What is a wave?
A wave is a body of water (a swell) moving along the surface of the ocean. Wind and storms at sea form pressure differences on the ocean surface and contribute to the creation of swells. These undulations travel thousands of miles and gather together to form waves. When the swell reaches the shore, these waves break as surf.
How swells form
The size of swells is determined by three factors: how hard the wind blows, the length of time it blows and the distance it blows. Generally speaking, if any of these factors increase, larger waves will be produced.
As swells begin to move out and away from where they were created, their crests (tops) become more rounded and take on a similar period (the time between the peak of one wave and the next) and height. As the swell approaches land and the sea bottom gets shallower, the waves become higher and narrower, and the distance between each wave becomes shorter. The wave continues to increase in height until it collapses and topples over as surf.
Types of waves
It is important to be able to recognise different types of waves, as some are suitable for swimming and some can be very dangerous.
A spilling wave occurs where the sea floor gradually gets shallower, and the wave crest tumbles down the face of the wave. They typically release their energy over a larger area, creating multiple lines of breaking surf. Spilling waves are typically not as powerful as plunging waves. These waves are safer for swimmers and learner board riders.
Plunging waves break where there is a sudden change in water depth. This can be on a steeper beach, sandbar, or reef. The steep slope causes the wave height to increase quickly, then the crest plunges forwards and downwards. These waves can be powerful and hazardous to water users, who may be injured if thrown onto the sea floor.
A plunging wave that breaks right on the beach is called a shorebreak. Shorebreaks can be particularly hazardous to children, people with limited mobility, and those entering and exiting the water.
Surging waves occur in areas where there is deep water very close to shore. This can be on the steepest beaches and around rocks. Surging waves do not increase substantially in height as they approach the shore, but can cause a powerful uprush and backwash that can knock people off their feet and sweep them into the water.