Oil Spill Series: Waves

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.

Formulas:

Wind stress magnitude is calculated from wind magnitude as τ = c_a ρ_a |u|²

where ρ_a = 1.2 kg m_-3 is the density of air, c_a = 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.

Reference:

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

Oil Spill Series: Tides

The physics of the ocean is essential to understanding the science of an oil spill. Besides the actual content of the oil and its effect on the environment, it is important to understand how the water behaves. This leads to processes which can determine the behavior or weathering of the oil. It plays a role in a responder’s ability to conduct clean-up.

When oil hits the water, earth’s forces immediately begin to go to work on it, both from above and below. For this segment, we look at how the tides work.

Tides

One of the cool things about the tides is that despite the constantly varying heights, they are completely predictable. We have predicted tides for a hundred years and more in the future (when I say “we” I don’t mean me…sounds like a completely tedious task, but I am glad someone out there enjoyed it enough to do it).

The tides are being influenced by three things: Gravity of the moon, the Earth’s rotation, and even a bit by the Sun’s gravitational pull. Each day, as the earth rotates, the gravitational pull of the moon creates a “bulge” on the earth and this causes a tidal change from low to high and high to low every 6 and a half hours.

Twice a month, the sun and moon are aligned (Full Moon and New Moon). When this occurs, the gravitational pull is even stronger and creates what are called Spring Tides. When the moon is at a right angle to the sun (half moon), the tidal change is less significant and is called a Neap Tide. The spring tide during the equinoxes have the highest tidal range (Owens, 1-17).

Equinox: the time when the sun crosses the plane of the earth's equator, making night and day of approximately equal length all over the earth and occurring about March 21 (vernal equinox or  spring equinox) and September 22 (autumnal equinox).

With these parameters laid out for us, the rule of thumb is that the tides at the equator (where the bulge is) are typically larger than those to the north or south. That is, until you take into account the geography.

Where your normal equatorial tide change is typically right around 2 meters, an example of a large tidal change due to tidal resonances is the Bay of Fundy (located between New Brunswick and Nova Scotia). The geography of this estuary supports a 15 meter tidal change (Bloomfield, 288)!

Resonance: is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others

Resonance is a fascinating topic for another post, but note: due to the design of the landscape which the wave is traveling through, this tidal resonance occurs and causes phenomena such as the Bay of Fundy, the Cook Inlet in Alaska, and a handful of others.

Now, keeping in mind that this massive tidal change must still take place within the span of 6.5 hours, you can imagine the speed at which the ocean must move. Don’t get caught out there! This is a good concern to keep in mind when an oil spill response occurs in these areas…

 

The tank vessel EXXON VALDEZ struck Bligh Reef on March 24^th, 1989. What is the significance of this date? Remember the effect that the equinox has on tides and when the equinox occurs? Also, what is the average tidal change for an area like Prince William Sound? These tidal effects all played a part in the behavior of the spill and the direction of the response to it.

 

Reference:

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

Series: Oil Spills

For the next few posts, I will be talking about subjects that I have become familiar with in my job: Oil Spills. Where they go, what happens to the ecosystem, the science behind response and response equipment, etcetera.

Keep in mind throughout the series that all the elements of oil spills are complex and variable; like fingerprints, no two oil spills are alike, even if they are in the exact same location. As I write and publish each of these posts, I will update this post with links to the posts that are part of the series. As an introduction, here are some pictures:

"This is a radar image of an offshore drilling field about 150 km (93 miles) west of Bombay, India, in the Arabian Sea. The dark streaks are extensive oil slicks surrounding many of the drilling platforms, which appear as bright white spots."

 Source: http://www.jpl.nasa.gov/radar/sircxsar//oilsk.html

The EXXON VALDEZ and the oil spill caused by the vessel's collision with Bligh Reef in1989.

Clean-up during the Exxon Valdez spill...this technique was not always better then leaving the oil in place, which I will address when I post about mechanical clean-up techniques.

This is a sattelite image of the Deepwater Horizon oil spill in the Gulf of Mexico, taken days after the intial oil-rig explosion in April, 2010. This turned into the worst environmental disaster in U.S. history. Photo from The Global Herald

Disclaimer: Any views and/or opinions expressed in The Masqerading Scientist are solely those of the author and not of the U.S. Coast Guard or any other U.S. federal agency. Thank you!

Plan to Save the Whales

This is a guest blog post written and illustrated by my daughter, Lily. She is 9 years old and has a passion for these wonderful creatures. These views are not all necessarily the views of anyone BUT a wonderful and curious little girl, so please, Enjoy!

Plan to Save the Whales

You know, lots of people love whales and they love to see whales; but the whales are endangered. To keep up the fun of whale watching, we need to do something before they are extinct.
Easy Home ways to save the whales:

• Cool down the earth by biking or walking to places that aren't far away
• Turn off electronics when not in use
• When you go to the bathroom, don't use a lot of toilet paper, because everything that goes down the toilet or drain ends up in the ocean
• Take short showers and don't make them that hot.

Whales sometimes get beached. So here is how you can save them if they do. If the whale is light enough, put wet blankets over it and wait for a helicopter to pick up the whale and move it back to the ocean. If the wale is like a Sperm Whale or a Blue Whale, put wet blankets on it and wait for the tide to come in and quickly pull off the blankets.

So keep your ocean clean and don't allow whaling!

References:
Mom & Dad propaganda

Briarcrest Elementary 3rd Grade Class