Saturday, August 27, 2011

Oil Spill Series: Waves


“Although there are many processes and factors that may need to be taken into consideration to assess operational safety or the fate of spilled oil, the dominant process on shorelines is that of wave action (Owens, 1-8).”

There are two aspects of waves I want to expand on here: the origination of waves out at sea, and the behavior of the wave when it reaches the shore.

Origination of Waves
That’s easy enough: Wind. Any questions? Just kidding…Though not about the wind part. Lets travel out to the open ocean.

At first, the wind doesn’t have much to work with…the surface of a calm sea provides little roughage for the wind to push on and create movement. It is the tiny ripples that form as the wind moves over the surface that begin to travel (as oscillating waves) and GROW the more surface the crest gives for the wind to push on…make sense? Let me put it this way:

The second and third pictures here are showing the effect of “wind stress” which is what is happening as the wave allows more surface area for the wind to push…therefore, wind stress is a square function (oh, imagine that with the natural parabola formed in the wave trough! I love when math shows up in nature…) that is conversely dependant on the roughness of the sea, which depends on the wind stress. Clear as mud? Good. Lets move on.

Fetch: The distance which wind blows over water.
This is what is causing fully developed seas to become swell.

Now, in case it hasn’t been made obvious yet, the water is NOT traveling with the wave…the wave is an oscillation of energy traveling through the medium of liquid, in this case the ocean water. In fact, as you travel deeper under the surface of the water (in the deep ocean) that oscillation disappears slowly. This aspect is what changes as you near the shore…where the waves “break.”

Waves, Shallow Water and the Shoreline

As the swell travels toward the shore, it encounters shallow water. Now the energy of the wave actually causes the water in place to travel in a circular motion (this is why it does not travel fully forward with the wave). As the circular motion of the water encounters the sea-bed, it becomes elliptical. This happens when the depth of the water is half the wavelength (Bloomfield, 293).

Now we have a wave that is slowing down due to obstacle (seafloor) and bunching together. In addition to this, the height of the crest and depth of the trough (or its amplitude) increases to keep up momentum despite the slowing. Now, as all this is happening, the wave begins to refract, or bend the direction of travel, to approach the beach or shore more directly.

Now, as the crest of the wave carries forward the circular motion, the bottom encounters the shore or land and this causes the momentum to create the breaking of the wave.


Wind stress magnitude is calculated from wind magnitude as τ = ca ρa |u|2
where ρa = 1.2 kg m-3 is the density of air, ca = 0.0015 a dimensionless drag coefficient, u the wind speed and |u| its magnitude.

As Pertains to Oil Spills
There are a couple of things to remember about waves during an oil spill response. There exist what are called wave generated beach cycles.
“When waves reach the shoreline, they dissipate their energy:
  • Spilling or surging breakers tend to build up a beach (accretion), and
  • Plunging breakers move sediments seaward causing erosion (Owens, 1-12).”
This difference in dissipation of energy needs to be recognized so that the responder can know where to look for the oil on a particular beach.
If you are dealing with commonly spilling or surging breakers, you will be looking up the beach and digging, whereas, with plunging breakers, be aware of oil coming ashore more slowly over time.

Bloomfield, Louis A. How Things Work. John Wiley & Sons, Inc. 2006.

Owens, Ed. Shoreline Operations and SCAT Surveys for Oil Spills on the West Coast. Polaris Applied Sciences, Inc. 2010

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