Waves Flashcards
How can you get water waves?
Landslides, earthquakes, moon/sun (tides) and wind
How does wind generation start?
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) .
Waves
High pressure =
Low pressure =
Push
Absorb
Where wind is blowing you get a range of wave sizes….
Short crests (messy, chaotic, sea waves) Swell (organised, longer crests, water waves)
Two fluid interactions…
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).
Bigger waves form
Stronger winds
Longer fetch (distance over which wind is blowing)
Longer duration
Limited by depth
What is wave pressure?
The weight of the water at a point when its still.
How do waves work?
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)
Wave motion and water motion are two totally seperate quantities…
Wave passes @ m/s; water moves within orbital @ cm/s.
Cycling inside the wave is independent of speed of wave
Period
Time taken from crest to crest
Orbital velocity is proportional to…
wave H at all depths
Propagation velocity depends on…
depth of “squeezing” -> falls off to some proportion of surface motion at depth around (half the wavelength)
Hill of water moves at rate directly proportional to…
how deep
Long wavelength you get…
Slower motion but you lose pressure gradient much more slowly
Travel faster because more total squeezing
Short wavelength you get…
Faster motion but you lose pressure gradient much more quickly
Travel slowly because less total squeezing (and lose a lot more energy to friction).
Wave superposition
Adding up different sinusoids
Constructive interference
Destructive interference
Mixed = groups, sets
Water parcel (reality)
Real waves in nature have complex shapes
Move forward more than move backward
Wave “drift” never really = 0 (get net displacement)
Wave drift increases with wave H (for 2 reasons):
Water velocity at bottom of orbit is less than velocity at top of orbit
More time moving with crest than with trough
What type of wave is in ocean wave?
A mechanical wave.
This means they have to travel through something (a medium) which, in the case of ocean waves, is water.
The crest and trough of a wave are…
the highest and lowest points of a wave respectively.
The amplitude is…
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.
A wave’s height is…
the vertical distance between a crest and a trough.
The wavelength of a wave is…
the horizontal distance between two crests or troughs.
On the run-up to a coastline, what happens to the water?
What is this process called?
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
During shoaling what happens to wave height?
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.
Depending on the properties of the wave, when it breaks on a coastline it can be classified as either…
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).
