Waves

Waves

What is a wave?

A periodic disturbance in a medium or in space, where energy is transferredfrom one place to another by vibrations.

Note that it is the energythat is transferred and not the medium.

There is almost no net movement of water asocean waves go by…Water simply moves inorbits. These waves areoften referred to asorbital waves.

WavesTerminology

Crest – highest point of the wave

Trough – lowest point of the wave

Wave height – distance between thewave crest and trough

Wavelength – distance between to adjacent crests (or troughs)

Still water level – level of the waterif there were no waves

Orbit – the orbital path of water aswave moves by…

Waves

As waves pass, water moves in an orbital path… this general movement continues intodeeper water, but the influence of the surface wave becomes smaller with the increasingWater depth…

By the time we get to a depth of halfthe wavelength, wave motion is almostnegligible… Circles are only 1/23 the diameter of those on the surface.

WavesSince water movement diminishes with depth, then we can see how wateron 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 oppositedirection 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 importantin creating the surface currents we discussed in the last lecture.

WavesSo 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

WavesDeep 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. Thisinteraction results in changes in the shape of the water orbits, as wellas the apparent shape of the waves themselves…

WavesDeep 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…

WavesWind 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.

WavesBreaking Waves

Waves breaking in areas with steep slopes create plunging waves, whereas areaswith gradual slopes create spilling waves… it is simply a question of the distanceover which the transition between deep water, to shallow water, to a 3:4 ratio of wave height to water depth is made.

WavesInterference 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, oneOut of every 1175 waves is 3 times the average height of waves observed, and oneIn 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 lineslow down at different rates, causing the wave lines to bend… this is whyyou see waves start to break at one end of the beach and then move towardsthe other side…

More Wave PhysicsWave Diffraction:

This is the propagation of waves around objects. Objects can interfere withThe 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 thatobject. Waves hitting an obstruction straight on will reflect back into the same direction that they came from, interfering with the oncoming wavesand 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 causesinternal waves?

Other Types of WavesIn addition to wind generated waves, there are other types of waves that can formin the oceans…

Seiches – this is the sloshing back and forth of water in an enclosed area, followingsome 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 causedby earthquakes, but tsunamis can also be caused by landslides, fallingicebergs, 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 WavesReview