Waves by yurtgc548

VIEWS: 8 PAGES: 18

									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. Wave-
  length 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

								
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