Why do some waves barrel?

I may have gotten ahead of myself last week, with the last article focusing on how wind affects the shape of a barreling wave. Let’s take a step back and talk about what causes a wave to barrel in the first place.

Many surfers know from experience that reefs and sandbars cause waves to break steeper. The science behind this goes back to 1968, a time when going leashless was just called surfing and breaks weren’t in need of the TSA to be orderly. Cyril J. Galvin Jr. published groundbreaking research classifying waves into four categories: spilling, plunging, collapsing, and surging. For surfers, spilling waves are what we call mushy, while plunging waves are what we call hollow or barreling.

Galvin's laboratory study revealed that wave types could be predicted using a combination of wave height (H_o), period (T), and bottom slope (m), related by the equation H_o/(gT²m²). As this parameter increases, waves progress from surging to collapsing to plunging to spilling.

A plunging breaker (barreling wave) on the left, a spilling breaker (mush) on the right.

The math reveals why waves behave as they do. Since wave height sits in the numerator, larger waves increase the parameter value. With other variables constant, extremely large waves tend to crumble – think Nazaré's giants. Both period and slope appear squared in the denominator, meaning their effects are amplified. A doubling of bottom slope reduces the parameter to ¼ of its original value. Similarly, waves with very short periods, like wind ripples, struggle to barrel.

Closeouts are barrels too?

This understanding gives us a recipe for steep walls:

Step 1: The slope has to be steep enough that the waves don’t crumble.

Step 2: The wave period has to be long enough to not crumble on the slope, but not so long that it surges like the tides.

Step 3: The wave height has to be big enough to not surge, but not so big that it crumbles.

Step 4: Combine and enjoy.

However, Galvin's 1960s research doesn't tell the complete story. Recent events challenge this paradigm – like Alessandro "Alo" Slebir's ~108ft barreling wave at Mavericks. While the above principles apply to gradually sloping sandy bottoms, reef breaks with sharp drop offs create different dynamics. Places like Teahupo'o produce caverns because the abrupt transition to shallow water forces the wave to pitch forward. Smaller scale examples can be found all over, and are easy to spot by the boils that the upward movement of water produces.

So, for a beach break that barrels like crazy - Ocean Beach SF, Blacks, etc. - the right combination of wave height, wave period, and bottom slope generate barreling waves. But for the reefy spots in the world, sometimes all you need is some swell and a well placed rock or coral head to catch some shade.

Further Reading:

The Paper (not open access unfortunately)

Record Mavericks Wave