An A-frame is an extreme version of a peak, or wave that is biggest in the middle and tapers off either side. As its name suggests, it forms the shape of a letter ‘A’.
Dunes, South Africa. One of the world's most consistent A-frames. Photo Ant Fox
It differs from other types of peak due to the rather unique way in which it is formed. Instead of a peak being formed because a shallow sandbar or reef focuses the swell onto that area and makes the wave break, it is formed because two swell lines approaching at slightly different angles interfere with each other.
A-frames can be distinguished from sandbar-generated peaks by the trained eye: they tend to form closer to the shore and, if you look carefully, you can see the waves coming in at varying angles before they break. The waves are usually shorter and more powerful than sandbar-generated peaks, often with a very steep take-off followed by a quickly-tapering wall.
The most common way to produce A-frames is with an offshore area of shallow water (a shoal), in the form of a reef or large semi-permanent sandbar. This is basically how it works:
The incoming waves hit the offshore shoal, but don’t break there because the water is too deep. Instead, they are refracted around it. Refraction is where one part of the wave slows down as it propagates over shallow water while the rest of the wave carries on at its original deep-water speed. The result is a bending of the wave towards the area of shallow water.
Then, as the waves move away from the shoal towards the beach, they do so at different angles. The angle is very sensitive to the original angle with which the wave hit the seaward part of the shoal, plus other characteristics of the wave itself, such as its wavelength and speed.
At some point, inside the shoal, two waves coming in at slightly different angles meet. When they do so they interact in a very simple way. If two crests or two troughs coincide, they come together to make a higher crest or a lower trough (constructive interference). However, if a trough and a crest coincide, they cancel each other out (destructive interference). At points where the waves interfere constructively, the height of this wave is suddenly quite large compared with the water depth, so the wave breaks. Since it only breaks in one spot, it does so in the form of an A-shaped peak.
The swell meets the offshore shoal but doesn't break. The broken swell now heads towards the beach at two different angles. When they meet, the A-frame peak is formed.
Perhaps the most famous beach where A-frames are found is Ocean Beach, San Francisco. Most normal beachbreaks reach a limit above which the waves close out for various reasons. But here, the waves almost never close out, even at 20 feet, thanks to a large offshore sandbar formed at the entrance to the nearby estuary.
Arial view of the refraction at Dunes, South Africa. Dunes works almost exclusively because of an offshore snadbar called 'The Mound' and works best on west swells.
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