Weather Watch

Waves’ anatomy begins with the wind

Special to The TribuneSeptember 14, 2013 

A surfer lays down a turn at Morro Rock in 2007. Wind from a storm spawned the waves, which are the result of wind over water.

JOE JOHNSTON — jjohnston@thetribunenews.com

I  received a few questions lately in regard to the terminology of “deep water” and “shallow water” waves in the Surf Report that appears in Monday’s edition of The Tribune.

To answer this question, let’s start from the beginning. The three main factors that determine wave height and length are wind speed, wind duration and the fetch — the distance over which the wind blows across the ocean. Waves are created by the friction or the dragging motion of the wind over the water.  These and a few other factors determine if ocean waves are small ripples or frighteningly large, cascading walls of water. 

As seas develop in the fetch, they can reach only a 7-to-1 ratio of wavelength to wave height. Specifically, a wave with a 7-foot length can reach only 1 foot in height before it breaks. When the wave breaks, longer wavelengths develop, allowing the seas to increase in height. When the seas move out from under these winds they become swells.

Depending upon where the fetch is located, for example in the Gulf of Alaska or a large storm off the coast of New Zealand or the seemingly ever-present Central Coast northwesterly winds, will determine where the seas or swells are coming from.

The longer the period of the swell, the lengthier its wavelength will be. The wavelength determines how far down in the water column the wave can be felt. Waves will begin to feel the ocean floor at half their wavelength. In other words, a wave with the 20-second period will begin to feel the ocean bottom at about 1,000 feet, while a wave with a 5-second period won’t feel the ocean floor until 80 feet.

If you were at the bottom of the ocean near the Diablo Canyon Waverider Buoy, which is moored in about 100 feet of water, you wouldn’t feel any surge from the locally generated short period seas overhead. However, if the period of the swell was 20 seconds, the surge from each passing wave would toss you back and forth along the ocean’s floor.

This is an important factor, as shallow water waves feel the bottom they bend toward shallower depths which changes their incoming direction and other characteristics. As any surfer will tell you, this refraction can have profound and unique effects at the different surf breaks along the California coast.

While stationed on San Clemente Island with HS-85, a U.S. Navy helicopter squadron, we would sometimes fly our trusty SH-3H Sea King helicopter out to the Cortes Bank located about 45 miles southwest of the island. When the conditions were right, enormous waves would suddenly erupt in the middle of the ocean.

Because of the unique bottom topography at different locations along our coastline, I use the deep-water incoming direction from the Harvest Platform buoy since it’s moored in 1,800 feet of water. You see, the waves haven’t felt the ocean’s floor at that location. This helps surfers to calibrate their favorite break.

In summary, a deep-water wave is a swell that hasn’t felt the ocean’s floor, while a shallow water wave has.

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Land stewardship is important to PG&E. The company manages 12,820 acres that surround Diablo Canyon Power Plant. This has allowed for coastal hiking trails open to public use, including the Pecho Coast Trail that leads to the restored Point San Luis Lighthouse. To learn more about these preserved lands, visit www.pge.com.

John Lindsey's column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist. He is president of the Point San Luis Lighthouse Keepers. If you have a question, send him an email at pgeweather@pge.com.

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