Having a ball with wave energy

Gary Kewley
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Friday - August 01, 2008
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The world's undisputed best paddler, Aussie Jamie Mitchell
The world’s undisputed best paddler, Aussie Jamie Mitchell

Aloha, surfers and beachgoers!

What a sweet last week of July. The Taz Sea delivered above the forecast and peaked at about 5 feet - yes, it did! But you had to be at the right spot at the right moment. The National Weather Service issued the high-surf advisory on Monday when 8-foot face observations rolled in.

Those SW swells are usually pretty fat with long lines - beautiful things. The periods of this last big one reached 20 seconds!

Any wave with more than about nine or 10 seconds is heading above wind swell, the least energetic type of wave when it’s small. We can get pretty serious surf at 16 seconds, so this last swell came from a whopper storm. The higher the winds and longer they blow one way, the bigger the wave heights and periods - two very different aspects.

It’s an over-simplification, but the longer the period, the bigger the waves. Picture two beach-balls sunk into the water side by side and rolling by. The bigger the beachballs, the longer it takes for their two tops to pass through the same point in space. Two beachballs of half the size would take 10 seconds to pass that same point. This also means that the big beachballs have more area underwater. So, they’ll feel the ocean bottom sooner than a small beachball and, therefore, “bounce” higher.


Now let’s switch to waves instead of balls. Wave height is the vertical distance between a wave’s top (crest) and bottom (trough). But wave period measures the size of the wave - through time! A wave period is measured by picking some stationary point and counting the seconds it takes for two consecutive crests (or troughs) to pass it.

Now for some reality. Beachballs don’t work in the real world of waves. You see, in deep water, a wave is simply forward motion of energy, not water! Indeed, water molecules don’t even travel with the wave. It will basically move up then down in the same spot as the “energy wave” moves by.

Malia Manuel, 14, of Kauai is the youngest girl to ever win the U.S. Open
Malia Manuel, 14, of Kauai is the youngest girl to ever win the U.S. Open

Try it. Place a ping pong or basketball on the surface of a non-breaking wave out at sea - remember, wind doesn’t count in this exercise. You’ll see them go up and down in about the same spot. Same deal with the actual water.

Back to the storm that created the 20 seconds. Fiji got slammed with over 15-footers from the same system. The two things that decreased the size for Hawaii were 1) distance traveled (4,000 miles) and 2) the “shadowing” effect (blockage by Fiji and other islands between us).

The interesting thing is that wave periods remain nearly constant over time and distance whereas wave heights drop about half every 1,000 miles (very rough estimate). This is why we can have 3- to 5-foot waves with 20 seconds while Fiji got 15-foot waves at the same 20 seconds.

As wave periods increase, so does the “amplification” of wave height once the bottom of the wave underwater “feels” the bottom of the ocean. The final effect is undeniable as the 20-second swell transforms into a thick breaking wave.


This weekend we have another SW swell, but nothing near this last one. At least August will be kicking off with something. Trades will be down a pinch, so the waves and weather should be real fine.

Last weekend was epic for competition. The girls of Hawaii are just going off the hook. I’ve got the story of Kauai’s history-making 14-year-old Malia Manuel at surfnewsnetwork.com. This adorable and talented ripper is the youngest girl ever to win the U.S. Open.

Also, guess who won the QS Edition Molokai to Oahu Paddle Board Race for the seventh straight time? He’s not from Hawaii. I wish some rich dude would pay our paddlers to go full time into training so we can catch this amazing man - Jamie Mitchell.

More on these athletes next week. I seem to have lost track of time on a wave.

GQ, Dropping in 4 U!

 

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