Waves

Currents transport

heat and water

Waves transport

ENERGY

What are waves?

A disturbance caused by the movement of energy through a medium

Ocean Waves

For an ocean wave the disturbing force can be:

Wind*

Earthquake (tsunami)

Landslide

Tidal forces

Change in atmospheric pressure

Anatomy of a Wave

Crest- 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

Motion in a Wave

Energy is moving forward, water is not!

Surfer example

Water particles move in circular orbits

Forward on the crest

Backward in the trough

Orbital waves- because particles move in circular orbits

At surface orbit = H (height)

Size of the orbit decreases rapidly with depth (dec. wind energy)

Wave motion negligible at depth of ~L/2 (1/2 wavelength)

Progressive wave- because the waveform progresses along the path

Wave Classification

Disturbing Force= energy input

Wind = wind waves*

Landslide/faulting = tsunami

Moon/sun gravitation = tides

Restoring Force = returns water to flatness

Tends to overcompensate leading to oscillations

Surface tension- for small waves (capillary waves)

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

Period of a Wave

Period (T) = time between wave crests

Is a constant for any wave

Function of the disturbing force

If wind energy decreases

Speed and wavelength can decrease
Period does not change

Wave Characteristics

Wavelengthà

Size of the orbits

Water Depth relative to wavelength à
Shape of the orbits

Deep vs. Shallow Water Waves

Deep Water Waves

Orbits don’t reach the seafloor

Water depth > _ wavelength

If L = 20m, depth > 10 m

Only wind waves can be deep water waves

Wavelengths for tsunamis and tides are so long (100’s — 1000’s km)

But the seafloor maximum depth = 11 km

Would need >50 m to equal deep water

Wave Speed (C)- speed the energy moves through the water

C = L/T

Speed (m/sec)

= wavelength (m)/period (sec)

(= function of the period)

Period is constant

Shorter wavelength = slower wave

Shallow Water Waves

Orbits reach and interact with the seafloor

Water depth < 1/20 wavelength

Orbits "feel" the bottom

Orbits flatten à elliptical

Water at seafloor moves back and forth

Shallow vs. Deep Water Waves

Shallow water waves

Water depth < 1/20 wavelength

Deep Water Waves

Water depth > _ wavelength

Intermediate Water Waves

In between depths

More difficult to characterize

Shallow Water Waves

Speed is a function of depth

S = ˆ gd

As wave enters shallow water it feels bottom more and slows down

History of a Wave

Wind waves= wind energy transferred to the water

Wind blows on the water — friction causes stretching of the surface

Ripples (capillary waves) form

Wind’s energy transferred to the water to drive the wave forward

Ripples create larger surface for interaction

More energy transferred to the water

Wave orbitals get larger

Height, wavelength increase

Seas- chaotic, irregular waves in the region of formation

Swell- waves that have moved away from the storm center

Sorted themselves out by speed (wavelength)

Become more regular and rounded

Wave Trains- groups of swell waves with same origin and wavelength (speed)

Wave Height

Factors controlling wave height:

Wind strength

Strong wind = more energy = taller waves

Wind duration

How 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 Seas

When the seas have reached the maximum wave size possible for the wind strength and fetch

Longer exposure to the wind will not increase the wave size

Biggest wave recorded ~34 m (112 ft)

Rogue waves- produced by constructive interference

Surf Zone

As waves approach the shoreline:

Become shallow water waves

Slow down due to friction

Become closer together

Steepen

Break

Lose energy

Change in properties

Period remains constant (function of generating force)

Speed (C) decreases — friction with seafloor

C = L/T

Wavelength decreases

Waves crowd together, steepen

When H/L > 1/7 wave breaks

Breakers

Types of breakers vary with:

Wave steepness

Slope of the seafloor

Wind direction relative to wave movement

Plungers

Narrow, steep beaches

Waves steepen quickly

Crest outruns trough

Curls (good surfing wave!)

Sudden energy loss

Destructive- backwash carries sand off the beach

Spillers

Wider, flatter beaches

Waves build more slowly

Wave face collapses and spills down front

More gradual energy loss

Constructive waves- carry sand onto the beach

Waves usually approach beach at a slight angle so the waves break to the right or the left

Angle of approach is generally less than expected due to the wind

Refraction- bending of the wave front so they approach almost parallel to the shore

Refraction

Waves approach the beach at an angle (from source region)

One end reaches shallow water first and starts to slow down

The end in deeper water moves faster and curves around