Beneath the surface of surf

At Pescadero Beach, a plunging breaker crashes against the rocks.

Californians are notorious for making waves. From flaunting alternative lifestyles on Castro Street to enthroning stars of the silver screen in Sacramento, we love to soak the unsuspecting body politic in the spray of our cultural cannonballs.

Maybe we’re taking the cue from the ultimate wave maker, a certain large body of water to our west. Year-round, along 840 miles of coastline, we get to see and smell the swell and scent of the mighty Pacific Ocean; get to hear waves, like breath, exhale onto the shore and inhale back to the dark lungs of the deep. And once in a while, tide and wind conspire to create that paragon of fluid physics: the plunging breaker. 

But what exactly is going on there? Why does a wave plunge? What physical forces produce the aesthetic event? And as a bonus puzzler: why do waves always roll in parallel to the shoreline?

To understand the inner workings of a wave, hold your breath; we’re taking it underwater. A wave isn’t primarily the movement of the surface of the water but the collective motion of water beneath the surface. If you’ve ever thrown a small piece of driftwood – or uncooperative Frisbee – onto a gentle incoming wave, you’ve seen this principle in action. The floating object rides up the wave crest’s leading edge and down its trailing edge with a slight forward, then backward motion. For its part, the wave rambles on toward the shore, leaving the object to bob atop the next incoming wave. In short, the wave moves toward the shore while the water on the surface moves up and down.

Boogie boarders test their mettle at Stinson Beach.

So what’s going on beneath the surface? Circular motion, that’s what. Once a wave gets organized out at sea, it resembles a long cylinder, like a roll of carpet in a warehouse. (The wave is actually a series of many rolls stacked on top of each other, decreasing in size the deeper they go.) A surfboarder knows this better than anyone. A breaking wave’s topmost roll forms the large tunnel along which the lucky surfer skims. When we say that waves “roll in” we’re speaking more than figuratively.

At a certain point in its tumble toward the shore, the deep water wave runs up against the ocean bottom, which causes two things to happen. First, as the wave brushes the bottom it’s pushed upward and its crest steepens. This causes the water at the crest to speed up. When the speed of the crest outruns the speed of the overall wave that supports it, the crest collapses as a plunging breaker. Imagine a slapstick comedian leaning nonchalantly on a cane. Someone sneaks up behind him and kicks away the cane, making him crash to the ground.

Parallel waves march in at sunset along the California coast.

The ocean bottom is also the culprit in our second cause of a wave’s collapse. When the water through which the wave spins becomes too shallow to allow the wave to complete a full rotation (that is, to get filled in with supporting water), the cane again gets kicked away and the wave falls on its face. But what a fall. If you’ve ever seen that translucent curved curtain strike a rocky cliff at the optimal gathering of energy, you’ll never forget it.

Finally, what explains the tendency of waves to approach the shoreline in parallel formation, like a well-disciplined marching band? Well, consider the marching band. That long, rolling cylinder zeroes in on the shoreline at, say, a 45-degree angle. One side of the cylinder’s length will feel the bottom first. Friction slows down that side, while the side out in deep water spirals along at its original clip. Like a marching band executing a wheel, the faster deep-water side rotates around the hinge of the slower shallow-water side till voilà! The wave marches home perfectly perpendicular to the shoreline.

Here’s wishing you a memorable escape at the coast, where breakers plunge and waves wheel – and your knowledge of the inner workings of these marvels makes them more dramatic.