How waves break: depth, steepness and the three kinds of break

6 min read

Out at sea a wave is a swing of energy, and the water barely moves forward. When the wave reaches water shallower than about half its wavelength, the bottom starts to drag on it. This is called shoaling, and three things follow:

• The wave slows down.
• The crests squeeze closer together.
• The same energy is packed into less water, so the wave grows taller and steeper.

Drawing: The wave grows as the bottom rises and breaks when its height is about three quarters of the water depth.

A wave breaks when the front of it is too steep to hold itself up. In practice that is when the wave height reaches about 0.8 times the water depth. A 1 m wave breaks in a little over a metre of water, a 3 m wave in about 4 m. This is why big waves break far from the beach, and why the same swell breaks in a different place at every tide.

Three ways to break

How quickly the bottom rises decides the shape of the break.

Drawing: Gentle bottom: spilling. Moderate: plunging. Steep: surging.

• Spilling. A gentle slope. The crest tumbles down the front in a long, slow roll of foam. Soft, forgiving waves, the ones to learn on.
• Plunging. A moderate or sudden rise. The lip throws forward in a curl and falls into the trough: the hollow wave, the barrel. Fast and powerful, and where the surfing gets serious.
• Surging. A very steep bottom, like a steep beach. The wave rushes up the slope without really breaking. Often a shorebreak that dumps right at the water's edge.

Wind changes the shape

Wind blowing off the land (offshore) pushes back against the face. The wave stands up steeper and holds its shape longer, so it breaks later and with more of a curl. Wind from the sea (onshore) pushes the top forward so the wave spills early and turns mushy. See Wind and tide.

Peeling: why some waves go left or right

A wave does not break all along its length at once. It breaks first where the bottom is shallowest, and the breaking part then travels along the crest in one direction. That line is the peel. A wave that peels slowly gives a long ride. One that breaks everywhere at once is a closeout: nowhere to go, fun only for the bravest.

• A peak that splits both ways gives a left and a right, common on beach breaks.
• A bottom that runs at an angle to the crest gives a long one-direction ride, as at a point break.

In the app: On the satellite pictures the white foam shows where the waves are breaking right now. Compare it with the spot's tide and you can see how the same sandbank behaves at different depths.

Open in the app →