Monday, August 22, 2011

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.

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 24th, 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.

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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