Wind Waves
Wave development
Most ocean waves wind-generated. Wave size depends on the fetch, the wind velocity, and the duration of wind.
- fetch - the area over which the wind is blowing
- wind velocity - the speed at which the wind is blowing
- duration - how long the wind has been blowing
As the wind begins to blow, energy is transfered from the air to the water, the surface begins to ripple. All waves start as capillary waves, very small waves less than 1.73 cm high. The wind pushes the water up, and gravity pushes it down. The wind pushes the wave forward, so the crest becomes a trough. This is what causes circular orbits of individual water molecules. So capillary waves are also forced waves as the waves are dependent upon the force that creates them to exist.
As the wind continues to grow, larger waves are formed. Sometimes the wind blows very strongly over a short distance; when this happens the wave becomes too tall and will break upon itself. Wave steepness is the ratio of the wavelength to height; waves with a ratio greater than 1:7, the wave will break and excess energy dissipated as turbulence. These waves are referred to as "white caps" and are an indication that a storm may be approaching.
Fully developed sea
- Maximum wave height, wavelength for particular fetch, speed, and duration of winds at equilibrium conditions
- Wind must blow continuously for 3 days
- Antarctic Circumpolar Current – largest waves
Swell Formation and Dispersion
Wave separation, or dispersion, is a function of wavelength. Waves with the longest wavelength move the fastest and leave the area of wave formation sooner. The smooth undulation of ocean water caused by wave dispersion is called swell. A wave train is a group of waves with similar characteristics. This sorting of waves by their wavelengths is wave dispersion The wave train speed is ½ speed of individual wave.
Interference There is more than one wave train in open ocean and they often interact. Waves will either add to or subtract from one another. This is called interference.
- Destructive interference - Converging waves cancel each other out. This is due to the two sets of waves being sufficiently different from one another. The result is either no or smaller waves.
- Constructive interference - Converging waves add to one another. This occurs when two sets of waves with similar characteristics (same wavelength and speed) meet. This results in larger waves.
- Mixed interference - Converging waves both cancel and add to each other. This is the most common, as there are wave trains of all sorts of speeds and wavelengths in the open ocean that can and do meet. The result is that some waves are canceled out (or lowered in height) and others are amplified.
Wave Generation And Motion
Wind waves are orbital waves. It is the orbital motion that causes the wave form to move. Orbital waves occur between two fluid media. They are also called progressive waves because the waveform moves forward.
Image source: Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
The Effects of deep water wave on movement of water particles
(Image from: By Kraaiennest - Own work, GFDL, https://commons.wikimedia.org/w/index.php?curid=3374567)
Wavelength determines the size of the orbits; Water depth determines the shape of the orbits. There are two broad categories:
- Deep-water waves are waves moving through water deeper than half their wavelength.
- Shallow-water waves are waves moving through water shallower than 1/20th their wavelength. The water molecule orbits “flatten” as they get close to the bottom. The water above seafloor cannot move in a circular path.
In the open ocean, the wave train begins to move towards the shore. So long as the wave is moving through water
(1) that is greater than 1/2 their wavelength the orbits will remain circular
(2). However, the circular motion of water molecules is interrupted and the wave slows as water becomes more shallow. Energy is no longer dissipated with depth, so the energy goes up and the wave becomes too high for its wavelength. The water at the top of the wave
(3) is now moving faster than that at the bottom of the wave. The orbits are still trying to form, and the wave breaks
(4) and forms the surf
(5) as it rushes on shore.
Types of breakers
Spilling breaker
Gently sloping seafloor Wave energy expended over longer distance |
|
 |
 |
| Click for image source | |
Plunging Breaker
Moderately steep seafloor Wave energy expended over shorter distance Best for board surfers |
|
 |
 |
| Click for image source | |
Surging breaker
Steepest seafloor Energy spread over shortest distance Best for body surfing |
|
 |
 |
| Click for image source | |
Big Wave Surfing
Mike Parsons surfing 77 foot wave at Cortes Bank. From the documentary "Billabong Odyssey"
|
Just where is Cortes Banks?  |
SurflineTV created a video, How Cortes Banks Works, to explain it all. |
| Garrett MacNamara surfs a 78 foot wave off the coast of Portugal (image) |
How is the Nazare Wave formed? It's different from Cortes Bank in California, yet similar. Off the coast of Nazare is not a bank, but a deep submarine canyon. Like in Cortes Bank, storm waves are fine until they reach the canyon's mouth, just offshore from Nazare. The sudden shallowness of the water creates a huge wave. This wave meets up with waves nearby, and constructive interferrance makes the large waves even larger. This video is in Portuguese but does an excellent job in explaining how the offshore bathymetry creates monster waves!
|
Wave Refraction, Reflection, and Diffraction
Waves help to erode the shoreline by physically pounding on the rocks and sediments that make up the coastline. Waves do not always travel in a straight line - they can bend in response to changes in the coastline or other factors.
- Refraction = bending of waves
- Reflection = bouncing back of waves
- Diffraction = radiating outward of waves
<< back |
||
