Waves

Laird Hamilton riding ‘the Wave’ (Riding Giants)

Waves• When undisturbed by wind (or some other

factor such as an earthquake), the sea surface is naturally smooth!

• Waves are moving energy and begin as a disturbance–Wind blowing across the surface of the

ocean generates most waves– Tides, turbidity currents, coastal landslides,

calving icebergs, and sea floor movement can also cause waves

Waves

• In an ocean wave, energy is moving at the speed of the wave, but water is not!

• Waves move energy, with very little movement of particles (including water particles!)

• The water ‘associated’ with a wave does not move continuously across the sea surface!

• Imagine a seagull resting on the ocean surface

• The bird moves in circles – up and forward as the tops of the waves move toward its position, and down and backward as tops of the waves move past

• Energy in the waves flows past the bird, but the gull and it’s patch of water move only a short distance

Each circle is equal in diameter to the wave height

Orbital Waves

• As a wave travels, the water passes the energy along by moving in a circular path, called an orbit

• An wave in which water particles move in closed circles is called an orbital wave

• Because the wave form moves forward, orbital waves are a type of progressive wave

Orbital Waves• The bigger the wave, the larger the size of the

orbit• The diameter of the orbit diminishes rapidly

with depth

Wave motion is negligible when orbits reach a diameter that is 1/23 of those at the surface

Wave motion is negligible below a depth of one half of the wavelength

Components of a Wave

• Ocean waves has distinct parts:– Wave crest: highest part of the wave above

average water level– Wave trough: lowest part of the wave below

average water level– Wave height: the vertical distance between a

wave crest and its trough– Wavelength: the horizontal distance between 2

successive crests, or troughs

Making Waves

• Ocean waves are classified by– the disturbing force that creates them– the extent to which the disturbing force

continues to influence the waves once they are formed– The restoring force that works to flatten

them– Their wavelength

Making Waves

• Energy that causes waves to form is called a disturbing force

• Wind blowing across the ocean surface provides the disturbing force to generate capillary waves (waves <1.73 cm) and wind waves

• Landslides and tectonic processes (volcanic eruptions, faulting of the sea floor) are the disturbing forces for seismic sea waves, or tsunamis

Making Waves

• The restoring force seeks to return the water to flatness after a wave has formed in it; gravity provides the restoring force on all waves >1.73cm

Wavelength is the most useful measure of wave size

Deep vs. shallow water waves

• Waves moving through water deeper than ½ their wavelength are deep water waves– Example: A wind wave with a 20m wavelength is

considered to be a deep water wave so long as it is passing through water >10m deep

• Waves in water shallower than 1/20 their wavelength are shallow water waves– Example: A wave with a 20m wavelength will act

as a shallow-water wave if the water is <1m deep

Shallow water waves

• When a wave approaches the shore, its proximity to the bottom flattens out the orbits of water molecules

• Causes the water at the bottom to move back and forth; no longer in a circular pattern

Ocean Motion

• In the ocean, only capillary and wind waves can be deep water waves

• Why???• Remember, deep water waves occur when

moving through water deeper than half their wavelength…

Gee, that’s just swell…• Generally speaking, the longer the

wavelength, the faster the wave• When waves move away from their area of

origination, wind speeds diminish and they eventually move faster than the wind

• Mature waves from a storm sort themselves into groups of waves with similar wavelengths and speeds as they outrun their smaller ‘relatives’

Gee, that’s just swell…• This results in swells; uniform, symmetrical

wind waves that have traveled out of their area of origination

When waves meet

• Because longer waves will outrun shorter waves, wind waves from different storm systems can interfere with one another

• When waves meet, they add to or subtract from one another

• Such interaction is called interference– Constructive: additive– Destructive: subtractive (cancellation)

Dude, constructive and destructive waves rule

• Surfers depend on constructive and destructive waves to generate their ‘wave sets’

• Constructive interference between waves of different wavelengths create the sought-after big waves

• Destructive interference diminishes the waves and makes it easier for the surfer to swim back out

• Constructive: crests of waves coincide• Destructive: crest and trough of waves coincide

Going Rogue

• Occasionally, wind waves of many wavelengths can approach a single point/spot from different directions

• A huge wave crest develops suddenly from the constructive interference, generated a rogue wave

• Rogue waves are much larger than surrounding waves and can be extremely hazardous

Just how big can waves be?

Just how big can waves be?• The size of waves depends on:– Wind strength– Wind duration– Fetch (distance over which the wind blows)

• A strong wind must blow continuously in one direction for ~3 days for the largest waves to develop

• The greatest potential for large waves occurs beneath the strong and nearly continuous winds surrounding Antarctica

Some personal experience…

A photograph of a wave taken from the bridge 50 feet above sea level

Photo taken here (bridge)

50 ft

~70-foot wave crossing the Southern Ocean

Encountering ‘The Perfect Storm’ just north of Antarctica (Ross Sea, Southern Ocean)

When ocean waves encounter land

• Deep water waves change to shallow water waves as they approach the shore

• Once the wave passes over water whose depth is less than one half its wavelength, the wave “feels” the bottom

• When this happens, the circular motion of the water molecules in the wave is interrupted

• Orbits flatten to ellipses near the bottom

When ocean waves encounter land

• The wave’s energy must now be packed into less water depth, and so the wave crests become peaked, rather than rounded

• Interaction with the bottom slows the incoming wave, but waves behind it continue toward shore at their original speed

• This results in a “bunching up” of the waves, which decreases their wavelength, but increases their height

Surf’s Up!• When the wave steepness reaches the 1:7

ratio (wave’s height is 7x it wavelength), the wave will break as surf

How to score an epic wave…

• Waves break along the shore in different ways• Waves are influenced by:– The bottom slope (the steeper the slope, the more

violent and toppling the wave)– Contour and composition of the bottom (gradually

shoaling bottoms sap waves of their strength as the wave loses energy interacting with the bottom)

– Localized winds and fetch

Why is surfing so much

better along the west

coast of the U.S. than the

east? (no offense…)

NOT Long Island

Waves refract when they approach a shore at an angle

• Waves usually approach the shore at an angle• Different parts of the wave is at different

depths, so the wave must bend, or refract as parts of the wave reaches shallower water and slows

• The slowing and bending of waves in shallow water is called wave refraction; the waves refract in a line nearly parallel to the shore

Waves refract when they approach a shore at an angle

Big Waves: Storm Surge

Big Waves: Tsunamis

• The Japanese term for large, often destructive waves that occasionally roll into their harbors is tsunami (tsu = harbor; nami = wave)

• Tsunamis originate from sudden changes in the sea floor caused by tectonic activity (undersea volcanic eruptions, faulting, collapse of large oceanic volcanoes) and even underwater avalanches such as those caused by turbidity currents

Tsunamis• The majority of tsunamis are caused by fault

movement• Underwater fault movement displaces the

earth’s crust, generates earthquakes, and if it ruptures the seafloor, produces a sudden change in water level at the ocean surface (up or down)

• The wavelength of a tsunami is ~125 miles, so it is a shallow water wave everywhere in the ocean

Tsunamis

• In the open ocean, tsunamis travel at speeds >435 miles per hour

• Tsunamis in the open ocean have heights of only ~0.5 meters (~1.6 feet)!

• However, once they approach the shore, they slow in the shallow water and increase in wave height

• Surges ashore; mistaken for an extremely high tide and so mistakenly called tidal waves

Abrupt vertical movement along a fault on the sea floor raises or drops water column creating a tsunami that travels from deep to shallow water

Killer Waves

• 86% of all tsunamis occur in the Pacific Ocean (Why?)

• On December 26, 2004, an enormous earthquake struck off the coast of Sumatra in Indonesia

• Occurred ~19 miles beneath the sea floor near the Sunda Trench, where the Indian Plate is being subducted beneath the Eurasian Plate

• Ruptured ~750 miles of sea floor!

Killer Waves• This thrusted the seafloor upward, displacing

>30 feet of water above it• The resulting tsunami spread across the Indian

Ocean, literally washing away many coastal villages and causing approximartely 300,000 human deaths in Indonesia (esp. Thailand) and along coastal India and Africa

• Although much smaller, the tsunami was also detected in the Atlantic, Pacific and Arctic Oceans!

Indonesian capital of Banda Aceh (before tsunami)

Indonesian capital of Banda Aceh (after tsunami)

Sequence of photos of tsunami inundating Chedi Resort in Phuket, Thailand on December 26, 2004