Summer Series #4: The science of the surf
Nothing says summer holiday quite like the rhythmical sigh of waves breaking gently on a sundrenched shore.
But waves are so much more than a soothing accompaniment to our afternoon nap: they’re the outcome of a fascinating combination of physical processes, they tell us a lot about the nature of the wind and the seabed, and they play a crucial role in shaping our coastal landscapes over the short and long term.
“Waves begin life when there is some sort of disturbance to the ocean surface,” explains NIWA coastal oceanographer Dr Scott Stephens. “Usually that disturbance is caused by the wind.
“Friction between the air and the sea surface transfers some of the wind’s energy into the water, and the water particles then carry that energy along until it is released onto a beach or rocky shore, which could be thousands of kilometres away.
“The size of a wave depends on the amount of energy that the wind transfers to the water surface; this is controlled by the strength, duration and distance – or ‘fetch’ – that the wind blows.”
In the deep ocean, the passing waves cause water particles to move in circles. For example, if you were lying in the water looking over the surface at a floating bottle, you would see the bottle move in a circle, on a vertical plane. The diameter of that circle would equal the wave height.
“The wave action is largest at the water surface but the circular motion decreases with depth, until eventually there is no particle movement at all.”
Oceanographers refer to two distinct types of ocean wave.
‘Sea waves’ are generated when the wind continues to blow in the area where the waves have been generated, and is still transferring energy to the waves. Sea waves are characterised by different shapes and sizes, short wavelengths and movement in different directions. They create what is commonly referred to as a ‘messy’ or ‘choppy’ sea.
‘Swell’ occurs in water some distance away from where the wind is blowing, or after the wind has stopped.
“Once generated, the waves move away from their source,” says Dr Stephens. “The largest waves move the fastest, and as they move they separate out into ‘clean’ waves of a single wavelength, moving in the same direction. This is what we refer to as ‘swell’, and these are the waves that surfers love.”
Coast shapes wave, wave shapes coast
Interesting things happen when a wave gets close to the shore.
“In deep water the speed of a wave is controlled by its wavelength,” Dr Stephens explains, “but in shallow water its speed becomes controlled by the water depth. The seabed effectively trips the wave up so the front of the wave slows down and the back catches up. The wave gets higher and steeper as a result, until it eventually collapses and the wave breaks. The steeper the seabed, the faster the wave will rear up and break.”
The nature of the coast can affect the wave in a number of other ways too.
“Sometimes a wave will ‘bend’ as it moves past a headland or into an enclosed cove. One or both ends of the wave get slowed down by the shallower water, causing the middle part, in the deeper water, to reach the shore first.
“Coastal features like headlands can also shelter a shoreline from waves, or focus waves so that they become much larger. Parts of the Portuguese coast are famous for focusing waves into absolute monsters, tempting thrill-seeking surfers.”
At the same time, waves are constantly shaping and reshaping our coastlines.
“When a wave meets the shore, the energy originally imparted by the wind is finally released,” explains Dr Stephens. “This causes currents to run on shore, move sideways, and return – supplying the energy to move sand – and also forming rip-currents that can trap unwary swimmers.”
“Steep, high-energy waves tend to erode away the sand, moving it offshore, while smaller, longer-period swells push the sand back onshore.
“It’s a constant cycle of ‘cut and fill’ as storms generate the energy to erode, then quieter periods of weather allow the sand to rebuild. That’s why our beaches are constantly changing.”
The angle at which waves reach the shore also determines how sand is transported. Waves arriving at an angle to the beach will move the sand laterally. In some enclosed coves, sand is continually being rotated from one end of the beach to the other as the angle of the incoming waves changes.
When waves reach a rocky shore, the processes of erosion and deposition are considerably more complex.
“As well as the energy of the waves, factors like the strength, structure and chemical make-up of the rocks determine how – and how quickly – the shore erodes,” says Dr Stephens. “Generally, wave-driven changes to a rocky coastline will take place over a much longer time period than on sandy shores.”
Wave shapes wave
Lines, or ‘trains’, of waves moving in different directions will often interfere with each other.
When the trough of one train meets the crest of another, the overall effect is to cancel the wave out. However, when two crests meet, a much higher wave may be created.
Mariners through the ages have recounted harrowing tales of ‘rogue’ waves: giants much larger than any others occurring around them at the time. They were, more than likely, encountering a wave enhanced by the interference of two or more wave crests.
As you rest on your beachside hammock, however, you’re unlikely to notice much more than a slight increase in the volume of breakers as a wave enhanced in this way makes landfall!
- On New Year’s Day 1995, a monitoring platform off the coast of Norway recorded a wave measuring a whopping 25.2m from trough to crest – officially the highest ever recorded.
- All waves have the same measurable characteristics. The highest part of a wave is the crest; the lowest part is the trough. The vertical distance between the wave crest and trough is the wave height. The amplitude of the wave is half of the wave height. The distance from a certain point on one crest or trough to the same point on the next crest or trough is the wavelength. The period is the amount of time it takes for succeeding crests to pass a specified point.
- The best surfing wave is a clean swell arriving at a beach when there is no wind to cause interfering sea waves. The shape of the seabed and angle of wave are important too. If the wave aligns with the slope of the seabed it creates the ideal surfing break. Often this scenario is created only when the tide is at a certain point.
- Storm surge is a single wave that comes in and out over about 12 to 48 hours, causing extra high sea levels that can be disastrous. Storm surge is created by the combination of abnormally low atmospheric pressure that sucks up the seas surface (scientists refer to the ‘inverse barometer effect’) and strong winds that force water to pile up against the shore.
- NIWA’s supercomputer in Wellington runs sophisticated numerical models that use global wave and weather data to forecast ocean levels and wave patterns that will affect New Zealand several days ahead.
- NIWA also operates a free service called Cam-Era, which is a series of webcams feeding live pictures from some of New Zealand’s most popular beaches. You can check out the wave conditions yourself without leaving home!
Dr Scott Stephens has been working at NIWA for 14 years as a coastal oceanographer and numerical modeller, undertaking environmental research and consulting. His projects include wave analysis and modelling, predicting and mapping extreme sea-level and waves, coastal morphological studies, and hydrodynamic and particle-tracking modelling.