Gravitation 32 ft/sec/sec
Water depth (ft)
Wave length (ft)
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h
L
Instability – when h > 1/7 LOR – when interior angle is less 120 degrees
120o
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Wind Generation of Waves
• The type of wave generated by wind is determined by:– Wind velocity– Wind duration– Fetch (distance over which wind blows)
• Simply put, wave size increases as the strength and duration of the wind, and distance over which it blows increases.
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Cat’s paw
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Fetch Conditions
• Time and distance• Small waves buildup, break• Larger waves begin – hold more energy before
breaking• Generally a range of wavelengths
– High wind velocity produces more uniform and longer wavelength waves
• Typically for NE waters – fully developed seas only for 10 knot winds– Larger seas in open ocean
• Swells travel huge distances unaffected
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Comments on Swells
• Product of distant storms– Can travel thousands of miles without losing energy– Period of swell indicates severity of storm –
• Longer period – more severe storm– 4 seconds – small– 8-10 seconds – hurricane
• Mid ocean – can have multiple swells crossing• In New England, sheltering of coast line limits
significant swell direction– E.g. Gulf of Maine typically will only see SE swells– Rhode Island catches a lot of Atlantic storms– Newport beaches/surfing
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Transformation of Shallow-water
Waves (Figure 7-7b)
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Reflecting Swells at Great Wass Island(Jonesport)
Angle of incidence equals angle of reflection
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• Bending of the wave crest as waves enter shallow water. It is due to– Drag along the
bottom.– Differential
speed along the crest.
Wave Refraction(Figure 7-8a)
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Wave Refraction at Chatham InletGradual transition between deep and shallow water
Shallow water
Deep Water
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Extreme refraction at Baker Island(Mt. Desert)
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Swell patterns around an atoll
reflections
Mainswell
Refractions
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Crossing swell patterns between islands
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Multi-swell patterns around island
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Polynesian stick chart – illustratingswell patterns from two islands
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Tides
• Tides are like very long period waves.• Caused by the divergence of the gravitational
lines of force from one body to another – Sun and moon on earth
• Moon’s tidal forces are five times larger than the sun’s
• Causes a distortion of the shape of the earth– The earth rotates underneath a tidal “bulge”– Horizontal forces push water up against the
continental shelves, causing the water level to rise and fall
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Moon
Gravitational field lines from the moon
Parallel lines, plus… Tidal forces
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Spring tides
Neap tides
Largest tides occur when pull from the sun andmoon are in the same direction.
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Why worry about tides?
• Affects navigation, particularly near land– Passages through shallow areas– Passages through constricted channels– Currents can be substantial (3-6 knots)
• Can frequently be clues to the presence of land– E.g. current draining from an atoll
• Tides in the middle of the ocean are small (a few inches)• Tides in embayments can be huge
– A result of resonance – Bay of Fundy – 40’ tides– Straits of Ungava
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Old sow whirlpool – Gulf of Maine
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Coriolis force causes rotary tidesPoints of zero disturbance called “amphidromic” points – high tidesrotate around these points
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Amphidromic system – rotary tides
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Amphidromic system in North Sea
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Diurnal and Semi-diurnal tides
Depends on location, the forcing function comesfrom the moon twice a dayand from the sun twice a day.(12 hours, 26 minutes, to beexact)
The harmonic properties of abody of water determine how it responds.
There is also a diurnal inequalityfrom the inclination of the
Moon’s orbit.
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Understanding local tides
• Local knowledge: tide chart (bait shops, marinas, web)
• Look at water line – observe it for 15 minutes– Is water dry above, or wet and drying out
• Incoming or outgoing tides
– From the structure of docks, height of seaweed zone, etc, can figure out maximum height of tide
• Look for necks of land where current flows quickly – use periods of slack tide to your advantage.
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Example: Sullivan reversing falls - Maine
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Currents
• Currents affect passage, must be taken into account for any voyage – near land or away from land.
• Many forces at play– Wind– Tide– Thermal gradients, salinity gradients– Gravity– Fluid dynamics– Geography
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Main features of ocean currents
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Franklin’s map of the Gulf Stream
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Satellite view of currents, and associated eddies
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Seasonal Variations – Summer vs. Winterin the North Pacific
SummerWinter
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Initial position
Position after drift
Current direction
Initial bearing
Final bearing
How Polynesians estimated currents
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Considerations for currents
• Without something stationary, like an island, you can’t really measure currents– Exception – when the wind is running against
a current, the waves become steeper facing into the wind
• Need to adjust heading for currents– Change angle of heading to take this into
account
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Sea Surface temperatures – some sharp boundariescan be aids to navigations