teahupoo anatomy of a monster

18 May 2009 16:05:00 AEST
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In our continuing series of analysing exactly what makes the greatest waves in the world what they are, we turn our high school science experiment minds towards Stop 5 of the ASP Tour, French Polynesia and the might of Teahupoo.

What makes Teahupoo the grinding eye of doom that it is? Why doesent your local reef break ever look like that?....why don't you paddle out one day to see jetskis and SUP riders going over the falls sideways while boatloads of rubbernecks laugh and hoot? Um...yeah...

Well, like people, it all helps to grow up in a good neighbourhood.

This graphic demonstrates the effect that the bathymetry of the complex seamount system to the south of the island of Tahiti has on long period swells. Some of the classic Teahupoo events of the past have been host to swell events with periods of 16 seconds or more. Waves of such period have an effective wavelength of over 256m. These swells begin to interact with the larger seamounts severla hundres miles before they reach the reef pass at Teahupoo, the effect is a 'funneling' of swell energy to the SE and SW of the main break. The seamounts immediately to the south of the break are substantially deeper than the surrounding formations and permit the energy to pass over them unhindered. So, even before the swell has begun to approach the vicinity of Teahuppo, it has been tuned and supercharged to deliver as much impact as possible.

The island of Tahiti is of pure volcanic origin, essentially the landmass consists of two substantial dormant volcanos. The south eastern mount immediately to the north of Teahupoo is called Tahiti Lti (small Tahiti).

Tahiti's ancient past was spotted with periods of upheaval that saw the landmass much higher than it presently is. Nutrient rich waters led very quickly to the establishment of significant coral reef systems, as these reefs sunk deeper underwater, future generations capitalised upon them to create almost an artifical continental shelf. The unique mature of upheaval and settling has blessed the reef pass at Teahupo with a very special reef formation that proceeds down into the deep oceanic waters to the south at a very steep angle, in fact the angle by sheer fluke of nature happens to correspond perfectly to the maximum incline a wave is able to follow before its lower portion slows substantially in relation to the building crest. At this point, the wave itself becomes unstable and...this happens...

Dr Blowfeld: Teahupoo; 2007

The unique shape, the effect of almost breaking below sea level is due to the specific shape of the reef head beneath the wave. Like a fingerprint, no two reefs in the world are unique. Many are similar which is why a lot of reef waves may look the same but to the trained eye there are distinguishing features that seperate them. The shape of the reef at Teahupoo is so unique that few can mistake a quality breaking wave here, quite simply there is nothing like it.

The semi circular nature of the leading edge of the reef drops down sharply to a second stage around 10 metres in depth. This upper profile of the reef creates the 'below water' effect of the breaking wave. The extreme angle creating instant instability in the wave. The second stage of the reef proceeds uniformly down to the 300 metres contour in about 50 metres of distance, or a ration of about 1/6 (.1667). The maximum steepness a wave is able to endure before it breaks is .17. So when height (h) is > .17 of wavelenght (λ) then the wave will break.

The reef at Teahupoo moves the entire available energy mass of the wave all the way from 300m to the 10m mark of the first stage of the reef at the maximum angle permissable prior to a wave breaking. Then at 10m prior to reaching the surface it puts up a steep wall of reef that causes the entire mass to fold onto a scalloped semi circle breaking arc. The fact that we are yet to find another Teahupoo proves that this combination of location, reef formation and swell window are a very rare find indeed.

Okay now for some wipeouts and associated carnage.

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