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 wave, as some are suitable for swimming and some can be very dangerous.
This wave occurs when the top of the wave tumbles down the face of the wave. These waves are good for swimmers and board riders.
A spilling wave at Piha beach.
Dumping wave (dumper)
This wave breaks with tremendous force and can easily throw a swimmer to the bottom. It usually occurs where the sea floor inclines steeply causing the wave height to increase quickly and dump sharply at the shore.
A dumping wave engulfs a surf boat.
This is a dumper that breaks on a steep beach face. Serious injuries can result when these waves throw swimmers violently onto the sand.
A dumping wave breaking as a shorebreak.
This wave may never actually break. This is because the water is very deep beneath the wave. These waves are common around rocks and can knock people off their feet and carry them back into deep water.
A surging wave between rocks.