Waves
Waves
What is a wave?
A periodic disturbance in a medium or in space, where energy is transferred
from one place to another by vibrations.
Note that it is the energy
that is transferred and
not the medium.
There is almost no net
movement of water as
ocean waves go by…
Water simply moves in
orbits. These waves are
often referred to as
orbital waves.
Waves
Terminology
Crest – highest point of the wave
Trough – lowest point of the wave
Wave height – distance between the
wave crest and trough
Wavelength – distance between to
adjacent crests (or troughs)
Still water level – level of the water
if there were no waves
Orbit – the orbital path of water as
wave moves by…
Waves
As waves pass, water moves in an orbital path… this general movement continues into
deeper water, but the influence of the surface wave becomes smaller with the increasing
Water depth…
By the time we get to a depth of half
the wavelength, wave motion is almost
negligible…
Circles are only 1/23 the diameter of
those on the surface.
Waves
Since water movement diminishes with depth, then we can see how water
on the top of the orbital paths on the surface (moving in direction of the wave)
would move slightly faster than the water on the bottom (moving in the opposite
direction from the wave).
Because of this very reason, there actually is a very small amount of transport of
surface water in the direction of the wave… this mass transport is very important
in creating the surface currents we discussed in the last lecture.
Waves
So what factors influence wind wave development?
1. Wind strength – the speed of the wind over water.
2. Wind duration - the uninterrupted time during which wind blows in the same
direction over the water.
3. Fetch – the uninterrupted surface over which the wind blows.
Waves
Waves
Deep Versus Shallow Water Waves
Deep water waves – waves moving through water that is deeper than half their
wavelength (i.e. the wave is not influenced by the bottom).
Shallow water waves – waves moving through water that is shallower than 1/20
of their wavelength.
Transitional waves – waves that are in water deeper than 1/20 but shallower
than ½ their wavelength.
As waves approach shallower areas, they begin to interact with the bottom. This
interaction results in changes in the shape of the water orbits, as well
as the apparent shape of the waves themselves…
Waves
Deep Versus Shallow Water Waves
Note how in shallow water,
the movement of the water
transitions from circular to
linear… Also, the waves
develop sharper peaks at
their crests…
Waves
Wind Waves Approaching The Shore
1. Waves transitioning from “deep” to “shallow” zones begin to interact with the bottom.
2. Circular motion of water changes to flattened eclipses, crests become peaks, the entire
waves energy needs to be packed into less water depth…
3. Interaction with bottom slows the wave, but waves from behind keep coming. Wavelength decreases, but period remains the same.
4. Wave becomes to high for its wavelength, approaching 1:7 ratio
5. Crest of the wave moves ahead of its supporting base (base is slowed by bottom), and the
wave breaks at a 3:4 ratio of wave height to water depth, creating surf.
Waves
Breaking Waves
Waves breaking in areas with steep slopes
create plunging waves, whereas areas
with gradual slopes create spilling waves…
it is simply a question of the distance
over which the transition between deep
water, to shallow water, to a 3:4 ratio
of wave height to water depth is made.
Waves
Interference and Rogue Waves
Waves can interact with each other and form larges or smaller waves…
Since different storms create different wave systems, what we see on our shore is
a combination of different wave systems interacting with each other.
That’s why there is a periodicity to waves and kayakers, surfers, or divers will spend
time counting the waves for the preferred larger or smaller waves to come…
This is also why we sometimes get rogue waves (sleeper waves)… on average, one
Out of every 1175 waves is 3 times the average height of waves observed, and one
In 300,000 is more than 4 times the average size…
More Wave Physics
Wave Refraction:
As waves lines approach shore at an angle, different parts of the wave line
slow down at different rates, causing the wave lines to bend… this is why
you see waves start to break at one end of the beach and then move towards
the other side…
More Wave Physics
Wave Diffraction:
This is the propagation of waves around objects. Objects can interfere with
The waves, creating new nodes from which the waves propagate from…
Diffraction of waves past chains of islands can cause areas of wave crest
reinforcement…
More Wave Physics
Wave reflection:
As waves approach and hit a large obstruction, they will reflect off of that
object. Waves hitting an obstruction straight on will reflect back into the
same direction that they came from, interfering with the oncoming waves
and creating standing waves…
Internal Waves
Waves can form in the transition between any two fluids (not just water and air).
Internal waves form between the transition of water masses of different densities
(a function of temperature, salinity, or both)…
What do you think causes
internal waves?
Other Types of Waves
In addition to wind generated waves, there are other types of waves that can form
in the oceans…
Seiches – this is the sloshing back and forth of water in an enclosed area, following
some form of disturbance. This could be in a bay, a bucket, or your
bathtub!
Tsunami – shallow water progressive wave caused by rapid displacement of
large quantities of ocean water. Seismic sea waves are tsunamis caused
by earthquakes, but tsunamis can also be caused by landslides, falling
icebergs, volcanic eruptions, etc.
Tides – Tides are for all practical purposes waves. The tide wavelength though is
so large (half of earth’s circumference) that it can never be free of bottom
influences and is always in form of a shallow water wave…
Other Types of Waves
Review