Behaviour of waves

All waves behave in certain characteristic ways. They can undergo refraction¹, reflection², interference³ and diffraction⁴. These basic properties define the behaviour of a wave – anything that reflects, refracts, diffracts and interferes is labelled a wave.

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Waves undergoing refraction

Refraction of waves as they enter the shallower water of the West Santa Cruz Island harbour (California). Because they refract in shallower water, water waves (including tsunamis) tend to ‘track around’ complex coastlines.

These behaviours of waves can help us understand how water waves interact with land. Out in the deep ocean, tsunamis⁵ and wind-generated waves settle to quite steady predictable wave patterns. However, as they approach the complex coastline of New Zealand, they can refract, diffract, be reflected and interfere with one another. Together, these behaviours direct the course and effects of waves around New Zealand’s coast.

Refraction: when waves slow down and change direction

Refraction is the change in direction of a wave as it slows down. In shallower water near the coast, waves slow down because of the force⁶ exerted on them by the seabed. If a wave is approaching the coast at an angle, the nearshore part of the wave slows more than the offshore part of the wave (because it’s in shallower water). This is why the wavefront changes direction.

Refraction is the reason why surf waves often line up parallel to the beach. Even if waves are coming in from deep water at an angle to the beach, the move to shallower water means that the waves will slow down and curve around (refract) so they are more parallel as the surf hits the beach.

Refraction is very important for tsunamis because (unlike other waves) they interact with the seabed even in deep water – so they are always undergoing refraction. This affects the direction that the tsunami⁷ travels through the ocean. Tsunamis also refract around land masses.

Reflection: when waves bounce back

Reflection of water waves at a coast is usually not an important part of their behaviour, unless the coast has a steep cliff or a seawall. However, reflected waves tend to interfere with the oncoming waves, and these patterns can be studied.

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Waves bouncing off cliffs

Reflection of water waves off cliffs at a coast is usually not an important part of coastal wave behaviour, unless the coast has a steep cliff or seawall.

A tsunami wave can reflect off continental shelves, ocean ridges and large reefs under the sea. Reflected tsunami waves off an ocean ridge to the west of Sri Lanka and south-west of India contributed to the damage on the western side of Sri Lanka during the 2004 Boxing Day tsunami.

Interference: when waves affect each other

When two waves travelling in different directions meet, they combine their energies and form interference patterns. This can result in regions of very high waves when they add up (constructive interference) alternating with regions of diminished or no waves when they cancel out (destructive interference).

Interference is important for surfers because it affects the size of surf waves. When two sets of swells with similar frequencies interact, they interfere with each other and form groups. Within the groups, interference means that the wave height will vary. Surfers can tell from the interference pattern which wave will be the biggest (and the best to surf!), for example, every 7th wave.

Resonance: when waves ‘slosh’ to and fro

A tsunami wave coming into a bay can cause the water in the whole bay to ‘slosh’ backwards and forwards. This is called resonance⁸, and it happens when the frequency⁹ of the tsunami wave is similar to the natural oscillating frequency (resonant frequency) of the body of water in the bay. Resonance can push the water level really high, making the effect of the tsunami greater.

Different bodies of water have different resonant frequencies – it depends on their size and shape. Lyttelton Harbour and Mercury¹⁰ Bay (Coromandel) are both prone to ‘sloshing’. Mercury Bay has a resonant frequency of 1 hour – once it gets disturbed by a tsunami, it will slosh backwards and forwards every hour, sometimes for several days, before it dissipates.

Bigger, more open areas of water like the North Canterbury Bight and the South Taranaki Bight can also amplify¹¹ tsunami waves through resonance, but with longer periods (for example, 2.5 hours in the North Canterbury Bight).

Diffraction: when waves bend

When waves get to a barrier such as an offshore rock or a small gap such as the opening to a harbour, they don’t go straight past the barrier or carry on straight after going through the gap. Instead, they bend – they curve outwards after passing through a gap and spread around an object. This is diffraction. It happens when the wavelength¹² of the wave and the size of the gap or barrier are similar.

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Diffraction and interference

Waves that pass through small gaps (such as between the island and the mainland) diffract – they bend outwards. This can also lead to constructive and destructive interference patterns on the far side of the island.

Diffraction is interesting to wave researchers because it means that wave energy reaches ‘shadow zones’ where you wouldn’t otherwise expect energy to be. It’s an important part of wave behaviour at ports, harbours, built structures and offshore islands. Diffraction is more important in shallow water than it is in deep water.