OCEAN WAVES Waves & Tides Unit 7

DEFINITION OF A WAVE: The disturbance of a medium (water in this case) caused by the movement of energy from a source (wind, seismic activity, etc.) through the medium (water). The disturbance of a medium (water in this case) caused by the movement of energy from a source (wind, seismic activity, etc.) through the medium (water).

MEDIUM DISTURBANCE  In an ocean wave the energy moves forward, not the water  Imagine doing “the wave” in a stadium – the “wave” moves through the crowd, but the participants do not

Wave diagram Direction of wave motion trough crest wavelength heightheight Orbital path of individual water molecule at water surface

 Frequency (f) Number of wave crests passing a point each second (f) Number of wave crests passing a point each second (f)

Period (P) Time required for a wave to pass by a given point Equations for solving for frequency & Period: f = 1/P P = 1/f f = 1/P P = 1/f

CLASSIFYING WAVES  Waves can be classified based on the: Disturbing force: the energy that creates the waves Disturbing force: the energy that creates the waves The force that tries to flatten the waveThe force that tries to flatten the wave is called the restoring force is called the restoring force Wavelength Wavelength

DISTURBING FORCES  1) Wind Friction between water and air pushes water to create waves Friction between water and air pushes water to create waves

 2) Seismic Activity Landslide Landslide Volcanic eruptions Volcanic eruptions Faulting of the seafloor associated with earthquakes Faulting of the seafloor associated with earthquakes

 3) Gravitational pull of Sun and Moon Gravitational forces among the Earth, moon and Sun combined with Earth’s rotation create tides Gravitational forces among the Earth, moon and Sun combined with Earth’s rotation create tides

Waves can also be classified based on WAVELENGTH  Capillary Waves – up to 1.73 cm  Wind Waves – more than 1.73 cm  Seiche – large, variable; in an enclosed basin  Seismic Sea Wave – 200 km  Tide – Half Earth’s circumference

 Capillary waves Waves of less than 1.73 cm Waves of less than 1.73 cm First waves to form when wind blows First waves to form when wind blows Restored by cohesion Restored by cohesion

 Wind Waves Most are less than 3 meters high Most are less than 3 meters high Form from capillary waves Form from capillary waves Wind wave formation is not uniform Wind wave formation is not uniform Waves of longer wavelength move faster Waves of longer wavelength move faster

 Fully Developed Seas – develop when an area has reached its maximum size waves (disturbing and restoring forces are balanced)

 Seiche (saysh): a type of standing wave that may be destructive in bays, lakes, and harbors; wave rocks back and forth with little movement at the node.  Caused by strong winds, seismic activity, or changes in atmospheric pressure Lake Superior seiche

 Seismic Sea Waves Waves typically have a wavelength > 200km Waves typically have a wavelength > 200km Formed from seismic activity that causes the Earth to move or shift Formed from seismic activity that causes the Earth to move or shift May be very damaging waves May be very damaging waves All seismic waves are considered tsunamis, but all tsunamis are not seismic sea waves. All seismic waves are considered tsunamis, but all tsunamis are not seismic sea waves. Ex. Tsunami caused by breaking of sea ice or a landslide is not seismicEx. Tsunami caused by breaking of sea ice or a landslide is not seismic

FREE AND FORCED WAVES  Free wave: A wave that is formed and then propagates across the sea surface without any further influence of the force that formed it (tsunami)  Forced wave: A wave that is maintained by the disturbing force (tide)

DEEP WATER WAVES  Water is deeper than half the wavelength  The water in the wave does not come in contact with the ocean floor

 Deep Water Wave Speed:   C = L / T C = L x f C = L x f C = 1.56 T 2 C = 1.56 T 2 Equation for Deep Water Wave

SHALLOW WATER WAVES  Water depth is less than 1/20 the wavelength  Orbits in water are flattened due to the proximity of the bottom  Water just above seafloor moves back and forth

Equation for Shallow Water Wave  C = gd or C = 3.1 d

WAVE TRAINS  Groups of swells with the same origin  The waves in front must go through undisturbed water and dissipate  Some energy is left behind and a new wave forms behind the group  Waves sort themselves or disperse and produce a swell or smooth undulation of water

FACTORS THAT AFFECT WIND WAVES  Wind Strength – Speed  Wind Duration – How long the wind blows  Fetch – The uninterrupted distance over which the wind blows

MAXIMUM WAVE HEIGHT  Steepness = the ratio between height : wavelength  Maximum = 1 : 7  Any higher and the wave will fall apart – excess energy will be dissipated as turbulence (whitecaps)

 Waves tend to be the largest around Antarctica  Common height = 11 meters

WAVE INTERFERENCE  When waves meet they add or subtract from one another  Constructive Interference (in phase) = add  Destructive Interference (out of phase ) = subtract

ROGUE WAVES  Near a storm many waves may approach from many directions  This can result in a sudden huge wave crest called a rogue wave

 This wave is larger than the ones before or after  Currents can contribute – Agulhas Current flows opposite waves Deadliest Catch Rogue Wave Monster waves from ship in storm Large waves in Antarctica

WAVES APPROACHING SHORE  When the depth is ½ the wavelength, wave “feels” bottom  Circles near bottom flatten to ellipses  Wave’s energy now packed into less depth

 Wave crest becomes peaked  Interaction with bottom slows wave; but waves behind continue at the original rate  Wavelength decreases, period stays the same  Wave becomes too high for wavelength  As water becomes shallower – bottom of wave slows due to friction

 Water at top is now moving faster than the energy in the wave  As crest moves ahead of supporting base, the wave breaks

 Waves break at a ratio of 3:4 (height to depth)  Water rushing shoreward is called surf

TYPES OF BREAKERS  Plunging – Form when waves approach a moderately sloping bottom Plunging Breaker

 Spilling – More gradual sloping bottom. Crest slides down wave face Spilling Breaker

 Surging – Very steep ocean bottom. Wave does not actually break, but moves up beach face lSurging Breaker :34-:55Surging Breaker :34-:55

WAVE REFRACTION  Waves rarely approach head on to a beach – they approach at an angle  Part of wave to enter shallow water first will slow down first  This bends the wave

 Sediment is transported along the beach – littoral drift  Obstructions, like jetties and groins, can interfere with the drift LONGSHORE CURRENTS OR LITTORAL DRIFT

Groins

Jetties

 Water in longshore currents may buildup between the shore and surf area and, like a river, overflow the “banks”  The water returns seaward quickly  Called a rip current Rip Currents Australian rip currents caused by La Nina conditions

DIFFRACTION  Propagation of a wave around an obstacle Diffraction of water

WAVE REFLECTION  Most objects reflect waves with little loss of energy  This can cause interference  Standing waves can result – wave appears to not move, but oscillate up and down Creating standing waves

INTERNAL WAVES  Internal waves move slowly because the density differences are not great between the water masses  Occur at the base of the pycnocline – especially if caused by a steep thermocline  Generated by Wind energy Wind energy Tidal energy Tidal energy Currents Currents Density Demo

 May mix nutrients into surface water and trigger plankton blooms  In 1963 a nuclear sub sunk perhaps because of an internal wave  In 1980 an oil platform rotated nearly 90 0 by a series of internal waves

Tsunamis Tsunamis  Caused by Sudden vertical movement of Earth along faults Sudden vertical movement of Earth along faults Landslides Landslides Icebergs falling from galciers (tsunami only, not seismic sea wave) Icebergs falling from galciers (tsunami only, not seismic sea wave) Volcanic eruptions Volcanic eruptions

 Shallow water waves due to long wavelengths – up to 200 km (deepest part of ocean 11 km)  Can move at speeds of 212 meters per second Japan Tsunami Approaching Shore Tsunami Video 2

 In deep water, the long period of a tsunami means that it would pass unnoticed underneath a ship  As it approaches shore Period remains constant Period remains constant Velocity drops Velocity drops Wave height greatly increases Wave height greatly increases

 Water rushes onshore – it does not break  Often a series of waves, not just one  As we know, many lives can be lost and much property destroyed if the tsunami hits certain areas

 A Tsunami Early Warning System has been set up in Pacific to help warn of tsunamis