Making Waves: Predicting Your Next Great Day

“Where do waves come from, Master? Why are there forty foot waves at Jaws when they’re just head high at Kenaha. Does swell period have anything to do with pregnancy or sanitary napkins? Why are there sets? How come we surf monks can’t get chicks.”

“Ah, grasshopper. These are all hopeless grommet questions, but then you are a hopeless grommet. Wax my board and polish my paddle and I will enlighten you..”

Wind makes waves. First ripples form, then the ripples give the wind something to push against and it shoves the ripples into chop, then waves, then big humping life-sucking victory-at-sea mackers if everything goes well.

Waves look like the water traveling across the surface, but the wave is just the energy of the wind, transferred by friction into energy in the water. The water mostly only moves up and down vertically, it’s the energy that travels.

The harder the wind blows, the longer it blows, and the distance the wind stays in contact with the water (fetch) all determine how much energy is transferred. As the waves grow larger the distance between the waves grows larger. This distance is called the wavelength and it’s directly proportional to the wave’s period, which is the time between wave peaks. As the wave travels it loses energy, so the more energy that gets stuffed into the waves when they are being formed, the further they can travel before they hold you down on a reef and break your board.

Long period waves (longer than 14 seconds) have much more energy than short period waves.

Most of the waves we care about come from storms. In the middle of a storm there is a confused mix of sea state. Various waves of different heights, directions and swell periods turn the ocean surface into a chaotic mess. All of these waves are the result of different cycles of the storm, with the short-period waves generated constantly and the longer period waves generated by winds earlier in the storm’s life that have had a longer time to develop.

As the waves move out of the storm area, they decrease greatly in size within the first thousand miles (more than 60 percent) and slowly thereafter. The short-period waves and chop dissipate rapidly once outside of the wind-generation area. All the waves get smaller as a result of losing energy from directional spreading of waves as they move away from the storm at different angles. The wave clusters separate as they travel forward at different speeds after leaving the storm area. These three factors allow the underlying long-period waves to move out from beneath the messy sea in the middle of the storm. Once these longer period waves break free from the storm’s confusion, they are easily identified as a more organized wave train, which we call swell.

Looking at the energy with a classical view of physics and wave energy we would expect the waves to dissipate rapidly since energy radiating from a point source diminishes inversely as the square of the distance, but this isn’t so with a wave traveling in water, for a number of reasons. First the storm is not a point source–it’s more like a line source until you get a long way away from it, and second, the waves have numerous energy-conserving factors operating, like traveling in trains, and the amplification of prevailing winds. At any rate, Northern swell trains routinely travel from the Aleutian Islands all the way to Peahi where they turn waverunners back into loose assemblages of parts.

Long-period swells sustain more energy because the face of the swell is not as steep, so it’s less affected by moving through the water–most of the energy is below the ocean surface. All swells travel as a group of waves or a “wave train” but long period waves benefit the most from the energy conserving nature of the train. The wave at the front of the train is slowed by friction and drops back to the rear of the group while the other waves move forward by one position. The benefit is similar to drafting by bicycle racers enabling wave trains to conserve their energy. The trains tend to separate somewhat and travel in bands, that also rotate the lead as the leading bands lose energy “breaking trail” for the bands behind them. This is why swell tends to stay fairly close together but still forms distinct sets.

Individual waves in open water travel at high speeds. As they start to make contact with the ocean floor they slow and get higher. Long-period waves move faster than short-period waves, so the really long period waves are the first to arrive. Very long period waves (18 seconds and up) often don’t build large peaks even though they contain a lot of energy–the energy is distributed through a long, long swell that doesn’t build into a face unless the ocean bottom has special features that enable it to. The wave train’s ridable surf generally comes in the 15- to 17-second range. Swell period steadily drops as the slower, shorter period swell arrives.

Swell period determines the depth that the waves begin to be affected by the ocean floor. A 20-second swell is affected by the ocean floor at about 1000 feet deep, while a six-second wave feels the bottom at about 100 feet deep. Since long period swells feel the bottom much further from shore, they can be turned by friction (actually refraction, but friction is the operating mechanism) with the bottom and wrap into a break while short period waves hardly wrap at all because the feel the bottom too late.

When waves feel the bottom the friction slows the bottom of the wave, focusing the energy upwards on the center of the swell, increasing the height of the wave. Long period waves have much more energy under the water so they will grow much more than short period waves. A four foot wave with a ten second swell may grow only a foot, while a three foot wave with a twenty second swell often grows to a fifteen foot face.

As waves reach shallow water, the energy is pushed upwards more until the waves become unstable and break at a water depth of about 1.3 times the wave height. A 6-foot wave breaks in 8 feet of water, a 20-foot wave in about 26 feet of water. If the ocean floor slopes gradually (like Peahi) the energy is dissipated over more of the wave so the top crumbles. When a wave hits a steep reef face (like Teahupoo) the energy is focused upwards sharply so the faces pitches outward faster as a hollower breaking wave.

Ocean waves refract just like light does. If one part of the wave is dragging over shallow water while another part stays deep, the wave in the shallow section slows down while the energy in the deeper part bends towards the shallow. This is why waves at point breaks curve down the point for a wide range of original wave directions. Deep-water canyons that run perpendicular to the shore can focus a great deal of energy into a section of the wave causing it to grow into a huge breaking wave as it nears shore. This is why you see giant waves like Peahi in one spot while a few hundred yards away the waves are much smaller. This only happens to long swell period waves, because the short period waves don’t feel the effects of refraction until it’s too late.

Swell, just like wind, is always described by the direction it’s coming from. When people use ordinal direction (degrees) instead of cardinal (north, south, etc), north is at zero degrees, east is 90 degrees, south is 180 degrees and west is 270 degrees. Swell height is not the same as wave height except in Hawaii (and officially not even there anymore). When someone says three foot waves in California they are referring to the breaking wave height–about waist high, whereas three feet in Hawaii might be waist to double overhead high depending on the particular location and the period of the swell.

“So Grasshopper, when surfers like you look at swell height and direction trying to predict the wave conditions at a break, you are looking at only part of the story. The ultimate surf-able wave size depends on the period and how the swell is affected by wave focusing elements of the ocean bottom in the vicinity of the break.

Grasshopper…Grasshopper…wake up you useless grom!”