Waves

1. Waves

2. 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.

3. WavesTerminology 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…

4. 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.

5. 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.

6. 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.

7. Waves

8. 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. This interaction results in changes in the shape of the water orbits, as well as the apparent shape of the waves themselves…

9. 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…

10. 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.

11. WavesBreaking 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.

12. 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, 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…

13. 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…

14. 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…

15. 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…

16. 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?

17. 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…

18. Other Types of WavesReview