It happens when the wavelength of the wave and the size of the gap or barrier are similar.A wave is a disturbance that moves along a medium from one end to the other. Instead, they bend – they curve outwards after passing through a gap and spread around an object. 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. 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.īigger, more open areas of water like the North Canterbury Bight and the South Taranaki Bight can also amplify tsunami waves through resonance, but with longer periods (for example, 2.5 hours in the North Canterbury Bight).

Lyttelton Harbour and Mercury Bay (Coromandel) are both prone to ‘sloshing’. Resonance can push the water level really high, making the effect of the tsunami greater.ĭifferent bodies of water have different resonant frequencies – it depends on their size and shape. This is called resonance, and it happens when the frequency of the tsunami wave is similar to the natural oscillating frequency (resonant frequency) of the body of water in the bay. Resonance: when waves ‘slosh’ to and froĪ tsunami wave coming into a bay can cause the water in the whole bay to ‘slosh’ backwards and forwards. Surfers can tell from the interference pattern which wave will be the biggest (and the best to surf!), for example, every 7th wave. Within the groups, interference means that the wave height will vary. When two sets of swells with similar frequencies interact, they interfere with each other and form groups. Interference is important for surfers because it affects the size of surf waves. 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). When two waves travelling in different directions meet, they combine their energies and form interference patterns. Interference: when waves affect each other 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. 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.Ī tsunami wave can reflect off continental shelves, ocean ridges and large reefs under the sea. However, reflected waves tend to interfere with the oncoming waves, and these patterns can be studied. 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. Tsunamis also refract around land masses. This affects the direction that the tsunami travels through the ocean. 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. 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 the reason why surf waves often line up parallel to the beach. This is why the wavefront changes direction. 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). In shallower water near the coast, waves slow down because of the force exerted on them by the seabed. Refraction is the change in direction of a wave as it slows down. Refraction: when waves slow down and change direction Together, these behaviours direct the course and effects of waves around New Zealand’s coast. However, as they approach the complex coastline of New Zealand, they can refract, diffract, be reflected and interfere with one another.

Out in the deep ocean, tsunamis and wind-generated waves settle to quite steady predictable wave patterns. These behaviours of waves can help us understand how water waves interact with land. Because they refract in shallower water, water waves (including tsunamis) tend to ‘track around’ complex coastlines. Refraction of waves as they enter the shallower water of the West Santa Cruz Island harbour (California).

Soundwave and water water background