waves, tides, currents

Waves, Tides, Currents

John HuthHarvard University

Wave Parameters(Figure 7-1a)

What Causes Waves?

• Wind

• Submarine disturbance

• Gravitational attraction of sun and moon (tides – very long wavelength waves)

Motion of Water Particles Beneath Waves

(Figure 7-3b)

Deep Water Waves(Figure 7-4a)

Waves do not interact with the seafloor

Orbits of the water molecules are circular.

Shallow Water Waves(Figure 7-4b)

Waves interact with the seafloor are known as Orbits of the water molecules become elliptical.

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

2

gLV

gdV

Deep

Shallow

g

d

L

Gravitation 32 ft/sec/sec

Water depth (ft)

Wave length (ft)

h

L

Instability – when h > 1/7 LOR – when interior angle is less 120 degrees

120o

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.

Cat’s paw

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

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

Transformation of Shallow-water

Waves (Figure 7-7b)

Reflecting Swells at Great Wass Island(Jonesport)

Angle of incidence equals angle of reflection

• 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)

Wave Refraction at Chatham InletGradual transition between deep and shallow water

Shallow water

Deep Water

Extreme refraction at Baker Island(Mt. Desert)

Swell patterns around an atoll

reflections

Mainswell

Refractions

Crossing swell patterns between islands

Multi-swell patterns around island

Polynesian stick chart – illustratingswell patterns from two islands

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

Moon

Gravitational field lines from the moon

Parallel lines, plus… Tidal forces

Spring tides

Neap tides

Largest tides occur when pull from the sun andmoon are in the same direction.

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

Old sow whirlpool – Gulf of Maine

Coriolis force causes rotary tidesPoints of zero disturbance called “amphidromic” points – high tidesrotate around these points

Amphidromic system – rotary tides

Amphidromic system in North Sea

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.

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.

Example: Sullivan reversing falls - Maine

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

Main features of ocean currents

Franklin’s map of the Gulf Stream

Satellite view of currents, and associated eddies

Seasonal Variations – Summer vs. Winterin the North Pacific

SummerWinter

Initial position

Position after drift

Current direction

Initial bearing

Final bearing

How Polynesians estimated currents

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

Sea Surface temperatures – some sharp boundariescan be aids to navigations