Waves and Water DynamicsWaves and Water Dynamics

Essentials of Oceanography

Pebble in Still Water

What happened when I dropped the pebble into the still tank?

Do the particles at the point of disturbance move outward too?

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Cork in the disturbance

How does the cork move?

What Causes Waves?

Waves are created by releases of energy (disturbances) including:

Wind

Movement of fluids of different densities

Mass movement into the ocean (splash waves)

Underwater sea floor movement (tsunami)

Pull of the moon and sun (tides)

Human activities

Most Ocean Waves Are Wind-generated

Anatomy of a WaveCrest- maximum elevation about still water level

Trough- max depression below still water level

Wavelength (L)- crest to crest, or trough to trough

Height (H)- vertical distance crest to trough

Amplitude- H/2, still water to crest or trough

Period (T) - time for one wavelength to pass a point

Frequency – number of wave crests passing a point in 1 second

Anatomy of a Wave

Wave Characteristics and Terminology (Continued)

If wave steepness exceeds 1/7, the wave breaks

Period (T) = the time it takes one full wave—one wavelength—to pass a fixed position

)( wavelength

)(height wave steepness Wave

L

H

fTf

1 (T) Periodor

1 )(Frequency

How do waves move?

1. The direction of propagation of wave, that is, the direction in which the disturbance travels.

2. The disturbance is transferred due to the oscillation of the particles of the medium involved. The direction of these oscillations is the second direction.

Types of Progressive Waves

LongitudinalBack-and-forth motion

TransverseSide-to-side motion

OrbitalCombination

Circular Orbital Motion

As a wave travels, the water passes the energy along by moving in a circular orbit Floating objects also follow circular orbits

Orbital Motion in Waves

Orbital size decreases with depth to zero at wave baseDepth of wave base = ½ wavelength, measured from still water level

What about Surfing?

Wave CharacteristicsPeriod (T) = time between wave crests

Is a constant for any waveFunction of the disturbing force

If wind energy decreasesSpeed and wavelength can decrease

Period does not change

Wavelength Size of the orbits

Water depth relative to wavelength

Shape of the orbits

Deep- and Shallow-water WavesDeep-water waves

Water depth > wave base

Shallow-water waves

Water depth < 1/20 of wavelength

Deep Water Waves

Orbits don’t reach the seafloorWater depth > ½ wavelength

If L = 20 m, depth > 10 m

Only wind waves can be deep water wavesWavelengths for tsunamis and tides are so long (100’s – 1000’s km)

Seafloor maximum depth = 11 km

Ocean depth would need to be >50 km for these to be deep water waves

Shallow Water Waves

Orbits reach and interact with the seafloorWater depth < 1/20 wavelength

Orbits “feel” the bottom

Orbits flatten elliptical

Water at seafloor moves back and forth

Wave Speed (S)General formula:

Deep-water waves:Wave speed (S) in meters per second = 1.56 T in seconds

Wave speed (S) in feet per second = 5.12 T in seconds

Shallow-water waves: (d = water depth)

)( period

)( wavelength )( speed Wave

T

LS

metersin 3.13 secondper metersin )( speed Wave dS feetin 5.67 secondper feet in )( speed Wave dS

Producing WavesDisturbing Force = energy input

Wind = wind waves*

Landslide/faulting = tsunami

Moon/sun gravitation = tides

Restoring Force = returns water to flatnessTends to overcompensate leading to oscillations

Surface tension- for small waves (capillary waves)

Gravity for larger waves- nearly friction-free travel long distances

History of a WaveWind waves = wind energy transferred to the water

Wind blows on the water – friction causes stretching of the surfaceRipples (capillary waves) formWind’s energy transferred to the water to drive the wave forwardLarger surface area more energy transferred = bigger waves

Factors Controlling Wave Height

Wind strengthStrong wind = more energy = taller waves

Wind durationHow long the wind blows in one direction

Waves produced by strong winds will dissipate when wind stops

Fetch - distance wind blows in one direction

Longer continuous input = taller waves

Fully Developed Sea

Factors that increase wave height:Increasing wind speed

Increasing duration (time) of wind

Increasing fetch (distance)

A fully developed sea is the maximum height of waves produced by conditions of wind speed, duration, and fetch

Largest Wind-generated Waves Authentically Recorded

In 1935, the vessel USS Ramapo experienced large waves while crossing the Pacific Ocean

Wave height was measured at 34 meters (112 feet)

Waves at the Shore

Waves Undergo Physical Changes in the Surf Zone

Surf Zone

As waves approach the shoreline:Become shallow water waves

Slow down due to friction

Become closer together

Steepen

Break

Lose energy