Waves

Question 1

How can you get water waves?

Answer 1

Landslides, earthquakes, moon/sun (tides) and wind

Question 2

How does wind generation start?

Answer 2

When the wind blows across the ocean, its speed varies with height being slower near the surface of the ocean.

This is because there is friction acting between the water and the air.

This friction produces turbulence in the air near the surface of the water while also transferring some of the air’s kinetic energy into the water = tiny ripples.

The transfer of energy into the water combined with the turbulence produces perturbations in the surface of the water that eventually become waves (piling = growth) .

Question 3

Waves
High pressure =
Low pressure =

Answer 3

Push

Absorb

Question 4

Where wind is blowing you get a range of wave sizes….

Answer 4

Short crests (messy, chaotic, sea waves)
Swell (organised, longer crests, water waves)

Question 5

Two fluid interactions…

Answer 5

A little perturbation (bump)
Higher pressure at A
B is sheltered by prevailing wind
High concentration to low concentration- high to low pressure direction
High pressure goes up at A, hill gets better (catches more wind).

Question 6

Bigger waves form

Answer 6

Stronger winds
Longer fetch (distance over which wind is blowing)
Longer duration
Limited by depth

Question 7

What is wave pressure?

Answer 7

The weight of the water at a point when its still.

Question 8

How do waves work?

Answer 8

Move in direction of pressure gradient
But water is an incompressible fluid
Squeezing- water surface elevation has to go up.

Peak horizontal at crest must come from somewhere…
Back here surface must go down.
Diverge (stretch) Converge (squeeze)

Question 9

Wave motion and water motion are two totally seperate quantities…

Answer 9

Wave passes @ m/s; water moves within orbital @ cm/s.

Cycling inside the wave is independent of speed of wave

Question 10

Period

Answer 10

Time taken from crest to crest

Question 11

Orbital velocity is proportional to…

Answer 11

wave H at all depths

Question 12

Propagation velocity depends on…

Answer 12

depth of “squeezing” -> falls off to some proportion of surface motion at depth around (half the wavelength)

Question 13

Hill of water moves at rate directly proportional to…

Answer 13

how deep

Question 14

Long wavelength you get…

Answer 14

Slower motion but you lose pressure gradient much more slowly
Travel faster because more total squeezing

Question 15

Short wavelength you get…

Answer 15

Faster motion but you lose pressure gradient much more quickly
Travel slowly because less total squeezing (and lose a lot more energy to friction).

Question 16

Wave superposition

Answer 16

Adding up different sinusoids
Constructive interference
Destructive interference
Mixed = groups, sets

Question 17

Water parcel (reality)

Answer 17

Real waves in nature have complex shapes
Move forward more than move backward
Wave “drift” never really = 0 (get net displacement)

Question 18

Wave drift increases with wave H (for 2 reasons):

Answer 18

Water velocity at bottom of orbit is less than velocity at top of orbit
More time moving with crest than with trough

Question 19

What type of wave is in ocean wave?

Answer 19

A mechanical wave.

This means they have to travel through something (a medium) which, in the case of ocean waves, is water.

Question 20

The crest and trough of a wave are…

Answer 20

the highest and lowest points of a wave respectively.

Question 21

The amplitude is…

Answer 21

If you imagined the surface of the water was flat and overlayed a wave on top of it, the amplitude of the wave would be the height of the crest above the stationary water.

Question 22

A wave’s height is…

Answer 22

the vertical distance between a crest and a trough.

Question 23

The wavelength of a wave is…

Answer 23

the horizontal distance between two crests or troughs.

Question 24

On the run-up to a coastline, what happens to the water?

What is this process called?

Answer 24

the depth of the water becomes shallower and a wave is forced to slow down.
As it slows, its wavelength decreases and its height increases.

This process is called shoaling

Question 25

During shoaling what happens to wave height?

Answer 25

It increases but the wave’s height can only increase so far however.
Above a height of around one seventh the wavelength of the wave, the wave becomes unstable and it breaks.

Question 26

Depending on the properties of the wave, when it breaks on a coastline it can be classified as either…

Answer 26

Constructive or destructive (also referred to as surging or surfing).
They differ in the strength of their swash (rush of water up a beach) and backwash (rush of water down a beach).