13.14: The Making of Surf

Interesting things happen when waves enter shallow water. When a wave begins to feel bottom, it begins to interact with the seafloor. Friction causes the wave to move slower. The geometry of wave orbitals becomes flattened as their vertical motion is hindered. With nowhere to go but up, the energy of the wave is directed upward. As a result, the wave gains height. Viewed from the shore or a pier, you can see the swell get bigger as it approaches the shore.

Eventually, the water particles in a wave can’t even complete an orbit, and they simply move back and forth. If you’ve ever dived beneath the waves or sat on the bottom as a scuba diver, you can attest to the strength of the back-and-forth motion of waves in shallow water. At some point in the wave’s progress into shallow water, the crest topples forward—sometimes with fury—and the wave breaks. This usually happens at a point when the wave height becomes too steep.

Oceanographers define the ratio between a wave’s height (H) and its wavelength (L) as wave steepness, which we’ll symbolize as Z (e.g., Bascom 1980):

Z = H / L

(Eq. 19.7)

In general, waves break when their height exceeds their wavelength by about 1/7 (0.142; Michell 1883; but see Perlin et al. 2013). A 7-foot wave will break when its height reaches 1 foot. Generally that happens when the water depth becomes less than about 1.3 times the wave height (e.g. Butt 2014). So a 3-foot wave will break when the water depth reaches 3.9 feet.

Types of Breaking Waves

Breaking waves are generally called surf or breakers. And as everyone who watches waves knows, no two breakers are alike. This is partly due to the characteristics of the waves themselves, but the slope of the seafloor also makes a difference. A gently sloping nearshore will slow down the wave gradually, and its energy will be expended along a greater distance in its progress toward the shore. A steeply sloping nearshore will permit a wave to approach a beach at near top speed. The differences between the waves breaking on a gradual versus a steep beach slope can be dramatic.

In general surf may be classified into one of four categories depending on the characteristics of its break. A wave whose lip trickles down the front of the wave as it approaches is called a spilling breaker. The top of the crest of the wave spills down the front of the wave. This type of wave generally occurs on a gently sloped bottom. Most of the small waves you’ll see in Florida are of this type. A wave that creates a tube with a lip that shoots over the face of the wave in a kind of waterfall is called a plunging breaker. This is the kind of wave most sought after by surfers. The waves at Pipeline in Hawaii and Teahupo’o in Tahiti are great examples of plunging breakers. These locations produce the much-coveted barrels. Plunging breakers can be found on shallow- to intermediate-sloped seafloors. More steeply sloped seafloors produce collapsing breakers, where the entire wave face disintegrates into foam. The steepest nearshore bottoms produce waves that don’t even break; they slide up onto the beach in a swoosh. Swooshy waves are called surging breakers (e.g., Butt 2014).

The speed and shape of the incoming wave, the dynamics of the nearshore that change the slope and depth of the seafloor, tides, and winds, among other factors, ultimately determine the way the wave breaks. Many waves are a combination of these types. For example, in Southern California, we often see breakers that spill and plunge. And really, when you’re out enjoying the waves, it doesn’t matter what you call them. They’re simply fun!

Wave–Seafloor Interactions

Often waves approach the beach at an angle, meaning the wave front doesn’t arrive parallel to the shoreline. This can occur because waves generated far out to sea come from different directions and because the direction a beach faces depends on where it’s located. Most beaches in Orange County face southwest. (The Huntington Beach pier points to about 220°, per Google Earth.) Further up the coast in the South Bay, Hermosa and Manhattan Beaches face almost due west. When waves approach at an angle, one part of the wave front may enter shallower water earlier than the rest. The part entering shallow water slows down, but the part in deeper water continues at full speed. The result is a bending of the wave, something known as wave refraction.

As a wave approaches a beach and refracts, the wave front will bend and align with the depth contours of the bottom—the isobaths. You can actually figure out which parts of the nearshore are deeper and which are shallower simply by watching the approaching waves. As waves approach the shore at an angle, the wave front bends and becomes more parallel to the beach, but rarely becomes perfectly parallel. If you watch carefully, you will see that one part of the wave begins to break before the other parts. The places where the wave breaks first are the shallowest parts of the nearshore. If the depth of the nearshore has a constant slope (no spots deeper or shallower than others), the wave will break right to left or left to right, depending on whether it’s coming from the right or the left of the direction the beach is facing, respectively.

Headlands—points of land that jut out into the ocean—are a great place to observe wave refraction. A wave approaching a headland straight on will slow down in the shallower water surrounding the headland, but because the extremities of the wave front are traveling faster, the wave will refract. The result is a wave front that bends toward the headland. If the wave approaches the headland at an angle, it will bend around the headland and produce something known as a point break. Point breaks produce some of the best surfing in Southern California. Dana Point once had the best point break in the world, until someone decided to build a jetty. (I hate it when that happens.) Similarly, a wave entering a bay will slow down at its extremities so that the wave bends and takes on the shape of the bay as it nears the shore.

Between islands or inside boat harbors, boat owners have to worry about something called wave diffraction, the lateral shifting of wave energy along the wave front. It gets complicated because it depends on the wavelength of the approaching wave relative to the width of the opening, and constructive and destructive interference can play a role, but wave diffraction will cause a wave front to arc and expand as it enters the opening. This can result in waves coming from an unexpected direction in what is supposed to be a safe harbor. Between islands the effects can be dramatic as wave patterns become fanlike and superimpose on each other.

When waves approach an underwater reef or seamount, wave refraction may cause the energy of the wave to converge on the reef or seamount. This concentration of energy is known as wave focusing (e.g., Butt 2014). Extreme surf spots such as Jaws, Mavericks, Cortes Bank, and Shark Park produce epic waves as a result of wave focusing around an underwater obstacle. Ranging in size from 50 to 100 feet (15–30 m; e.g., Smith 2006), these are waves to die for—literally, as most of us would be dead if caught in a wave of this magnitude. Next time you’re standing next to an eight-story building, look up. The wall next to you is about the same height as a wave you might experience at one of these places. Except that in an extreme wave, the wall is moving 20 to 30 mph and falling down on top of you. Yikes!

One Southern California extreme wave spot—featured in Bruce Brown’s classic film The Endless Summer (1966)—serves as a perfect example of wave reflection—the change in direction of a wave front when it encounters an obstacle such as an island, beach, seawall, or jetty. If swell period and swell direction are ideal, the Wedge, at the south end of Newport Beach on the Balboa Peninsula, can produce 20-foot (6 m) waves (e.g., Surfline 2023). Waves at the Wedge result from the interaction of incoming waves with waves reflected off the Newport Inlet jetty. When waves approach from the SSW, they bounce off the jetty, which extends from the shore about 1,800 feet (549 m), and reflect 90° (NNW), where they interfere with the incoming waves. Where the crests of the incoming and reflected waves meet (constructive interference), a towering “wedge” of water is produced, hence the name. Next to Hawaii’s Banzai Pipeline, the Wedge rates as one of the best places to watch extreme wave surfing. On a summer day with a large SSW swell, hundreds of onlookers will gather at the Wedge to watch bodyboarders, bodysurfers, and even board surfers take their chances on the waves.

All of the science aside, ocean waves captivate our attention, whether we’re at the beach or simply watching a surf cam from home. Waves reflect the moods of the ocean—perhaps our own, as well. As the fictional teen “surfer girl” Gidget puts it, “Surfing is out of this world. . . . It positively surpasses every living emotion I’ve ever had” (Wendkos 1959).